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The Reply Message

Figure 1: SSR down link format

Figure 1: SSR down link format

Figure 1: SSR down link format

CodeModusMeaning
77003/A, BGeneral air emergency
76003/A, BLoss of radio
4· frame1, 2, 3/A, BMilitary emergency call
75003/AHijacking

Table 1: examples of different Codes

The Reply Message

The SSR down link format consists of a number of pulses, nominally 0.45 µs (±0.1 µs). F1 and F2 are always present and separated by 20.3 µs (±0.1 µs) – they are often referred to as a bracket or framing pair. Other pulse positions within this framing pair are spaced by 1.45 µs and are used to convey the required reply information in answer to the specific interrogation (e.g Mode A identity or Mode C flight level values). The pulses are identified to give the bits of an octal code (ABCD). The X pulse at the centre of the reply is not used. The three blank positions may not be occupied by pulses, otherwise some decoders may reject the entire answer as interference.
Note that the reply information itself does not contain any information to indicate which mode it is a reply to. The interrogator will assume that the replies received are in answer to it latest mode of interrogation.

In the case of Mode A, the octal code (ABCD) is set by a control panel in the cockpit. In the case of mode C, the flight level is encoded in a special way (by a special form of Gray code known as Gillham code - this has the characteristic of only one bit changing for each change in flight level).

The SPI (Special Purpose Identification) pulse is used by air traffic controllers to confirm the identity of certain aircraft. The controller will ask the pilot to squawk ident – the pilot pressing a button on the control panel which adds the SPI pulse to SSR replies for a certain period (18±1 s). The display system will then highlight aircraft with SPI. (The SPI pulse may have been appropriate to distinguish aircraft on older display systems before fully plot extracted displays became available). The out of frame position of the SPI pulse is somewhat strange and, as will be seen later, the SPI pulse position chosen introduces rather unfortunate complications for automatic decoding purposes. According to ICAO the SPI-pulse will be added to Mode A reply only.

By international standards it is possible to assign defined questions and answers to certain standard situations:

Each answer receives its meaning only in connection with the respective question. For example: