SSR Frequencies and Transmissions

(SSR) Radar Surveillance Secondary 14

Figure 14.4 SSR operates in the UHF band

The ground station transmits/interrogates on 1030 MHz and receives on 1090 MHz. The aircraft receives on 1030 MHz and transmits/transponds on 1090 MHz after a delay of 50 microseconds. The SSR ground antenna transmits a narrow beam in the horizontal plane while the aircraft transmits omni-directionally i.e. the radiation pattern is circular around the aircraft.

Modes

The aircraft is interrogated from the ground station by a predetermined series of pulses on the carrier frequency of 1030 MHz; its transponder then transmits a coded reply on a carrier frequency of 1090 MHz. The two main modes of operation are:

•Mode A - an interrogation to identify an aircraft

•Mode C - an interrogation to obtain an automatic height read-out of an aircraft.

To differentiate between the interrogations three pulses (P1, P2 and P3) are always transmitted.

The spacing between P1 and P2 is fixed at 2 µs. The spacing between P1 and P3 is 8 µs for a Mode A and 21 µs for a Mode C interrogation.

2 µs

Mode determined by spacing

Figure 14.5 Modes

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

1.45

µs

20.3 µs 4.35 µs

Figure 14.6 SSR reply pulse patterns

The aircraft transponder will reply correctly to a Mode A or C interrogation provided the pilot has correctly selected the mode and code allocated by ATC. On receiving a valid interrogation, the aircraft transponder transmits two framing pulses, F1 and F2, 20.3 µs apart. Between the framing pulses there are 12 usable information pulses (pulse X is for Mode B which is at present unused). A pulse can be transmitted or not, i.e. there are 212 = 4096 possible combinations of pulses or codes which are numbered 0000 to 7777; the figures 8 and 9 are not available.

A further pulse called the Special Position Identification (SPI) pulse may be transmitted together with the information pulses when the “Ident” button on the pilot’s transponder is pressed, usually at ATC’s request. This pulse is after the last framing pulse and will be automatically and continuously transmitted for about 20 seconds. It produces a distinctive display so that a controller can pick out a particular aircraft by asking the pilot to “Squawk Ident”.

Figure 14.7 Transponder controls & indicators

Secondary Surveillance Radar (SSR) 14

(SSR) Radar Surveillance Secondary 14

When the aircraft receives a Mode C interrogation the transponder will produce an ICAO determined code that corresponds to its height, referenced to 1013 hPa, regardless of the pressure setting on the altimeter and the code selected on the transponder. The Mode C code is determined by an encoder which is mechanically actuated by the altimeter’s aneroid capsule and is thus totally independent of the altimeter’s pressure setting.

The system provides Automatic Altitude Telemetering up to 128 000 ft, with an output change (based upon 50 ft increments or decrements) every 100 ft and provides the controller with the aircraft’s Flight Level or Altitude e.g. If an aircraft is flying at an allocated level of FL65, then 065 will be displayed on the screen. If the aircraft now drifts downwards, as it passes from 6450 ft to 6445 ft the coded transmission changes and results in 064 being indicated at the controller’s consol.

SSR Operating Procedure

Pilots shall:

•if proceeding from an area where a specific code has been assigned to the aircraft by an ATS unit, maintain that code setting unless otherwise instructed.

•select or reselect codes, or switch off the equipment when airborne only when instructed by an ATS unit.

•acknowledge code setting instructions by reading back the code to be set.

•select Mode C simultaneously with Mode A unless otherwise instructed by an ATS unit.

•when reporting vertical levels under routine procedures or when requested by ATC, read the current altimeter reading to the nearest 100 ft. This is to assist in the verification of Mode C data transmitted by the aircraft.

Note 1: If, on verification, there is a difference of more than 300 ft between the level read-out and the reported level, the pilot will normally be instructed to switch off Mode C. If independent switching of Mode C is not possible the pilot will be instructed to select Code 0000 to indicate transponder malfunction. (Note: this is the ICAO specification)

Note 2: A standard of ± 200 ft is applied in the UK and other countries.

Conspicuity Code

When operating under VFR rules at and above FL100, in European airspace, pilots should select code 7000 + Mode C (in North America the code is 1200 + Mode C) except:

•when given a different setting by an ATS unit.

•when circumstances require the use of one of the special purpose codes.

When operating below FL100 pilots should select the Conspicuity Code and Mode C except as above.

MODE C should be operated with all of the above codes.

Disadvantages of SSR

Air Traffic Services in Europe have increased their reliance on SSR (which provides data on an aircraft’s position, identification, altitude, speed and track) but the existing civil Mode A (identification) and Mode C (altitude reporting) system is reaching the limits of its operational capability. It has the following disadvantages:

Garbling

This is caused by overlapping replies from two or more transponders on nearly the same bearing from the ground station and within a distance of 1.7 NM from each other measured on a line from the antenna. [The reply pulses from the aircraft are transmitted over a period of 20.3 µs which relates to a distance of just under 1.7 NM in terms of radar miles.]

Fruiting

This is interference at one interrogator caused by replies from a transponder responding to interrogations from another.

Availability of Codes

Only 4096 identification codes are available in Mode A.

Secondary Surveillance Radar (SSR) 14