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ADS/CPDLC CONCEPTS

TABLE OF CONTENTS

CHAPTERNO. TOPIC Page No.

1 Requirements of ADS & CPDLC System 2

2 Automatic Dependent Serveillance (ADS) 10

3 Controller Workstation Overview 43

4 VHF Datalink & CPDLC 52

5 Communication Service Provider 83

Annexure

i. Explanation of Terms 106

ii. Glossary 108

iii.ADS Message Data Glossary 112

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Requirements of ADS & CPDLC System

1.1 DATA LINK APPLICATIONS

1.1.1 DATA LINK BASED AIR TRAFFIC SERVICES SYSTEMS

One of the overall objectives is to harmonize the different air traffic control systems among

the regions, irrespective of the communications, navigation and surveillance systems in use.

Data link communications can support direct controller-pilot communication, the passing of

automatic dependent surveillance data, the implementation of a request/reply data link flight

information service to the aircraft, and exchanges between aircraft and ATC systems. This will

overcome the shortcomings of the current systems by providing for global communications,

navigation and surveillance coverage from (very) low to (very) high altitudes, for digital data

interchange between the air-ground systems to fully exploit the automation capabilities of

both, and for the development of a fully integrated CNS end-system which will operate in a

normalized manner throughout the world.

The data link applications based system will improve the handling and transfer of information

between operators, aircraft and ATS units. The system will provide extended surveillance

capabilities by using ADS and advanced ground-based data processing and display systems tothe controller, thus allowing advantage to be taken of the improved navigation accuracy in

four dimensions and accommodating the preferred flight profile in all phases of flight, based

on the operator's objectives.

In oceanic areas and remote land airspaces with limited ground-based air navigation facilities,

surveillance of air traffic is envisioned to be provided by ADS position reporting through

satellite communications. Surveillance of low-altitude traffic operations, including helicopters,

will be conducted in a similar manner. In continental airspaces, surveillance of air traffic may

be achieved by ADS reports integrated with ground-based radar systems. CPDLC and the

interchange of ATS messages will be carried out by satellite, SSR Mode S, VHF, high frequency(HF) or other suitable data link(s) available.

In order to ensure that higher priority messages, including time critical messages, will be

transmitted before lower priority messages, a message priority capability will be included in

the data link system.

1.2 SYSTEM COMPONENTS

There are six major components which combine to form an integrated data link based ATC

system. Implementation of data link must allow incorporation of system enhancements to be

made without any disruption to operations. The six main components of a data link based ATS

are:

a) pilot interface;

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b) aircraft (including airborne automation);

c) air-ground and ground-ground data link communications;

d) communication interface;

e) ATC automation; and

f) controller interface.

1.2.1 Pilot interface

The pilot interface to the data link system must be efficient an easy to operate. Pilot-controller

messages require some rapid entry mechanism. Use of data link for pilot-controller

communications will result in changes to cockpit procedures, since messages currently

transmitted by voice will require system input by the pilot, and receipt of a message willrequire reading text. Procedures and systems should be developed to minimize system input

errors.

1.2.2 Aircraft equipment

Data link applications must be supported by aircraft equipment which is able to gather the

data from pilot interface, appropriate sensors and flight management computers, format the

data and direct it to the appropriate air-ground data link within the appropriate time scales.

This on-board equipment should also have the capability of receiving messages originated by

the controlling and other authorized ATS units. Avionics should make maximum use of data

link equipment already in place in the aircraft.

1.2.3 Air-ground and ground-ground data link communications

The required air-ground data link will be ATN compatible for most applications and could be

satellite data link, VHF digital data link, Mode S data link, or any other medium which meets

the operational requirements. The ATC and aircraft systems will select the most suitable path

based on time-varying considerations such as geographical location, cost, delay, throughput

and link availability. For example, in oceanic airspace, satellite data links will most likely be

used, while in domestic airspace VHF or Mode S could be used.

The resulting communications links will appear seamless from the user's perspective (i.e.

independent of the communications systems in use).

Voice communication will be available to complement data link system operation.

1.2.3 Ground communication interface

The air-ground data link will be connected to the ATC system through a terrestrial

communications network. The network will conform to the protocol suite defined as part of the

ATN concept. For messages from controller to pilot, the ground ATN routers must choose the

most suitable data link device available and route the message to that transmitting station.

1.2.4 ATC automation

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The ground system must be capable of supporting position reporting and communications

procedures with minimal controller input. Conformance monitoring, confliction avoidance,

automatic transfer of control, controller alerting, and many other functions concerned with

safe and efficient ATS management will result from the incorporation of advanced levels of

automation that will take advantage of the data link applications' functionality. CPDLC will

require some level of message processing that should be included in the ATC automation

component.

Error detection and correction, and, where appropriate, alerting mechanisms should be

implemented.

In addition, the ATC system will allow for safe recovery from response delays, non-response,

system failures, system management errors, or other errors which impede operation, such as

unauthorized access and unauthorized transmission. Systems will be capable of delivering

messages associated with error notification and recovery within the time required for safe

recovery.

Use of data link will not impose undue competition for display or control resources. Systemswill not preclude access to other functions or unduly conflict with higher priority functions.

1.2.5 Controller interface

The controller interface will contain the required tools for the composition of air-ground data

link messages. ATS providers will define and develop specific controller interfaces tailored to

their particular needs. The human-machine interface will be left to the individual service

provider. The controller interface should be efficient, easy to operate and provide a rapid

message input mechanism. The interface should also provide a means to display air-ground

messages.

1.3 DATA LINK INITIATION CAPABILITY (DLIC)

1.3.1 DLIC OVERVIEW

The DLIC provides the necessary information to enable data link communications between

ATC ground and aircraft systems. It is an aircraft-initiated application. The DLIC encompasses

the following functions:

a) logon: data link application initiation and, if required, flight plan association,

b) update: updating of previously coordinated initiation information,

c) contact: instructions to perform data link initiation with another specified ground system,

d) dissemination: local dissemination of information, and

e) ground forwarding: ground-ground forwarding of logon information.

The ADS Panel has developed specific operational requirements for the establishment of data

link communications between an aircraft and ground systems. These requirements, and the

method of operation, are outlined below.

1.3.2 DLIC HIGH-LEVEL OPERATIONAL REQUIREMENTS

i. The ground system must be able to identify an aircraft's data link capabilities from thefiled flight plan.

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ii. Data link ground units need advance notification of aircraft equipage in order to assignappropriate ADS contracts. Prior to the aircraft entering ADS airspace, the relevant ATCunit's ground system database will be updated to reflect the aircraft equipage fromdata included in the received flight plan.

iii. The pilot will include details about data link capabilities in the flight plan.iv. Procedures must be in place to allow timely establishment of data link between aircraft

and the ground system.

v. Before entering airspace where the data link applications are provided by the ATCautomation system, a data link connection will need to be established between theaircraft and the ground system, in order to register the aircraft and allow the start of adata link dialogue when necessary. This will be initiated from the aircraft, eitherautomatically or by pilot intervention.

vi. At a time parameter before a data link equipped aircraft enters data link airspace, thepilot or the aircraft will need to initiate the DLIC logon procedure. The aircraft will thengenerate and transmit the logon request message which contains the aircraft-uniqueidentifier and the data link applications it can support. The ground system responds tothe aircraft's logon request.

vii. The ground system should be able to correlate the aircraft-unique identifier with theaircraft identification stored in its database.

viii. During the initial establishment of a data link connection with a ground system thatground system must be able to register the data link capabilities supported by theaircraft.

ix. The ground system will identify the communications and surveillance capabilities of aircraft in order to establish appropriate ADS contracts.

x. The ground system initially contacted by the aircraft should be able to pass thenecessary aircraft address information to another ground station via ground-groundcommunications links.

1.4 AUTOMATIC DEPENDENT SURVEILLANCE (ADS)

1.4.1 ADS APPLICATION OVERVIEW

The implementation of ADS, through reliable data link communications and accurate aircraft

navigation systems, will provide surveillance services in oceanic airspace and other areas

where non-radar air traffic control services are currently provided. The implementation of ADS

will also provide benefits in en-route continental, terminal areas and on the airport surface.

The automatic transmission of the aircraft position through ADS will replace present pilot

position reports. In non-radar airspace, the effective use of ADS in air traffic services will

facilitate the reduction of separation minima, enhance flight safety and better accommodate

user-preferred profiles. The ADS application and associated communications will have to besupported by advanced airborne and ground facilities and data link communications with

proven end-to-end integrity, reliability and availability. It is recognized that safety aspects of

radio navigation and other safety services require special measures to ensure their freedom

from harmful interference; it is necessary therefore to take this factor into account in the

assignment and use of frequencies.

In addition, there is the emergency mode, a special periodic reporting mode of operation

initiated by the pilot (or exceptionally, the aircraft system) specifically tailored to providing

the essential position and information data at a specific reporting rate.

The ADS application allows the implementation of reporting agreements, which, with theexception of an aircraft in an emergency situation, are established exclusively by the ground.

An ADS agreement is an ADS reporting plan which establishes the conditions of ADS data

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reporting (i.e. data required by the ATC system and the frequency of the ADS reports which

have to be agreed upon prior to the provision of the ADS services). The terms of an ADS

agreement will allow for information to be exchanged between the ground system and the

aircraft by means of a contract, or a series of contracts. An ADS contract specifies under what

conditions an ADS report would be initiated, and what data groups will be included in the

reports. There are three types of contract "demand", which provides a single report,

"periodic", which provides a report at a regular periodic interval determined by the groundsystem, and "event", which provides a report when or if a specified event or events take

place.

ADS contracts necessary for the control of the aircraft will be established with each aircraft by

the relevant ground system, at least for the portions of the aircraft flight over which that

ground system provides ATS. The contract may include the provision of basic ADS reports at a

periodic interval defined by the ground system with, optionally, one or more additional data

blocks containing specific information, which mayor may not be sent with each periodic

report. The agreement may also provide for ADS reports at geographically-defined points such

as way points and intermediate points, in addition to other specific event-driven reports.

The aircraft must be capable of supporting contracts with at least four ATS Units (ATSU)

ground systems simultaneously.

An ADS application can only be provided by an ATSU having appropriate automation and

communication facilities. The ADS application should be supported by direct two-way

controller-pilot data link and voice communications.

Implementation of ADS will overcome limitations found today in procedural ATC systems

based on pilot-reported position reports. The introduction of air-ground data links through

which the ADS reports and associated messages will be transmitted, together with accurate

and reliable aircraft navigation systems, presents the opportunity to improve surveillance of

aircraft in those airspaces. It offers the potential for increasing flight safety and airspace

utilization by reducing ATC errors in air-ground communications and by providing ATC with

accurate aircraft position information. The exchange of ATS messages by digital data link will

alleviate the overloading of ATC radio frequencies and support ATC automation, as well as the

implementation of other ATS data link applications.

The processing of automated position reports will result in improved automatic monitoring of

aircraft operations. Automatic flight plan data validation will facilitate the early detection by

ATC of on-board system flight and route data insertion errors. Conflict prediction and

resolution capabilities will be enhanced. The display of the traffic situation as derived fromADS reports and the automated processing of ATS safety messages will significantly improve

the ability of the controller to respond to pilot requests and to resolve traffic situations.

With a combination of improved ATC automation, reliable communications and accurate

navigation and surveillance, it will be possible to increase the level of tactical control and to

reduce separation minima on the basis of controller intervention capability and other ATM

improvements, thereby leading to possible increases in airspace capacity.

As with current surveillance systems, the benefit of ADS for ATC purposes requires supporting

complementary two-way controller-pilot data and voice communication (voice for at least

emergency and non routine communication). Where VHF coverage exists, the communicationrequirement is envisaged to be met by VHF voice. In areas where HF communications are

currently used (e.g. oceanic airspace), the provision of an ADS service during the en-route

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phase of flight will be supported by the routine use of Controller Pilot Data Link

Communication (CPDLC).

1.4.2 ADS HIGH-LEVEL OPERATIONAL REQUIREMENTS

1.4.2.1 The ground system will be able to identify the ADS capability of the aircraft and

allocate the appropriate ADS contracts.

1.4.2.1.1 Based on the current flight plan information obtained from the aircraft, the ADS

capability of the aircraft and ATM requirements, an appropriate ADS contract will be identified

by the ground system. The necessary contract requests will be transmitted to the aircraft for

acceptance.

1.4.2.1.2 ADS reports will be made available to facilities other than the controlling ATC

unit on the basis of mutual agreement and local procedures.

1.4.2.1.3 At a parameter time or distance prior to the ATS airspace boundary, the ground

system will generate and allocate an appropriate ADS contract for the aircraft, based on the

current flight plan information obtained from the aircraft and the ATM requirements in effect.

1.4.2.1.4 The ground system will transmit the relevant ADS contracts to the aircraft. The

aircraft will confirm acceptance of the ADS contract to the ground system.

1.4.2.2 The aircraft must be able to provide automatic position reporting in accordance

with ADS contracts allocated by the ground system.

1.4.2.1.1 The aircraft with ADS capability will generate and transmit ADS reports to the

appropriate ground system in accordance with the ADS contracts in force.

1.4.2.2.2 The controller will be capable of replacing the ADS contract as required by thecircumstances. The ground system will generate appropriate messages to the aircraft to

initiate such modifications to existing ADS contracts.

1.4.2.3 The aircraft must be capable of identifying any changes to position

determination capability and of notifying the ground system accordingly.

1.4.2.3.1 Based on parameters established in the ADS contract, the aircraft will

automatically report to the ground system when the aircraft's navigation capability (figure of

merit) has changed.

1.4.2.4 Both the aircraft and the ground system must be capable of providing anemergency mode of ADS operation to support ATC alerting procedures and to assist search

and rescue operations.

1.4.2.4.1 The system should provide for a pilot-initiated emergency. The pilot will use

simple action to initiate an emergency mode. It would also be permissible for aircraft to

automatically establish the emergency mode. The aircraft system will alert the pilot to an

auto-establishment of the emergency mode.

1.4.2.4.2 The aircraft system will generate and transmit the basic ADS report at a pre-set

initial reporting rate together with the state of emergency and/or urgency. This pre-set

reporting period will be the lesser of 50 per cent of the existing periodic contract reportingperiod, or 1 minute. However, the emergency reporting period will not be less than 1 second.

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A single default value of 1 minute may be used in initial implementations. Aircraft

identification and ground vector group will be included in every fifth report.

1.4.2.4.3 The ground system will recognize the emergency mode and alert the controller.

The ground system will be able to modify the emergency reporting rate if necessary.

1.4.2.4.4 When an emergency mode is declared, any existing periodic contract between

the ground system and that aircraft should be modified to a default emergency periodcontract. While there is an emergency mode in effect, any request for a normal periodic

contract should be deferred. An emergency mode should not affect an event contract. The

periodic contract in effect when emergency mode ends should be reinstated.

1.4.2.4.5 The pilot will have the ability to cancel the emergency mode.

1.4.2.5 The controller must be provided with the most up-to-date traffic situation

available using ADS-derived information.

1.4.2.5.1 In an ADS environment, the controller must be provided with the most up-to-date

ADS-derived information to permit the provision of effective air traffic control. The groundsystem will process the ADS position information sent by ADS-equipped aircraft. The ground

system will generate warnings (and alternative clearances, where conflict resolution

algorithms are incorporated) to the controller when it identifies a potential conflict.

1.4.2.6 The ADS application will have to allow for the comparison of the four-dimensional

profile stored in the aircraft system with flight data stored in the ground system.

1.4.2.6.1 Many operational errors today in non-radar airspace are due to waypoint

insertion errors in aircraft flight management systems. To minimize the possibility of such

blunders and to permit advanced strategic planning in a data link based ATS, the ground

system will verify that the aircraft's planned four-dimensional profile is the same as the profile

that ATC is expecting the aircraft to follow.

1.4.2.7 The aircraft must permit self-monitoring and automatic reporting of significant

flight variances, when called for by an appropriate event contract.

1.4.2.7.1 The ground system will determine the flight conformance criteria applicable to

the airspace and phase of flight. The ground system will include within the ADS contract the

values that trigger these reports.

1.4.2.7.2 The aircraft will recognize when one of the reporting criteria is satisfied or

exceeded. The aircraft will generate and transmit an appropriate ADS report for the specificflight variance. The ground system will generate an alert to the controller if any parameter is

exceeded. If a variance parameter is exceeded, the report will comprise an indication of which

parameter has triggered the report, the basic plus the air or ground vector block as

appropriate, based on the current ADS contract.

1.4.2.8 The ground system will have the ability to monitor the flight of the aircraft before

it enters the airspace under its control.

1.4.2.8.1 As a consequence of the ADS contracts accepted by the aircraft, the aircraft will

begin to send ADS reports to the appropriate ground system to initiate flight-following for

planning purposes. The ground system will use ADS information to update its database to

ensure entry conditions into the airspace remain acceptable.

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1.4.2.8.2 The position information of the aircraft will be made available to the controller.

1.4.2.9 The ground system must be capable of recognizing that the aircraft has entered

the airspace over which it has controlling authority.

1.4.2.9.1 In non-radar airspace, especially when transiting from an uncontrolled airspace

to airspace where ADS applications are available, the ground system of the controlling ATC

unit must recognize that the flight has entered its airspace. A set of data as specified by theADS contract will then be sent by the aircraft to the ground system.

1.4.2.10 The ground system must be able to confirm that the aircraft's projected profile

coincides with that stored in the ground system.

1.4.2.10.1 Whenever the ground system receives an aircraft's projected profile information,

the ground system will check and verify that it is consistent with that already held. The

ground system will generate and display an appropriate alert to the controller if any value of

the specified parameters delta(latitude), delta(longitude), delta(level) or delta(time) are

exceeded.

1.4.2.11 The ground system must be able to verify that the aircraft is proceeding in

accordance with the ATC clearance.

1.4.2.11.1 In the data link based ATS, the ground system will use the ADS position reports

and other ADS message group data to provide automated flight-following and conformance-

monitoring.

1.4.2.11.2 The aircraft will generate and transmit ADS data to the appropriate ground

system according to the current ADS contract. The ground system will compare the aircraft's

ADS-reported position with the position predicted by the ground system. The ground system

will generate and display appropriate messages to the controller if the ADS position report

does not conform, within the given parameters, to the position predicted by the ground

system.

1.5 CONTROLLER-PILOT DATA LINK COMMUNICATIONS

1.5.1 CPDLC APPLICATION OVERVIEW

One of the keys to the future air traffic management system lies with the two-way exchange

of data, both between aircraft and the ATC system and between ATC systems. CPDLC is a

means of communication between controller and pilot, using data link for ATC

communications.

ICAO has developed a communication systems architecture that provides a range of

capabilities to suit the needs of ATS providers and their users. Various air-ground

communication data links will be integrated through ATN based on an open system

interconnection (OSI) architecture. Eventually, the ATN will allow worldwide connectivity and

an established quality of service which will provide optimum routing and delivery.

During the transition towards the ICAO CNS/ ATM systems, the number of data link

applications which require a globally uniform approach and standardization will increase.

The CPDLC application provides the ATS facility with data link communications services.Sending a message by CPDLC consists of selecting the addressee, selecting and completing, if

necessary, the appropriate message from a displayed menu or by other means which allow

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fast and efficient message selection, and executing the transmission. The messages defined

herein include clearances, expected clearances, requests, reports and related ATC

information. A "free-text" capability is also provided to exchange information not conforming

to defined formats. Receiving the message will normally take place by display and/or printing

of the message.

CPDLC will remedy a number of shortcomings of voice communication, such as voice channel

congestion, misunderstanding due to bad voice quality and/or misinterpretation, and

corruption of the signal due to simultaneous transmissions.

In the future, it is expected that communications with aircraft will increasingly be by means of

digital data link. This will allow more direct and efficient linkages between ground and cockpit

systems. At the same time, extensive data exchange between ATC systems will allow efficient

and timely dissemination of relevant aircraft data, and will cater for more efficient

coordination and hand-over of flights between ATC units. In turn, this will reduce controller

and pilot workload and will allow an increase in capacity.

Implementation of CPDLC will significantly change the way pilots and controllerscommunicate. The effect of CPDLC on operations should be carefully studied before deciding

the extent to which voice will be replaced by data link.

Among others, the following aspects of CPDLC are to be taken into account in considering its

application and in defining procedures:

a) the total time required for selecting a message, transmission of the message, and reading

and interpretation of the message;

b) the head-down time for the pilot and controller;

c) the inability of the pilot to listen to other transmissions in the same area of operation;

d) unauthorized access; and

e) unauthorized transmissions.

1.5.2 CPDLC HIGH-LEVEL OPERATIONAL REQUIREMENTS

1.5.2.1 A data link based ATS system must provide for the reduction of routine

communication tasks which contribute to the saturation of voice frequencies.

1.5.2.2 The ADS Panel has identified specific operational requirements relating to thecapabilities of the CPDLC application. These are outlined below:

1.5.2.2.1 The system must be capable of providing CPDLC when this application is

required by the ATM system in force.

1.5.2.2.2 When required, the data link ATS will support the exchange of data link

messages between the pilot and controller to support the effective provision of the data link

based ATS service.

1.5.2.2.3 The pilot or controller may initiate a data link message using either the defined

message set, a free-text message, or a combination of both. The ground system will make themessage available to the appropriate controller, or the aircraft system will make the message

available to the pilot.

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Automatic Dependent Serveillance (ADS)

2.0 PROBLEM STATEMENT

Whilst Primary and Secondary Surveillance Radar have been the core systems providing ATM

Surveillance Services for over 30 years, the continuous growth in air traffic has led to a need

to enhance these surveillance systems to help support increased airspace capacity. Moreover,

it has long been recognised that there are parts of airspace where rotating SSR systems are

not feasible or are too costly. An emerging technology that may resolve the above issues is

Automatic Dependent Surveillance (ADS): ADS is a surveillance technique in which an aircraft

transmits onboard data from avionics systems to ground-based and/or airborne receivers. The

data may include: aircraft identity, position, altitude, velocity, and intent.

There are two forms of ADS:

i. ADS Contract (ADS-C) - also known as Addressed ADS (ADS-A) andii. ADS Broadcast (ADS-B).

ADS-C comprises air-to-ground transfer of data. ADS-B comprises air-to-ground and air-to-air

(i.e. transmitted from one aircraft and received by another) data transfer. ADS is seen as

being a key element in the surveillance infrastructure.

Essentially, ADS can be defined by its constituent parts. ADS is:

Automatic in that an aircraft reports its own ship’s position to a suitably equippedground or airborne participant according to some defined communication standards,

Dependent in that the position is derived from data sources onboard the aircraft, and Surveillance in that the purpose of ADS messages is to report the identification and

location of the aircraft to others.

ADS is made possible by two necessary elements:

i. reliable data communications and

ii. accurate position location.

Air/ground or air/air data link provides the former, and global navigation satellites, particularly

GPS, provide the latter. Without these, ADS would not be feasible.

Furthermore, air/ground data link must provide coverage in the airspace of interest. The

potential for air/ground, ground/air, and air/air data links has given rise to a number of data

link operational concepts. Some of these concepts fall within the definition of ADS while

others do not. In order to differentiate between ADS system concepts discussed and other

data link concepts not covered within the scope of this effort, the following discussion is

presented.

Results & Benefits

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ADS will improve the availability of real-time flight data from aircraft, support airport

surveillance, provide surveillance services in areas with none or only limited radar coverage

and contribute to the compatibility of air and ground systems. In addition, it enables new

airborne and ground ATM functions with the potential to bring extra capacity and increased

safety.

Note: Hereafter, wherever ADS-Contract (ADS-C) is discussed it is mentioned as

ADS and for ADS-Broadcast it is mentioned as ADS-B.

ADS - C

2.1 INTRODUCTION

ADS is a surveillance technique for use by air traffic services in which aircraft automatically

provide, via a data link, data derived from on-board position-fixing and navigation systems.

ADS will allow controllers to obtain position data and other information from ADS-equipped

aircraft in a timely manner in accordance with their requirements, and will allow the aircraft to

be tracked in non-radar airspace.

The primary objective of the ADS application is to provide automated aircraft position data for

ATC. The ADS application may also be useful in air traffic flow management (ATFM) and

airspace management (ASM).

ATM benefits from the use of the ADS application may include separation minima reduction,

and more efficient use of airspace.

Although the application of ADS does not specifically encompass ATC communications,

automation or procedures, all of these elements must be tailored to support the ADS

application and to make meaningful use of the data. Thus, it is critical to consider the ATC

automation and communications systems as the foundation upon which an ADS-based ATC

system is built. The implementation of ADS into air traffic systems will be an evolving process.

There will be a gradual transition from procedurally oriented strategic air traffic control

towards a more tactical control environment.

Figure - 2.1ADS System Concept

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The ADS application and associated communications will have to be supported by advanced

airborne and ground facilities and data link communications with proven end-to-end integrity,

reliability and availability. ADS is one of the applications supported by the ATN. Figure-2.1

depicts a general overview of several components of an ADS system.

2.2 USE OF ADS IN ATS

The implementation of ADS, through reliable data link communications and accurate aircraftnavigation systems, will provide surveillance services in oceanic airspace and other areas

where non-radar air traffic control services are currently provided. The implementation of ADS

will also provide benefits in en-route continental, terminal areas and on the airport surface.

The automatic transmission of the aircraft position through ADS will replace present pilot

position reports. The content and frequency of reporting will be determined by the controlling

ATC unit. In non-radar airspace, the effective use of ADS in air traffic services will facilitate the

reduction of separation minima, enhance flight safety and better accommodate user-

preferred profiles.

2.2.1 Use of ADS outside of radar coverage

In oceanic and other areas which are beyond the coverage of land-based radar, ADS reports

will be used by ATS to improve position determination, resulting in improvements in safety,

efficient utilization of airspace and improved controller efficiency. This is expected to increase

airspace capacity and allow more economical routing and spacing of aircraft.

Further, the introduction of ADS in non-radar airspace will better enable controllers to identify

potential losses of separation or non-conformance with the flight plan and to take the

appropriate action.

2.2.2 ADS transition airspace

In transition airspace where other means of surveillance become available, provisions are

required to integrate ADS and other surveillance information. In AAI the integration of ADS

targets in Radar display is in progress.

2.2.3 Within radar coverage

ADS will be beneficial in areas where it may serve as a supplement to or for back-up for radar.

2.3 FUNCTIONAL DESCRIPTION

ADS information can assist ATC in performing the following functions:

1) Position monitoring. The ground system processes the incoming ADS information to verify

its validity and to compare the information with that held for the aircraft.

2) Conformance monitoring. The ADS reported position is compared to the expected aircraft

position, which is based on the current flight plan. Longitudinal variations which exceed a pre-

defined tolerance limit will be used to adjust expected arrival times at subsequent fixes.

Horizontal and vertical deviations which exceed a pre-defined tolerance limit will permit an

out-of-conformance alert to be issued to the controller.

3) Conflict detection. The ADS data can be used by the ground system automation to identifyviolation of separation minima.

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4) Conflict prediction. The ADS position data can be used by the automation system to

identify potential violations of separation minima.

5) Conflict resolution. ADS reports may be used by the automation system to develop possible

solutions to potential conflicts when they are detected.

6) Clearance validation. Data contained in ADS reports are compared with the current

clearance and discrepancies are identified.

7) Tracking. The tracking function is intended to extrapolate the current position of the

aircraft based on ADS reports.

8) Wind estimation. ADS reports containing wind data may be used to update wind forecasts

and hence expected arrival times at waypoints.

9) Flight management. ADS reports may assist automation in generating optimum conflict-

free clearances to support possible fuel-saving techniques, such as cruise climbs requested by

the operators.

2.4 ADS FUNCTIONAL CAPABILITIES

The ADS application is designed to give automatic reports from an aircraft to a ground

system. The aircraft provides the information to the ground system in four ways:

1) on demand;2) when triggered by an event;3) on a periodic basis; and4) in an emergency.

The system is capable of distinguishing each of the four ways listed above.

2.4.1 OPERATING METHOD

The ADS application comprises the following functions:

1) establishment and operation of a demand contract;2) establishment and operation of an event contract;3) establishment and operation of a periodic contract;4) cancellation of contract(s);5) establishment and operation of emergency mode;6) modification of the emergency mode; and7) cancellation of the emergency mode.

2.4.1.1 Establishment and operation of a Demand Contract

The demand contract provides the capability for a ground system to request a single ADS

report from an aircraft and specify which optional ADS data is required (if any) in addition to

the basic ADS report. Any number of demand contracts may be sequentially established with

an aircraft.

If the avionics can comply with the demand contract request, it sends the requested report. If

there are errors in the contract request, or if the avionics cannot comply with the request, itsends a negative acknowledgement indicating the reason for rejection. If the avionics can

partially comply with the contract request, it sends a message which includes:

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1) a non-compliance notification indicating those parts of the contract it cannot complywith;

2) the basic ADS report; and3) the information requested which can be supplied.

2.4.1.2 Establishment and operation of an Event Contract

The event contract allows the ground system to request the avionics to send ADS reports

when the specified events occur, principally for the purpose of conformance monitoring by

ATC. The event contract states the event types that are to trigger reports and also any

required threshold values delimiting the event types.

An ADS event report consists of a basic ADS report and any additional information required by

the triggering agent. Only one event contract may exist between a ground system and an

aircraft at anyone time, but this may contain multiple event types. Each time an event

contract is established it replaces any event contract already in place.

If the avionics can comply with the event contract request, it sends an ADS report with basicinformation, any additional required information if required by the event type, and a positive

acknowledgement. Should the contracted event occur, the required ADS report(s) is/are sent.

If there are errors in the event contract request, or if the avionics cannot comply with the

request, it sends a negative acknowledgement to the ground system indicating the reason for

its inability to accept the contract.

If the avionics can partially comply with the request, it sends a non-compliance notification

indicating those parts of the contract with which it cannot comply. Event reports are

subsequently sent only for those events with which the aircraft can comply.

Should an event for lateral deviation change, altitude range deviation, or vertical rate

change occur, a report is sent once every minute while the limit(s) specified in the contract

are exceeded. The reports will cease when the event parameters return within the specified

thresholds. However, they will resume as soon as the event parameters are exceeded again.

For all other events, a single report is sent every time the event occurs. If more than one of

the events described below occurs at the same time, the avionics sends separate ADS event

reports for each event.

The system will put a tolerance box round the aircraft, see Figure 2.2, and provided that the

aircraft reports indicate that the aircraft is adhering to the thresholds set by ATC, the

controller will be not be passed any information, unless he/she specifically requests it.

Event based contracts will be used as a means of spotting flight conformance errors, and at

that point the controller will be informed. One possible concern of the pilots is that the

controller may know of an aircraft deviation before the pilot does. This may enhance safety,

but not reputations.

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Figure 2.2 Flight Conformance

(Lateral, Longitudinal and Vertical Deviation Monitoring)

Event types

The following event types have been defined for ADS:

1) vertical rate change;2) way point change;3) lateral deviation change;4) level change;5) level range deviation;

6) airspeed change;7) ground speed change;8) heading change;9) extended projected profile change;10) FOM (Figure of Merit) field change; and11) track angle change.

Vertical rate change

The vertical rate change event can be triggered in two ways. For positive vertical rate, the

event is triggered when the aircraft's rate of climb is greater than the vertical rate threshold,

i.e. its rate of climb is greater than planned. For negative vertical rate, the event is triggeredwhen the aircraft's rate of descent is greater than the vertical rate threshold, i.e. its rate of

descent is greater than expected. The ADS vertical rate event report is sent once every

minute whenever the aircraft's rate of climb/descent exceeds the value of the vertical rate

change threshold. The avionics will cease sending ADS reports when the aircraft's rate of

climb/descent is less than or equal to the value of vertical rate change threshold. An ADS

report sent as a result of the occurrence of a vertical rate change event will contain the basic

ADS information and ground vector information. Figure-2.3 illustrates a vertical rate change

event.

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Figure – 2.3 Illustration of vertical rate change event

Waypoint change

Waypoint change event is triggered by a change in the next way point. This change is

normally due to routine way point sequencing. However, it will also be triggered by a change

in a waypoint which is not part of the ATC clearance but is entered by the pilot for operational

reasons.

The ADS report resulting from a waypoint change event is sent once each time the event

occurs. An ADS report sent as a result of the occurrence of a waypoint change event contains

the basic ADS information and the projected profile information. Figure -2.4 illustrates the

waypoint change event.

Figure- 2.4 Illustration of waypoint change event

Lateral deviation change

The lateral deviation change event is triggered when the absolute value of the lateral distancebetween the aircraft's actual position and the aircraft's expected position on the active flight

plan becomes greater than the lateral deviation threshold.

The ADS lateral deviation change report is sent once every minute while the aircraft's lateral

deviation is greater than the value of the lateral deviation threshold. The avionics will cease

sending ADS reports when the lateral deviation of the aircraft is less than or equal to the

value of lateral deviation change threshold. An ADS report sent as a result of the occurrence

of a lateral deviation change event contains basic ADS information and ground vector

information. Figure- 2.5 illustrates the lateral deviation change event.

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Figure-2.5 Illustration of lateral deviation change event

Level change

The level change event report is triggered when the aircraft's level differs negatively or

positively from its value in the previous ADS report by an amount exceeding the level change

threshold specified in the event contract request. If there has been no previous report, a basic

ADS report is sent.

The ADS report resulting from a level change event is sent once each time the event occurs.

An ADS report sent as a result of the occurrence of a level change event contains basic ADS

information and ground vector information. Figure- 2.6 illustrates a level change event.

Figure- 2.6 Illustration of level change event

Level range deviation

The level range deviation is triggered when the aircraft's level is higher than the level ceiling

or lower than the level floor. The ADS level range deviation event report is sent once every

minute when the aircraft's level is greater than the value of the level ceiling or less than the

value of the level floor. The avionics will cease sending ADS reports when its level is less than

or equal to the value of level ceiling and greater than or equal to the value of the level floor.

An ADS report sent as a result of the occurrence of a level range deviation event report

contains basic ADS information and ground vector information. Figure-2.7 illustrates a level

range deviation event.

Figure -2.7 Illustration of level range deviation event

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Airspeed change

The airspeed change event is triggered when the aircraft's airspeed differs negatively or

positively from its value at the time of the previous ADS report containing an air vector by an

amount exceeding the airspeed change threshold specified in the event contract request. If

there has been no previous report containing an air vector, a report is sent. The ADS report

resulting from an airspeed change event is sent once each time the event occurs. An ADS

report sent as a result of the occurrence of an airspeed change event contains basic ADS

information and air vector information.

Ground speed change

The ground speed change event is triggered when the aircraft's ground speed differs

negatively or positively from its value at the time of the previous ADS report containing a

ground vector by an amount exceeding the ground speed threshold specified in the event

contract request. If there has been no such previous report containing a ground vector, a

report is sent. The ADS report resulting from a ground speed change event is sent once each

time the event occurs. An ADS report sent as a result of the occurrence of a ground speedchange event contains basic ADS information and ground vector information.

Heading change

The heading change event is triggered when the aircraft's heading differs negatively or

positively from its value at the time of the previous ADS report containing an air vector by an

amount exceeding the heading change threshold specified in the event contract request. If

there has been no previous report containing an air vector, a report is sent. The ADS report

resulting from a heading change event is sent once each time the event occurs. An ADS

report sent as a result of the occurrence of a heading change event contains basic ADS

information and air vector information. Figure- 2.8 illustrates the heading change event.

Figure-2.8 Illustration of heading change event

Extended projected profile change

The extended projected profile change event report is triggered by a change to any of the setof future way points that define the active route of flight. The number of waypoints covered in

the contract is either defined by a specified time interval or by a selected number from the

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time of the request. The ADS report resulting from an extended projected profile change

event is sent once each time the event occurs. An ADS report sent as a result of the

occurrence of an extended projected profile change event contains basic ADS information and

extended projected profile information with the waypoints covered by either the specified

time interval or within the specified number of future waypoints.

Figure of Merit (FOM) field change

The FOM field change event is triggered by change in the navigational accuracy, navigational

system redundancy or in the airborne collision avoidance system (ACAS) availability. The ADS

report resulting from a FOM field change event is sent once each time the event occurs. An

ADS report sent as a result of the occurrence of a FOM field change event contains only basic

ADS information.

Track angle change

The track angle change event is triggered when the aircraft's track angle differs negatively or

positively from its value at the time of the previous ADS report containing a ground vector by

an amount exceeding the track angle change threshold specified in the event contract

request. If there has been no previous report containing a ground vector, a report is sent. The

ADS report resulting from a track angle change event is sent once each time the event

occurs. An ADS report sent as a result of the occurrence of a track angle change event

contains basic ADS information and ground vector information. Figure- 2.9 illustrates the track

angle change event.

Figure-2.9 Illustration of track angle change event

2.4.1.3 Establishment and operation of a Periodic Contract

The periodic contract provides the capability for a ground system to request periodic reports

from an aircraft. The ground specifies which optional ADS data is required (if any) in addition

to the basic ADS data. It also specifies the rate at which the basic ADS information is required

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and a modulus (multiple of the basic reporting rate) on the basic rate for each (if any) optional

data required.

Only one periodic contract may exist between a given ground system and a given aircraft at

anyone time. Each time a periodic contract is established, it replaces any periodic contract

already in place. If the avionics can comply with the periodic contract request it sends the

requested ADS reports. If there are errors in the periodic contract request, or if the avionics

cannot comply with the periodic contract request, it sends a negative acknowledgement to

the ground system indicating the reason for its inability to accept the contract. If the avionics

can partially comply with the request, it sends a non-compliance notification indicating which

parts of the periodic contract cannot be complied with. Periodic reports are subsequently sent

containing only the requested information that the avionics can supply. If the avionics cannot

meet the requested report rate, it will send periodic reports.

CANCELLATION OF CONTRACT(S) OPERATION

Cancellation of contracts allows the ground system to cancel a contract or all contracts

currently in operation. The ground system specifies which contracts will be cancelled. Theavionics acknowledges the cancellation and ceases sending the ADS reports for the cancelled

contract(s).

2.4.1.4 Establishment and operation of Emergency Mode

This function allows the avionics to initiate emergency mode, either on instruction from the

pilot or automatically. Emergency mode is entered between the aircraft and all ground

systems that currently have periodic or event contracts established with that aircraft.

Any existing periodic contract is suspended during operation of the emergency mode. Neither

an event nor a demand contract is affected. The emergency reporting rate on initiation of theemergency mode is the lesser of 1 minute or half of any existing periodic contract rate.

The position, time and FOM are sent with each ADS emergency mode report, and the aircraft

identification and ground vector sent with every fifth message.

MODIFYING AN EMERGENCY MODE

This capability allows the ground system to send an emergency mode modification messageto the avionics. The avionics modifies the reporting rate of the emergency mode, and then

sends the emergency reports at the new interval. This only affects the emergency mode

reports to the ground system making the request.

CANCELLATION OF EMERGENCY MODE

This function allows the pilot to cancel the emergency mode, or the ground system to cancel

the emergency mode indication.

When the pilot cancels emergency mode, the avionics sends a cancel emergency mode

message to each ground station receiving the emergency mode reports. If there was aperiodic contract in place before the emergency was declared, it is reinstated.

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When the ground system cancels the emergency mode indication, the avionics continues to

send ADS reports to the ground system as in emergency mode, but the reports are no longer

designated as emergency reports by the ground system.

Table 2.1 ADS functionality summary

2.5 FIGURE OF MERIT (FOM)ADS is a DEPENDENT system. This means that it depends, for its position reporting and

general navigation information, upon the equipment carried on board, whether this be a

Global Navigation Satellite System (GNSS), Inertial Navigation or Reference System (INS/IRS)

or a combination of terrestrially based short range navigation systems (VOR/DME). The

ground needs to know the truthfulness of this data being passed from an aircraft, in order to

be able to assess an area which is likely to contain the aircraft's most probable position.

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This implies that the aircraft must have a self-monitoring of navigation capability, and be able

to pass this inform to the ground system. The aircraft's self monitoring capability gives rise to

a Figure of Merit (FOM), and this figure is included in the basic position report sent by the

aircraft in every contracted response.

The aircraft will normally be navigating using the best system it has available, and it is

therefore unlikely that the FOM will be other than statistically derived. For example, if an

aircraft is navigating using a fully operational GNSS, the FOM will be consistent and reflect a

high navigational capability. Navigation using ground-based aids could well have a FOM

dependent on the area of operation, reflecting the coverage of the aids. Navigation based on

inertial reference systems alone inherently loses accuracy from the moment they are moved

from their stm1-up reference location. However, this rate of deterioration is to a large extent

time dependent and in most cases well known for particular equipment. Therefore a FOM for

such a system would be expected to show a time dependent change for the worse for the

duration of the flight and this can be reported to the ground.

This figure of merit is taken as an index of the aircraft’s navigational accuracy.

It should be noted that the figure -2.9 given is exemplary. This figure illustrates the case

where the GNSS fails for some reason. Although in the future GNSS will be approved as a

stand-alone system for aircraft in Remote and Oceanic areas, it is also likely that aircraft

operators will retain some form of IRS on board. The GNSS will have a relatively fixed FOM,

and reversion to an IRS, particularly if this occurs some way into a long flight, is liable to

result in a fairly abrupt change in the FOM.

Figure 2.9 Figure of Merit (FOM) change

2.5 ADS CONTRACT REQUIREMENTS –GROUND STATION OPERATIONS

The aircraft will be capable of supporting contracts with at least four ATS ground

systems simultaneously.

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The aircraft will be capable of supporting one demand, one event and one periodic

contract with each ground system simultaneously.

The aircraft will have to differentiate between ATS and non-ATS communications - it could be

possible for an Aircraft Operational Communications (company based) link to log on to an ADS

application. Procedures will therefore have to be established to ensure that only appropriate A

TC ground systems initiate ADS contracts with a given aircraft.

While at least four ADS contracts may be simultaneously in force, appropriate local

procedures will need to be in place to ensure that non-current contracts are dropped in

sufficient time to allow the receiving ground system to set up the controlling ADS contract.

Such procedures will also take care of the case where aircraft are crossing from airspace

where ADS service is provided into non-ADS airspace, to ensure closure of all ADS contracts,

and thus efficient use of resources

.

Figure 2.10

ADS ARCHITECTURE

As discussed in Chapter- 02 para 2.2 there are six major components which combine to form

an integrated data link based ATC system are-

a) pilot interface;

b) aircraft (including airborne automation);

c) air-ground and ground-ground data link communications;

d) communication interface;

e) ATC automation; and

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f) controller interface

Figure 2.11 Elements of ADS system

2.7 Aircraft Avionics The aircraft avionics consists of the Flight Management Computer (FMC), FANS -1/A

package, Aircraft Communications Addressing and Reporting System Management Unit

ACARS-MU, SATCOM and VHF systems. The datalink is provided Aircom Service Providers

(ASP) either SITA or ARINC (discussed later Chapter-09). The ground system is provided by

Air Traffic Service Units (ATSU) that is discussed later, Chapter-14 onwards. GlobalNavigation Satellite System (GNSS) is for aircraft navigation (discussed in Chapter-11), and

INMARSAT & VHF are used by the aircraft/ASP for datalink purpose.

Flight Management Computer (FMC)

• processes navigational information• monitors equipment malfunction• runs cockpit instrument• manages & processes all this information and presents it clearly to the pilot

FANS Package

• a modification to FMC• combines datalink, GPS navigation and ADS

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INMARSAT



• ARINC 745 processes the navigation data & also sends the automatic surveillancereports

• RTCA DO-219 this software runs the FMC datalink system (to encode and decode bit-oriented messages in accordance with a standard protocol) and holds the CPDLCpreset messages

• ARINC 622 converts the ADS and datalink data to the correct protocols for transmission ACARS MU

Aircraft Communications Addressing and Reporting System Management Unit

• switches between VHF and SATCOM• addresses all messages• ensures message format is correct

GPS interface

• converts all information from the GPS receiver, so that it can be used by the FMCGPS receiver

• receives GPS signals from navigational satellites

Figure 2.12 Aircraft avionics

2.8 ADS MESSAGE DESCRIPTION

2.8.1 Basic ADS information. Every ADS report contains the following information:

a) the 3-D position of the aircraft (latitude, longitude, and level);b) the time; andc) an indication of the accuracy of the position data information figure of merit.

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Aircraft

Flight

Management

Computer

FANS

GPS

interfac

e

GPS

receiver

SATCOM

avionics

ACAR

S MU

VHF

system

ARINC 745

ADS

function

DO 219CPDLCfunction

ARINC

622

protocol

function



2.8.2 Optional ADS information. In addition to the basic information included in each ADS

report, an ADS report may contain any (or all) of the following information:

a) aircraft identification;b) ground vector;c) air vector;d) projected profile;

e) meteorological information;f) short-term intent;g) intermediate intent; andh) extended projected profile.

2.8.3 The aircraft identification is contained in field 7 of the ICAO model flight plan.

2.8.4 The ADS ground vector is composed of the following information:

a) track;b) ground speed; and

c) rate of climb or descent.

2.8.5 The ADS air vector is composed of the following information:

a) heading;b) Mach or IAS; andc) rate of climb or descent.

2.8.6 The ADS projected profile is composed of the following information:

a) next waypoint;b) estimated level at next way point;c) estimated time at next waypoint;d) (next + 1) waypoint;e) estimated level at (next + I) waypoint; andf) estimated time at (next + I) waypoint.

2.8.7 The ADS meteorological information is composed of the following:

a) wind direction;

b) wind speed;c) temperature; andd) turbulence.

2.8.8 The ADS short-term intent is composed of the following information:

a) latitude at projected position;b) longitude at projected position;c) level at projected position; andd) time of projection.

2.8.9 If a level, track or speed change is predicted to occur between the aircraft's current

position and the projected position (indicated above), additional information to the short term

intent data would be provided as intermediate intent (repeated as necessary) as follows:

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a) distance from current point to change point;b) track from current point to change point;c) level at change point; andd) predicted time to change point.

2.8.10The ADS extended projected profile is composed of the following information:

a) next waypoint;b) estimated level at next waypoint;c) estimated time at next waypoint;d) (next + I) waypoint;e) estimated level at (next + I) waypoint;f) estimated time at (next + 1) waypoint;g) (next + 2) waypoint;h) estimated level at (next + 2) waypoint;i) estimated time at (next + 2) waypoint ...

j) ... [repeated for up to (next + 128) waypoints].

2.8.11A positive acknowledgement indicates acceptance of a requested contract and contains

no further information.

2.8.12A negative acknowledgement indicates rejection of the requested contract and may

contain information on the cause for rejection.

2.8.13A non-compliance notification contains an indication on which part of a requested

contract cannot be complied with.

2.8.14A demand contract message indicates the contract type and which of the optional ADS

information is to be included in the ADS report.

2.8.15A demand ADS response message contains the basic ADS data and the optional ADS

data required in the demand contract.

2.8.16An event contract message indicates the contract type, contains an indication of the

events to be reported on, together with thresholds (as required) for each event specified.

2.8.17An event contract response message contains an identification of the event type and

the required ADS data for the particular event.

2.8.18A periodic contract message indicates the contract type, the required report interval,

an indication of which of the optional ADS information is to be included in the periodic reports,

and the modulus from the basic interval for each optional field to be included.

2.8.19A periodic ADS response message contains the basic ADS data and the optional ADS

data required in the periodic contract.

2.8.20A cancel contract message contains an indication of the contract (i.e. periodic or event)

to be cancelled. A cancel contract message without a contract type parameter indicates that

all ADS contracts with the ground system are to be cancelled.

2.8.21An emergency mode message indicates the position, time and FOM. In addition to the

above, the aircraft identification and ground vector are sent with every fifth message.

2.8.22A modify emergency mode message contains only a new reporting rate.

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2.8.23A cancel emergency mode message indicates that the pilot has cancelled the

emergency mode.

ADS message data glossary is provided in Annexure.

2.9 MESSAGE DATA STRUCTURE

There is a standard structure to the uplinks and downlinks which are sent and received

by the ground system.

International Airline Transport Association

Airline Transport Association

• This structure is defined in the ATA/IATA ICM

Interline Communications Manual

• The ICM structure is used for all messages in the ground network.• The other parts of the communications network also have standard message

structures.For example:

Air Ground messages use the ARINC 618 message structure. The SITA ASP uses the ARINC 620 structure to convert between Ground and Air-

ground messages.

• The Ground network message structure was developed over 30 years ago. It wasdesigned to transmit text-based messages.• Here is an example of a downlink received by the Interim OCS, and the uplink which

was sent in reply. The messages have several lines of characters.

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Time = <19960522225319>

DOWNLINK

QU SYDEIQF AKLCBYA

.QXSXMXS 222253

FML

FIQF0119/AN VH-OJC

DT QXT POR1 222253 F27A

-AFN/FMHQFA119,.VH-OHC,,

225236/FPOS36466E164207,

1FC

0ADS,01/FCLATC,015795

545

Time = <19960522225323>

UPLINK

QU FANS1XS

.AKLCBYA 222253

FMD

AN VH-OJC/MA 001A

-/A0

AKLCBYA,AFN/FMHQFA119,.

.VH-

OHC,,225321/FAK0,NZZO/

FARADS,0/FARATC,0BABB

001

Ground

SystemAFN

ADS

CPDLC



• These messages have two main parts:1. Several lines which have a strict format.2. Free text lines, which were traditionally used for messages which could not be sent

using the standard structure.

2.9.1 Downlink details

Downlinks have five lines of structured text (Uplinks have only four lines). The

example above shows a typical downlink message.

Let us have a closer look at lines three, four and five

1. Line Three

This line is the 'Standard Message Identifier' (SMI).

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The SMI is a three letter code which is like a header. It identifies the type of message which will follow.

There are many of these SMI codes.Here are three which you will see often.. .

• FML Flight Management Computer Left• FMD Selected Flight Management Computer• MAS Message Delivery Assurance

2. Line Four

This line is the 'Text Elements' (TE) line.• A Text Element is a unique two-character code which identifies the message

element which will follow. The TE code is often followed by several other characters in a standardformat.e.g. AL is always followed by a letter, then three digits. ALA370 stands for an

Altitude of 37,000 ft.

The TE line may have several of these TE codes. Here are two which youwill see often.FI Flight Identifier

AN Aircraft Registration

3. Line Five

This is the DT line.• It always starts with the text element code DT.• The DT line is only found in Downlinks. Uplinks have only four lines. The DT line contains…• The datalink service provider..

.- this will be either QXT (SITA Singapore) or

DDL (ARINC Annapolis).

• The receiving ground station. This will be...a Ground Earth Station e.g. PORl (Pacific), IORl (Indian Ocean), or a VHF

station e.g. AKLl, CCLl, DAKl.

• Date and time when the message was received by the ground station.• The message sequence number - used for network housekeeping.

The 'free text' lines

Traditional use of freetext

• The ATA/IATA ICM message structure included free text lines, so that non-struc-tured messages could be sent.

•

FANS uses these freetext lines

• FANS is able to send datalink messages through the existing network• This is only possible because of the free text lines. FANS uses the free text lines

to send AFN, CPDLC and ADS messages.

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Even bit-based (binary) data can be sent over the existing character-based network. This

data is ‘translated’ by the ARINC 622 software in the aircraft.

4.9.2 Uplink details

• The uplink structure is the same as for downlinks, except for line five. Uplinks don’t

need line five.• Uplink messages can be delivered to the aircraft without confirmation. The sender

assumes the message got through to the FMC.• To know for sure that the message gets through, the sender can include a 'message

assurance' in line four of the uplink.• Here is an example of an uplink, with a message assurance text element in line four..

.

Line 1 QU FANS1 XS

Line 2 .AKLCBYA 222253

Line 3 FMD

Line 4 AN VH-OJC/MA 001 A

Free text - /A0

AKLCBYA.AFN/FMHQFA 119,. VH-

OJC,,2 25321/FAKO,NZZO/FARADS,0/F

ARATC,0BABB

BATAP001

• In the above example, the message assurance text element is MA 001A.001 is a number provided by the sender of the uplink. The first message sent has the 3-

digit number 000. For each message after that, the number is increased by one. When

999 is reached, the number starts at 000 again.

.9.3 Message examples

2.9.3.1 Initial AFN Logon

FN_CON Time = <19960522225319>

DOWNLINK

(

QU SYDEIQF AKLCBYA

.QXSXMXS 222253

FML

FI QF0119/AN VH-OJC


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This is the message

assurance text

element. It says that

the message needs to

be confirmed by the

ACARS MU.



- AFN/FMHQFA119,.VH-

OJC,,225236/FPOS36466E164207,1/FC

0ADS,01/FCOATC,015795

545

FN_AK Time = <19960522225319>

UPLINK

(

QU FANS1XS

.AKLCBYA 222253

FMD

AN VH-OJC/MA 001A

- /A0

AKLCBYA,AFN/FMHQFA119,.VH-

OJC,,225321/FAK0,NZZO/FARADS,0/F

ARATC,0BABB

001

AFN (Air Traffic Services Facilities Notification) Logon procedure: The Pilot initiates a

logon message (FN_CON) to the ground ATS Unit. When the ground system receives a contact

message – data from the message appears on a screen in a special AFN Window. It registers

the aircraft’s datalink (CPDLC) and ADS capability and OKs the aircraft’s software version. The

ground system sends an automatic acknowledgement (FN_AK) message. This completes the

Logon procedure.

2.9.3.2 ADS Request and ReportTime = <19960522225447>

UPLINK

(

QU FANS1XS

.AKLCBYA 222254

FMD

AN VH-OJC/MA 002A

- /A6 AKLCBYA,ADS.VH-

OJC07000BCE0C010D010E0110010F0100BD

002

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Time = <19960522225726>

DOWNLINK

(

QU SYDEIQF AKLCBYA

.QXSXMXS 222257

FML

FI QF0119// AN VH-OJC


-

ADS.VJ-

OJC030207E5C5F3AC0C0A0335BD9D0C446071C798200DE5A963B4760A0281C8E56E83CAE

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The Controller sends a request for ADS Report. The aircraft system sends an automatic ADS

Report.

The ADS Contract is initiated by the Controller by initiating a ADS Contract from a menu in the

AFN Window.

2.9.3.2 Initiating a CPDC connection {Datalink connection request (CR.1) andconfirm (CC.1)}

CR.1 Time = <19960522225450>

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The Controller sends a CPDLC connection request (CR.1), this is done by selecting from a

menu on the screen. The aircraft system sends an automatic CPDLC Connection Confirm

(CC.1) message.

FANS1 aircraft can have two CPDLC connections (to two different ATS Units), but only one

connection can be active. The active connection links the aircraft and the controller who has

‘data authority’. The non-active connection is between the aircraft and the ‘next data

authority’. It becomes active as soon as the other connection is terminate

2.10 BENEFITS FROM THE USE OF ADS

ADS is an enabling technology. Major benefits will not accrue from its implementation alone,

but its incorporation, either alone or in conjunction with other data link technologies, will

result in the ability to develop and change ATM techniques and technology in such a way that

airspace capacity will increase, and financial benefits will result.

Nevertheless, either by itself or in conjunction with other technologies, ADS can be expected

to benefit the future ATS through -

1) Provision of surveillance in remote and oceanic areas;2) Reduction of standard separation minima;3) Enhanced flight safety;4) Improved flight economy through better accommodation of user-preferred

trajectories;5) A significant reduction in R/T traffic, and hence reduction in frequency congestion;6) A corresponding reduction in pilot and controller workload, and,7) The enabling of a greater degree of ATS automation.

2.11 ADS IMPLEMENTATIONS

The first operational aircraft equipped with ADS capability was the Boeing 747-400. This

aircraft was equipped with a flight management system (FMS) which included three air traffic

control (ATC) data communication applications: ADS-C, CPDLC, and ATS Facilities Notification

(AFN). Collectively these features, along with others, were knows as the FANS -1 Package. The

FANS -1 package was certified in June of 1995 and went into service in the South Pacific on

flights between the US and Australia and New Zealand. ADS-C, as defined by ARINC 745-2,

has been used in some of the flight information regions (FIRs) in place of high frequency (HF)

voice position reporting.

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Airbus Industries has developed the FANS -A avionics for implementation of the same ADS-C,

CPDLC, and AFN applications in accordance with the same standards documents. The

international community has coined the term FANS 1/A to indicate either airframe

manufacturer’s implementation of these applications.

Since that time, ADS-C alone or ADS-C and CPDLC have already been implemented at many

sites throughout the world. Countries with sites using these capabilities to support airspace

operations include: Australia, Fiji, Indonesia, India, Japan, Malaysia, Mongolia, New Zealand,

Russia, Singapore, South Africa, Sweden, Tahiti, Thailand, United States (Oakland and

Anchorage Centers). Other sites using these capabilities in demonstrations include: Canada,

China, Hong Kong, Iran, Latvia, Norway (North Sea), South Korea, and United Kingdom (North

Sea). The ground system is as defined in ARINC 745 document, contracts for ADS-C.

In India, AAI have been actively involved in studies and planning for implementation of ADS

systems since 1995. The first engineering demonstration was done by M/s ECIL at Chennai.

The first trial system, a standalone IBM system (CNS/ATM Interim system) was installed and

commissioned at Kolkata. The system was later replaced with a full functional system from

M/s ECIL. Today, we have four ADS/CPDLC systems in operation at Kolkata, Chennai, Mumbaiand Delhi; two systems from M/s ECIL at Kolkata & Chennai and two more systems from M/s

Raytheon at Mumbai & Delhi.

ADS - B

2.12 INTRODUCTION

ADS-B is a surveillance application that allows the transmission of parameters, such as

position and identification, via a broadcast-mode data link for use by any air and/or ground

users requiring it. This capability will permit enhanced airborne and ground situationalawareness to provide for specific surveillance functions and cooperative pilot-controller and

pilot-pilot ATM.

The ADS-B application is not limited to the traditional roles associated with ground-based

radar systems. ADS-B will provide opportunities for new functionality both on board the

aircraft and within the ground ATC automation systems. Depending on the implementation,

ADS-B may encompass both air-ground and air-air surveillance functionality, as well as

applications between and among aircraft on the ground and ground vehicles. ADS-B will have

many benefits in extending the range beyond that of secondary surveillance radar,

particularly in airport surface and low-altitude airspace, and in air-to-air situational awareness. The ADS-B application supports improved use of airspace, reduced ceiling/visibility

restrictions, improved surface surveillance, and enhanced safety. ADS-B equipage may be

extended to vehicles on the airport surface movement area, and non-powered airborne

vehicles or obstacles.

As per ICAO Doc 9694-AN/955 (Part VII Ch. 1 para 1.4), the following definitions have been

adopted:

a) ADS-B emitter is a source, which is equipped with an ADS-B transmitter and continuallybroadcasts its identification, position, and other defined parameters via a data link.

b) ADS-B receiver receives and processes ADS-B data.c) Air-ground operation is a transmission from an ADS-B emitter used by a ground

receiver for the purpose of surveillance and monitoring.

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d) Air to air operation is a transmission from an ADS-B emitter used by another ADS-B airreceiver.

Each ADS-B capable emitter will periodically broadcast its position and other required data

provided by the on-board navigation system. Any user, either airborne or ground-based,

within range of this broadcast may choose to receive and process this information. The

emitter originating the broadcast need have no knowledge of what system is receiving itsbroadcast. Because broadcast data might be received by the ground station at a rate in

excess of the requirements of the ATC system, some filtering and/or tracking may be

necessary.

The requirements and performance characteristics for ADS-B information may differ between

airborne emitters and emitters on the airport surface. They may also differ depending on the

class of airspace within which the emitters are intended to operate, and the level of service

offered in such classes of airspace. This will enable appropriate benefits to be offered to all

categories of users in a cost-effective manner, and will minimize the requirement for over-

sophistication of equipage for general aviation and other non-revenue producing users.

2.13 SCOPE

ADS-B consists of several services, including those designed for both air-ground and air-air

use. This version of the manual addresses ATC surveillance only. Other potential services

using ADS-B derived data are being investigated including:

a) airborne situational awareness;

b) conflict detection (both airborne and ground based);

c) ATC conformance monitoring; and

d) ADS-B lighting control and operation.

It is anticipated that other services will be added in future.

Many other forms of broadcast data may become available, including flight information

services (e.g. NOTAM and weather information). These services are inherently different from

ADS-B in that they require sources of data external to the aircraft or broadcasting unit,

broadcast information other than encompassed in ADS-B, and independently defined

performance requirement.

2.14 FUNCTIONAL CAPABILITIES

2.14.1 BROADCAST REQUIREMENTS

Each ADS-B emitter will periodically broadcast its position and other required data. Any

receiver within range of the broadcast may receive and process the information. The emitter

originating the broadcast need have no knowledge of what system is receiving its broadcast.

2.14.2 MESSAGE ELEMENTS

The following message elements shall comprise the minimum set of information to be

transmitted by an ADS-B emitter:

emitter category;

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emitter identifier; latitude; longitude; level; aircraft identification, if applicable; and FOM.

System design should allow for inclusion of additional message elements for future use in

airspace where air-to-air applications of ADS-B are envisaged. This will also entail appropriate

enhancement of aircraft equipment. Potential message elements may include:

ground vector, containing ground track, ground speed and vertical rate; or air vector, containing heading, IAS or Mach, and vertical rate; and short-term intent, containing next waypoint and target altitude; rate of turn; and aircraft type.

2.14.3 ADS-B MESSAGE DATA GLOSSARY

The following data are used as the ADS-B message element variables, and are shown here in

alphabetical order.

Aircraft identification. A group of letters, figures or a combination thereof which is identical

to or the code equivalent of the aircraft call-sign. It is used in field 7 of the ICAO model flight

plan.

Aircraft type. Refers to the particular classification of the aircraft, as defined by ICAO.

Air speed . Provides air speed as a choice of the following. Mach, lAS, or Mach and IAS.

Air vector . A sequence of Heading, Air speed and Vertical rate.

Distance. Specifies distance.

Emitter category . Refers to the characteristics of the originating ADS-B unit. It should be

listed as one of the following:

1. Light aircraft - 7 000 kg (15 500 Ib) or less

2. Reserved

3. Medium aircraft - more than 7 000 kg (15 500 Ib) but less than 136000 kg (300000 Ibs)

4. Reserved

5. Heavy aircraft - 136000 kg (300 000 Ib) or more

6. High performance (larger than 5G acceleration capability)

7. Reserved

8. Reserved

9.Reserved

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10. Rotocraft

11. Glider/sailplane

12. Lighter-than-air

13. Unmanned aerial vehicle

14. Space/transatmospheric vehicle

15. Ultralight/ handglider/paraglider

16. Parachutist/skydiver

17. Reserved

18. Reserved

19. Reserved

20. Surface vehicle - emergency vehicle

21. Surface vehicle - service vehicle

22. Fixed ground or tethered obstruction 23. Reserved

24. Reserved

Note.- 2, 4, 7-9, 17-19, 23 and 24 reserved for future assignment.

Emitter identifier . Refers to the unique 24-bit address specific to an airframe, vehicle or

other emitter.

ETA. Estimated time of arrival at a waypoint.

Figure of Merit (FOM). Indicates the FOM of the current ADS-B data. The information

consists of the Position accuracy and indications I) whether or not multiple navigational units

are operating, and 2) whether or not ACAS is available.

Ground speed. Provides ground speed.

Ground vector . A sequence of Track, Ground speed and Vertical rate.

lAS. Indicated air speed.

Intermediate intent . Set of points between current position and the time indicated in the

Short-term intent. Consists of a sequence of the following: Distance, Track, Level and

Projected time.

Latitude. Latitude in degrees. minutes and seconds.

Level . Specifies level as either altitude or flight level in feet.

Longitude. Longitude in degrees, minutes and seconds.

Mach. Air speed given as a Mach number.

Mach and lAS. Air speed provided as both Mach and lAS.

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Next waypoint . Specifies the next waypoint in the avionics.

Position accuracy. An indication of the navigational accuracy.

Projected time. Predicted time at a particular point.

Short-term intent . A sequence of Latitude, Longitude, ETA, and Intermediate intent

(optional) data structures.

Timestamp. Date and time that an ADS-B message is generated.

Track . Provides track angle in degrees.

Turn rate. Refers to the aircraft rate of turn.

Vertical rate. Rate of climb/descent (climb positive, descent negative).

2.15 AIR TRAFFIC SURVEILLANCE

2.15.1 SCOPE AND OBJECTIVE

Enhanced navigation and communication capabilities will permit the use of aircraft-derived

position information to be transmitted to the ground. These data will be used as surveillance

data to supplement ground-based ATC surveillance, and may also be used as a sole means of

surveillance data, particularly in areas in which there is no radar coverage.

2.15.2 EXPECTED BENEFITSOnce the ground infrastructure is deployed, the increased accuracy, the update rate and

additional parameters available with ADS-B should result in the following benefits:

i. improved services in airspace not having radar coverage;ii. improved airspace utilization;iii. improved conflict prediction and detection;iv. improved airport surface movement, guidance, and control;v. improved runway incursion prevention; andvi. improved automated conformance monitoring

2.15.3 ANTICIPATED CONSTRAINTS

In order to provide surveillance based exclusively on ADS-B information, all aircraft operating

in the airspace must be ADS-B equipped.

ATS providers will require an independent means of validation of the ADS-B position

information.

2.15.4 OPERATING ENVIRONMENT WITHOUT ADS-B

The surveillance environment comprises primary/secondary radar, with differing update rates,

which support the provision of specified separation minima.

The nature and expense of radar limits the area of coverage where these surveillance

services can be provided. As a result, large separation minima and procedural methods of air

traffic control are used where no surveillance coverage is available.

2.15.5 OPERATING ENVIRONMENT WITH ADS-B

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ADS-B wiII enhance ATC surveillance in the following ways:

i. in a mixed ADS-B/radar surveillance environment, ADS-B data will complement orsupplement radar data; and

ii. ADS-B wiII extend surveillance services into non-radar airspace, such as low-altitudeairspace, remote airspace and coastal waters.

As ADS-B is implemented to different initial levels of capability, with mixed aircraft equipage,

ATS providers must ensure efficient levels of service to all airspace users.

2.15.6 GENERAL OPERATIONAL REQUIREMENTS

To provide a basis for the design of ADS-B systems for ATC surveillance, the following general

operational requirements have been determined:

i. an ATSU will be capable of knowing that an aircraft is ADS-B equipped;ii. all aircraft operating in an ADS-B airspace will broadcast as required by the ATS

provider;

iii. the ground system will receive, process and display the ADS-B information; andiv. procedures and/or systems must be in place to validate the ADS-B information.

A summary of ATC specific performance requirements using ADS-B is presented in Table 4-1.

Parameter

Operational domain

En route Terminal Airport

surface/vicinity

Maximum update period 10 seconds 5 seconds1 second (see

Note)

Probability of update within

period

98 per

cent98 per cent 98 per cent

Position accuracy 350 m 150 m 3m

Instantaneous number of

aircraft to be supported per

ATSU

1250450 in a 60 NM

radius

100 in motion;

150 stationary

Message latency 2 seconds 1 second I second Note.- A less frequent update rate may be permissible for stationary

emitters.

Table 2.1 ATC Specific surveillance requirement using ADS-B

2.15.7 EXCEPTION HANDLING

The ADS-B application will be capable of providing a warning to pilot and controller whenever

the navigation accuracy is degraded below that required to operate in the airspace, as thiswilI affect the application of separation.

Back-up procedures should be developed for ADS-B complete and partial system failure.

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Figure 2.13

Figure 2.14

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2.16 SUMMARY: ADS-B

Aircraft determines its position using GPS Broadcasts automatic, accurate routine reports - position, identity, altitude and velocity

information (ADS-B out)

Ground stations receive the broadcasts and relay the information to air traffic control Other aircraft receive broadcasts & display to pilot (ADS-B in) Enhanced “See &

Avoid” Air-Air Surveillance High update rate ~ (eg: every 0.5 seconds) Rate determined by avionics Line of sight coverage – No satellite International “standardised” DATALINKS VDL Mode 4/ ModeS 1090 Mhz Extended

Squitter (1090ES)/ UAT (Universal Access Transceiver) Worldwide Consensus to use 1090ES datalink as initial link Applications

ground based radar-like services in areas not covered by radar; support surface movement surveillance; operational control for operators -surveillance data to airlines; improve military-civil coordination based on common surveillance; SAR support ; and provide enhanced pilot situational awareness.

Benefits move from procedural to radar-like service; reduction in the cost of the provision of air traffic services through operational

efficiencies; enabling a seamless “gate-to-gate” surveillance service, not only to international

civil aviation but should include general aviation and military operations;

Increased safety and efficiency through the use of aircraft-derived data in avariety of systems; and Increasing airport safety and capacity, especially under low visibility conditions; Changes to airspace sectorisation and route structure resulting from improved

surveillance; Reduced infrastructure costs; Cost savings achieved from ADS-B based surveillance system rather than the

lifecycle expenses associated radar-based surveillance; Possibility of overall savings if associated with relevant navigation changes; Improved SAR efficiency; Reduced impact on the environment.

1090 MHz ADS-B equipped aircraft broadcasting position by state of registry - France, Japan, Thailand, Switzerland, Luxemburg, Vietnam, Iceland, USA, UK, UAE, NewZealand, Singapore, Korea, Mauritius, Malaysia, Chain, Australia

Ground based equipment demonstration have been made by M/s Raytheon, Senses, Thales

ADS-B demonstration was done in India at Chennai by M/s Thales in November 2005 Domestic Airlines

All Kingfisher Airlines and Air Deccan Airbus A320 aircraft in theChennai TMA vicinity were ADS-B equipped

Selected Jet Airways and Spice Jet Boeing 737 aircraft in the Chennai TMA vicinity were ADS-B equipped

International Airlines

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Selected Singapore Airlines, Thai Airways, Ethiad Airways, Gulf Air,Lufthansa, British Airways, Sri Lankan, Yemini Air and Emirates aircraftin the Chennai TMA vicinity were ADS-B equipped

A total of 10.5% of movements in Chennai airport from 28-Nov-05 (8pm IST) to 2-Dec-05 (9am IST) were ADS-B equipped aircraft.

2.17Combined ADS-A/C and ADS-B Benefits

After both ADS-A/C and ADS-B concepts have been fully proven, distributed ATM functions

may become operational. In this scenario, pilots will assume some responsibility for

separation of their aircraft from nearby aircraft through the use of ADS-B with CDTI (Cockpit

Display of Traffic Information). The controller, using ADS-A/C, will be in a monitor role to

assure that aircraft remain separated. Such procedures are part of the “Free Flight” concept

envisioned as the future ATM system.

Controller Workstation Overview

3.1 INTRODUCTION

This is chapter is to give an idea about the Controller Workstation peripherals andWindows for various operation. This may not match to your system in the station. This

covers a generalized idea of Controller Workstation.

3.1 A standard workstation is likely to comprise of the following –i. One or two computers. Two computers incase it has a separate computer for the

Aircraft Situation Data Display (ASDD).ii. Two displays. One for the ADS & CPDLC Windows, generally a 19” CRT Monitor

and the second for ASDD, generally a 29” CRT or LCD 2k x 2k monitor.iii. Mouseiv. Keyboard

v. Printersvi. There are special graphic cards for the 29” ASDD monitorsvii. LAN cards

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The details of the peripherals are dealt in later chapters. What we are going to discuss

in this chapter to familiarize you is about the various Windows and its functions in brief.

Figure 3.1 Controller Work Station

3.1 Windows OverviewFigure 3.2 below will help you to recapitulate the parts the windows you already know.

It shows you what the parts are called, and what they are.

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Figure 3.2 Sample Window

3.1 Workstation Window This window has two main functions –

To display various pieces of information such as date, time, system status,message priority and number of Uplink, Downlink, Adjacent Center andSystem messages

To open other important windows, using the Windows menu and the queuebuttons.

FIgure 3.3 describes a typical Workstation window.

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Figure 3.3 Workstation Window

Figure 3.4 shows the various window links from the workstation window.

3.1 AFN Window

1. From the Open menu we can open the AFN window (Figure 3.5). This is a veryimportant window which allows you to initiate and terminate datalink contracts oncean aircraft has logged ON to your system, Figure 3.6 shows the AFN menus. In otherwords you should also be able to –

select and ADS and/or CPDLC connection for an aircraft on the list (Figure 3.7explains the contents of each column of the listed aircraft that has loggedON)

edit an ADS contract edit or send a CPDLC message

2. From the Uplink menu you can view all your uplink messages.3. From the Downlink menu you can view all your downlink messages and cal also

process downlink messages.4. From the System menu you can view all your system messages.5. Adjacent center communication is not used.

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Figure 3.4 Workstation window links

Figure 3.5 AFN Window

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Figure 3.6 AFN Window menus

Figure 3.7 AFN list

Figure 3.8 and 3.9 gives an overview of the CPDLC and ADS windows respectively.

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Figure 3.8 CPDLC Window overview

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Figure 3.9 ADS Window overview

3.6 Aircraft Situation Data Display (ASDD)

This window is sometimes also called Aircraft Situation Display (ASD) or Situation Data

Display (SDD). This window is a pictorial display of the traffic situation in the Controller’s area.

It includes a map of the area, aircraft position symbols to show the position of the aircraft and

details of the aircraft. The Controller can adjust the way the information is displayed. The

window menus and pop-up windows provide a wide range of adjustment display formats. This

window is on a different 2k x 2k 29” CRT or LCD monitor. Figure 3.10 gives an overview of the

window.

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Figure 3.10 ASDD Window overview

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VHF Datalink & CPDLC

4.1 Historic

Since the early 70’s, airlines have operated the ACARS ( Aircraft Communications Addressing

and Reporting System) network for their Maintenance, flight and Cabin operations. Initially

based on VHF Data Link (VDL) communication only, this ACARS network has been gradually

expanded to other communication means like SATCOM and HF.

Furthermore, the ACARS network has been updated with AOA (ACARS Over AVLC – Aviation

VHF Link Control) using VDL Mode 2. The ACARS Network Access is provided through

Communication Service Providers (e.g. ARINC, SITA,…) to Airlines.

Since the late 80’s, Air Navigation Services Providers (ANSPs) have been pushed by airlines to

consider the use of the Airline Data Link development for the delivery of some continental

operations such as Pre-Departure Clearance (PDC/DCL), Oceanic Clearance (OCL) and Digital-

Airport Terminal Information Service (D-ATIS). These initial datalink services are today in

operational use by major ANSPs (USA, Europe, North-Atlantic, Australia,…). Aircraft providethese services generally as an option, in equipment such as the Communication Management

Units (CMU) or Air Traffic Service Unit (ATSU).

Since the late 90’s, Data Link for has been deployed and used for CPDLC (Controller and

Pilot Data Link Communication) and ADS (Automatic Dependant Surveillance) operations over

oceanic airspace, following an industry initiative led by Boeing and Airbus. These Datalink

services are called FANS -1 for Boeing and FANS -A for Airbus. The Boeing FANS -1 and the

Airbus FANS -A are functionally equivalent and fully interoperable with ground systems

designed to support the FANS-1/A capability. ANSPs, initially in the South Pacific and now in

the majority of the Oceanic Airspaces and some remote continental airspaces (e.g. Pacific,Indian, North-Atlantic, Northern-Canada, Australia,…), as well as the Maastricht Upper

Airspace Center congested en-route airspace are providing the FANS-1/A services.

International airlines operate the FANS-1/A services mainly on their long-range aircraft.

Recently an upgraded package has been standardised and has been certified on some

aircraft. This version mitigates some of the FANS/1/A identified issues related to late message

delivery. Interoperability with existing ground systems has been preserved.

Aeronautical VHF Band

All VHF communications occur in the Aeronautical Mobile (Route) Service (AMRS). The lowest

assignable channel in this band is 118.000 MHz and the highest is 136.975 MHz. The size of

the band has grown from 118-132 MHz up to its present size to meet demand for more

capacity.

The total number of assignable channels in the AMRS is 760. Co-ordination is required

between states when allocating a channel to a particular service volume, since other users

outside of the service volume, or on the same or adjacent channels can disrupt the services

inside the volume.

The channel assignments for services has reduced from 100 kHz to 25 kHz at present, and8.33 kHz channel spacing has recently been introduced in some areas of European upper

airspace for some R/T services. Again, the reason for the reductions in channel spacing has

been the need for more capacity. Note that the underlying modulation scheme has never

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changed and the reductions have been possible because of improvements in analogue radio

technology.

Current Services in VHF band

There are two types of operational service that currently exist in the aeronautical VHF band:

VHF Radio/Telephony (R/T) voice communications, which is currently a centralcomponent of ATC services. VHF R/F is based on Double Side Band - AmplitudeModulation (DSB-AM) operating in 25kHz channel assignments. VHF R/T is used for,amongst other applications, pilot-controller voice communication, flight informationservices, meteorological services and aerodrome terminal information services(ATIS).

Data communications using the ACARS system. ACARS is commonly used for airlineoperational communications (AOC) applications, but is only used for a small numberof ATS services, e.g .pre-departure clearances (PDCs) at a few airports, ADS-C,CPDLC Work is underway to improve the quality of service of ACARS to allow moreATS services to be supported.

4.2 VHF DataLink (VDL)

ICAO has recognised a need for transition to digital data links for ATM communications and

has supported the development of standards for mobile VHF data communications. In the

long-term future it is expected that most ATM communications will be based on data and

voice will only be used as a fall back or for emergencies.

VDL is an ICAO acronym standing for VHF Digital Link, referring to the future generation of

ICAO standardised digital mobile communication systems that will operate in the VHFAeronautical Mobile (Route) Service Band. VDL is defined by ICAO as:

A constituent mobile subnetwork of the aeronautical telecommunication network (ATN),

operating in the aeronautical mobile VHF frequency band. In addition, the VDL may

provide non-ATN functions such as, for instance, digitized voice.

The VDL standards will provide mobile subnetworks within the Aeronautical

Telecommunication Network (ATN) and these will operate in parallel with other air-ground

links, e.g. those provided by Mode S and satellite (AMSS). Strictly, a VDL subnetwork provides

connectivity in the form of switched virtual circuits (SVCs) between two ISO 8208 DTE entities

of an airborne and ground ATN Intermediate System (IS). However, VDL Modes can also

provide non-ATN services and these services differ between the Modes.

(Note: It is often assumed that VDL stands for 'VHF Data Link', but ICAO uses the term 'VHF

Digital Link' because one of the standards carries digitised voice as well as data.)

There are 4 different VDL 'Modes' under standardisation. Each Mode represents a different

system and there is limited interoperability between the Modes. The VDL Modes are also at

different stages of standardisation. There is still controversy over the actual status of the

modes, especially Mode -4. Mode -4 is currently being developed in support of navigation (i.e.

for ADS-B) but clearly has the ability to support data transfer in the longer term.4.2.1 Overview of VDL Modes

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There are 4 VDL 'Modes' that are currently being considered by ICAO. SARPs for VDL Modes 1

and 2 were included in Annex 10 in November 1997. However, SARPs for Modes 3 and 4

remain under development at present.

The 4 VDL Modes are very different and offer different services and protocols. All the Modes

provide an ATN-compatible mobile subnetwork, but two of them also offer non-ATN services.

(Such services are sometimes referred to as 'Specific Services'.)

The VDL Modes have limited interoperability and support different services. These services

may be characterised as:

ATN data: All the VDL Modes can act as a mobile subnetwork to the ATN.

Non-ATN data: VDL Mode 4 allows data to be transported without using the ATN

protocols. Two examples of non-ATN applications that VDL Mode 4 supports are air-to-

air communications and Automatic Dependent Surveillance Broadcast (ADS-B). VDL

Mode 3 can also support air-to-air communications albeit under the control of a groundstation.

Voice: VDL Mode 3 integrates digitised voice with data communications.

4.2.2 VDL Architecture Overview

The basic architecture for VDL involves a VDL subnetwork that provides a seamless data

connection between a ground and airborne ATN router as part of an end-to-end data

connection between two End Systems (ES).The various components in the architecture are

(see Figure 4.1):

The end systems, for example pilot interface and controller interface for the controller-pilot datalink communications (CPDLC) application. The end systems house theapplication software and services for the transport layer and above.

The ATN routers, which manage the various inter-network routing functions in the ATN. The A TN routers are not part of the VOL subnetwork but interface to it at the AirborneNetwork Interface (ANI) and Ground Network Interface (GNI).

The airborne VDL radio (or 'mobile station') which provides one side of the mobile dataexchange protocols. One radio may incorporate functionality for several VDL Modes andalso analogue VHF R/T capability.

The ground VDL radio (or 'ground/base station') which provides the other side of themobile data exchange protocols. One radio may incorporate functionality for severalVDL Modes and also analogue VHF R/T capability.

The ground VDL network which connects one or more ground radios to one or morerouters. There is no airborne network since there is only one airborne radio and onerouter which are simply connected together.

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Figure 4.1

4.2.3 VDL Subnetwork Architecture

Each VDL standard has broadly the same subnetwork architecture for ATN communications,

although 2 of the Modes have additional non-ATN services see figure 4.2).

The functions within the subnetwork are:

Physical Layer: Responsible for data transmission and reception in the correct VHFchannel.

Link Layer functions which include the following sublayer functions: Media Access Control (MAC) sublayer, which provides the following services:

• Data reception by the transceiver/receiver• Data transmission by the transceiver/transmitter · Notification services (e.g.

channel idle/occupied). The MAC sublayer manages the transmit queue, keeping data queued until it is

ready for transmission.

Data Link Service (DLS) sublayer, which provides connection-oriented point-to-point Jinks and connectionless broadcast services (i.e. this sublayer manages thetransfer of data). The DLS sublayer protocol is based on the ISO HDLCasynchronous balanced mode protocol for data transfer but has been optimised to

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make most efficient use of capacity-limited links. In Modes I, 2 and 4 the optimisedprotocol is known as aviation VHF link control (A VLC) and performs the samefunctions as HDLC, i.e.:• Link activation and release• Frame synchronisation and sequencing · Error detection and control• Address identification• Data transfer

In Mode 3, a slightly different form of link layer protocol is employed, known as

Acknowledged Connectionless Data Link (A-CLDL)

Link Management Entity (LME), which establishes and maintains link layerconnections, including the hand-off between ground stations. Strictly, the SARPs referto a VDL Management Entity (VME) which is the superset of all LMEs in each station (astation must have one LME for each peer connection.) The LME also controls thefrequency of the transceiver, instructing it to tune to channels to locate particularservices.

Subnetwork Layer, which is a connection-oriented protocol, based on the ISO 8208packet layer protocol providing a DTE/DCE connection. This provides the interface tothe mobile subnetwork dependent convergence function (SNDCF) at the ATN router. InModes 1, 2 and 4, connections may be initiated only from the aircraft DTE.

A variant of the subnetwork architecture is proposed for VDL Mode 3 which includes a DCE in

the aircraft side of the subnetwork, which is not present in the other modes.

The differences between the VDL Modes are mostly at the lower layers - particularly in the

physical layer and the MAC sublayer. The higher parts of the stack are more similar for each

VDL Mode.

Figure 4.2

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4.2.4 VDL Modes 1 and 2

VDL Modes 1 and 2 provide mobile data communications only as ATN-compatible

subnetworks. Originally they were intended as upgrades to the Aircraft Communications

Addressing and Reporting System (ACARS). VDL Mode 2 was known as 'AVPAC'.

VDL Mode 1 ICAO saw the need to adopt a VHF data link system that would be bit-oriented,

would offer greater message integrity, and would be suitable for ATS. ICAO developed the VHFdigital link (VDL) Mode I based on the ACARS physical layer (modulation scheme, data rate

and channel access protocol) to enable the early introduction of VHF data services and

introduced SARPs into Annex 10. These Standards became applicable in 1996, but were later

withdrawn from Annex 10. At the same time, a higher level of performance was under study,

and another mode of the VDL was developed. ACARS and VDL Mode I is a lowspeed bit-

oriented data transfer system. It uses the CSMA methodology. The new developments have

overtaken VDL Mode 1 and it is no longer in use.

VDL Mode 2 The VDL Mode 2 data link is an evolution from Mode 1 that uses a digital, 8-

phase shift keying (D8PSK) modulation scheme at a data rate of 31.5 kilobits per second. It isATN-compliant, providing a bit-oriented protocol that may also handle character-oriented

messages that are compatible with non-ATN infrastructures. It has limitations in its support of

time-critical applications in high air traffic density areas because of its CSMA channel access

protocol that exhibits a non-deterministic behaviour. It does not support message priorities

and it cannot guarantee the message transfer time. VDL Mode 2 employs a globally dedicated

common signaling channel at 136.975 MHz. ICAO SARPs for this air ground data link were

applicable in 1997.

Guidance material is provided in ICAO's Manual on VHF Digital Link (VDL) Mode 2 (Doc 9776).

Limited VDL Mode 2 commercial services are available at this time, as aircraft operators and

service providers are able to introduce new equipment.

4.2.4.1 Physical Layer (VDL Mode 1)

VDL Mode 1 uses Amplitude Modulated Minimum Shift Keying (AM-MSK) operating at a bit rate

of 2.4 kbps. This is the same modulation scheme used in ACARS. It is a constant-phase,

frequency shift keying technique using two audio tones. The presence of the lower tone

indicates that there is a bit change from the previous bit, and the presence of the higher tone

indicates that there is no bit change. The phases of the two tones are chosen so that the

minimum phase discontinuity occurs at the interface between bits and so that the amplitude

of each tone is zero at the bit transition. The audio tones are Amplitude Modulated onto the

RF carrier.

A particular feature of this modulation scheme is that it is compatible with existing analogue

voice radios. The audio tones may be fed into the radio in place of the line level audio signal

conventionally connected to the aircraft intercom system.

AM-MSK is a very low data rate scheme. It has an instantaneous data rate of 2.4 kbs per

channel, which is further reduced by the inefficiency of the media access technique, and then

must be shared amongst all the competing users. It is unlikely to satisfy requirements for

improved capacity. However, it is a robust system, and much experience has been gainedthrough its deployment in the ACARS system, and it was specified for VDL Mode 1 in order to

provide a fall back to utilise VDL Mode 2 protocols with a well validated physical layer.

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The following table summarises the likely performance characteristics of AM-MSK modulation:

AM-MSK

Channel data ratePoor

(Low data rate)

Frequency re-useGood

(Low DUR# ratio)

Interference

characteristics

Good

(ACI expected to be low)

#DUR (desired/undesired signal ratio) refers to the ability of a receiver to successfully decode

a desired signal in the presence of an interfering (undesired) signal.

4.2.4.2 Physical Layer (VDL Mode 2)

Mode 2 uses Differentially encoded 8-Phase Shift Keying (D8PSK) operating at a bit rate of

31.5 kbps. 8-Phase shift keying means that each transmitted symbol represents one of eight

phase change states of the carrier (0, π /4, π /2, 3π /4 radians, etc).

The availability of 8 states means that each symbol can represent 3 bits of data. The symbol

rate is 10.5 kbps, giving a net bit rate which is three times higher, i.e. 31.5 kbps. Data is

gathered into 'triples' of bits before transmitting and padding is used to ensure that

transmissions always include an integer number of 3-bit symbols (see figure 4.3).

The phase encoding of the carrier is such that each symbol is represented by the difference

between the previous phase and the current phase, i.e. the signal is differentially encoded.

This is because the receiver has no phase reference for the carrier signal, i.e. the receiver

does not know what the signal should look like with a phase shift of 0 radians. Instead the

receiver can measure the difference between two phases and use this to determine the

symbol being transmitted.

Following encoding of the phase changes the signal is filtered with a raised-cosine filter (with

parameter α = 0.6) prior to transmission. The raised-cosine filter reduces the spectral

sidelobes of the D8PSK signal, reducing the amount of energy that spills outside of the

channel.

Note that data is also scrambled before being transmitted on the datalink, as a result of

multiplication by a pseudo random binary sequence. The scrambling is used to aid clock

recovery in the receiver and to stabilise the shape of the transmitted spectrum.

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VDL Mode 3 provides both ATN data and digital voice services. VDL Mode 3 works by

providing four logically independent channels in a 25 kHz frequency assignment. Each

channel can be used for voice or data transfer. A design driver for VDL Mode 3 was the aim

that a single radio should be able to provide voice and data services simultaneously.

There are seven configurations defined for VDL Mode 3. They offer a range of static voice and

data channel assignments, as well as standard or long-range operation. One of the

configurations provides dynamic channel assignment, in which a channel can be switched

dynamically between voice and data.

ICAO SARPs became applicable in November 2001. Guidance material is provided in ICAO’s

Manual on VHF Data Link (VDL) Mode 3 ( Doc 9805)

4.2.5.1 Physical Layer (Mode 3)

Mode 3 uses Differentially encoded 8-Phase Shift Keying (D8PSK) at a bit rate of 31.5 kbps.

The same physical layer was selected for VDL Modes 2 and 3 to ease transition between the

two Modes. It is hoped that the upgrade from VDL Mode 2 to 3 can be achieved using a

software upgrade and the addition of the voice module. This strategy is intended to

encourage early equipage of VDL Mode 2.

Note that the high data rate of 31.5 kbps provided by D8PSK is needed to be able to provide 4

digital voice channels in a 25 kHz frequency allocation.

The physical layer has the same functions as the Mode 2 physical layer. Note that although an

FEC is applied to the DLS data, it is not interleaved with the data as in Mode 2. Instead, the

FEC is put at the end of the slot.

The following table summarises the likely performance characteristics of the VDL Mode 3

physical layer (note that it is identical to VDL Mode 2):

VDL Mode 3

Channel data rateGood

(High data rate)

Frequency re-usePoor

(High DUR# ratio)

Interference

characteristics

Not fully tested.

(Some concerns raised about

interference to voice.)

#DUR (desired/undesired signal ratio) refers to the ability of a receiver to successfully decode

a desired signal in' the presence of an interfering (undesired) signal.

4.2.5.2 Media Access Control

VDL Mode 3 MAC operation is based on time division multiple access (TDMA) which allows alarge number of users to share a broadcast channel without transmitting simultaneously and

causing mutual interference. The technique involves dividing the channel into pre-defined

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time intervals and allowing different users to transmit only at these intervals. The pre-defined

intervals are known as timeslots or just slots.

The MAC is based on a 4-timeslot structure for normal (standard range) operations. Each slot

consists of 2 ‘sub-channels’ each one offering the opportunity for a burst of data to be

transmitted. One sub-channel in each slot is used for the transmission of management data

(eg commands from the ground station) and one is used for transmitting user information -

either voice or data.

A MAC Cycle is used to describe a standard timing cycle that consists of an even and odd

TDMA frame, ie in the standard range configuration this consists of 8 timeslots:

Figure 4.4 MAC cycle in VDL Mode

3 (standard range configuration)

In the extended range configurations, a 3 slot structure is used. The length of each burst in

the slot (M Burst or V/D Burst) is the same, but the guard bands between each burst are

increased to allow longer range operation. In extended range configurations, each slot lastsfor 40 ms. The length of TDMA frame and MAC cycle is the same as for standard range

configuration.

Each subchannel is used to transmit management information or user information (see figure

4.5):

Management data is transmitted in the 'M burst'. User data is transmitted in the 'V/D burst', which can only contain vocoder data or ATN

data. The MAC sublayer segments messages which are longer than one V /D Burst.

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Figure 4.5

4.2.5.3 Voice Services

The vocoder provides the following services:

assembly and disassembly of TDMA voice bursts, i.e. analogue to digital conversion; squelch window rejection of co-channel interference (i.e. voice transmissions from

distant users can be muted. The distance of users is measured using the propagationdelay of the transmissions);

detection and handling of vocoder frame errors; ground pre-emption of the voice channel (i.e. a ground user can stop the transmissions

of an airborne user and start transmitting instead. In the present voice system there isno way that a ground controller can force an airborne radio to stop transmitting.);

truncation of voice bursts to increase available guard time under specified conditions.

The last service is used if an airborne user loses the timing reference from the ground station,

e.g. because it moves out of range or the ground station fails. In this case, the aircraft does

not have confident knowledge of the times in which to transmit voice data. To prevent the

aircraft transmitting at the wrong time, and therefore blocking other transmissions on the link,

the vocoder shortens the transmissions by removing some bits. This also reduces the quality

of the digitised voice.

The service is half-duplex i.e. only one user can transmit on the link at a time without

corrupting the transmissions of other users. At this time, all other users can only receive. This

is the same as the analogue voice R/T service.

The digital voice coder (vocoder) must operate at a new rate of 4800 bps, including the coded

voice and any error detection/correction coding that is applied. The vocoder must provide

satisfactory voice quality with an error rate on the data link of 1 in 10-3. (Note that the FEC

coding that is applied to data bursts at the physical layer is not applied to voice bursts - FEC

for voice remains the responsibility of the vocoder.)

This low rate is demanding when compared to, for example, the GSM mobile phone standard

06.10 RPE-LTE which uses a data rate of 13kbps for voice, rising to 22.8 kbps when FEC is

added. The required low data rate for the vocoder is technically challenging and has been

perceived as a risk to the timely completion of SARPs.

VDL Mode 3 incorporates features to improve the service compared to the analogue R/T. In

the case of basic voice operation, such features are:

o Pre-emption of airborne users by the ground. This allows the groundcontroller to stop airborne transmissions and gain access to the communicationschannel. This can be used to overcome problems of 'stuck transmitters' andsimultaneous transmission by two aircraft.

o Rejection of transmissions from long-distance aircraft. The squelchfunction allows the radio to reject transmissions from aircraft more than a certainrange from each other.

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However, where an aircraft has performed net-entry and a local user ID has been allocated,

additional features, known collectively as enhanced voice services, become available. Such

features include:

• Discrete voice addressing capabilities, including selective calling uplink (instead of broadcasting to all users) and caller identification (downlink) to reduce the possibility of confusion for the ground controller.

• Call waiting/urgency indication, to identify when another station is waiting to pass amessage, or to emphasize the urgency of a message

The use of digital voice also offers increased security for pilot-controller communications. With

the use of digital voice instead of the current analogue voice system, it will become harder for

unauthorised users to transmit on the aviation band and this will reduce incidents of ‘spoof

controllers’.

Strengths and Weaknesses

VDL Mode 3 is able to integrate voice and data services, and enjoys higher link utilisation

efficiency and support for priority at the MAC layer. However, it could represent a single point

of failure and a proprietary vocoder has been recommended for incorporation. Furthermore,there are questions surrounding the impact of interference from D8PSK modulation on

adjacent analogue voice channels.

Strengths:

VDL Mode 3 will integrate all VHF communications into a single radio unit, avoiding theneed to carry separate VHF voice and data equipment.

The centrally managed TDMA protocol supports prioritisation readily at the MAC layerover all stations competing for access to the channel, thus facilitating use of the link fortime critical applications.

The TDMA access scheme is capable of significant improvements in link utilisationefficiency, in comparison with the CSMA techniques available with VDL Mode 2.

Weaknesses:

All VHF communications (i.e. voice and data) will pass through a single system. Failureof equipment might result in the loss of all VHF communications (i.e. high integrityvoice and data links will require careful engineering design, possibly with increasedcosts).

A proprietary vocoder design has been proposed, which will require a licence to benegotiated by any manufacturer.

The performance issues associated with D8PSK modulation remain to be resolved fully.

At the time of writing, ICAO has proposed a modification to the spectral mask in anattempt to control ACI effects. It has yet to be proven in a typical operatingenvironment.

4.2.6 VDL Mode 4 (Mode 4)

VDL Mode 4 SARPs specify a general data communication system for a range of applications.

The system is based upon the Swedish STDMA datalink, which supports "navigation and

surveillance" applications using the Self-Organising Time Division Multiplex Access protocol.

However, VDL Mode 4 now also supports full data communications functionality (ATN and non-ATN) and offers a potential upgrade path for Mode 2 with substantially improved performance

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and, in particular, support to time critical ATM communication. “Navigation and surveillance”

applications refer primarily to 2 applications:

The uplink of Differential GNSS (DGNSS) augmentation messages, to providecorrections and integrity messages to GPS/GLONASS receivers.

The transmission of Automatic Dependent Surveillance - Broadcast (ADS-B) data. This isan application in which all users broadcast their position to all other users. This data

can be used for some surveillance applications and in the case of the VDL Mode 4 isused to assist communication management. One of the main applications of ADS-B is togive pilots a traffic situation display that shows all surrounding air traffic. (ADS-B differsfrom TCAS in that it has much greater range and shows all air traffic is fully labelledwith flight ID and velocity. ADS-B is a strategic planning tool for pilots, compared to

TCAS which is a tactical safety-net.)

ICAO SARPs for VDL Mode 4 became applicable in November 2201. Guidance material is

provided in ICAO’s Manual on VHF Digital Link (VDL) Mode 4.

2.6.1 VDL Mode 4 Features

VDL Mode 4 supports both ATN communications and non-ATN communications. ADS-B is one

application that makes use of non-ATN communications. Another is direct air-to-air

communications, i.e. communications between aircraft that do not pass via a ground station.

VDL Mode 4 uses Gaussian-filtered Frequency Shift Keying - GFSK modulation, selected for its

CCI performance, with an option for D8PSK. It is based on self organising TDMA using a short

timeslot structure. Time slots are only 13.3 ms or 9.1 ms (depending on the physical layer

option selected). All transmissions are synchronised to the start of a timeslot. Transmissions

can continue across many slots without a break, although a significant number of

transmissions will be one slot long.

VDL Mode 4 uses a set of reservation protocols for managing access to the data link. These

are designed to minimise occurrences of random access (in which two users' transmissions

may interfere with each other), and are intended to support broadcast transmissions (e.g. the

basic ADS-B function) as well as point-to-point communication for the ATN.

VDL Mode 4 requires a source of navigation data and precise time to operate. The source of

this data is not specified in the SARPs. A GPS receiver can be used to provide the necessary

data, but other sources can also be used, e.g. the output from the FMS for position and an

accurate clock for timing. One suggestion is to obtain position data via the data link, by

ranging from other ADS-B users. Ranging can be performed because the precise time of transmission and the location of other users is known. By measuring the propagation delay of

transmissions, a user could determine his own location using similar algorithms to GPS.

However, this approach has not been practically validated.

4.2.6.2 Physical Layer (Mode 4)

The physical layer of VDL mode 4 is specified with two options stated in draft SARPs for the

modulation scheme:

D8PSK operating at a bit rate of 31.5 kbps.

Gaussian-filtered Frequency Shift Keying (GFSK) operating at a bit rate of 19.2 kbps.GFSK uses two tones, alternating between them when a zero is transmitted. Thechange between the tones is not abrupt, but instead is smoothed using a Gaussianfilter (Bandwidth-Time product = (0.28±0.03: the BT parameter defines the shape of

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the Gaussian filter). Gaussian Minimum Shift Keying –GMSK is a special case of GFSK (with filter parameter BT = 0.5 and modulation index = 0.5) which is widely used formobile communications. GFSK or GMSK are used in inter alia the following mobilecommunication standards: GSM, DECT, CT/CT2, Wireless LAN standard IEEE 802.11 andGlobalstar.

Apart from the actual modulation schemes, the two options only differ in two significantareas:

1. The lengths of slots for data transmissions. Since D8PSK has a higher data rate, itcan transmit the same number of bits as GFSK in a shorter time, thus D8PSK can use ashorter slot whilst transmitting the same information. When transmitting ATN data,D8PSK takes about 2/3 of the time of GFSK to transmit the same data.

2. The transmitter ramp-up times. DSPSK ramp-up time is specified as 380µscompared to 832µs for GFSK. The shorter ramp time of D8PSK makes more efficient useof the available channel but it is harder to implement while maintaining low levels of adjacent channel interference.

GFSK is a form of frequency modulation (FM) compared to D8PSK which is a form of phase

modulation (PM). D8PSK has the advantage that it is the same modulation scheme as VDL

Modes 2 and 3 and has a higher data rate. However, GFSK has been proposed because it may

be more suitable for navigation and surveillance applications because it operates at a lower

desired/undesired signal ratio (DUR) than D8PSK. (DUR refers to CCI and ACI performance.)

The DUR determines how much stronger a desired signal must be than an undesired signal in

order for the desired signal to be correctly decoded (if the desired signal is not sufficiently

stronger than the undesired signal, then neither can be decoded by the receiver). Flight trials

have shown that the DUR of GFSK is approximately 7dB compared to approximately 16dB for

D8PSK(with a single interfering source.)

In the VDL Mode 4 surveillance application ADS-B, there will be many aircraft transmitting

ADS-B reports. Some of these aircraft will transmit their reports in the same slot because they

are out of line of sight of each other and therefore each does not know that the other is using

that slot. A third aircraft may be able to receive both ADS-B reports and will want to decode

the report from the closer aircraft. Whether this aircraft can do so depends directly on the

relative receiving powers of the reports and the DUR of the modulation scheme. This is

illustrated in the following figure 4. 6.

Figure 4.6 Decoding ADS-B reports in the presence of undesired signals

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Aircraft A must decode the signal from the nearest aircraft (B) over

a 'background' signal from the more distant aircraft (C).

The ability of aircraft A to decode the desired signal in the presence

of an undesired signal depends on the relative powers of the two

signals (which is dependent on the square of the ratio of the ranges

of the transmitters) compared to the DUR.



Using GFSK instead of D8PSK could improve the performance of the ADS-B function for VDL

Mode 4, and could also change the performance of communications applications. However,

further flight validation of GFSK is required.

A possible solution to the modulation scheme debate may be for VDL Mode 4 to use GFSK for

non-ATN communications and D8PSK for ATN communications. The different services would

operate on different VHF channels.

4.2.6.3 Media Access Control

VDL Mode 4 requires all users to be time synchronised so that their transmissions stay within

their allocated slots and do not overlap unintentionally. The time standard specified for VDL

Mode 4 is Universal Co-ordinates Time (UTC). Whilst this is system independent, the easiest

way to get UTC is from a GPS receiver.

VDL Mode 4 has an Integrated Timing Concept (ITC) which refers to the ways in which UTC

can be obtained. Five techniques have been proposed for obtaining UTC time in the ITC

(although in practice any method could be used since the source of time is not specified in the

SARPs):

1. GNSS: A user equipped with a GNSS receiver can determine the UTC time to within340ns (2 signal). This is the most likely time source for VDL Mode 4, making itindependent from ground stations.

Synchronisation from ground station: VDL Mode 4 ground stations transmit time

synchronisation messages on a regular basis. Ground stations can remain time

synchronised by using GNSS time transfer, atomic clocks or other techniques. These

should allow a mobile user to estimate time with sufficient accuracy to maintain UTC, by

measuring the time of transmission between the stations.

2. Atomic clocks: In the near future, low cost atomic clocks may be available forairborne use. Present hardware configuration does not include on-board atomic clocks.Sufficient time synchronisation might be achievable using quartz clocks.

3. Synchronisation from other mobile users: A user, who is unable to acquire GNSSor a VDL Mode 4 ground station (in any combination), could rely on other synchronisedusers in airspace. This is the same principle as timing fr6tn a ground station, but islikely to be less accurate because ground stations will probably have higher precisionclocks.

4. Floating network: This functionality is similar to alternative (4), with the difference

that all users have lost the GNSS or ground derived UTC time synchronisation. In thiscase users will continue to broadcast synchronisation bursts and attempt tosynchronise on other users. In the absence of synchronisation each user's clock willtend to drift. Users will tend to correct their own clocks toward the "average drift rate"of the user population as a whole.

In order to co-ordinate access onto the physical medium, all VDL Mode 4 stations maintain a

slot map which identifies the status of each slot for the next four minutes ahead. For each slot

in the map, the table identifies the station(s) which have reserved the slot, for each such

reservation also the intended recipient (for a point-to-point transmission) as well as the

reservation protocol used to make the reservation. Whenever a station wishes to find a slotfor its own transmission, it consults the table to choose either a slot which is unreserved, or to

re-use a slot under the prescribed rules for slot re-use.

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A single transmission may require a single slot (e.g. an ADS-B report) or else may continue

over several slots (e.g. a point-to-point ATN transmission). In principle, broadcast and point-

to-point transmissions may co-exist on the same channel, but on a channel which is heavily

occupied by single slot broadcast reports such as ADS-B, it may prove difficult to select

contiguous blocks of slots required for A TN transfers. In this case there would be some

advantage in using different channels for these different types of transmission.

Figure 4.7

4.2.6.4 Reservation Protocols

Dithering of a periodic broadcast stream is important because it prevents two stations

reserving the same slot when they are out of range of each other, and then garbling each

other because they approach each other without knowing that they have selected the same

slots.

The periodic broadcast protocol carries two parameters with each burst, known as the

periodic timeout (pt) and periodic offset (po). Normally, a periodic broadcast will reserve slots

in the following 3 superframes for the stream to continue, and the pt parameter specifies the

number of superframes into the future for which the same slot as the current transmission isto be reserved (pt can take a value between 0 and 3). If pt is less than 3, then the po

parameter defines an offset to a new slot to which the stream will dither to a nearby slot in

the following superframe, and continue in that slot. In this way, the slots used by a station for

periodic broadcasts exhibit a slow random dither about the nominal reporting frequency.

A stream may continue in the same slot for up to 16 superframes, but typically it will do so for

around 8 superframes (i.e. for 8 minutes) before dithering. Certain events may provoke a

stream to dither; it may detect during slot selection that another station has a reservation in a

later superframe for the slot it is using, in which case it plans to dither prior to reaching that

slot. Alternatively it may hear a conflicting reservation from another station, which can be

resolved by making a dither to a new slot. If either of these applies, then the time to make thedither will be determined stochastically.

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In an incremental broadcast reservation, only the next reservation in a sequence is

announced in each transmission, by means of an incremental offset (io) parameter. However,

the io parameter is multiplied by 8 to yield the actual offset from the current to the reserved

slot, and so only slots which are an exact modulo 8 difference from the transmission slot may

be reserved by this process.

The incremental broadcast may be combined with the periodic broadcast when the latter is

not announcing a dither, since under these conditions the po parameter is redundant, and the

field may be used to carry an incremental offset instead.

Figure 4.8

Strengths and Weaknesses

VDL Mode 4 offers a very flexible communications service, but it has not been fully validated.

Strengths:

The communications system specified in the VDL Mode 4 SARPs is the most flexibledata communications system of all the VDL Modes.

VDL Mode 4 includes a 'built-in' ADS-B function. This means that the system canprovide surveillance and communications services.

Weaknesses:

The integration of surveillance and communications services could lead to a problem of common mode failures.

Although extensive trials and demonstrations have been conducted, these have beenwith the simpler STDMA system, rather than the full VDL Mode 4 SARPs. As a result,some aspects of the system concept are undemonstrated, for example the operation of ATN and the derivation of precise time from other users.

There is also a possible patent issue with the use of GPS for time synchronisation.

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Figure 4.9 VDL 4 in air-air and air-ground modes of operation

6.2.7 ICAO Standards:

ATN SARPs, ICAO ANNEX 10 - Aeronautical Telecommunications – Volume

III, Part I (Digital Data Communication Systems) – Chapter 3 -Aeronautical Telecommunication Network;

ICAO DOC 9705-AN/956 – Manual of Technical Provisions for theAeronautical Telecommunication Network; Edition 2, December 1999

VDL SARPs, ICAO ANNEX 10 - Aeronautical Telecommunications – VolumeIII, Part I (Digital Data Communication Systems) –Chapter 6 - VHF Air-Ground Digital Link (VDL); Date: March 2000

Manual on VHF Digital Link (VDL) Mode 2, Doc 9776/AN970; First Edition2001

Table 4.1 Characteristics and modulation schemes of differentVHF data links

Mode Modulation

Modulation

rate

(Kbits/sec)

Access

controlApplication

ACARS AM MSK 2.4 CSMA Data

VDL I AM MSK 2.4 CSMA Data

VDL2 D8PSK 31.5 CSMA DataVDL3 D8PSK 31.5 TDMA Data & voice

VDL4 D8PSK 31.5 STDMA Data for SUR

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VHF digital link is compatible with ATN. Mode 2 has higher capacity; successor to Mode I. Mode 3 is suitable for high-density areas and areas experiencing frequency congestion.

Mode 3 provides up to 4 voice and/or data circuits. Mode 3 and Mode 4 are capable of transmitting time-critical messages and can accept

prioritization of messages. Mode 4 - allocation of time slot without external unit. Single modulation scheme permits a single VHF radio to operate all the modes with a

minimum addition of circuits. With VDL, aircraft is not involved in any manual frequency tuning for any station

change. Mode 4 is a candidate technology for ADS-B operations.

SUMMARY:

i. VDL Modes 1 and 2

VDL Modes 1 and 2 are data links that were develop initially as an upgrade to ACARS. They

differ only in the definition of the physical layer and some associated parameters; VDL Mode 1

has a much simpler physical layer with lower capacity. VDL Mode 1 was standardised in case

the VDL Mode 2 physical layer could not be developed in the required timescale. Since ICAO

has accepted VDL Mode 2 validation, VDL Mode 1 is unlikely to be implemented.

VDL Modes 1 and 2 are based on a media access protocol in which they monitor the VHF

channel and transmit if it is free with a certain probability. This protocol does not require anymanagement (e.g. control from the ground) but is generally unsuitable for prioritisation of

services. Hence, low priority communications could block higher priority communications and

this explains why neither Mode is likely to be used for time-critical safety-critical applications.

However, Mode 2 may be used for applications such as delivering pre-departure clearances

(POCs) and Aerodrome Terminal information Service (ATIS) messages. This could make it

suitable as a transition system to longer term higher-performance data links that would

support more time-critical applications.

Standardisation of VDL Modes 1 and 2 is complete.

ii. VDL Mode 3

The FAA has proposed VDL Mode 3 as the long-term solution to VHF frequency congestion. It

integrates voice and data services into single VHF radio equipment on the aircraft.

VDL Mode 3 offers four logically independent voice or data channels in a single 25 kHz

channel assignment. This is achieved using a TDMA structure based on a repeating pattern of

four data slots and four shorter management slots. There are seven configurations defined

which offer various combinations of voice and data slot assignments, as well as some for long-

range operation (these use a slightly different TDMA structure). One of the configurations

provides dynamic channel assignment, in which a channel can be switched between voice and

data services according to demand.

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The use of centrally managed TDMA can be expected to improve the link utilisation efficiency,

and also supports readily the implementation of priority at the MAC layer, in comparison with

the CSMA technique of VDL Mode 2.

VDL Mode 3 uses the same physical layer definition as VDL Mode 2, which is based on the

D8PSK modulation scheme. However, ACI performance issues relating to use of this

modulation have been raised, and ICAO have adopted a revised spectral mask in an attempt

to control them. It remains to be determined that these issues have been fully resolved.

iii. VDL Mode 4

VDL Mode 4 is a potentially powerful data communication system with more flexibility for data

communications than the other VDL Modes. It was originally conceived as a data link for

"navigation and surveillance" applications, but the system described in the SARPs could

support many different types of applications, The primary driver of the development of VDL

Mode 4 to date has been the surveillance application ADS-B.

VDL Mode 4 has been promoted by the Swedish CAA and is based on a Swedish-developed

technology known as "STDMA". The VDL Mode 4 standards have emerged from the trials

using the STDMA equipment. However, the draft SARPs describe a much more powerful

communications system than has been demonstrated so far in STDMA trials.

VDL Mode 4 is based on a self organising TDMA protocol that uses a large number of short

timeslots. The timeslots are reserved for transmissions by users, so that several users do not

try and use the same timeslot simultaneously. Transmissions occupy at least one timeslot and

may extend across many.

The SARPs specify two options for the physical layer, one based on a D8PSK modulation

scheme and one based on a GFSK modulation scheme. D8PSK would provide commonalitywith VDL Modes 2 and 3, while GFSK is proposed as more suitable for surveillance

applications.

4.3 CONTROLLER-PILOT DATA LINK COMMUNICATIONS (CPDLC)

Controller-pilot data link communications (CPDLC) is a means of communication between

controller and pilot, using data link for ATC communication.

The CPDLC application provides air-ground data communication for ATC service. This includes

a set of clearance/information/request message elements which correspond to voice

phraseology employed by ATC procedures. The controller is provided with the capability toissue level assignments, crossing constraints, lateral deviations, route changes and

clearances, speed assignments, radio frequency assignments, and various requests for

information. The pilot is provided with the capability to respond to messages, to request

clearances and information, to report information, and to declare/rescind an emergency. The

pilot is, in addition, provided with capability to request conditional clearances (downstream)

and information from a downstream ATSU. A "free text" capability is also provided to

exchange information not conforming to defined formats. An auxiliary capability is provided to

allow a ground system to use data link to forward a CPDLC message to another ground

system.

Controllers and pilots will use CPDLC in conjunction with the existing voice communication. Itis expected to be used for routine or frequent types of transactions. Although initial

implementation is intended to conform to existing procedures, it is anticipated that future

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evolution of the system and procedures will result in the greater automation of functions for

both aircraft and ground systems.

The introduction of CPDLC does not affect the principle that there is only one controlling

authority for a given aircraft at a given time. The capability for the pilot to request

downstream clearances does not affect this principle.

Sending a message by CPDLC consists of selecting the recipient, selecting the appropriatemessage from a displayed menu or by other means which allow fast and efficient message

selection, and executing the transmission. The received message may be displayed and/or

printed. A message sent by a downstream ATSU will be distinguishable from a CPDLC

message sent by the current ATS unit.

CPDLC may be used to remedy a number of shortcomings of voice communication, such as

voice-channel congestion, misunderstanding due to poor voice quality and/or

misinterpretation, and corruption of the signal due to simultaneous transmissions.

Implementation of CPDLC will significantly change the way pilots and controllers

communicate. The effect of CPDLC on operations should be carefully studied before deciding

the extent to which voice will be replaced by data link.

The following aspects of CPDLC should be taken into account in considering its application and

in defining procedures:

a) the total time required for selecting a message, transmission of the message, andreading and interpretation of the message;

b) the head-down time for the pilot and controller; andc) the inability of the pilot to monitor other data link transmissions to and from other

aircraft in the same area of operation.

4.3.1 CPDLC definitions

Current data authority (CDA). The ground system which is permitted to conduct a CPDLC

dialogue with an aircraft.

Downstream clearance (DSC). A clearance issued to an aircraft by an ATC unit that is not

the current controlling authority of the aircraft. Unless coordinated, downstream clearances

shall not affect the aircraft's original flight profile in any airspace, other than that of the ATC

unit responsible for the delivery of the down-stream clearance.

Downstream data authority (DDA). The ground system which is permitted to conduct a

DSC dialogue with an aircraft.

Next data authority (NDA). The ground system so designated by the CDA.

4.3.2 Use of CPDLC in ATS

CPDLC is expected to be used for routine operations in areas where the use of voice

communication is considered not efficient or unnecessary, thereby reducing voice-channel

use and possibly reducing the number of required voice channels.

Where CPDLC is used as the primary method of communication between an aircraft and theCDA, voice communication will continue to be required. Voice is still particularly suited where

a rapid-exchange, short-transaction communication style is required. It is recognized

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however, that the use of voice alone negates the capability of simultaneously updating the

flight data processing system (FDPS) or flight management system (FMS) coincident with the

entry and acknowledgement of CPDLC messages.

CPDLC messages are classified according to uplink and downlink categories. Each message

has associated urgency, alerting and response attributes.

The CPDLC application has three primary functions:

a) the exchange of controller-pilot messages with the current data authority;b) the transfer of data authority involving current and next data authority; andc) downstream clearance delivery with a downstream data authority.

CPDLC links

To accomplish the CPDLC application, three CPDLC links are defined:

- CDA link : the CPDLC link with the current data authority;

- NDA link : the CPDLC link with the next data authority; and

- DDA link : the CPDLC link with a downstream data authority.

4.3.3 GENERAL REQUIREMENTS

Message transmission

The CPDLC application requires:

a) that messages are generated and sent in a time ordered sequence; andb) that messages are delivered in the order that they are sent.

The system will ensure that messages are sent to the specified recipient.

When a ground system receives a message requesting an unsupported function or service,

the ground system will respond indicating that the requested service is unsupported.

The system will be capable of supporting up to 64 unfinished message exchanges between

one ground system and each of the aircraft with which it is linked.

Quality of service

The ground system will have the ability to specify its required QoS based on a user-preferred

combination of message delay, cost, and permissible error rate.

Time requirements

Wherever time is used in the CPDLC application, it will be accurate to within 1 second of UTC.

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Time stamping will be available for all messages. The timestamp will consist of the date

(YYMMDD) and time (HHMMSS). The timestamp will be the time the message is dispatched by

the originating user.

CPDLC Priority

The priority for all CPDLC messages will be "high priority flight safety message" as determined

by the ATN Internet Protocol Priority categorization.

4.3.4 DATA-LINK REQUIREMENTS FOR CPDLC

4.3.4.1 Controller pilot message exchange with the Current Data Authority

This function allows the establishment of a CDA link between an aircraft and the CDA for the

exchange of CPDLC messages. This function provides messages for the following:

a) general information exchange;

b) clearance:

1) delivery,

2) request, and

3) response;

c) level/identity surveillance;

d) monitoring of current/planned position;

e) advisories:

1) request, and

2) delivery;

f) system management functions; and

g) emergency situations.

4.3.4..2 Transfer of Data Authority

The transfer of data authority function provides the capability for the CDA to designate

another ground system as the Next Data Authority (NDA). Once a ground system becomes the

NDA, an NDA link can be established. This capability is intended to prevent a loss of

communication that would occur if the NDA were prevented from actually establishing any

CPDLC link with an aircraft until it became the CDA.

4.3.4.3 DownStream Clearance message exchange

The DSC function provides the capability for the aircraft to establish a Downstream Data

Authority (DDA) link with an ATS unit which is not the current data authority for the purpose of

receiving a downstream clearance.

Although the entire CPDLC message set will be available for the DSC function, it is

recommended that regional agreements be established to govern the use of the CPDLC

message elements for the DSC function. It is expected that the primary use of the DSC

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function will be for clearance delivery messages. Other uses of the DSC function may be

identified.

There will be procedures that prevent the pilot from executing a clearance received on a DDA

link until the aircraft enters the airspace of that DDA. If the information a pilot receives on the

DDA link requires action while still in the airspace of the CDA, the clearance for such action

must be obtained from the CDA.

4.3.5 COMPOSITION OF A CPDLC MESSAGE

A CPDLC message is composed of a message header, and from one to five message elements.

The message header for air-ground message exchange is composed of a message

identification number, a message reference number if required, a timestamp, and an

indication if a logical acknowledgement is required (optional).

A message element consists of a message element identifier, data as indicated by the

specified message element, and associated message element attributes.

Free text messages may contain the IA5 character set, consisting only of the following

characters: (0...9) (A..Z) (,) (.) (I) (-) (+) ( ( ) ( ) ) and the space character.

Message identification numbers

Message identification numbers provided by a CPDLC ground system for messages to/from an

aircraft have no relationship to the message identification numbers provided by the same

ground system for another aircraft.

Similarly, message identification numbers provided by an aircraft on a given CPDLC link for

messages to/from a ground system have no relationship to the message identificationnumbers provided by the same aircraft with another ground system.

The message identification number provided by the ground user will be different from any

other message identification number currently in use with that particular aircraft.

The message identification number provided by the avionics will be different from any other

message identification number currently in use with that particular ground system.

A message identification number will be deemed currently in use until:

Scenario a): the message does not require a response: the message has been sent; or

Scenario b): the message requires a response: the closure response has been received.

When a CDA or DDA link is established, all message identification numbers will be considered

available.

Message identification numbers should be provided sequentially.

Message reference numbers

All response messages will contain a message reference number.

The message reference number will be identical to the message identification number of the

initiating message to which it refers.

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Message attributes

Message attributes dictate certain message handling requirements for the CPDLC user

receiving a message. Each CPDLC message has three attributes: urgency, alert and response

attributes.

Urgency

The urgency (URG) attribute delineates the queuing requirements for received messages that

are displayed to the end-user.

Table 4.2. Urgency attributes (uplink and downlink)

Alert

The alert (ALRT) attribute delineates the type of alerting required upon message receipt.

Table 4.3. Alert attributes (uplink and downlink)

Resonse

The response (RESP) attribute mandates response requirements for a given message

element.

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Type Description Precedence

D

U

N

L

Distress

Urgent

Normal

Low

1

2

3

4

Type Description Precedence

HM

L

N

HighMedium

Low

No alerting

required

12

3

4



Type Response required Valid responsesPreceden

ce

W/U

Yes

WILCO, UNABLE, STANDBY

permitted, LOGICAL

ACKNOWLEDGEMENT (only if

required), ERROR (if necessary)

1

A/N

Yes

AFFIRM, NEGATIVE, STANDBY

permitted, LOGICAL



2

R

Yes

ROGER, UNABLE, STANDBY

permitted LOGICAL



3

Y

Yes

Any CPDLC downlink message,

LOGICAL ACKNOWLEDGEMENT

(only if required)

4

N

No, unless logical

acknowledgement

required

LOGICAL ACKNOWLEDGEMENT

(only if required), ERROR (if

necessary, only when logical

acknowledgement is required)

5

Table 4.4. Response attributes (uplink)

Type Response required Valid responses Precedence

Y Yes

Any CPDLC uplink message, LOGICAL


required)

1

N No, unless logical

acknowledgement

required

LOGICAL ACKNOWLEDGEMENT (only

if required), ERROR (if necessary,only when logical acknowledgement

is required)

2

Table 4.5. Response attributes (downlink)

Attribute association

For each message element, urgency, alert, and response attribute types are associated with it

are specified in Doc 9694-AN/955 Part IV Appendix A to Chapter 3.

When a message contains a single message element, the message attributes are the

message element attributes.

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When a message contains multiple message elements, the highest precedence message

element attribute type becomes the attribute type for the entire message. Message element

attribute table entries are listed in order of precedence (Le. a precedence value of 1 is

highest, followed by 2, etc.). For example, this means that a message containing multiple

message elements, where at least one element has a W/U attribute, the whole message then

has a W/U attribute.

Response messages

A message containing the ERROR message element will always be permitted as a response

message.

Any message that is considered a response message (i.e. it contains a message reference

number) will have message urgency and alert attributes not less than the message to which it

refers.

If the CPDLC user sends a message containing the ERROR message element instead of the

expected response message, the ERROR message will contain the initiating message

identification number as the message reference number. This ERROR message will be a

closure response message.

Logical acknowledgement messages

The logical acknowledgement provides confirmation from a receiving system to the message

originator that the message has been successfully received and is acceptable for display to

the responsible person, if this is required. The logical acknowledgement in no way replaces

any required operational response.

A ground system will determine if the use of the logical acknowledgement (either air or

ground) is permitted/ required within its airspace.

The logical acknowledgement message element must not be combined with any other

message element in a message.

A logical acknowledgement response message, if required, will be sent prior to sending any

other related response message(s), other than an ERROR message, if necessary.

Message differentiation

A CPDLC message intended for transmission on a DDA link must be clearly distinguishable

from a CPDLC message intended for transmission on a CDA link.

4.3.6 CPDLC MESSAGE RECEIPT REQUIREMENTS

CDA/DDA link messages

When a CPDLC user places all messages received from both a CDA and a DDA in the same

queue, the messages from a CDA will be placed ahead of messages from a DDA, regardless of

message urgency.

A CPDLC message received on a DDA link must be clearly distinguishable from a CPDLCmessage received on a CDA link.

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Logical acknowledgement prohibited

Upon receipt of the CPDLC message USE OF LOGICAL ACKNOWLEDGEMENT PROHIBITED the

aircraft will be prohibited from requiring a logical acknowledgement for any message

exchanged with that ground system for the duration of a CDA or DDA link.

Urgency requirements

When a CPDLC user queues received messages, messages with the highest urgency type will

be placed at the beginning of the queue.

When a CPDLC user queues received messages, messages with the same urgency type will be

queued in order of receipt.

Alerting requirements

CPDLC will provide three distinct alerts determined by the received message alert attribute.

Message response requirements

A CPDLC user will only respond to a received message in its entirety. This means, for

example, that if three message elements are concatenated in a single message, any response

given applies to the whole message, and not to any individual message element.

If a message is received that requires a response, the CPDLC user will:

a) be allowed to send any permitted response messages; and

b) send one and only one closure response message.

For a given message, once the CPDLC user has sent the closure response message, no otherresponse messages will be sent.

Table-4.6 contains the closure response messages permitted for each message response

category.

The CPDLC application must provide the message initiator the capability to provide closure for

a CPDLC message, independent of CPDLC closure message receipt.

Permitte

d

respons

e

Closure response

W/U

A response message containing one of the following:

WILCO, UNABLE, STANDBY, NOT CURRENT DATA AUTHORITY,

NOT AUTHORIZED NEXT DATA AUTHORITY,

LOGICAL ACKNOWLEDGEMENT (only if required), ERROR

A/N


AFFIRM, NEGATIVE, STANDBY, NOT CURRENT DATA

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AUTHORITY, NOT AUTHORIZED NEXT DATA AUTHORITY,


R


ROGER, UNABLE, STANDBY, NOT CURRENT DATA AUTHORITY,

NOT AUTHORIZED NEXT DATA AUTHORITY,


Y

The first response message sent from the aircraft that does

not contain a STANDBY or a LOGICAL ACKNOWLEDGEMENT

(only when required)

The first response message sent by the ground that does not

contain a STANDBY, a LOGICAL ACKNOWLEDGEMENT (only

when required), or REQUEST DEFERRED message element

from the ground system.

N

The only permitted response from the aircraft will be one of

the following:

LOGICAL ACKNOWLEDGEMENT (only if required), NOT

CURRENT DATA AUTHORITY, NOT AUTHORIZED NEXT DATA

AUTHORITY, or ERROR

The only permitted response from the ground will be one of

the following:

LOGICAL ACKNOWLEDGMENT (only if required), SERVICEUNAVAILABLE, FLIGHT PLAN NOT HELD, or ERROR

Table 4.6. Permitted closure responses by response category

4.3.7 ESTABLISHMENT OF THE CDA OR NDA LINK

Either the airborne system or the ground system can request CPDLC. Acceptance by either

the airborne system or the ground system of a request for CPDLC establishes a CDA or NDA

link.

Upon acceptance of a CDA or NDA link, the CPDLC application will have the capability of

informing both the controller and pilot of this link establishment.

An aircraft will be permitted to request CPDLC with any ground system, if the aircraft has no

existing CDA or NDA link. If the ground system accepts the CPDLC request, that groundsystem will become the CDA.

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Only if an aircraft has received a message from the CDA designating an NDA will the aircraft

be permitted to request CPDLC with the specified ground system.

In general, ground acceptance of an airborne request for CPDLC is determined by local

procedures.

However, if a ground system receives a request for CPDLC from an aircraft, for which it

currently has a CDA or NDA link, it will:

a) accept the request; and

b) cancel the first NDA or CDA link.

Note.- The aircraft could realize that a CDA or NDA link has been lost, and request CPDLC

before the ground is aware of the loss of the CDA or NDA link. By allowing the ground to

accept a "second" CDA or NDA link from the aircraft, the potential for loss of communication

is minimized.

If the ground requests CPDLC with an aircraft and the aircraft does not have a CDA or NDAlink, then the aircraft will accept the CPDLC request and consider the ground system as the

CDA.

If the ground requests CPDLC with an aircraft and the aircraft already has a CDA link, the

aircraft will accept the CPDLC request if

a) the request is from the ground system that is the CDA; or

b) the request is from the NDA.

If the aircraft accepts a "second" CDA or NDA link, the "first" CDA or NDA link with that

ground system will be terminated.

Note.- The ground could realize that a CDA or NDA link has been lost, and request CPDLC

before the aircraft is aware of the loss of the CDA or NDA link. By allowing the aircraft to

accept a "second" CDA or NDA link from the ground, the potential for loss of communication is

minimized.

The aircraft will reject a request for CPDLC from any other ground system, and will indicate to

the requesting ground system what ground system is the CDA.

The aircraft will disregard CPDLC messages over the NDA link and indicate to the originator

that it is not the CDA.

Only the CDA can designate a ground system as the NDA.

The CDA can designate only one ground system as the NDA at a time (i.e. only one per CPDLC

message).

An airborne system will only consider a ground system as the NDA if it has received such an

indication from the CDA.

Any indication from the CDA designating an NDA will replace any previously received NDA

designation for another ground system.

If an NDA message element is received without specifying a facility (null), any previously

specified NDA is no longer valid.

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If the ground system rejects a request for CPDLC, it will provide a reason for the rejection

using a CPDLC message.

4.3.8 ESTABLISHMENT OF THE DDA LINK

Only the airborne system can request DSC. Acceptance by the ground system of a request for

DSC establishes a DDA link.

Upon acceptance of a DDA link the CPDLC application will have the capability of informing

both the controller and pilot of this link establishment.

If an aircraft has no DDA link, that aircraft will be permitted to request DSC with any ground

system that is not its CDA. The ground system may only accept a request for DSC if it has a

filed flight plan for the requesting aircraft. If the ground system accepts the DSC request, that

ground system will become the DDA.

Generally, ground acceptance of an airborne request for DSC, even when the ground has a

filed flight plan for that aircraft, is determined by local procedures.

However, if a ground system receives a request for DSC from an aircraft, for which it currently

has a DDA link, it will:

a) accept the request, and

b) cancel the first DDA link.

Note.- The aircraft could realize that a DDA link has been lost, and request DSC before the

ground is aware of the loss of the DDA link. By allowing the ground to accept a "second" DDA

link from the aircraft, the potential for loss of communication is minimized.

If the ground system rejects a request for DSC, it will provide a reason for the rejection using

a CPDLC message.

4.3.9 LINK TERMINATION AND TRANSFER

Once normal link termination is initiated only CPDLC closure response messages may be

exchanged over the CDA or DDA link being terminated.

Once termination is initiated, the system will have the capability of informing the pilot or

controller of this action.

When normal link termination is initiated and there are still outstanding responses required,the pilot and controller will be informed of any message for which closure is outstanding.

If a CDA or NDA link is terminated for any reason, any DDA link will not be affected.

Normally, CPDLC service termination with the CDA is initiated by the ground system to end

service or transfer service to the next ATS facility.

The ground system will not perform a normal termination of the CDA or DDA link while there

are any CPDLC messages for which closure is outstanding.

Any NDA link will be terminated by the aircraft if it receives a subsequent designation of NDA.When terminating an NDA link in this situation the aircraft will indicate to the ground system

being terminated that it is no longer the NDA.

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When the CDA link is terminated normally, the aircraft will recognize the ground system

currently designated NDA as the CDA.

If the CDA link is terminated for any reason other than under instruction from the CDA, any

designation of a ground system as an NDA will be deleted, and any NDA link in place will be

terminated.

In the event of an unexpected termination of the CDA link, the CDA should again send theNDA information to the aircraft, if an NDA is in place.

Only an aircraft can normally terminate a DDA link.

DDA normal link termination will be automatically initiated if a DDA becomes a CDA, and the

pilot will be informed of this action.

4.3.10 MESSAGE PRESENTATION

The presentation of messages is a local implementation.

For more information on CPDLC message formats you may refer to Doc 9694-AN/955 Part IV

Chapter 3 - Appendix A for the CPDLC message element description, Appendix B contains a

data glossary and Appendix C provides data range and resolution.

4.3.11 CPDLC MESSAGE SET

The CPDLC message set includes uplink and downlink messages. The list of messages is

described in ED100A/DO258A.

The CPDLC application allows the controller to uplink messages from a set of

around 180 CPDLC

Uplink messages. These messages cover:

Responses and acknowledgments to downlink CPDLC Messages (e.g. UNABLE, STANDBY, ROGER,…); Vertical Clearances (e.g. CLIMB TO AND MAINTAIN [altitude]); Crossing Constraints (e.g. EXPECT TO CROSS [position] AT [altitude]); Lateral Offsets (e.g. OFFSET [direction][distanceoffset] OF ROUTE); Route Modifications (e.g. PROCEED DIRECT TO [position]); Speed Changes (e.g. INCREASE SPEED TO [speed]); Communications management (e.g. CONTACT [icaounitname][frequency]); Surveillance request (e.g SQUAWK [beaconcode]); Report/Confirmation request (e.g. REPORT REACHING [altitude]); Negotiation request (e.g. WHEN CAN YOU ACCEPT [level]); Air traffic Advisory (e.g. RADAR SERVICE TERMINATED); System Management Messages (e.g. NEXT DATA AUTHORITY [Facility designation]); Additional messages (e.g. DUE TO TRAFFIC); Freetext messages, allowing the uplink of messages not covered by the original set.

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The CPDLC applications allows the pilots to downlink messages from a set of

around 80 CPDLC

Downlink messages. These messages cover:

Responses and acknowledgments to uplink CPDLC Messages (e.g. WILCO, UNABLE,STANDBY, ROGER,…);

Vertical requests (e.g. REQUEST [altitude]); Lateral Offset requests (e.g. REQUEST OFFSET [direction][distanceoffset] OF

ROUTE); Speed requests (e.g. REQUEST [speed]); Voice contact requests (e.g. REQUEST VOICE CONTACT [frequency]); Route modification requests (e.g. REQUEST DIRECT TO [position]); Several types of Reports (e.g. LEAVING [altitude]); Negotiation request (e.g. WHEN CAN WE EXPECT [speed]); Emergency messages (e.g MAYDAY MAYDAY MAYDAY);

System Management messages (e.g. NOT CURRENT DATA AUTHORITY); Additional messages (e.g. DUE TO AIRCRAFT PERFORMANCE); Freetext messages, allowing the downlink of messages not covered by the original

set.

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COMMUNICATIONS SERVICE PROVIDER

5.1 COMMUNICATIONS SERVICE PROVIDER

Data link communications between aircraft and air traffic control (ATC) require an interfacebetween air/ground data link networks and ATC systems that recognizes different message

formats—both current and future. Communication Service Providers SITA and ARINC meet the

Communications, Navigation, Surveillance/Air Traffic Management (CNS/ATM) Gateway needs.

In other words AFN, ADS and CPDLC related uplink and downlink messages between the

ground system and the aircraft are all routed through the ARINC’s CNS/ATM Gateway or SITA’s

AIRCOM Data Link Traffic handling application (ADLT). The SITA Host Processor is in Singapore

and ARINC’s is in Annapolis. SITA and ARINC they are basically the two ATS communications

service provider

The Gateway supports full automatic dependent surveillance (ADS) and controller-pilot datalink communications (CPDLC) message sets, in accordance with existing standards and

specifications. For CPDLC, the Gateway allows the ATC end system to encode and decode bit-

oriented messages in accordance with a standard protocol (RTCA DO-219). These encoded,

two-way data link messages are then converted to ARINC 622 format and sent to the aircraft

over the Aircraft Communications Addressing and Reporting System (ACARS) network.

About ARINC

Since in 1929, ARINC has been a leader in aviation communications. ARINC is a private

company owned by many of the world's airlines including; American Airlines, Continental

Airlines, British Airways, Air France, and SAS. Other non-airline companies also own a share of ARINC including the Ford Motor Company.Today, a company that’s recognized as the leading

provider of transportation communications and systems engineering solutions for five major

industries: aviation, airports, defense, government, and transportation with its Headquartered

in Annapolis, Maryland, having two regional headquarters - Singapore, established in 2003 for

the Asia Pacific region, and London, established in 1999 to serve the Europe, Middle East, and

Africa region.

About SITA

SITA is a leading service provider of IT business solutions and communications services to the

air transport industry. SITA manages complex communication solutions for its air transport,government and GDS customers over the world’s most extensive communication network,

complemented by consultancy in the design, deployment and integration of communication

services. Provides the air transport industry with the information and communications

technology (ICT) it needs to operate seamlessly in every corner of the world. Their main office

is in Geneva, Switzerland. Their Geneva office provides services for Europe, Middle East &

Africa, and the other two regional offices are at Sydney, Australia for Asia Pacific region and

Montreal, Canada for America. As a partner with airlines, airports and the many related air

transport organizations, SITA has worked closely with the community as it has evolved over

the last 55 years. They've also evolved in that time to be a very different organization today

than when they were founded in 1949 as the 'Société Internationale de TélécommunicationsAéronautiques', now using only the shortened version SITA

SITA is the ATS communications service provider for Airports Authority of India.

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5.1 BACKGROUND The International Civil Aviation Organization (ICAO) ADS Panel (now called Air Traffic

Management Operational Concept (OPLINK) Panel) has produced an ATS Datalink Applications

Manual which specifies the Automatic Dependent Surveillance (ADS) and Controller Pilot Data

Link Communications (CPDLC) applications of Future Air Navigation Systems (FANS).

The FANS 1/A ADS/CPDLC applications uses Aircraft Communication Addressing and Reporting

System (ACARS) which is now installed in most widebody civil aircraft and some military

aircraft. FANS 1/A uses the existing ACARS datalink network infrastructure for CPDLC and

ADS.

Rather than wait for the ATN, in 1992 Boeing developed the processing of the CPDLC and ADS

applications in the Honeywell Flight Management System on the Boeing 747-400 as FANS-1,

and certified it in 1995. Airbus followed with its development of FANS-A, certified in 2000.

The SITA AIRCOM network provides communications services for ACARS equipped aircraft.

The ATS AIRCOM FANS 1/A Service enables ATS provider ground systems to communicate

with FANS-1/A equipped aircraft via the VHF and Satellite AIRCOM ACARS data link service.ATS Internetworking enables aircraft equipped with HF data link to also use the FANS 1/A

Service.

FANS-1/A Standards

Boeing specified the FANS-1 system in an "Air Traffic Services (ATS) System Requirements

and Objectives (SR&O)" document which specified the Honeywell package. The FANS-1

requirements were first frozen at the beginning of 1993 but the document has continued to be

updated and Boeing has issued upgrades to the initial FANS-1 package for various airframes.

The development by Airbus of a FANS-A "System Requirements and Objectives (SR&O)" fortheir implementation led to the creation of the joint RTCA SC189 and Eurocae WG-53 group to

develop industry standard requirements for FANS-1/A services.

The FANS-1 SR&O specifies the implementation of the ADS and CPDLC which are based on

standards which the aeronautical industry bodies developed and which formed the basis for

the ICAO ADS Panel’s development of the ICAO Standards.

The FANS-1 ADS application conforms to the Airlines Electronic Engineering Committee (AEEC)

Characteristic 745 for ADS avionics. This is very similar to the RTCA Minimum Operational

Performance Standard (MOPS) for ADS which was issued as RTCA DO-212.

The FANS-1 CPDLC application conforms to the RTCA MOPS for Two-Way Data Link (TWDL)

which was issued as DO-219.

The SITA AIRCOM network was used in the US Federal Aviation Administration approval of the

first FANS-1 installation which was on a QANTAS B747-400. The operational experience gained

since that approval in mid 1995, has enabled SITA to continually enhance the ATS AIRCOM

FANS-1/A service performance within the limitations of ACARS.

The ATN specification of ADS includes all the capability the FANS-1/A ADS application provides

and some additional capability. The ATN specification of CPDLC includes all the FANS-1/A

CPDLC messages and some additional messages.

In the FANS-1 package the ATS Facilities Notification (AFN) function provides the data link

initiation capability provided by the ATN Context Management Application (CMA). The AFN has

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been demonstrated to work and its replacement by the CMA will not significantly change the

service provided to the ADS and CPDLC users.

ATS AIRCOM FANS -1/A Service

The SITA ATS AIRCOM service enables Air Traffic Service providers to implement ATS data link

communications with aircraft using the SITA VHF AIRCOM and Satellite AIRCOM networks.

The ATS AIRCOM FANS-1/A Service enables ATS providers to operate the FANS AFN (ATS

Facilities Notification), and FANS 1/A ADS and CPDLC applications via the SITA AIRCOM

network with aircraft which are equipped with FANS-1/A avionics.]

The VHF AIRCOM network support data link communications via direct VHF radio links. The

Satellite AIRCOM service supports data link communications via links provided by the

INMARSAT Aeronautical Mobile Satellite System (AMSS). The SITA VHF AIRCOM service

provides VHF data link service to aircraft equipped with ACARS avionics.

The propagation characteristics of VHF radio signals require line-of site coverage. SITA has an

extensive global VHF data link ground station network. SITA has VHF internetworkingagreements which allow VHF AIRCOM user aircraft to access the service via other networks

such as the AVICOM network in Japan and DATACOM network in Brazil. SITA provides a data

link service to aircraft through the Satellite AIRCOM network, made up of the INMARSAT

satellites and two double Ground Earth stations operated by France Telecom and Telstra

Australia.

SITA provides a FANS Service to over 1,000 FANS- equipped aircraft. Some aircraft establish

AMSS links to GES other than the Satellite AIRCOM GES and connect the ACARS Datalink

Service provided by ARINC. SITA and ARINC have an ATS Internetworking connection for

FANS-1/A communications which enables ATS AIRCOM users to communicate with theseaircraft thus ensuring that they can communicate with all the FANS-1/A equipped aircraft in

their airspace.

AIRCOM ACARS network

The SITA AIRCOM ACARS data link network is made up of a collection of VHF and AMSS ground

stations connected to a data link processor.

The VHF AIRCOM ACARS ground stations consist of a standard VHF transceiver designed for

voice communications connected to a computer specifically developed to convert between

VHF ACARS signals on the radio link and data messages on the link to the AIRCOM Datalink Traffic (ADLT) handling application.

The Satellite AIRCOM GES are much more powerful and costly than the VHF AIRCOM stations

because they need to support the AMSS data and voice communications of the larger number

of aircraft which can be in the coverage of one of the INMARSAT geo-stationary satellites.

The AIRCOM Datalink Traffic (ADLT) handling application interconnects the air-ground network

to the SITA terrestrial network. ATS Provider ground systems access the ATS AIRCOM service

by connecting to the ADLT.

The ICAO standard ADS and CPDLC applications require a network which can transport binarydata with any content and not just character codes. A FANS-1/A ADS/CPDLC avionics package

has been implemented which uses a special interface defined in AEEC Specification 622 which

enables the use ACARS networks.

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This requirement is independent of the ground system access methods offered by AIRCOM to

help customers gain access to AIRCOM services:

A Direct Host Processor (DHP) connection between the customer host and the ADLTacross the SITA X.25 or IP service, ensuring reliability and high performance.

Indirect access via the MegaSWitch (MSW), for storing and forwarding type B messagesincluding ACARS.

Figure 5.2 -a

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Figure 5.2 –b AIRCOM Datalink architecture and protocol

Direct X.25 or IP access for optimal performance

When VHF ACARS networks were first introduced, the only network that provided a messaging

service among airline ground systems was the Type B network.

The Type B service requires the MSW to guarantee that all messages received are deliveredto their destination. If the MSW cannot deliver the message immediately, it keeps a copy until

it can do so.

The MSW was not specifically adapted to air-ground data communication and its use could

limit AIRCOM performance. Therefore, as airlines began to use AIRCOM for applications that

were critical to timely flight operations, SITA introduced a solution – to provide the option of

bypassing the MSW using a direct connection through the SITA X.25 network to the ADLT

instead. This requires an adaptation of software in the customer’s ground system to address

the AIRCOM traffic directly to the ADLT, rather the MSW.

The spread of Internet Protocol (IP) networking throughout the industry currently allowsairlines to integrate their various IT systems, providing benefits and synergies across their

organization. With SITA’s new IP access service known under the name of ACARS Over IP,

aircraft too can be integrated into this infrastructure.

An AIRCOM end-to-end solution

To help customers implementing Datalink, AIRCOM has also developed customer premises

and SITA hosted solutions (see figure 5.3):

AIRCOM Server offers data processing and – installed as a front end of the user mainframe –

may avoid the software adaptation required in the user’s ground system to configure adedicated connection X.25 or IP with the service provider.

AIRCOM Service Bureau is a SITA hosted solution designed for reformatting messages as per

customer needs, supporting X.25 and IP communication with ground applications.

SITA also offers an ACARS Access Gateway customers can install on their premises purely to

handle the direct connection to the ADLT, enabling them to quickly implement improved

interfaces with data link networks.

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Figure 5.3 AIRCOM ground access solution

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5.3 Fast and slow message paths for uplinks

Figure 5.4

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5.4 Fast and slow message paths for downlinks

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Figure 5.5

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S. Kobiraj IT (IDP -25) 32

Satellite(contd.)

4 satellites provide global coverage



Figure 5.6 INMARSAT Satellite Coverage

Table 5.1 INMARSAT Satellite Service Status

i. POR – Pacific Ocean Regionii. IOR – Indian Ocean Regioniii. AOR(E/W) – Atlantic Ocean Region (East/West)

Figure 5.7 SITA VHF Aircom coverage (Remote Ground Stations – RGS)

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S. Kobiraj IT (IDP -25) 38

Datalink in over 165 countries and over 800 ground stations,

forming the world's largest VHF data network

VHF AIRCOMcoverage



Figure5. 8 SITA VHF Remote Ground Stations – India

5.5 Communication Problems

There are many places in the network where a communications problem can occur.

The ATS Unit Software Hardware

The communications service provider. The satellite provider. The aircraft avionics (ACARS MU, FMC, etc) etc.

This flowchart provides some ideas about the different communications problems, and what

needs to be done to solve them…..

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Communication

problem?

What’s

the

problem

like?

Look for ‘patterns’ –

location, time of day,

distance from FIR

boundary, type of

message …..

Likely to be an

aircraft procedure or

system problem.

You will need to

investigate:

i. Commn. software

ii. Commn. Hardware

Check for errormessages.



Problems with just Some of the time,

one aircraft? With some of the

aircraft?

Problem with several aircraft,

or all aircraft?

Figure 5.9 showing messages associated with uplinks

(OCS here is the New Zealand ground system)

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Provider



5.5.1 System Queue Error messages

5.5.1.1 No Delivery Confirmation.

If system does not receive the delivery confirmation message within 2 minutes of the uplink

being initiated, it generates a SITA_NL no delivery confirmation message and sends it tothe system queue. The message contains the aircraft's registration and the message for

which no delivery confirmation has been received.

Examples of typical messages

CPDLC Messages

SITA_NL - No delivery confirmation from : <.ZK-NBT>

For <DESCEND TO REACH F350 BY 301605>

SITA_NL - No delivery confirmation from : <.ZK-NBS>

For <CLEARED TO DEVIATE UP TO L 030 NM,

REPORT BACK ON ROUTE,

Freetext : TEST>

CPDLC Connection Request

SITA_NL - No delivery confirmation from : <.9V-SPF>

For <CPDLC Connection Request : NZZO Label B>

ADS Uplink

SITA_NL - No delivery confirmation from : <.VR-HOZ>

For <ADS Uplink message>

AFN ACK Message

SITA_NL -. No delivery confirmation from

For </FMHSQ002, .N106UA,,014606/ : <.9V-SPF>FAKO,NZZO/FARADS,O/FARATC,O>

Note:

Because this error message is generated by the ground system at a VSP (Vertical Seismic

Profiling) time it is still possible to get the response from the aircraft to the original uplink

after the error message is generated.

5.5.1.2 Delivery failure message.

At times the communications service provider is unable to deliver the message and the

delivery confirmation message will contain a failed delivery code.

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When the ground system receives this message it generates a SITA_NL - Delivery failure

message in the system queue referencing the message assurance number of the failed

message.

For example:

SITA_NL - Delivery failure message received for Seq : <91>

5.5.1.3 Downlink failures.

Occasionally a downlink message will be received and rejected by the system.

When this happens ground system generates a MSSW_TL - Downlink message ACARS/

PER layer failed in the system queue referencing the message.

Here are two examples:

MSSW_TL - Downlink message PER layer failed: < AT1.VH-

OJQ14F4588482DE480E9B889DODF60B6……..>

MSSW_TL - Downlink message ACARS layer failed < R U READY FOR TEST SCRIPT /Q/>

5.5.1.4 Uplink message failure.

Occasionally a software problem may prevent the ground system being able to send a uplink.

When this occurs the ground system will generate an error message in the system queue.

For example:

MSSW_TL - Uplink message PER layer failed : <CLEARED

Route:

29S175W 14S155W OlN145W lON139W 16N135W 22N130W 28N125W FFICKY >

5.5.1.5 ADS Report Overdue.

When an ADS report is overdue the OCS will generate a warning message to the controller in

the system queue.

For example:

From : DF (16926) ON nzd01

Date : 96 MAY 18 03:29:09.776

ADS report overdue, Aircraft tail number : .N187UA

5.5.1.6 Unexpected ADS Data.

When an ADS downlink is received but the aircraft is not in the AFN window the ground

system will generate an error message. This can occur if the controller uses the 'Delete

contact' button in the AFN window without first canceling the ADS contract. It can also occur if

for some reason the ground system fails and is then reloaded.

For example:

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From: DF (17845) on nzd01

Date: 96 MAR 3 20:59:21.135

Unexpected ADS downlink data was received.

Aircraft not found in ADS list, aircraft tail number: .VH-OJJ

NOTE: These messages will continue until the aircraft terminates ADS or until the aircraft

carries out another AFN logon.

5.5.1.7 ADS data received not in conformance with contract.

Occasionally it is possible to receive an ADS downlink which contains groups not specified in

the current contract held at the ground system. When this occur ground system generates a

warning to the controller in the system queue.

For example:

From : DF (17457) on nzd01

Date : 95 JUN 21 06:46:35.976

Aircrafts tail number: .VH-OJQ, contract request number: 3 The following received groups are not

conform with contract requirements:

Flight ID group is missing.

Predicted route group is present, but was not asked for. Airframe ID group is missing.

NOTE: To overcome this problem, establish another contract with the aircraft.

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5.5.2 AFN Errors in Downlink or Uplink Queues

AFN messages may contain a status indicator to indicate the success of a message exchange

or provide a reason for its failure.

5.5.2.1 Six reason codes

Reason Code Meaning

0 Successful

1 Protocol Error

2 Version Mismatch

3 Max resources exceeded

4 Could not match ill/position to flight plan

5 Unknown

Reasons 1-5 indicate failure of the message exchange in some way.

5.5.2.2 Examples of Initial Logon.

Example of a successful sequence:

FN_CON AFN Contact Message has been received

Flight ID: UAL841, Aircraft Registration: .N187UA

Time initiated: 20:48 :00 UTC

Position 370312S1744648E, Active flag set

Applications requested:

ADS version 01

ATC version 01

FN_AKAFN Acknowledgment Message has been sent

Flight ID: UAL841, Aircraft Registration: .N187UA

Time initiated: 20:48:04 UTC

Overall Status: Successful

ICAO address: NZZO

ADS,Status: Successful

ATC,Status: Successful

For a version mismatch on ADS, the overall status would remain successful, but the ADS

status line would read:

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ADS, Status: Version mismatch

5.5.2.3 Examples of Address Forwarding

A successful FN_ COMP message is as follows:

FN_COMP ATS facilities Notification Complete message has been received

Flight ID: ANZ18, aircraft Registration: .ZK-NBT


Next ATC center address: PPTCDYA, Status: Successful

An unsuccessful message:

FN_COMP ATS facilities Notification Complete message has been received

Flight ID: ANZ18, aircraft Registration: .ZK-NBT


Next ATC center address: PPTCDYA, Status: Protocol error

NOTE: It is important to read the Status fields of AFN messages to confirm that the

exchanges have been successful.

5.5.3 CPDLC Errors in the Downlink Queue

The following CPDLC error messages may be seen in the downlink queue:

Reason Code Meaning

0 application error

1 duplicate message identification number

2 unrecognised message reference number

3 end service with pending messages

4 end service with no valid response

5 insufficient message storage capacity

6 no available message identification number

7 commanded termination

8 insufficient data

9 unexpected data

10 invalid data

When the FMC detects a message error it sends a response message containing one of the

error codes above. The format of this message is

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ERROR [Error information].

5.5.3.1 Meaning of Error codes

Meanings of these error codes are as follows...

0 Application Error

Sent by FMC when error does not correspond to one of the other specific error

codes.

1 Duplicate Message Identification Number (MIN)

Sent by FMC when an uplink is received that contains a message identification

number corresponding to the message identification number of a previous

message which is still open.

2 Unrecognised Message Reference Number (MRN)

Sent by FMC when an uplink is received with an MRN and there is no open

downlink with a matching MIN.

3 End service with pending messages

Sent by FMC when a disconnect request is received and there are open uplinks in

the FMC. Note that this message means that all connections will have been

disconnected.

4 End service with no valid response

Sent by FMC when disconnect request is received together with another element

which requires either no response or a response other than wilco / unable.

5 Insufficient message storage capability

747-400 receives uplink with freetext element containing more than 80

characters, or uplink received when FMC log is full.

6 No available message identification number

Cannot occur on 747-400, means that FMC has only one MIN available that is notbeing used for a pending downlink.

7 Commanded termination

Sent when pilot turns ATC datalink off or changes the entered flight identification

in the FMC.

8 Insufficient data

Insufficient bits in uplink to define header, or element, or variable.

9 Unexpected data

Uplink contains more data than is specified.

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10 Invalid data

Uplink contains a variable outside valid range or contains a character FMC

cannot display.

NOTE: Some of these responses will automatically disconnect the CPDLC connection.

Details are contained in the Boeing ATS SR&O, Appendix G.

5.5.3.2 Here are some example messages...

ERROR: Command termination ERROR: Unrecognised Message Ref Number ERROR: End Service With Pending Messages Message:

Cpdlc disconnection due to Duplicate Message ID Number

Message:Cpdlc disconnection due to Application Error

5.5.3.3 FMC generated error messages

During the establishment of a CPDLC connection the FMC will check that it does not exceed its

limit of two connections and that the ATC facilities attempting to make the connection are

permitted in terms of the next data authority protocols. The FMC can generate various error

messages during this phase. These are described below:

Current Data Authority [facility designator]

If one CPDLC connection is established in the FMC it will only permit a secondconnection if the second connection is the next data authority (NDA) as specified by the

first (active) connection. If the second facility attempting the connection is not the NDA

then the FMC will send this error message. The message contains the ICAO facility

designator of the ATC facility that the FMC sees as current data authority. e.g Current

Data Authority

: YBBB

Not Current Data Authority The FMC will originate this message if the non-active CPDLC connection attempts to

send a CPDLC uplink to the aircraft.

5.6 ATS Unit addressSITA uses a 7-character address eg. Kolkata ADS SITA address is CCUCBYA; Chennai is

MAACAYA and so on. The first three-letters “CCU” identifies the city/ station code (you

must have noticed in your airlines baggage tags i.e. Chennai as MMA, Mumbai as BOM

etc.). The last four-letters “CBYA” identifies the nature of service/ facility code.

However, the pilot is not concerned with these 7-character addresses. The pilot must

know the unique four-letter datalink/ FANS address of each ATS Unit having datalink

capabilities. The logon address for Chennai is VOMF, Kolkata – VECF and so on. The

logon addresses are configured in SITA Server, which recognizes the address, convertsto 7-character SITA address and forwards the message to the respective ATS Unit.

Historical development –

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The airline industry has been making use of teletype technology since the early 1920s

using radios stations located at 10 airfields in the United States. The US Post Office and

other US government agencies used these radio stations for transmitting telegraph

messages. It was during this time period that the first federal teletype system was

introduced in the United States to allow weather and flight information to be exchanged

between air traffic facilities.

In 1929, Aeronautical Radio Incorporated (ARINC) was formed to manage radio

frequencies and licence allocation in the United States, as well as to support the radio

stations that were used by the emerging airlines, a role ARINC still fulfils today.

In 1949, the Société Internationale de Télécommunication Aeronautique (SITA) was

formed as a cooperative by 11 airlines: Air France, KLM, Sabena, Swissair, TWA, British

European Airways, British Overseas Airways Corporation, British South American

Airways, Swedish A. G. Aerotransport, Danish Det Danske Luftfartselskab A/S, and

Norwegian Det Norske Luftfartselskap. Their aim was to enable airlines to be able to

use the existing communications facilities in the most efficient and cost-effective

manner.

Morse code was the general means of relaying information between air

communications stations prior to World War II. Generally, it was only necessary to relay

a message between one or two stations. After World War II, there was an increase in

the number of commercial aircraft operating, and these aircraft were capable of flying

greater distances than in the past. As a result, the Aeronautical Fixed

Telecommunications Network (AFTN) was implemented worldwide as a means of

relaying the necessary air traffic communications, sometimes through the use of radio

teletype

Today, the airline industry continues to use teletype messages over ARINC,SITA or AFTN networks as a medium for communicating via messages. Most

teletype messages are machine-generated by automatic processes. IATA

standardizes teletype message formats throughout the airline industry. SITA/ARINC

networks uses 7-character addresses and in AFTN network 8-character addresses are

used viz. AMSS <VIDDZTZX> 1ST 4-letters ‘VABB” are station code (Mumbai) and the

last 4-letters “ZTZX” are facility code (Control Tower).

5.6 Type B Messaging

Type B Messaging Service has evolved over 50 years to become one of the world’s largest,

fastest and most reliable messaging services. It supports the world’s largest Type B

messaging community and regularly exchanges over 25 million messages a day.

• Assured delivery of Type B messages through the use of serial numberingor end-to-end protocols.

• Flexible addressing and routing options using multiple deliveries for asingle message, multiple destinations for a single address and groupcoding for multiple addresses.

Type B messaging is a proprietary communication standard defined by IATA and used

in the air transport and travel-related industry. Type B messages are recognised asvery reliable and highly performing and therefore deployed for mission-critical

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applications like booking seats, tracking cargo, issuing flight plans or providing

aerospace parts.

Standard Type B messages have a very strict pattern, i.e. a maximum length of 60 lines

of 63 characters each, and a limited set of allowed characters, i.e. only capital

characters A to Z, the numbers 0 to 9 and the signs “/”, “-“ and “.”. Besides, Type B

messages are transmitted in global private networks. As a consequence, participants of

the Type B world need access to these private networks via dedicated nodes and

require an internal infrastructure compatible with this proprietary communication

standard

SITA’s managed IP connectivity to Type B enables airlines to exchange messages

through IBM’s Message Queue (MQ) or Mapping Airline Traffic over IP (MATIP protocol).

You can connect to the service through one TCP/IP network connection and gain access

to all of SITA’s messaging systems. Operational hosts, business messaging systems and

open messaging solutions can all talk to SITA using the same network connection,

reducing costs and increasing functionality.

Type B Messaging Service can be accessed in a number of ways - X.25, AX.25, P1024,

P1124, IP VPN, ATeX and LAN Access.

5.8 Communications satellite

Communications satellite artificial satellite that functions as part of a global radio-

communications network. Echo 1, the first communications satellite, launched in 1960, was

an instrumented inflatable sphere that passively reflected radio signals back to earth. Later

satellites carried with them electronic devices for receiving, amplifying, and rebroadcasting

signals to earth. Relay 1, launched in 1962 by the National Aeronautics and Space

Administration (NASA), was the basis for Telstar 1, a commercially sponsored experimentalsatellite. Geosynchronous orbits (in which the satellite remains over a single spot on the

earth's surface) were first used by NASA's Syncom series and Early Bird (later renamed

Intelsat 1), the world's first commercial communications satellite.

In 1962, the U.S. Congress passed the Communications Satellite Act, which created the

Communications Satellite Corporation (Comsat). Agencies from 17 other countries joined

Comsat in 1964 in forming the International Telecommunications Satellite Consortium

(Intelsat) for the purpose of establishing a global commercial communications network.

Renamed the International Telecommunications Satellite Organization in 1974 and a private

corporation since 2001, Intelsat now has a network of 28 satellites in geosynchronous orbits

that provides instantaneous communications throughout the world. It has orbited several

series of Intelsat satellites, beginning with Intelsat 1 (Early Bird ) in 1965.

Inmarsat was established in 1979 to serve the maritime industry by developing satellite

communications for ship management and distress and safety applications. Inmarsat was

originally an intergovernmental organization called the International Maritime Satellite

Organization but later changed its name to the International Mobile Satellite Organization to

reflect its expansion into land, mobile, and aeronautical communications. In 1999 it became a

private company as Inmarsat, and the International Mobile Satellite Organization became

responsible for overseeing Inmarsat's public service obligations. Inmarsat's users now include

thousands of people who live or work in remote areas without reliable terrestrial networks.Inmarsat presently has ten satellites in geosynchronous orbits.

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In addition to the Intelsat and Inmarsat satellites, many others are in orbit, some managed

by private companies and others by government-owned operators. These are used by

individual countries, organizations, and commercial ventures for internal communications or

for business or military use. A new generation of satellites, called direct-broadcast satellites,

transmits directly to small domestic antennas to provide such services as cablelike television

programming.

Annexures

Explanation of Terms

Aircraft address. A unique combination of 24 bits available for assignment to an aircraft for

the purpose of air-ground communications, navigation and surveillance.

Aircraft identification. A group of letters, figures or a combination thereof which is identical

to or the code equivalent of the aircraft call sign. It is used in Field 7 of the ICAO model flightplan.

Air traffic services interfacility data communication (AIDC). A data link application that

provides the capability to exchange data between air traffic service units during the

notification, coordination and transfer of aircraft between flight information regions.

Automatic dependent surveillance (ADS). A surveillance technique in which aircraft

automatically provide, via a data link, data derived from on-board navigation and position-

fixing systems, including aircraft identification, four-dimensional position, and additional data

as appropriate. ADS is a data link application.

Automatic dependent surveillance (ADS) agreement. An ADS reporting plan which

establishes the conditions of ADS data reporting (i.e. data required by the air traffic services

unit and frequency of ADS reports which have to be agreed to prior to the provision of the

ADS services ).

Note.- The terms of the agreement will be exchanged between the ground system

and the aircraft by means of a contract, or a series of contracts.

ADS contract. A means by which the terms of an ADS agreement will be exchanged between

the ground system and the aircraft, specifying under what conditions ADS reports would be

initiated, and what data would be contained in the reports.

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Note.- The term "ADS contract" is a generic term meaning variously, ADS event

contract, ADS demand contract, ADS periodic contract or an emergency mode.

Ground forwarding of ADS reports may be implemented between ground systems.

Automatic dependent surveillance-broadcast (ADS-B). ADS-B is a surveillance

application transmitting parameters, such as position, track and ground speed, via a

broadcast mode data link, at specified intervals, for utilization by any air and/or ground users

requiring it. ADS-B is a data link application.

Availability. The ability of a system to perform its required function at the initiation of the

intended operation. It is quantified as the proportion of the time the system is available to the

time the system is planned to be available.

Baseline information: Required information upon which to measure certain type of ADS

events (altitude change event, air speed change event, ground speed change event, heading

change event and track angle change event).

Continuity. The probability of a system to perform its required function without unscheduled

interruptions during the intended period of operations.

Controller-pilot data link communications (CPDLC). A data link application that provides

a means of communication between controller and pilot, using data link for ATC

communications.

Data link application. A data link application is the implementation of data link technology

to achieve specific air traffic management (ATM) operational functionalities. For example, in

this context the current functionalities are DLIC, ADS, CPDLC, DFIS, AIDC, and ADS-B.

Data link flight information services (DFIS). A data link application that allows the

exchange of pertinent flight data between air and ground users.

Data link initiation capability (DLIC). A data link application that provides the ability to

exchange addresses, names and version numbers necessary to initiate data link applications.

Data link service. A data link service is a set of ATM-related transactions, both system

supported and manual, within a data link application, which have a clearly defined operational

goal. Each data link application service is a description of its recommended use from an

operational point of view.

End-to-end. transfer delay. The period elapsed from the time at which the originating user

initiates the triggering event until the time the transmitted information has been received bythe intended recipient.

Integrity. The probability that errors will be mis-detected. This may be when a correct

message is indicated as containing one or more errors, or when a message containing one or

more errors is indicated as being correct.

Note.- Integrity relates to the trust which can be placed in the correctness of the

information provided.

Operational requirement (OR). A statement of the operational attributes required of a

system for the effective and/or efficient provision of air traffic services to users.

Reliability. The probability that the system will deliver a particular message without errors.

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Note.- Explanations of other terms are provided in the Glossary and in the Data

Glossaries for data link applications.

Glossary

ACARS Aircraft communications addressing and reporting system

ACAS Airborne collision avoidance system

ADS Automatic dependent surveillance

ADS-A Automatic dependent surveillance- addressable

ADS-B Automatic dependent surveillance-broadcast

ADS-C Automatic dependent surveillance- contract

AEEC Airlines European Engineering Committee

AIDC ATS interfacility data communication

AIP Aeronautical information publication

AIRCOM AIRcraft COMmmunication service (SITAs proprietary Air-Ground Datalink

Service)

ALRT Alert

AMSS Aeronautical mobile satellite service

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AOC Aeronautical Operational Control

ARINC Aeronautical Radio Inc.

ASM Airspace management

ATC Air traffic control

ATFM Air traffic flow management

ATIS Automatic terminal information service

ATM Air traffic management

ATN Aeronautical telecommunications network

ATS Air traffic service(s)

ATSU Air traffic services unit

C-ATSU Controlling ATS unit

CDA Current data authority

CDTI Cockpit Display of Traffic Information

CNS Communications, navigation and surveillance

CPDLC Controller-pilot data link communications

D-ATSU Downstream ATS unit

DC Departure clearance

DDA Downstream data authority

DFIS Data link flight information services

DLIC Data link initiation capability

DSC Downstream clearance

EAEC European Airline Electronic Committee

EOBT Estimated off-block time

ETA Estimated time of arrival

FANS Special Committee for the Monitoring and

(Phase II) Co-ordination of Development and Transition Planning for the Future Air

Navigation System (Phase II)

FASID Facilities and Services Implementation Document

FDPS Flight data processing system

FIR Flight information region

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FIS Flight information service

FMS Flight management system

FOM Figure of merit

GNSS Global navigation satellite system

GPWS Ground proximity warning system

HF High frequency

IAS Indicated air speed

ID Identification

IFR Instrument flight rules

LACK Logical acknowledgement

LOA Letter of agreement

METAR Aviation routine weather report

Mode S Mode select

MOPS Minimum Operation Preference Standards

MOU Memorandum of understanding

MSG Message

NDA Next data authority

NIM Navigational integrity monitoring

NM Nautical miles

NOTAM Notice to airmen

OCM Oceanic clearance message

OLDI On-line Data Interchange

OR Operational requirement

OSI Open systems interconnection

PANS-RAC Procedures for Air Navigation Services -

Rules of the Air and Air Traffic Services

(Doc 4444)

PIREP Pilot report

Note.- Not specified in other ICAO documents.

QoS Quality of Service

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R-ATSU Receiving ATS unit

RESP Response

RGCSP Review of the General Concept of Separation Panel

RNP Required navigation performance

RTCA Radio Technical Commission of Aeronautics

RVR Runway visual range

SARPs Standards and Recommended Practices

SID Standard instrument departure

SSR Secondary surveillance radar

STCA Short-term conflict alert

TAF Aerodrome forecast

T-ATSU Transferring ATS unit

TWS Terminal weather service

URG Urgency

UTC Coordinated universal time

VFR Visual flight rules

VHF Very high frequency

VMC Visual meteorological conditions

WILCO Will comply

WMO World Meteorological Organization

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ADS MESSAGE DATA GLOSSARY

1. ADS MESSAGE DATA GLOSSARY

1.1 The following data are used as the ADS message variables, or components of the

variables, and are shown here in alphabetical order:

ADS emergency report. ADS information consisting of the following sequence:

- position;

- time;

- FOM;

- aircraft identification (optional); and

- ground vector (optional).

ADS event report . ADS information consisting of a sequence of event type and ADS

report.

ADS report. ADS information consisting of the following sequence:

- position;

- time;

- FOM;

- aircraft identification (optional);

- projected profile (optional);

- ground vector (optional);

- air vector (optional);

- meteorological information (optional);

- short-term intent (optional); and

- extended projected profile (optional).

Aircraft identification. A group of letters, figures or a combination thereof which is

identical to or the code equivalent of the aircraft call-sign. It is used in field 7 of the ICAO

model flight plan.

Air speed. Provides airspeed as a choice of the following:

Mach, IAS, or Mach and IAS.

Air speed change.Provides the threshold of change for either Mach speed or indicated airspeed that requires that the avionics generates an ADS report when the current aircraft speed

differs more than the specified threshold from the air speed in the last ADS report.

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Air vector. Provides the air vector as a sequence of heading, air speed, and vertical rate.

Cancel contract. Allows the ground to cancel event and/or periodic contracts in effect.

Contract type. Indicates which type of ADS contract is specified: demand, event, or

periodic.

Demand contract. Indicates that an avionics is to generate an ADS report containing theindicated data upon receipt of the contract. The data that can be indicated includes: aircraft

identification, projected profile, ground vector, air vector, meteorological information, short-

term intent, and extended projected profile.

Distance. Distance in non-SI units.

ETA. Estimated time of arrival at a waypoint.

Event contract. Indicates event types and the threshold for the specified event types.

Event type. An indication of what type of ADS event is specified:

- vertical rate change;

- waypoint change;

- lateral deviation change;

- level change;

- level range deviation;

- airspeed change;

- ground speed change;

- heading change;

- extended projected profile change;

- FOM field change; and

- track angle change.

Extended projected profile. Provides a sequence (1-128) of waypoint position data and

ETA at the specified waypoint.

Extended projected profile change. Indicates that an ADS report is to be generated when

there is a change in the extended projected profile.

Extended projected profile modulus. Sequence of modulus and extended projected

profile request.

Extended projected profile request . A choice indicating whether the extended projected

profile information is to be provided on a time or waypoint interval, and the interval of the

specified choice.

Facility designation. Specifies the ICAD four-letter location indicator or the ICAD eight-

letter combined location indicator, three-letter designator and an additional letter.

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Following waypoint . Indicates the waypoint after the next waypoint as a Position.

FOM. Indicates the figure of merit of the current ADS data. The information consists of the

position accuracy and indications 1) whether or not multiple navigational units are operating,

and 2) whether or not ACAS is available.

FOM field change. Indicates that an ADS report is to be generated when any FOM field

changes.

Ground speed. Provides ground speed in non-SI units.

Ground speed change. Provides the threshold of change for ground speed that requires the

avionics to generate an ADS report when the current aircraft ground speed has differed by

more than the specified threshold from the last ADS report.

Ground vector. Provides the ground vector of an aircraft provided as a sequence of track,

ground speed, and vertical rate.

Heading. Provides aircraft heading in degrees.

Heading change. Provides the threshold of change for heading in degrees that requires the

avionics to generate an ADS report when the current heading has differed by more than the

specified threshold from the last ADS report.

IAS. Indicated air speed.

Intermediate intent. Set of points between current position and the time indicated in the

short term intent. Consists of a sequence of the following: distance, track, level and projection

time.

Lateral deviation change: Provides the threshold of change for lateral value that requiresthe avionics to generate an ADS report when the current lateral deviation exceeds the

specified threshold.

Latitude. Latitude in degrees, minutes, and seconds.

Level. Specifies level in non-SI units.

Level ceiling. The level above which a level deviation event is triggered. Provided as a level.

Level change. Provides the threshold of change for level that requires the avionics to

generate an ADS report when the current level differs by more than the specified thresholdfrom the level in the last ADS report.

Level floor. The level below which a level deviation event is triggered. Provided as a

level.

Level range change. Threshold of change permissible between levels in consecutive ADS

reports.

Longitude. Longitude in degrees, minutes, and seconds.

Mach. Airspeed given as a Mach number.

Mach and IAS. Airspeed provided as both Mach and indicated airspeed.

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Meteorological information. A sequence of wind direction, wind speed, temperature and

turbulence.

Modulus. Provides a multiplier on the basic ADS report interval.

Next time. Time at next waypoint.

Next waypoint. Specifies the next way point in the avionics.

Non-compliance notification. Used to indicate partial compliance to a contract.

Periodic contract. Provides the requirements for the generation of ADS reports. The

periodic contract provides the reporting interval, and the modulus for when and what optional

data to be included in an ADS periodic report.

Position. Provides aircraft position information using a sequence of latitude, longitude, and

level.

Position accuracy. An indication of the navigational accuracy.

Projected profile. A sequence of next waypoint, next time, and following waypoint.

Projected time. Predicted time at a particular point.

Reporting interval. Provides the required ADS reporting interval.

Report type. Indicates which type of ADS report is provided: demand, event or periodic.

Request type. A choice indicating which type of ADS request is being up linked. The

choices are as indicated below:

- cancel event contract;

- cancel periodic contract;

- demand contract;

- event contract;

- modify emergency reporting rate;

- periodic contract; or

- cancel all contracts.

Short-term intent . A sequence of position, ETA, and intermediate intent (optional) data

structures.

Temperature. Temperature in degrees Celsius.

Time. Time at position in HHMMSS format.

Time stamp. In every report in YYMMDD and HHMMSS format.

Track . Provides track angle in degrees.

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Track angle change. Provides the threshold of change for track angle in degrees which

triggers avionics to generate an ADS report when the current track angle differs by more than

the specified threshold from the track angle in the last ADS report.

Turbulence. Indicates severity of turbulence.

Vertical rate. Rate of climb/descent (climb positive, descent negative).

Vertical rate change. The threshold of change for vertical rate that requires the avionics to

generate an ADS report when the current vertical rate differs by more than the specified

threshold from the vertical rate in the last ADS report.

waypoint change. Change in the next waypoint information.

Wind direction. Wind direction in degrees.

Wind speed . Wind speed in knots.

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