The Reply
Replies to interrogations consist of 15 time slots, each 1.45 μs in width. The reply is encoded by the presence or absence of a 0.45 μs pulse in each slot. These are labeled as follows:
F1 C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4 F2 SPI
The F1 and F2 pulses are framing pulses, and are always transmitted by the aircraft transponder. They are used by the interrogator to identify legitimate replies. These are spaced 20.3 μs apart.
The A4, A2, A1, B4, B2, B1, C4, C2, C1, D4, D2, D1 pulses constitute the "information" contained in the reply. These bits are used in different ways for each interrogation mode.
For mode A, each digit in the transponder code (A, B, C, or D) may be a number from zero to seven. These octal digits are transmitted as groups of three pulses each, the A slots reserved for the first digit, B for the second, and so on.
In a mode C reply, the altitude is encoded by a Gillham interface, Gillham Code, which uses gray code. The Gillham interface is capable of representing a wide range of altitudes, in 100-foot (30 m) increments. The altitude transmitted is pressure altitude, and corrected for altimeter setting at the ATC facility. If no encoder is attached, the transponder may optionally transmit only framing pulses (most modern transponders do).
In a mode 3 reply, the information is similar to the mode A reply in that there are 4 digits transmitted between 0 and 7. The mode 3 reply differs from the mode A reply in that the transmitted code is assigned by a military air traffic controller, not a civilian air traffic controller.
The X bit is currently only used for test targets. This bit was originally transmitted by BOMARC missiles that were used as air launched test targets. This bit may be used by drone aircraft.
The SPI pulse is positioned 4.35μs past the F2 pulse (3 time slots) and is used as a "Special Identification Pulse". The SPI pulse is turned on by the "identity control" on the transponder in the aircraft cockpit when requested by air traffic control. The air traffic controller can request the pilot to ident, and when the identity control is activated, the SPI bit will be added to the reply for about 20 seconds (two to four rotations of the interrogator antenna) thereby highlighting the track on the controllers display.
Read more about this topic: Air Traffic Control Radar Beacon System, Theory of Operation
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