29 June 2010


These two articles describe the transponder interrogation/response protocol in a little more detail than my course materials. I'd always wondered how the transponder worked and had been under the mistaken belief that the primary air traffic control (ATC) radar pulses triggered the transponder to reply. Not so. It turns out that separate directional and omni-directional antennas transmit groups of three interrogation pulses on 1030 MHz. If an aircraft transponder receives these pulses with proper relative amplitudes, it will transmit the encoded squawk code and altitude on 1090 MHz after a 3 microsecond delay. The three interrogation pulses ensure that transponders will only be triggered by the main lobe of the rotating ATC antenna. The system can compute the distance of the aircraft from the ground antenna by measuring the time difference between the interrogation transmission and reception of the aircraft's reply. It is interesting that the transponder squawk codes use the octal number system. So there are 8^4 = 4096 possible 4-digit octal squawk codes. This makes efficient use of the log2(4096) = 12 bits that are needed to represent the squawk code in the response. If the codes were instead represented using 4 decimal digits, ceil(log2(10^4)) = 14 bits would be required and not all of the possible bit combinations would be usable.

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