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Informal paper

EUROCONTROL

An Overview of ADS

- Principles, Drivers, Activities, Technology and Standards -

1 June 1999, v1.0

1 June 1999, v1.0 An Overview of ADS

Table of Contents

1. Introduction ............................................................................................................4

1.1. General....................................................................................................................................4

1.2. How to get more information ....................................................................................................4

2. Principles of ADS ...................................................................................................5

2.1. Overview .................................................................................................................................5

2.2. Information content of messages..............................................................................................6

2.3. Operation in different phases of flight.......................................................................................6

2.4. ADS in relation to other surveillance approaches .....................................................................6

2.5. ADS-Contract and ADS-Broadcast ...........................................................................................8

2.6. The ADS System ...................................................................................................................12

3. Drivers for ADS.....................................................................................................14

3.1. The Historical Development of ADS.......................................................................................14

3.2. ADS Requirements ................................................................................................................14

3.3. New Applications Enabled by ADS-B......................................................................................14

3.4. Use of ADS in ATM and Non-ATM Functions .........................................................................15

4. ADS-C Data Link Technologies...........................................................................17

4.1. Introduction ............................................................................................................................17

4.2. FANS-1/A ADS-C...................................................................................................................17

4.3. ICAO ADS-C..........................................................................................................................18

4.4. The ADS-C protocol ...............................................................................................................18

4.5. Subnetwork issues .................................................................................................................19

5. ADS-B Data Link Technologies...........................................................................20

5.1. Introduction ............................................................................................................................20

5.2. Mode S Extended Squitter .....................................................................................................20

5.3. VDL Mode 4 ...........................................................................................................................21

5.4. UAT .......................................................................................................................................22

6. Supporting Technologies....................................................................................23

6.1. Introduction ............................................................................................................................23

6.2. Navigation and other onboard data sources............................................................................23


6.3. Cockpit Display of Traffic Information (ADS-B).......................................................................23

6.4. Changes to SDPD and ground functions ................................................................................24

7. A Selection of ADS Activities ..............................................................................26

7.1. Introduction ............................................................................................................................26

7.2. EUROCONTROL ADS Programme........................................................................................26

7.3. Operational use of FANS-1/A in the South Pacific..................................................................26

7.4. NEAN/NEAP/NUP..................................................................................................................27

7.5. Safeflight 21...........................................................................................................................27

8. Standards Development ......................................................................................29

8.1. Introduction ............................................................................................................................29

8.2. ICAO......................................................................................................................................29

8.3. AEEC.....................................................................................................................................29

8.4. EUROCAE/RTCA...................................................................................................................29

8.5. Summary of ADS Standards ..................................................................................................31

9. Abbreviations .......................................................................................................32


1. Introduction

1.1. General

This document gives an overview of ADS and in particular covers these topics:

§ The principles behind ADS,§ The drivers for ADS,§ ADS technologies,§ A selection of ADS activities,§ ADS standards.

In this document, the term “ADS” embraces both ADS Contract and ADS Broadcast. The terms“ADS-B” / “ADS Broadcast” or “ADS-C” / “ADS Contract” are used explicitly where only oneconcept is being discussed. This differs from ICAO usage where “ADS” refers to “ADSContract”, “ADS-B” refers to “ADS Broadcast” and there is no general term including bothconcepts.

1.2. How to get more information

For more information on EUROCONTROL’s ADS Programme, contact Pieter van der Kraan atEUROCONTROL on:

Telephone: +322 729 3359

Fax: +322 729 9086

Or visit the EUROCONTROL ADS website:

www.eurocontrol.be/projects/eatchip/ads


2. Principles of ADS

2.1. Overview

ADS is defined by ICAO as:

“A surveillance technique in which aircraft automatically provide, via a datalink, data derivedfrom on-board navigation and position-fixing systems, including aircraft identification, four-dimensional position, and additional data as appropriate.” (ICAO Circular 256-AN/152)

Although the ICAO definition refers explicitly to aircraft, ADS may also be used by groundvehicles, although this is less mature than aircraft use of ADS. Together, aircraft and groundvehicles may be referred to collectively as ‘mobile platforms’.

Using ADS, a mobile platform will send information in a surveillance message (an ‘ADS report’)to other systems via the datalink. ADS-C and ADS-B are different techniques built on thisprinciple.

Figure 1 illustrates data transfer between the mobile platforms (aircraft and ground vehicles)and ground systems that may occur using ADS.

Figure 1: ADS data transfer

ATM/Non-ATMfunctions

ADS requires a data link and, reliable and accurate navigation system to be available on boardof the mobile platform. As a result of these requirements, ADS is highly dependent onsophisticated airborne systems. This is different from conventional surveillance systems whichrequire less sophisticated aircraft equipment, if any. Another difference is that aircraft positionmeasurements are made at the ground sensor with conventional surveillance radar, but with


ADS the position measurements are made on the aircraft itself and are based on theinformation used to navigate the aircraft.

On the ground ADS requires ground communications equipment (known as ‘data acquisitionunits’) to receive ADS information and to pass it to the appropriate surveillance systems.

ADS does not usually require any input or co-operation from the aircrew or vehicle driver.

2.2. Information content of messages

A mobile platform’s ADS surveillance message is usually known as an ADS report and containsinformation such as the following:

§ mobile platform identity;§ 3-D position;§ a timestamp;§ indication of navigation figure of merit (this is a measure of the accuracy of the onboard

navigation system).

Other information may also be contained in the ADS report, such as:

§ ground track and speed;§ airspeed and heading;§ vertical rate;§ next waypoint;§ meteorological information.

2.3. Operation in different phases of flight

ADS has the potential to be used for:

§ Air-to-ground surveillance, i.e. surveillance of aircraft in flight by ground systems. This is aconventional ATC role and would complement conventional techniques.

§ Air-to-air surveillance, i.e. surveillance between two aircraft, potentially without thepresence of a ground infrastructure. This is a unique feature of ADS-B and also givesmobile platform-to-mobile platform surveillance.

§ Ground movement surveillance, i.e. surveillance of aircraft or ground vehicles when on theground by ground systems. This would provide for surveillance when on the airport surfaceand again would complement conventional techniques.

ADS can potentially provide surveillance coverage from ‘gate-to-gate’, i.e. from the aircraft’sfirst movement on the ground through all phases of flight and back onto the ground.

2.4. ADS in relation to other surveillance approaches

The following list describes the different approaches that presently used for civil aviation:

• Primary radar (PSR). Primary radar provides surveillance of all aircraft, independent of theirequipage. With primary radar, only aircraft 2-D position is known and other information suchas identity and altitude is not available. It is used largely by the military and as a back-up incase SSR fails. Primary radar coverage is required for high-complexity TMAs by the


EUROCONTROL surveillance standard. Primary radar is also used to provide surveillanceused on the airport surface.

• Secondary radar (SSR) Mode A/C. Secondary radar provides surveillance of all aircraftequipped with a functioning SSR transponder. The SSR transponder is used to report theaircraft’s measured barometric altitude and an identity code via a simple data link, thus theaircraft 3-D position and identity is known. Dual SSR coverage is required for use in en-route airspace and major terminal areas by the EUROCONTROL surveillance standard.

• Mode S. This is an enhancement to SSR Mode A/C in which more data transfer capabilitiesare added to the aircraft. It also overcomes certain limitations of SSR Mode A/C, forexample, increasing the number of aircraft identity codes available and providing greateraltitude precision. Mode S is not presently operational in ECAC airspace. Mode S includesseveral capabilities:

• Elementary Surveillance: Basic identity and altitude reporting. The aircraft’s callsign canalso be made available to the ground via the data link.

• Enhanced Surveillance Reporting of intent, velocity and other information through theuse of the downlink of airborne parameters (DAPs). This makes information from theavionics systems available to ground systems and in this respect provides a similarfunction to ADS. The implementation of enhanced surveillance in the core area ofECAC is the target of the EUROCONTROL IIMSES program.

• Data link: Mode S can potentially act as a communications subnetwork of theAeronautical Telecommunications Network (ATN). It also can also act as a non-ATNdata link with broadcast and other functions.

• Extended Squitter ADS-B: This is a potential ADS-B data link and is described inSection 5.2.

• ACAS/TCAS is also related to Mode S/SSR operation. This airborne collision avoidancesystem operates in the same frequency bands as Mode S/SSR and with compatibleprotocols.

• Manual position reporting. Aircrew reports data from navigation system via a voicecommunication link (usually HF radio). It is used in the North Atlantic and other areas wherethere is communications coverage but no surveillance infrastructure. It may also be used inthe event of a failure of surveillance infrastructure in continental areas. Manual positionreporting is not generally used in ECAC airspace.

In the long term, a multi-surveillance environment could evolve in ECAC airspace. This wouldsee different types of surveillance system, including ADS, in operation in overlapping andcomplementary coverage. Figure 2 illustrates the concept of the multi-surveillance environment.


Figure 2: Multi-surveillance environment

PSR SSR/Mode S

ADS-C

ADS-B

2.5. ADS-Contract and ADS-Broadcast

There are two types of ADS: ADS-Contract and ADS-Broadcast, described below.

2.5.1. ADS-Contract

ADS-Contract (ADS-C, also known as ADS-Addressed, or ADS-A) involves the transmission ofposition, identity and other information from a mobile platform (aircraft or ground vehicle) to asingle ground function. All ADS-C communications are point-to-point communications betweensystems on the aircraft and the ground. ADS-C relies on a two-way communications flow fromground functions to and from the mobile platform.

Figure 3 illustrates ADS-C information being passed from an aircraft to a surveillance functionon the ground.


Figure 3: ADS-Contract


The rate at which the mobile platform transmits information, the information sent and theconditions under which the mobile platform transmits data are all controlled by the groundfunction using a ‘contract’ with the aircraft which specifies the reporting conditions. The contractis initiated by the ground function and must be agreed between the ground function and theADS application on the mobile platform. An aircraft may have several different contracts withdifferent ground functions at the same time and they may be changed during the course of theflight.

The ground function that establishes the contract and uses the data can include ATM and non-ATM functions, and these are described in Section 3.4.

Because a point-to-point (connection oriented) data link is used, the reception of each ADS-Cmessage is guaranteed and reliable unless there is a total loss of data link communications inwhich case the sender would be notified of this fact. ADS-C users may be confident that eitherdata will be delivered to the receiving party or they will be notified if communications havefailed.

Early implementations of ADS-C have focussed on satellite communications as the data linkbecause the greatest initial benefits of ADS are in areas where there is no surveillanceinfrastructure at present, e.g. oceanic or continental areas with low traffic density.

Although it is technically possible for ADS-C to be used to provide position reporting in busyTMAs and on the airport surface, the presently available data links are not suitable for this. TheEUROCONTROL ST-15 study, which investigated data link suitability for ATS applications,


determined that there was insufficient capacity from presently available data links to meet thehigh reporting rate requirements that are defined for TMAs and surface movement. Hence ADS-C will probably not be used by ground vehicles in the intermediate term and will be restricted toaircraft applications where low reporting rates are required (eg low density areas).

ADS-C may be implemented at the same time as other data link applications, in particularController-Pilot Data Link Communications (CPDLC). In fact, it is foreseen by ICAO that “theADS[-C] application should be supported by direct two-way controller pilot data link and voicecommunication”, i.e. there will be CPDLC and voice communications also available.

2.5.2. ADS-Broadcast

ADS-Broadcast (ADS-B) is defined by ICAO as:

“ADS-B is a surveillance application transmitting parameters, such as position, track and groundspeed, via a broadcast mode data link, and at specified intervals, for utilisation by any air and/orground users requiring it.”

Figure 4 illustrates the transfer of ADS-B information from an aircraft to other users in thevicinity.


Figure 4: ADS-Broadcast

ATM/Non-ATM functions

For example,cockpit display


ADS-B relies on the regular and frequent transmission of ADS reports via a broadcast data link.The ADS-B reports are sent periodically by the mobile platform with no intervention from theground function. ADS-B reports may be received and processed by any recipient in range of thetransmitting mobile platform. If received by a ground data acquisition unit, the ADS-B report willbe processed with other surveillance data and used by ATM or non-ATM functions.

ADS-B offers surveillance data delivery from air-to-air or from air-to-ground. Transmitting datadirectly from air-to-air means that there is no need for a ground infrastructure to be present forairborne surveillance to be performed. Using ADS-B reports received from surrounding aircraft,a traffic surveillance picture can be generated in the cockpit of all aircraft. Direct air-to-airsurveillance, as offered in ADS-B, is not available from ADS-C.

ADS-B reports are not acknowledged. Therefore, the transmitting mobile platform does notknow which, if any, recipients are receiving and processing the ADS-B reports. There will be anoccasional loss of messages (ie ADS reports transmitted but not received correctly) due to thenature of this broadcast approach.

ADS-B transmissions lie outside of the ATN or ACARS, which only provide point-to-point(connection oriented) communications.


2.6. The ADS System

Figure 5 illustrates the components in the overall ADS System on an aircraft and groundsystems. Note that the ground components will always be present when using ADS-C, but theymay not be present in an ADS-B environment. Some of the components may have multipleroles, for example the crew display may support other systems in addition to ADS.

Figure 5: ADS system components

ADSapplication

Data link equipment/Data acquisition unit


Crew interface/display *

Data link equipment/Data acquisition unit

ADSapplication

* = Depends on implementation

Navigationsystem

Surveillance dataprocessing and

distribution (SDPD)

The components are described below:

§ Navigation/avionics systems (mobile): These are the systems that provide the necessarydata to the ADS system, including user’s identity, position and other data.

§ ADS Application (mobile): This manages the processing, encoding and transmission ADSdata. It may also manage the reception of ADS-B information from other mobile units. In thecase of ADS-C, it must manage the contract that has been established with the ground ADSapplication.

§ SDPD (mobile): This processes surveillance data, if it is received on the mobile unit, for useby on-board systems.

§ Data Link Equipment/ Data acquisition unit (mobile): For ADS-B, this transmits the ADS-Breports to other users and, possibly, receives ADS-B reports from other mobile units. In the


case of ADS-C, it provides a two-way communications functions with the ground systems,including the transmission of the ADS reports to the ground.

§ Crew interface/display (mobile): This may have several functions, including the display ofother ADS information received from an ADS-B system, or the input of ADS data (forexample, aircraft callsign or next waypoint).

§ Data Link Equipment/ Data acquisition unit (ground): For ADS-B, this receives ADS-Breports from mobile platforms and possibly transmits broadcast information to those users.In the case of ADS-C, it provides a two-way communications functions with the mobilesystems.

§ ADS Application (ground): This manages the reception of ADS information from mobileplatforms. In the case of ADS-C, it manages the establishment and change of the ADScontracts.

§ SDPD (ground): This processes the received surveillance data for use by ground systems.


3. Drivers for ADS

3.1. The Historical Development of ADS

ADS has been driven, to a greater or lessor extent, by the availability of new technology.Through the new technology, the possibilities of dependent surveillance have become visible.The main two technological developments that have led to ADS are:

§ Data links which are used to transmit digital information from an aircraft to a ground system(possibly via a satellite) or to another aircraft.

§ Navigation systems which can provide the aircraft location in terms of latitude/longitude co-ordinates. Such systems have been around for some time (inertial navigation systems areused for navigation in remote and oceanic areas) but low costs systems based on theGlobal Positioning System (GPS) have only recently become available.

In ADS, these two technologies are integrated with appropriate protocols to provide the ADSapplication.

The rapid pace of technology development, combined with the relatively slow pace ofinternational standardisation, has meant that the first systems to be available are based onairline industry standards.

3.2. ADS Requirements

Development of operational requirements for ADS-C and ADS-B is undertaken at the highestlevel by ICAO. The ICAO ADSP has produced a manual of ATS data link applications whichprovides guidance on the implementation of data link applications for aviation authorities,airspace users and service providers. It defines requirements for ADS-C and ADS-Bperformance for air-to-ground purposes (not air-to-air).

The RTCA, an industrial organisation in the US, has also defined a ‘MASPS’ for ADS-B,including requirements for air-to-air range for different applications.

3.3. New Applications Enabled by ADS-B

The ‘unique’ feature of ADS-B is the capability that it provides for air-to-air (or mobile-to-mobile,eg aircraft-to-ground vehicle) situation awareness without a requirement for a groundinfrastructure. This will enable new applications and the RTCA ADS-B MASPS identify suchapplications. Some of the RTCA ‘near-term’ airborne applications are summarised in Table 1,although the operational details of these applications are not yet internationally agreed.


Table 1: New applications enabled by ADS-B

Application Description

In-trail climb/descent orlateral passing manoeuvres

Limited autonomous passing manoeuvres. Likely to be firstintroduced in remote/oceanic areas.

Station keeping Airborne separation maintained by one aircraft followinganother. Separations may be set by a ground controller whomaintains control.

Final approach spacing Pilot takes some responsibility for achieving optimum finalapproach spacing.

Enhanced visual acquisition Low-criticality application. ADS-B is used to help pilot identifyother aircraft in the vicinity.

Note that ADS-B is not necessarily the only enabling technology that could support theseapplications.

3.4. Use of ADS in ATM and Non-ATM Functions

The EATCHIP operational concept document defines the following ground ATM functions, all ofwhich could potentially benefit from ADS data:

• Controller situation display which presents an accurate and timely indication of aircraftposition, identification and associated data.

• Safety nets (such as short term conflict alert, STCA, or minimum safe altitude warning,MSAW) which alerts controllers of potential conflicts time, while minimising the number ofnuisance alerts.

• Medium term conflict detection (MTCD) which detects conflicts between approximately 2and 20 minutes ahead of current time of the aircraft and makes them available for displayor to other functions

• Trajectory prediction tool which predicts the relevant trajectory data items of an aircraft overa given time horizon ahead of current time.

• Flight data processing and distribution (FDPD) which checks and fuses all incoming flightdata, determines the route that each flight will follow and the sectors/terminal airspacethrough which the flight will pass. FDPD also calculates and updates the estimated times atwhich flights will be overhead points on the route, on the basis of Surveillance Data.

• Monitoring aids which has two functions:

• Conformance monitoring functions which obtain trajectory prediction data and compareit with current Surveillance data. If the current data deviates from the predicted data bymore than a parameter then a deviation is declared and the controller warned.

• Automatic reminder functions which reminds the controller of actions he should performat a certain time or position.

• Departure and arrival management (DMAN/AMAN) which assists a controller in optimisingthe sequence of arrivals and departures from one, or more, runways).

• Meteorological data management which collects the latest and forecast meteorological data,and provide integrated meteorological information to functions which require it.

• Air traffic flow management which balances demand against capacity for the ATM system.

Non-ATM functions could also make use of ADS data. These functions include:

• international airports,


• aerodromes and heliports,

• weather service providers to aviation,

• air defence systems,

• search and rescue,

• law enforcement, and

• accident investigation authorities.


4. ADS-C Data Link Technologies

4.1. Introduction

There are two implementations of ADS-C: FANS-1/A and ICAO ADS-C. They are described indetail in the following sections, but the primary characteristics are summarised in Table 2.

Table 2: Primary characteristics of ADS-C technologies

FANS-1/A ATN

Standardised by Industry ICAOCommunications networkused

ACARS ATN

Use of subnetworks Any ACARS subnetwork(Satellite, VHF, HF)

Any ATN subnetwork(Satellite, VHF, Mode S, HF)including those defined in thefuture

Availability and operation Available and operationalnow

Available and operational inthe future

Although it is technically feasible for a ground vehicle to support ADS-C, it is not widelyforeseen that this would happen due to the relatively high costs of ADS-C equipment and thelack of a suitable subnetwork at present. Therefore, the following ADS-C description is restrictedto aircraft.

4.2. FANS-1/A ADS-C

This is a general name for the industry-standardised implementations of ADS-C. FANS-1 is theBoeing implementation and AIM-FANS is the Airbus implementation (commonly known asFANS-A). Generically, they are known as FANS-1/A. Implementations by other airframemanufacturers are also under development.

FANS-1/A operates over the ACARS data link and can use any of the ACARS mobilesubnetworks, i.e. satellite, VHF or HF. The ACARS (Aircraft Communication Addressing andReporting System) is a data link that is operated mainly for airline purposes. It provides air-to-ground and ground-to-ground communications and has been operational for more than 20years.

There are two service providers that provide ACARS services: ARINC, which operatesprincipally in the United States and China, and SITA, which operates mainly in Europe and therest of the world.

FANS-1/A has been widely trialled by many states and is presently operational in the SouthPacific.

FANS-1/A implementations are based on the ARINC 622 and ARINC 745 standards defined bythe AEEC (Airlines Electronic Engineering Committee), which is an airline industry standardsorganisation.


4.3. ICAO ADS-C

This is ADS-C implemented to ICAO standards and operating over the AeronauticalTelecommunications Network (ATN). ICAO ADS-C can use any of the mobile ATN sub-networks that are available to the aircraft and ground. It may also be known as ATN ADS-C.

At present the following ATN mobile subnetworks are defined by ICAO: satellite (AMSS), VHF(VDL Modes 1 and 2), Mode S and HF. More subnetworks may be defined in the future.

ATN ADS-C is not used operationally and is presently the subject of trials and development.

The ICAO ADS-C standards are defined in SARPs (Standards and Recommended Practices) ofAnnex 10. Various SARPs are defined, including those that define the ADS application andthose that specify the ATN communications systems and its subnetworks. The SARPs are atdifferent levels of maturity – not all are finalised yet. They are also supported by relevantindustry standards, such as MOPS.

The ICAO ADS-C application is similar, but not identical, to the FANS-1/A application.

Whilst ICAO ADS-C is generally accepted as the long term solution for ADS-C, there will needto be a transition for those aircraft that are already equipped with FANS-1/A to ATN. Thetimescales for such a transition are not clear and may take many years.

4.4. The ADS-C protocol

ADS-C relies on the establishment of contracts between an aircraft and the ground system. Acontract is an agreement between ground and aircraft systems as to how and when the aircraftwill transmit ADS reports to the ground. A contract is used in ADS-C to allow a ground system tospecify what information an aircraft reports and when it reports it. Contracts can be established,modified and cancelled at any time during the flight.

There are three types of contract:

• Periodic: The aircraft transmits ADS-C reports at a regular rate.

• Event: The aircraft transmits ADS-C reports when certain events occur. An event could be,for example, a change in altitude or speed for the aircraft or a deviation from the route.

• On-demand: The aircraft transmits a single ADS-C report when requested by the groundATCC (a ‘one-shot’).

An aircraft may also report in emergency mode. This is initiated by the airborne systems and isnot a contract. In this mode, the aircraft transmits ADS-C reports at a high rate to all groundfunctions that it has a contract established with.

All ADS-C reports consist of a ‘basic ADS group’ to which optional groups may be added (Notethat there are small differences between FANS-1/A and ATN messages.) The basic ADS groupconsists of:

• Latitude/Longitude,

• Altitude,

• Time,

• Figure of Merit.(Identity is also provided, although it is not part of the ADS group.)


The optional data can include: the projected profile, ground vector, air vector, weatherinformation, short term intent and extended projected profile.

4.5. Subnetwork issues

The performance of ADS-C is critically dependent on the performance of the mobile subnetworkthat provides the air-to-ground communications because this is often performance or capacitylimited. Therefore, subnetwork performance can place a practical limit on the overallperformance of the ADS-C application.

The primary characteristics of each ATN subnetwork are summarised in Table 3.

Table 3: Primary characteristics of ATN subnetworks

Satellite(AMSS)

VHF *(VDL)

Mode S * HF *

Capacity(relative)

Low High High Low

Latency (relativeend-to-enddelays)

High Low Medium Low

Cost (relative) High Low Medium LowCoverage Almost global

forgeostationaries:up to lat 80-90°excluding poles

Within line ofsight of groundstation

Within line ofsight of groundstation

Hundreds of NMfrom groundstation

* = Expected or predicted


5. ADS-B Data Link Technologies

5.1. Introduction

There are two primary implementations of ADS-B under discussion: Mode S extended squitterand VDL Mode 4. A third implementation, UAT, is at a less mature stage of development. Allthree technologies are discussed in the following sections and a summary of their primarycharacteristics is given in Table 4.

Table 4: Primary characteristics of ADS-B technologies

Mode S ExtendedSquitter

VDL Mode 4 UAT

System Development of ModeS system

New VHF data link New data link

Communications bandused

1090 MHz (SSR band) 118-137 MHz(VHF band)

around 960 MHz(proposed)

Communicationsprotocol

Random ‘squitter’ Co-ordinated use oftimeslots

’Squitter’ in a randomtimeslot

Communications typesoffered

Broadcast and point-to-point

Broadcast and point-to-point

Broadcast

5.2. Mode S Extended Squitter

Mode S Extended Squitter is an enhancement to the Mode S SSR in which aircraft regularlytransmit ‘extended squitter’ messages containing aircraft position, identity and other information.The extended squitters are transmitted on the SSR frequency of 1090 MHz. They can bereceived by any suitably equipped mobile platform or ground station.

In theory, ground vehicles could also transmit Mode S Extended Squitter, but this application ismuch less mature than aircraft equipage.

Transmission of extended squitters should not be confused with the other functions of the ModeS system: elementary surveillance, enhanced surveillance, and datalink, or with ACAS/TCAS.All of the Mode S functions, and also ACAS/TCAS, operate using the same protocols andmessage formats and on the same frequencies (1030 MHz for interrogations and 1090 MHz forreplies.)

Extended squitters are transmitted at random intervals with certain average transmission rates.The transmission of extended squitters is not synchronised with the transmissions from anyother user.

A summary of the extended squitters contents and transmission rates is given in Table 5.


Table 5: Mode S extended squitters

Extended squitter type Message contents Average transmission rate

Airborne position(transmitted whenairborne)

latitudelongitudealtitudestatus (i.e. alert conditions)CPR format/time marker *

0.5 s

Surface position(transmitted on surface)

latitude/longitudeground speedground trackCPR format/time marker *

(when moving) 0.5 s(when stationary) 5.0 s

Status altitude type (barometric/GNSS)transmission rate

Only transmitted on demandfrom a ground stationinterrogator

Identification aircraft typeaccuracy of position informationcallsign (8 character ICAO code)

(when moving) 5.0 s(when stationary)10.0 s

Velocity east-west velocitynorth-south velocityvertical rateintent change flag

0.5 s

Event driven variable Only transmitted as result ofcertain data changes

* CPR = Compressed Position Reporting, which is the position encoding system used. TheCPR format/time bit provides information for the CPR algorithm and time information.

A transponder capable of transmitting extended squitters may retain its original functionalityassociated with Mode S SSR. Alternatively, an equipment might transmit/receive only extendedsquitters, and not perform any other Mode S functions.

5.3. VDL Mode 4

VDL Mode 4 is being developed from a system known as STDMA (Self-Organising TimeDivision Multiple Access) which was invented and developed in Sweden.

VDL Mode 4 is intended to support a range of broadcast and point-to-point applications of whichADS-B is the most important. Like Mode S SARPs, VDL Mode 4 SARPs also include ATNcompatibility, so that VDL Mode 4 can act as an ATN subnetwork at the same time as providingADS-B (and other applications).

VDL Mode 4 operates in the Aeronautical Mobile (Route) Service (AMRS) band that is a VHFband extending from 118.000 MHz to 136.975 MHz. Each VDL Mode 4 channel occupies one25 kHz channel and the system is intended to operate with a minimum of two channelssimultaneously. More channels may be used locally to provide sufficient capacity for theapplications being supported by VDL Mode 4.

VDL Mode 4 is based on a VHF data link that uses a ‘timeslot’ structure for data linkcommunications. All transmissions are synchronised to the start of a timeslot and each timeslotis a nominal period in which one user transmits a single ADS-B report. Timeslots are used inturn by different mobile or fixed platforms.


The data link uses a set of ‘reservation protocols’ for managing media access which allow atransponder to reserve a later timeslot for another message. They are designed to minimiseoccurrences of two transponder’s transmissions interfering with each other and they also supportthe basic ADS-B function.

A VDL Mode 4 transponder requires a source of precise time to maintain synchronisation to thetimeslots. The source of this data is not specified in the SARPs. A GNSS receiver can be usedto provide the necessary synchronisation, or another source might be suitable, e.g. an accurateclock. Accurate time sources are presently under investigate through EUROCONTROL.

5.4. UAT

The Universal Access Transceiver (UAT) is an experimental data link system being developedas a research and development project at The MITRE Corporation's Centre for AdvancedAviation System Development in the US. The aim of the project is to illustrate and evaluate theconcept of a multipurpose broadcast data link architecture to meet aviation requirements.

UAT is a single-channel system, using a channel bandwidth of approximately 2 MHz. It operateson the same frequency for transmit and receive in order to allow full air-air connectivity forADS-B with a minimum of new hardware. All UAT stations access the channel autonomously,using random transmissions in a timeslot structure. UAT trials have been conducted at thefrequency of 960 MHz, which is a frequency in the DME band.

UAT supports only broadcast communications and does not provide point-to-pointcommunications or operate as an ATN subnetwork. There are no draft SARPs available for UATand it is not under development in any ICAO panel.


6. Supporting Technologies

6.1. Introduction

This section describes some of the supporting technologies that, in addition to the data linksdescribed in the previous section, are required to implement ADS.

6.2. Navigation and other onboard data sources

Suitable navigation systems (and other onboard systems) are required to provide the necessarydata for ADS. They must also have the necessary integrity, availability and other performanceparameters required for the functions/applications which are using ADS.

However, existing navigation systems were not designed with the requirements of ADS in mindand therefore may not the requirements of some ADS surveillance applications.

Possible navigation systems that can provide ADS data on an aircraft include:

§ GPS/GNSS,§ Inertial reference systems (IRS),§ Multi-DME systems.

Combinations of these systems may also be used to improve overall performance.

For a ground vehicle, the most pragmatic system in the near-term is GPS. This can give highaccuracy when used with differential corrections and is a low cost system.

6.3. Cockpit Display of Traffic Information (ADS-B)

Some applications may require a cockpit display of traffic information (CDTI) that gives anaircrew a surveillance picture of surrounding traffic. The data for a CDTI can be obtain byreceiving and processing ADS-B reports from surrounding mobile platforms or informationreceived from a ground station. The range of traffic on the display may be up to 200 NM.

Using a CDTI, an aircrew may be able to undertake new applications. These include airborneself-separation and passing manoeuvres. Another benefit of CDTI is the increased situationawareness for aircrew. This may be a particular benefit in the future as data linkcommunications replace the voice radio/telephony (R/T) that presently provides situationawareness through the ‘party line’. Thus ADS-B may assist the introduction of data linkcommunications.

However, the introduction of these applications is not without difficulty since it raises questionsof integrity/availability of data and also of control/responsibility residing in the cockpit or with theground controller.

The NEAN project, and other VDL Mode 4/STDMA projects, are testing prototype CDTI’s (asshown below) in various commercial aircraft and also in ground vehicles. The NLR hasundertaken flight simulations of a ‘free flight’ environment in which all aircraft perform selfseparation using a CDTI.


Figure 6: Prototype CDTI’s tested in the NEAN project

In order to use surveillance data on the aircraft, some surveillance data processing (SDP) willbe required. This is largely undefined at present.

CDTI could also be provided using the Traffic Information Service (TIS or broadcast TIS, TIS-B)data link application. This involves transmitting surveillance information from the ground to theaircraft for display in the cockpit. The transmitted surveillance picture is derived from SSR.

6.4. Changes to SDPD and ground functions

Figure 7 shows how ground surveillance data processing and distribution (SDPD) systems willbe required to cope with more surveillance data sources in a multi-surveillance environmentthat include ADS. In addition to previous activities, the SDPD system(s) will be required to:

§ fuse data from very diverse surveillance sources,§ undertake integrity checks between different surveillance systems,§ manage ADS-C contracts,§ process ADS-B data,§ cope with the new information (e.g. next waypoint) that will become available through ADS

and other similar systems.


Figure 7: Multi-surveillance environment

PSR/SSR/Mode SADS-BADS-C

Surveillance Data Processing andDistribution (SDPD)

ATNsubnetwork

ATM Functions Non-ATM Functions

In addition to the changes to SDPD, changes to the downstream functions relying on SDPDdata may be required. Examples of these changes include:

• Enhancements to controller displays to show additional information, eg callsign, nextwaypoint.

• Enhancements to the performance of ground functions, eg improvements to trajectoryprediction using next waypoint and weather forecasts using aircraft wind measurements.

• The development of new functions making use of newly available information.


7. A Selection of ADS Activities

7.1. Introduction

This selection describes a selection of ongoing ADS activities. There are too many to describein full, so a small number of influential activities have been selected.

7.2. EUROCONTROL ADS Programme

At the end of 1998, the first stage of the EUROCONTROL ADS Programme was formallyapproved. The ADS Programme includes all necessary actions to achieve the initialimplementation and operational use of ADS in Europe. The programme relies on considerableco-ordination and sharing of resources with States.

The following text describes the programme objectives:

“The first objective of the ADS Programme is to determine if ADS, as either sole means or inconjunction with other surveillance sources, can meet the operational requirements forsurveillance for the medium or long-term. In parallel with this it has to be assessed whether ADSas a concept and the various corresponding candidate ADS-B and ADS-C technologies are safeand cost-beneficial in ECAC airspace.

The second objective of the Programme, subject to a positive outcome of a correspondingbusiness case, is the local implementation of operational systems (i.e. deployment of ADSinfrastructure in ECAC) and the maintenance and support activities for those systems.”

The programme is in 4 stages, of which the first (Stage 0) will last for 1999. Programmeactivities are foreseen up to 2007.

The activities in the Stage 0 include:

§ Development of an ADS strategy,§ Initial safety analysis,§ Initial cost-benefit analysis,§ Development of ADS initial technical requirements,§ Development of ADS initial specifications.

7.3. Operational use of FANS-1/A in the South Pacific

Of the ADS technologies described here, only FANS-1/A is operational. As of March 1999,FANS-1/A systems are operational in the South Pacific, between the US west coast and Sydneyand Auckland. Much of the trials of FANS-1/A systems has been in the Pacific area.

Three airlines are flying the ‘FANS routes’ that have been established: United, Qantas and AirNew Zealand. Of the bordering control authorities, only the New Zealand authority has anoperational ADS ground system. Air Services Australia are presently testing an ADS system.

The primary certification of FANS-1 took place in June 1995 in conjunction with Qantas. Tomeet the requirements for Pacific operation an aircraft must equip with the FANS-1 packageconsisting of GPS, CPDLC, ADS and ACARS upgrades.


In the area of FANS-1 operations, two new operations have been built on the ADS-C/CPDLCinfrastructure: flex tracks and Dynamic Aircraft Re-route Planning (DARP). Flex tracks aretracks that can move based on changing wind and temperature conditions, instead of the fixedtracks previously in place. DARP allows a new track to be uplinked to an aircraft in flight. Theaircrew can choose whether to adopt the new track.

United Airlines have reported that flying FANS-1/A routes over the South Pacific has giventhem typical fuel savings of 1,500 to 1,800 kg, and flight time reductions of 15 minutes perflight.

One of the aims of the FANS-1/A Pacific work is to reduce separation standards. Currentseparation standards in the Pacific are generally 100NM, although 50NM is used in some areas.One quoted target is to reduce the standards to 30NM lateral separation.

7.4. NEAN/NEAP/NUP

Prototype VDL Mode 4 equipment, known as STDMA, has been trialled in several projects suchas the EC-sponsored North European ADS-B Network (NEAN) and its sister project NEANApplications Project (NEAP).

The NEAN project involved the implementation of a trials infrastructure for STDMA equipmentinvolving ground stations, aircraft and ground vehicles on the airport surface. The trialsinfrastructure spreads across Denmark, Germany and Sweden. Participants include Lufthansa,Scandinavian Airways System (SAS), German CAA, Swedish CAA and Danish CAA. In theproject, STDMA equipment and cockpit displays (as shown in Figure 6) were installed onrevenue aircraft including: SAS Fokker 28s, SAS DC9s and Lufthansa 747s.

The NEAP project testing applications of the NEAN infrastructure. The applications included:

§ air and ground situation awareness,§ surface movement surveillance and runway incursion,§ ATIS-broadcast,§ TIS-broadcast,§ DGNSS-supported approaches.

Although the STMDA trials are widespread and fairly extensive (they are the largest ADS-Bactivities presently underway), the equipment used is simpler than that described in the draftSARPs. An extension to NEAN/NEAP has recently started. This is known as NUP (NEANUpdate Programme) and it will include a transition of the NEAN infrastructure to equipment thatis compatible with the VDL Mode 4 draft SARPs.

7.5. Safeflight 21

Safeflight 21 is an FAA programme which will conduct and operational evaluation of nineoperational enhancements that ADS-B and other systems could deliver. The nineenhancements have been defined by the RTCA:1

§ “Provide weather and other information to the cockpit,

1 “Joint government/industry roadmap for free flight operational enhancements”, RTCA, August1998.


§ Affordable means to reduce controlled flight into terrain,§ Improved capability for approaches in low visibility conditions,§ Enhanced capability to see and avoid adjacent traffic,§ Enhanced capability to delegate aircraft separation authority to the pilot,§ Improved capability for pilots to navigate airport taxiways,§ Enhances capability for controller to manage aircraft and vehicular traffic on airport surface,§ Provides surveillance coverage in non-radar airspace,§ Provides improved separation standards.”

The Safeflight 21 programme involves the trial of ADS-B (and associated) technologies toevaluate the delivery of the above operational enhancements.


8. Standards Development

8.1. Introduction

The section summarises the main ADS standards and activities of organisations working onADS standards.

8.2. ICAO

There following list describes the main ADS activities undertaken by ICAO Panels:

§ Automatic Dependent Surveillance Panel: The ADSP deals with the operational aspects ofADS and Air Traffic Management data link services in general. The panel is investigatingthe implementation of ADS-C and ADS-B with particular reference to requirements andprocedures. It has produced a ‘Manual of ATS Data Link Applications.’

§ Aeronautical Telecommunications Network Panel: The ATNP is responsible for developingthe ATN SARPs and the ICAO ADS-C application. These SARPs (known as CNS-ATM/1SARPs) are already published.

§ Airborne Mobile Communications Panel: The AMCP is developing SARPs for the mobilesubnetworks of the ATN. SARPs have already been developed for the satellite subnetwork(AMSS), VHF data links 1 and 2 and the HF data link. The panel is also developing SARPsfor VDL Mode 4 for surveillance applications.

§ SSR Improvements and Collision Avoidance Panel: The SICASP has developed SARPs forMode S as an ATN subnetwork and also for extended squitter.

8.3. AEEC

FANS-1/A ADS-C is defined by a set of ARINC standards, which are industry standards definedby the AEEC (the Airlines Electronic Engineering Committee). The following standards defineFANS-1/A ADS-C:

• ARINC 745: This defines the ADS application, airborne equipment requirements andinterfaces to other airborne systems (e.g. navigation).

• ARINC 622: This is used to provide a suitable interface for the ADS application and toenhance communications integrity of the ACARS data link.

8.4. EUROCAE/RTCA

EUROCAE (The European organisation for Civil Aviation Equipment) and RTCA (Requirementsand Technical Concepts for Aviation) are developing MASPS (Minimum Aviation SystemPerformance Standards) and (Minimum Operational Performance Standards) related to ADS.EUROCAE is a European standards organisation and RTCA is the US equivalent.

The following list summarises the main EUROCAE and RTCA activities in the area of ADS:

§ EUROCAE Working Group 51: EUROCAE WG-51 is developing MASPS for ADS-B and forMOPS for both Mode S Extended Squitter and VDL Mode 4.


§ RTCA Special Committee 186: RTCA SC-186 is developing ADS-B MASPS and ADS-BMOPS for Mode S Extended Squitter. It is also developing CDTI MOPS for ADS-B andAirborne Surveillance Processing MOPS (ASSAP).

§ EUROCAE WG-49: EUROCAE WG-49 is responsible for the specifications for Mode S datalink transponders and processors. The ED-73 equipment characteristics documentpublished in 1998 includes the extended squitter functionality and is currently being updatedby the group.

§ EUROCAE WG-55: EUROCAE WG-55 is analysing the capabilities of next generationsatellite systems communications (NGSS). Part of its considerations include the use ofNGSS for ADS-C applications.


8.5. Summary of ADS Standards

Document Developed/Published by Status

General ADS Standards (not data link technology specific)

ICAO Manual of ATS Data LinkApplications

ICAO ADSP Published 1999

ADS and Air Traffic Services DataLink Applications Circular

ICAO ADSP Published 1995

RTCA ADS-B MASPS RTCA SC186 Published 1998.Update expected early 2000

ADS-B CDTI MOPS RTCA SC186 WG-1 Being drafted, expected in 1999.Airborne Surveillance ProcessingMOPS (ASSAP)

RTCA SC186 WG-4 Draft expected late 1999/early2000.

ATN ADS-C Standards

ATN SARPs, Sub-Volume II – Air-Ground Applications (‘CNS/ATM-1SARPs)

ICAO ATNP Published

ATN Subnetwork SARPs and MOPS(Satellite, VDL, Mode S, HF andothers)

Various, includingEUROCAE, RTCA, andICAO AMCP and SICASP

Various, including some publishedand some in draft stage

FANS-1/A ADS-C Standards

ARINC 745 AEEC PublishedARINC 622 AEEC Published

Mode S (1090MHz) Extended Squitter (ADS-B) Standards

Mode S Extended Squitter SARPs ICAO SICASP Published July 1998Manual on Mode S Specific Services ICAO SICASP Issue 1 published 1997. Updates

and modifications continue.Mode S Extended Squitter ADS-BMOPS

WG51 Sg1 & SC186 Wg3 Being drafted, expected early 2000.

VDL Mode 4 (ADS-B) Standards

VDL4 SARPs ICAO AMCP Under validation. Expected 2000VDL4 ADS-B MOPS EUROCAE WG51 SG2 Being drafted, expected early 2000

(dependent upon VDL4 SARPs)


9. AbbreviationsADS Automatic Dependent SurveillanceADS-B Automatic Dependent Surveillance - BroadcastADS-C Automatic Dependent Surveillance – ContractADSP Automatic Dependent Surveillance PanelAEEC Airlines Electronic Engineering CommitteeAMAN Arrival ManagerAMCP Aeronautical Mobile Communications PanelARINC Aeronautical Radio IncorporatedATN Aeronautical Telecommunications NetworkATNP Aeronautical Telecommunications Network PanelCDTI Cockpit Display of Traffic InformationDGNSS Differential Global Navigation Satellite SystemsDMAN Departure ManagerEUROCAE European Organisation for Civil Aviation EquipmentFAA Federal Aviation AdministrationFDPD Flight data processing and distributionGNSS Global Navigation Satellite SystemsGPS Global Positioning SystemMASPS Minimum Aviation System Performance StandardsMOPS Minimum Operational Performance SpecificationMSAW Minimum Safe Altitude WarningMTCD Medium Term Conflict DetectionNEAN North European ADS-B NetworkNEAP North European ADS ApplicationsPSR (or PR) Primary Surveillance RadarRTCA Requirements and Technical Concepts for AviationSARPs Standards and Recommended PracticesSICASP Surveillance Improvements and Collision Avoidance PanelSSR Secondary Surveillance RadarSTCA Short Term Conflict AlertSTDMA Self-Organising Time Division Multiple AccessTIS-B Traffic Information Service - BroadcastUAT Universal Access TransceiverVDL VHF Digital Link

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