introduction to radar

South East RTS

Introduction to RADAR

Principle of RADAR operation and interpretation of presented data

REVISION 7 (Sunday, 26 February 2012 at 20:17)

PUBLISHED Sunday, 12 February 2012

Introduction to RADAR Principle of RADAR operation and interpretation of presented data

2 / 16

Contents Contents .................................................................................................................................................. 2

Section - 1 Introduction ...................................................................................................................... 3

Section - 2 Primary Surveillance Radar (PSR) ...................................................................................... 3

2.1 Methods of Operation ........................................................................................................... 4

2.2 Limitations of PSR .................................................................................................................. 5

Section - 3 Transponders .................................................................................................................... 6

Section - 4 Secondary Surveillance RADAR (SSR) ................................................................................ 6

4.1 Limitations of SSR .................................................................................................................. 7

4.2 Data processing and presentation ......................................................................................... 9

4.3 Mode S transponders .......................................................................................................... 11

Section - 5 Methods of identification, validation and verification ................................................... 12

5.1 Identification ....................................................................................................................... 12

5.1.1 Identification using PSR ....................................................................................................... 12

5.1.1.1 Turn Method ................................................................................................................... 13

5.1.1.2 Departure Method .......................................................................................................... 13

5.1.1.3 Position Report Method .................................................................................................. 13

5.1.2 Identification using SSR ....................................................................................................... 13

5.1.2.1 Code Change ................................................................................................................... 14

5.1.2.2 Validation of previously identified data .......................................................................... 14

5.1.2.3 IDENT method ................................................................................................................. 14

5.2 Validation of Mode A codes ................................................................................................ 14

5.3 Verification of Mode C ......................................................................................................... 14

Section - 6 Squawk Codes ................................................................................................................. 15

6.1 Flight Plan code ................................................................................................................... 15

6.2 Approach unit codes ............................................................................................................ 15

6.3 Conspicuity codes ................................................................................................................ 16

6.4 Listening codes .................................................................................................................... 16

Section - 7 Acknowledgements ......................................................................................................... 16

Section - 8 Further Reading ............................................................................................................... 16


3 / 16

This document is written for VATSIM-UK approach controllers and is for the use on the VATSIM

network only! It goes without saying – this document should never be used outside of this

environment.

Section - 1 Introduction Radio detection and ranging (RADAR) was, famously, developed in secret during World War Two in

order to detect enemy aircraft approaching. It has since been exploited in a variety of capacities –

including radio astronomy, motor vehicle detection, weather and terrain information gathering,

outer space rendezvous systems and guided missile targeting systems. Of course our primary

interest is in the application for air traffic control systems. This document will detail the operation

and interpretation of RADAR systems for air traffic services.

Section - 2 Primary Surveillance Radar (PSR) Primary surveillance RADAR is the most basic of RADAR systems applied in an aviation capacity. It is

classic radar – reflecting all echoes – including clouds, aircraft and other object alike. This has some

advantages – all targets can be observed by RADAR – whether they want to or not – and objects

wishing to be detected require no additional equipment. However – this system provides only

position information and by monitoring change of position or detecting Doppler shift - speed

information.

Figure 1 Primary Surveillance RADAR (PSR) in use at Solent.


4 / 16

2.1 Methods of Operation The RADAR system consists of two key components. A transmitter which emits radio waves in

defined directions and a receiver which detects radio waves from specific directions. These are often

co-located (but sometimes they are not) and are always pointing in the same direction. This principle

detects objects because the radio waves from the transmitter are reflected by the object and

detected as reflected waves by the detector.

Distance information is ascertained by response time – the further away an object is – the longer the

return pulse takes to be detected. There are considerations when using RADAR for detecting range –

the pulse needs to be powerful enough to reach the object and return to the transmitter. To

increase the range of a RADAR system requires compromises in accuracy of the returns detected.

Direction of the object is determined by the direction that the detector and transmitter are pointing

when they receive the signal. In order to build up a picture of the sky in a reliable and consistent

manner both the detector and transmitter are usually rotating. The rotation speed is an important

variable when designing RADAR systems.

Speed of an object is calculable by two methods – either by memory systems within a computer

interpreting the results and comparing each blip to the previous position – or by analysing the

returning signal – which will have some Doppler shift associated with it. This is a phenomenon that

results in the change in frequency of a wave when it is emitted from a moving object. Since we know

the transmitted frequency then by measuring the frequency of reflected waves – we can gain an

understanding of the velocity of the source.

RADAR systems spend relatively small amounts of their time transmitting – instead taking in as much

data as possible in the form of reflected waves. The system performs “flip-flopping” – transmitting a

pulse – and then listening for reflections. After a defined time – it transmits a second pulse and

listens for further reflections. This is repeated until the head has completed a full revolution to build

up a picture of the sky.


5 / 16

Figure 2 Typical RADAR head arrangement combining PSR and SSR. Credit (FAA)

2.2 Limitations of PSR PSR is limited in the following ways

By line of sight – to avoid obstacles RADAR sites are often on hills and mounted above the

surrounding landscape. This means RADAR systems have a “floor” below which nothing can

be seen – usually several hundred feet above the surrounding terrain. Similarly – in

mountainous environments – RADAR systems may suffer from loss of signal due to the

relative positioning of the object, the transmitter, detector and terrain.

RADAR overhead – the RADAR head is unable to transmit directly upwards. This results in a

small area overhead the RADAR where reflections are distorted or are entirely non-existent.

This forms a cone the diameter of which is approximately 3nm at FL100 and expands the

higher it gets.

The maximum non-ambiguous range. That is the range a pulse can travel and return to the

detector prior to the next pulse being transmitted. This is a result of the “flip-flopping”

system (discussed above) with defined listening periods. On a rotating detector – this is the

maximum range a pulse can travel, be reflected, and still be detected before the RADAR

begins transmitting again. Longer range RADAR systems have a very slow RPM in order to

collect data from the further targets – this allows the transmission and detection of longer

pulses (to improve accuracy) – however – the accuracy of the range information is reduced

by having longer blips.

RADAR sensitivity (and power of the return target). While planes, being made of metal are

very good reflectors – the smaller and more composite based aircraft will return weaker

signals. Coupled with high noise levels and environmental factors – the RADAR system will

eventually be unable to deconvolute the signal from the background.


6 / 16

Heavy or widespread precipitation can cause strong returns to appear on a PSR which

reduces information on aircraft within that area. This information can be filtered by reducing

the range of the RADAR head or by reducing the gain on the signal.

Section - 3 Transponders Aircraft transponders are electronic devices that provide a defined response when it detects a

specific radio pulse – known as an interrogation. The transponder has a settable code – which assists

in identification and monitoring of specific aircraft as well as functionality which allows for the

reporting of altitude to the air traffic system.

Under mode A, the transponder responds with the transponder code only. These are four digit, octal

numbers (digits 0 through 7) which allows for the transmission of the code in 12 binary “bits”. A

thirteenth bit is reserved for the “ident” function. A bit is recorded on the transponder as 12

readable and writable electronic states – either 1 (on) or 0 (off). The transponder then converts the

“on” or “off” to a radio signal – which is then transmitted in 0.45µs pulses. The RADAR head then

decodes these pulses back into 1’s and 0’s which can then be interpreted by the equipment on the

ground. This results in 212 codes available for selection by a transponder. When you divide that by

the 4 digits of the transponder – we are limited to 8 selectable numbers on the panel. (Since 212=84)

Where rated as mode C, the transponder returns the pressure altitude, encoding using a Gilham

code - occupying another four-bit octal code as well as the transponder code.

Section - 4 Secondary Surveillance RADAR (SSR) Secondary surveillance RADAR provides not only range and position information (as PSR does, see

above in section 3) but also additional information such as aircraft squawk code (and thus the

identity of the aircraft), pressure altitude, and (where aircraft and RADAR systems are equipped with

Mode S) supplementary information.

SSR relies on the same equipment as PSR – with one exception – each target must be equipped with

a transponder. SSR detects a different frequency than it transmits in order to gather the secondary

information. None of the transmitted (and thus reflected) waves are detected by the SSR. Pulses

transmitted by SSR are much weaker than PSR because they do not need to be detected as

reflections.

Interrogation of a transponder by a RADAR sweep allows for improved understanding and increased

information about aircraft in the vicinity of the RADAR head. As the interrogation pulse is detected

by the transponder – it sends its own signal back to the RADAR receiver. This signal contains a

variety of information depending on the mode of operation that the transponder is equipped with

and the information requested by the RADAR head. This interrogation and response method forms a


7 / 16

network of computers talking to each other via radio frequencies. PSR is, by comparison, a primitive

system – which bounces a significant amount of energy through the airspace seeking responses.

Figure 3 Secondary Surveillance RADAR (SSR) (only) in use at Solent

Just as the PSR spends much of its time receiving – so does an SSR. It is also only designed to capture

either the mode A information or the mode C information at any one time. It transmits two pulses

(one requesting mode A information and the other requesting mode C information) of about 0.2μs

separated by a gap of 21μs. The 21μs gap is used to receive information from the transponders that

have been interrogated by the 0.2μs transmission. The RADAR head receives 21μs of mode A

information followed by a short period of transmission followed by another 21μs of receiving mode

C information after which the sequence then repeats. The RADAR head communicates all of this

information to the data processing system which combines it and displays it on the situation display

for the controller.

For backup purposes – PSR and SSR are often combined in order to provide information about non-

transponder equipped objects and to prevent any SSR malfunctions (at either the aircraft or ground

station end) causing unwanted loss of information.

4.1 Limitations of SSR The limitations of SSR are detailed below

While 4096 (212 or 84) squawk codes might seem like a sufficient number – once specific use

codes have been allocated – many areas of congested airspace suffer from a lack of available


8 / 16

squawk codes. For this reason – they have to be very carefully managed not only nationally

but across continents. For this reason – in many ACCs (for example Maastricht) some flights

are becoming solely reliant on Mode S – which eliminates the need for unique squawk

codes.

Since all aircraft will “reply” to SSR when it is detected – the replies can sometimes merge

together – meaning traffic can be detected in the wrong position – or its signal can be

superimposed on another – resulting in the traffic disappearing entirely. This process is

called Garbling. Further to these problems traffic within 2nm of each other can sometimes

get their responses intertwined – this results in aircraft swapping datatags.

While traffic is being interrogated by one ground signal – it cannot reply to another request

– this means that responses to second RADAR heads can be delayed by up to 0.1s – resulting

in further inaccuracies.

FRUITing (False Replies Unsynchronised to Interrogator Transmission). This occurs where

one or more of the targets involved is in the main beam of at least two interrogators. This

results in replies to a RADAR head which didn’t request a response – several of these can

build up to present a target on a controller display which does not exist.

SSR requires traffic to have a transponder installed and functional. Using SSR alone produces

an incomplete picture – as traffic may be permitted to operate within airspace without a

transponder.

Many (if not all) of these deficiencies are addressed by the implementation of Mode S systems.


9 / 16

Figure 4 RADAR floor and slope definitions. Image credit: Todor Atanasov

4.2 Data processing and presentation As discussed above (in sections 4 and 5) – with mode C transponders (and even mode S

transponders) the information returned to a RADAR site by a transponder is limited to live

information (position, pressure altitude, speed (rate of change of position), vertical speed (rate of

change of pressure altitude) and track. All other information such as flightplan, assigned heading,

assigned speed, co-ordination point, exit level, scratchpad information and aircraft type data is all

held on the ground. This is presented to the controller as either as part of a datablock (and

associated functionality within) or by flight strip (or some combination of the two).


10 /

16

Figure 5 Example of data processing sources as presented on a situation display

In its raw form Mode A/C SSR data is displayed as a blip, a pressure altitude and a squawk code. To

improve situational awareness and to take full advantage of SSR systems are designed to perform an

operation known as code/callsign conversion. This is the process of matching a squawk code from a

SSR return to an allocated squawk code in a flight plan. For this to work effectively traffic which

needs to be code/callsign converted needs to be allocated a unique squawk code. Duplicated

squawk codes can result in erroneous results being displayed to the controller. The process of

combining RADAR information and flight plan information is known as correlation


11 /

16

Figure 6 Demonstration of data presentation examples for a) PSR only; b)PSR+SSR together and c) PSR, SSR

with code/callsign conversion.

4.3 Mode S transponders Mode S (select) transponders are able to return an even greater level of information. Each aircraft,

when manufactured, is given a unique 24-bit fixed address, the system now interrogates this code,

and a Mode S transponder returns information such as the full callsign, pressure altitude, indicated

airspace, indicated heading, track, QNH setting, level selected on the MCP. The system periodically

checks for all Mode S aircraft in the vicinity – allowing for any new targets to be interrogated. . Given

that Mode S is not (currently) used on VATSIM this section is intentionally brief.

Parameter PSR SSR Mode S

Who is visible Everyone Only those with a

transponder

Only those with a

Mode S transponder

Human Error potential Nil – being in the air

renders you visible

Reliant on entry of

correct SSR code

Reliant on entry of

callsign to transponder

Transmitted beam

strength Strong (to reflect)

Weak (to trigger

transponder)

Weak (to trigger

transponder)


12 /

16

Range Lower and dependant

on aircraft

Higher - Independent

of target cross-section.

Higher - Independent

of target cross-section

Interrogation Nil Broad - asking for all

Mode A / C

Selective – asking for

each aircraft

Reply limitation Nil –aircraft reflects

every wave

Cannot be interrogated

at same time (1.5μs)

Is told not to reply by

the RADAR head

Duplication limit Nil 4096 codes available 16.7 million

What information is

returned Position and Speed

As PSR & transponder

code & altitude

As SSR & a multitude

of information

Side Effects Tradeoff between

range and accuracy

FRUITing, Garbling and

other issues Nil

Figure 7 Table detailing the advantages and disadvantages of primary and secondary RADAR as well as

Mode S.

Section - 5 Methods of identification, validation and verification The concepts in this section can be easily muddled. In many situations these three procedures

happening simultaneously;

Identification: The situation which exists when the position indication of a particular aircraft is seen on a situation display and positively identified. (ICAO) Validation: Confirming that any assigned mode A code is set correctly by the pilot. Verification: Confirmation of the accuracy of the mode C readout by the controller. (MATS Pt 1)

5.1 Identification Identification is required for both PSR and SSR data returns to corroborate the information on the

situation display. Depending on whether the controller is able to utilise PSR or SSR will determine

which method they are able to use.

A pilot must be informed as soon as their aircraft is identified when outside of controlled airspace or

where identification if completed using the method detailed in 5.1.1.1.

5.1.1 Identification using PSR One of the following methods is to be employed when PSR is used to identify aircraft. Other

equipment (where installed and approved) may be used. These methods are rarely used on VATSIM


13 /

16

as it is always possible to access SSR information. They are included for those controllers simulating

primary only facilities.

5.1.1.1 Turn Method The turn method is commonly used for PSR identification, particularly where it has not departed

from nearby aerodrome. To identify a particular aircraft on a situation display where all returns

appears identical requires one of those returns to perform a unique action. As we only have very

basic amount of data (no transponder or altitude information) we identify by observing heading and

track changes.

This method requires the controller to ascertain the heading of an aircraft and, following a period of

observation, changing the track of the aircraft by at least 30⁰. This turn can be:

an instruction from the controller

an instruction from another controller

reported by the pilot.

Traffic must always be informed when identified by turn method and must always be given its

position.

5.1.1.2 Departure Method Identification can be achieved by correlating a PSR return within 1nm of the end of the departure

runway with a known airborne time. Care must be taken to avoid mistaking the aircraft for one

conducting a missed approach, overflying the airfield, holding overhead the airfield or departing

from an adjacent runway.

5.1.1.3 Position Report Method This method can be used where a pilot reports:-

Over an exact reporting point displayed on the situation display

At a particular distance(not exceeding 30nm) on a particular radial from a co-located VOR/

DME or TACAN (DME). The source facility must be displayed on the situation display.

over a VRP or other prominent geographical location displayed on the controller display as

long as the pilot is clear of cloud, visual with the surface and below a height of 3000ft.

This method should be reinforced using an alternative if there is any doubt about its validity.

5.1.2 Identification using SSR Identification of transponder equipped aircraft can be accomplished using three methods (as well as

the methods detailed in 5.1.1.). Controllers allocating any mode A code must also validate that code.


14 /

16

5.1.2.1 Code Change By observation of compliance with the instruction to select a discrete four digit code transponder

code. Note:- This method requires the controller to see the Mode A code change for it be a valid

method.

5.1.2.2 Validation of previously identified data Recognising a validated mode A code previously assigned to that aircrafts callsign. Where

code/callsign conversion is in use, recognising the callsign on the display is enough to maintain

identity provided the code has been validated.

5.1.2.3 IDENT method Observing the IDENT feature when it has been requested by the controller. Caution should be used

for simultaneous requests for IDENT in the same area. Aircraft squawking 7000 are not to be

identified using this method.

5.2 Validation of Mode A codes A controller assigning any mode A code must validate the code by checking as soon as possible that

the data displayed corresponds with the code that has been assigned. The code must be checked by

one of the following methods:

Instructing the aircraft to squawk the assigned code and observing that the correct numbers

appear on the situation display;

Instructing the aircraft to squawk IDENT and simultaneously checking the code numbers (or

callsign when code/callsign conversions are in use) associated with the SSR response;

Matching an already identified PSR return with the assigned code for the flight.

Controllers may deem mode A codes have been validated when they have been assigned by another

controller who is capable of validating the code (e.g. APP or CTR with SSR) except where the code in

use is a conspicuity code.

5.3 Verification of Mode C Controllers are to verify the accuracy of Mode C data, once the aircraft has been identified and the

Mode A validated, by checking that the readout indicates 200 feet or less from the level reported by

the pilot.

Traffic with a deemed validated mode A code is assumed to have had its mode C readout verified.


15 /

16

Figure 8 Example of simultaneous identification, verification and validation on outbound traffic.

Section - 6 Squawk Codes Squawk codes are essential for controllers to correlate traffic using SSR. They are sorted and

managed on a European level by Eurocontrol – this prevents any overlap of squawk codes and a

large usable database. There are several types of squawk code discussed below.

6.1 Flight Plan code This code is allocated to all airways traffic around 30 minutes before the flight plan off-block time. In

Euroscope these codes are allocated using the UKA plugin. On VATSIM this code is often kept for the

entire flight but in the real world – the code may be changed each time traffic crosses an ORCAM

boundary. The world is split up into ORCAM regions in which no duplication of squawk code is

permitted. When an aircraft crosses an ORCAM boundary, it may be allocated a new squawk code in

order to prevent duplication in the new region.

6.2 Approach unit codes Each RADAR unit is provided with a selection of codes which are available to non-airways traffic

joining and leaving controlled airspace for the purposes of identification and provision of services

inside and outside of controlled airspace. There are a limited number of these codes – so it is

important to manage them carefully – know which codes are already allocated – and to “retrieve”

each code at the conclusion of the RADAR service by allocating a new code (e.g. 7000). Each aircraft

should be given a unique code which should not be repeated within the range of operation for that

approach unit. Each unit is allocated unique codes and because these will often extend to being

unique within the FIR.


16 /

16

6.3 Conspicuity codes Conspicuity codes are codes which are used to indicate the purpose of the flight and not its identity.

These codes include 7000 (general flight outside CAS), 7010 (aerodrome circuit), 0033 (para-

dropping) and other codes which are used by individual airports. Conspicuity codes cannot be

validated and the Mode C height information associated with them cannot be verified.

Usually one conspicuity code is provided for each unit. It should be set by pilots who are receiving a

service from the unit – but do not need to be identified (for example under a basic or procedural

service). This code signifies to other controllers which unit the aircraft is talking to and therefore

allows coordination or other interactions to take place. The same code will be used by many aircraft

– to indicate the ATC unit they are in communication with.

6.4 Listening codes Some conspicuity codes are used to signify to the controller that the aircraft is on frequency, but do

not require a service from that unit. This enables the controller to establish communications if

necessary, though this is still challenging as the controller does not know the callsign of the aircraft.

Listening codes are for use within the vicinity of that unit or its airspace. The same code will be used

by many aircraft and (as above), cannot be validated and Mode C height information cannot be

verified.

Section - 7 Acknowledgements The author wishes to thank Callum Presley for provision of excellent background reading, Ben

Hunwicks for useful discussion on mode S operations as well as to Dan Parkin, Kieran Hardern and

especially George Wright for their excellent feedback, suggestions and amendments.

Section - 8 Further Reading MATS Part 1 (http://www.caa.co.uk/docs/33/CAP493Part1corr.pdf )

Eurocontrol Mode S overview

(http://www.eurocontrol.int/msa/public/standard_page/modes_operational_overview.html)

CAA Squawk tables

(http://www.ead.eurocontrol.int/pamslight/pdf/4e415453/EG/C/EN/AIP/ENR/EG_ENR_1_6_en)

In depth RADAR information (http://www.radartutorial.eu/index.en.html)

introduction to radar

Documents