10 secondary surveillance radar - pilotafutar.hu · secondary surveillance radar 10-1 ......

Secondary Surveillance Radar 10-1

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As we have just noted, the primary radar element of the ATC Surveillance Radar System provides detection of suitable targets with good accuracy in bearing and range measurement but with the following limitations:

•Targets that are too small, are built of a poor radar reflecting material or have a poor aspect may not be detected

•Targets cannot be identified directly•Radar energy suffers from attenuation

(losses) both on the path out to the target and on the return path of the reflections

To overcome these problems, a Surveillance Radar installation will often consist of both a primary radar and a secondary radar,

the latter being known as a Secondary Surveillance Radar (SSR). The role of the SSR is to complement the primary radar element. Fig. RN 10.1 shows the aerials of a primary radar (the big reflector) and an SSR radar (the long, flat aerial on top).

10.1 PrinciplesSSR operates on secondary radar principles. An SSR “link” uses one ground-based transmitter and receiver, called the interrogator and one airborne transmitter and receiver, referred to as the ATC transponder, or simply ‘transponder’. The interrogator transmits pulse pairs. A receiver within the interrogator’s beam receives these pulses and decodes them. The transponder then responds by transmitting a pulse train (many pulses in a stream) back to the interrogator. The pulse train contains information according to what the interrogator requested.

All interrogations are transmitted at a frequency of 1030 MHz and all transponder responses are transmitted at a frequency of 1090 MHz. The SSR aerial consists of a radiator and reflector similar to that used in the primary radar but, because the return is much stronger than that of a primary radar reflection, it is much smaller.

10 Secondary Surveillance Radar

Fig. RN 10.1 Primary radar aerial with SSR antenna on top

Radio Navigation E5 - Final.indb1 1 06.05.2008 10:30:54

10-2 Secondary Surveillance Radar

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Fig. RN 10.2 shows the polar diagram, in the horizontal plane, for such an aerial.

The aerial illustrated at fig. RN 10.1 is known as a large aperture aerial. Side lobes can create a problem since an aircraft’s receiver, especially at closer range, will detect them and this could trigger false responses from aircraft outside the main beam. To counteract this, a process known as “side lobe suppression” (SLS) is introduced.

Side Lobe Supression (SLS)Interrogations are sent in the form of a group of three pulses that we will identify as P1, P2 and P3, see fig. RN 10.3. P1 and P3 are sent

via the directional radar antenna, P2 via an omnidirectional aerial. The spacing between P1 and P2 is constant at 2.0 ms. Pulse “P2” is used in the electronic side lobe suppression. The P1 and P3 signals in the main beam are stronger than the omnidirectional P2 pulse, but P2 is stronger than the P1 and P3 pulse received from the side lobes. If the P1 pulse is weaker than the P2 control pulse, then the P1, P3 replies are suppressed.

Pulse SpacingThe spacing between P1 and P3 is set at a value dependant upon the type (mode) of response required from the aeroplane transponder. There are two current modes and their applications and ‘P1 to P3’ intervals are in table RN 10.1.

Fig. RN 10.2 Polar diagram

Side lobe Main beam

Fig. RN 10.4 SSR radar screen

Table RN 10.1 Modes of interrogation

A

Mode Transmission of P1 to P3 spacing

C

code

pressue altitude. report

8 µs

21 µs

Fig. RN 10.3 Interrogation

Mode Use

IdentificationA

P1 P2P3

8µs

Altitude reporting C

P1 P3

21µs



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Modes B and D are not currently in use and the conventional aeroplane transponder is designed to use only modes A and C.

The pilot sets the transponder to the mode and code as instructed by ATC. If the transponder is set to the “Alt RPTG OFF” or on some other types “ON” position, the unit will respond to Mode A interrogations. If set to “XPDR” or on some other types “ALT”, the transponder will respond to Mode A and C interrogations, sending identification and automatic altitude information.

A typical control panel for the airborne unit is shown in fig. RN 10.5. This panel controls 2 transponders, XPDR1 or XPDR2. The modes are: •Test •Stand by

•ALT RPG OFF (only code no altitude reporting) •XPNDR (TCAS functions disabled) •TA only (ATC transponder + TA active) •TA/RA (ATC transponder +TA/RA active)

The transponder’s response will be in the form of a pulse train as shown in fig. RN

10.6. It consists of two “framing” pulses separated by 20.3 ms. Between those two framing pulses there is a facility for up to 12 coding pulses to be transmitted. The “A”

Fig. RN 10.5 Boeing 737-400 transponder control panel, including TCAS selection

Fig. RN 10.6 Pulse train

12 Pulse system 4096 codes available

F1 C1 A1 C2 A2 C4 A4 B1 D1 B2 D2 B4 D4 F2

Example: code 5637

F1 C1 A1 C2 A4 D1 B2 D2 B4 D4 F2

3 5 7 6

Ident



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pulses form the first digit of the four-figure code. “B” the second, “C” the third and “D” the fourth. Fig. RN 10.6 shows the possible arrangement of A, B, C, and D pulses for sending the digits. You will note that, for each digit, there are 8 possibilities ranging from 0 to 7. This leads to a total of 4096 selectable codes (84).

The selected pulses for code 5637 are indicated on the lower part of the figure. The Mode “C” altitude reporting facility transmits information direct from a pressure altitude sensor (such as an encoding altimeter). The altitude information is relative to the 1013.2-hPa level, i.e. it is pressure altitude.

10.2 Use of TransponderPre-departure the transponder should be set to “Standby”. The test function should then be activated in order to establish the operational status of the equipment. When instructed, set the mode and code given by ATC, and when told to “Squawk”, set the code as appropriate. In order to avoid causing interference, do not change the code without first selecting “STBY” on the control panel. However modern transponders automatically revert to “STBY” mode for a short period of time when the code is changed. When in an abnormal situation, there are three codes that you may set to alert the controllers. These codes have their predefined meaning and, with one of these selected, a signal indicating a “special condition” will be triggered on the controller’s screen. The aircraft symbol may

change colour to attract his attention. On some radar systems, a sound alarm will be triggered together with the visible alarm.•Code 7500 Unlawful interference•Code 7600 Radio failure•Code 7700 Emergency

From time to time the ATC controller may ask you to “SQUAWK IDENT”. By pushing the “IDENT” button, the transponder is activated to transmit the additional pulse. This is shown on the radar display as a flashing target or symbol. This function, when first enabled, will continue for approximately 20 seconds. Never press the “IDENT” button unless you are instructed to by the air traffic controller.

10.3 Presentation and InterpretationIn general, the SSR information is presented together with the primary radar information. The difference between the two is that the primary information is very accurate in bearing and range, but doesn’t consist of any other information. The secondary radar information provides reliable information that can identify every aircraft and provide altitude information.

The primary radar element only provides the necessary bearing and range and the use of computer generated displays allows calculated information, such as track and ground speed, to be shown. Fig. RN 10.7 illustrates a common style of displaying combined (primary and secondary surveillance radar) information on the air traffic controller’s radar screen.



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10.4 LimitationsSince all SSR units operate at the same frequency, this can result in an aeroplane’s response to one interrogator being detected by other ground units. Such responses will be out of synchronisation and will cause random responses to appear. This is called ‘Fruiting’ (FRUIT= False Replies Unsynchronised to Interrogator Transmission).

Electronic circuits are employed (de-fruiters) to remove this effect but they do not remove all random responses and the situation becomes worse as traffic density increases. Another problem is known as ‘garbling’. This occurs when targets are close to one another; e.g. in a holding pattern or progressing along an airway one above the other. If they are in the interrogation beam at the same time and are close to one another within 1.7 NM, the ground interrogator will be unable to

differentiate between them and will ‘see’ only one confused return.

10.5 Operation of Mode S (Selective Addressing)This is a development of the basic SSR. The ‘Mode S’ ground interrogators and airborne transponders are fully compatible with the conventional Mode A and C units. However, Mode S units working together have much greater capabilities.

The Mode S interrogator and transponder, see fig. RN 10.8 and fig. RN 10.9, operate on the same frequency as standard SSR. The initial part of the interrogation signal is such that the standard SSR modes will be recognised by the normal airborne transponder unit. The second part of the Mode S interrogator signal consists of a message of up to 112 data bits within which 24 bits are allocated to aircraft address. This permits the controller to interrogate a specific aircraft. The 24 bits allocated are sufficient to provide for over 16 million individual addresses, which is thought to be sufficient for the registration of all aircraft in the world. In order to detect further Mode S transponders, a special feature known as SSR/Mode S “ALL CALL” is broadcast at intervals. Normal SSR transponders respond to this in Mode A or C (dependent on the P1/P3 relationship). Mode S transponders will recognise the special character of the “ALL CALL” interrogation as a roll call request and will transmit a response which will include the aircraft’s identity/address along with details of the capability of the relevant on board equipment.

Fig. RN 10.7 Information on the radar screen

Flight levelor altitude inhundreds of feet

Aircraft call sign Squawk code

Ground speedin tens of knots

Arrow indicatesclimb (or descent)

Radar blip

SK 12344772280- 48



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The interrogation is different for the kind of call that is made. For an mode A/C only all call a P1, P2, P3 and short P4 pulse are sent by the interrogator to mode S receivers. For an mode A/C/S all call a longer P4 pulse is sent. The length of P4 is important for the kind of all call.

The altitude echo function in the surveillance interrogation is intended to indicate (to the pilot) the flight level that ATC is being given by the aeroplane’s transponder. Comm A and Comm C can be used to send longer messages by breaking the messages up into suitably sized blocks and transmitting on successive cycles. Comm D, from the airborne transponder, has a similar capability. Comm D cannot be used for position update, as the messages contain no altitude information.

The increased use of Mode S will have the following benefits over standard SSR:•Elimination of synchronous garbling•Elimination of ‘fruiting’•Increased traffic capacity•Improved safety.

In addition, the ability to send messages will allow for a reduction of congestion on the current R/T communication frequencies. Transmitted data will be presented to the pilot on a CDU either integral with the Mode S transponder or on the FMS screen.

Mode S information, transponder to transponder, can also be integrated with the airborne Traffic Alert and Collision Avoidance system allowing the systems of conflicting aircraft to communicate and resolve convergent situations.

Fig. RN 10.9 “State of the art” digital Mode S transponder for general aviation

Fig. RN 10.8 Mode S transponder designed for light aircraft use



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ATC ServicesMode S data link can serve as a back up to many ATC services that are provided today by VHF ‘voice communications’. This data link back up will improve system safety by reducing communications-related errors within the ATC system. Many types of messages are potential candidates for data link back up and other ATC services. These include:•Flight identification, altitude

clearance confirmation•Take-off clearance confirmation•New communication frequency for

sector hand-over•Pilot acknowledgement of ATC clearance•Transmission to the ground of aircraft

flight parameters, and•Minimum safe altitude warning.

There are two steps in interrogation:•Elementary Surveillance (ELS)•Enhanced Surveillance (EHS).

The difference is that EHS extracts further aircraft parameters known as Downlink Airborne Parameters (DAPs).ELS foresees in the use of the unique address of the aircraft and its identity. It also enables to read out the altitude in 25 ft vertical resolution. Transponder capability and air to ground flight status. These parameters are the ELS-DAPs.

Mode S on AirportsModern airports are equipped with a mode S surface movement system. Aircraft operators should ensure that the mode S transponders are able to operate when the aircraft is on the ground according to

ICAO specifications (Annex 10, volume IV, 3.1.2.8.5.3 and 3.1.2.10.3.10). Pilots shall select the assigned mode A (squawk) code and activate the mode S transponder:•From request of push-back or taxi

whichever is earlier • After landing, continuously until the

aircraft is fully parked on stand. The transponder shall be deactivated immediately after parking.

Activation of the mode S transponder means selecting AUTO mode, ON, XPNDR, or the equivalent according to specific installation. Selection of the STAND-BY mode will NOT activate the mode S transponder. Depending on the hardware configuration, selecting ON could overrule the required suppression of SSR replies and mode S all-call replies when the transponder is on the ground.

Whenever the aircraft is capable of reporting aircraft identification (i.e. call sign used in flight), the aircraft’s identification should be entered before the activation of the transponder. Pilots must use the ICAO defined format for entry of the aircraft identification. To ensure that the performance of systems based on SSR frequencies (including airborne TCAS units and SSR radars) is not compromised, TCAS should not be selected before receiving the clearance to line up. It should then be deselected after vacating the runway. For aircraft taxiing without flight plan, mode A code 1000 should be selected.



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QUESTIONS

1 Long range surveillance radar may typically use a frequency of:

a) 1000 MHz b) 600 MHz c) 3000 MHzd) 10 GHz

2 In ATC SSR how many codes/modes are currently in use, excluding Mode S?

a) 4096 codes/2 modes b) 1028 codes/2 modes c) 4096 codes/4 modes d) 1028 codes/2 modes

3 Why does the aircraft transponder system not respond to its own transmissions when reflected from the ground?

a) Different frequencies are used 60 MHz apartb) Pulse repetition frequency changedc) The transponder system does reply to its own reflected signals, but

these responses are rejected by the transponder system at the sited) The aircraft signal is not reflected

4 What frequency does an SSR interrogator transmit on?

a) 1030 MHz b) 1030 kHz c) 1000 MHzd) 118.4 MHz



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5 An SSR antenna polar diagram:

a) Shows the polarization of the signal from the antennab) Shows how efficiently the antenna receives

or transmits in different directions c) Shows how efficient an antenna is at high latitudesd) Is always shown in the vertical plane

6 A primary radar serving as Terminal Approach Surveillance Radar (TAR):

a) Will be able to provide continuous glide slope correction to an approaching aircraft

b) Is well suited to give a picture of the ground traffic along taxiways and runways

c) Will have an aerial speed of rotation of about 2 – 3 RPMd) Provides guidance in azimuth only

7 The advantage of the use of slotted antennas in modern radar technology is to:

a) simultaneously transmit weather and mapping beamsb) virtually eliminate lateral lobes and, as a consequence, concentrate more energy in the main beamc) have a wide beam and as a consequence better target detectiond) eliminate the need for azimuth slaving

8 Complete the following statement. Aircraft Surface movement Radar operates on frequencies in the: (i) .......... band employing an antenna that rotates at approximately (ii) .......... revolutions per minute; it is (iii) ......... possible to determine the type of aircraft from the return on the radar screen.

a) (i) SHF (ii) 10 (iii) alwaysb) (i) EHF (ii) 30 (iii) neverc) (i) SHF (ii) 60 (iii) sometimesd) (i) EHF (ii) 100 (iii) never

The answers can be found at the end of the book.


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