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The aircraft DME receiver is able to accept replies to its own transmissions and reject replies to other aircraft interrogations because..
  • A
    the time interval between pulse pairs is unique to that particular aircraft.
  • B
    aircraft interrogation signals and transponder responses are 63 MHz removed from each other.
  • C
    transmission frequencies are 63 MHz different for each aircraft.
  • D
    pulse pairs are amplitude modulated with the aircraft registration.

The aircraft transmits a stream of pairs of pulses to the ground station (the two pulses in each pair are separated by 12 micro seconds). After a short delay of 50 microseconds the ground station, called transponder, retransmits them. The time delay between sending and receiving the pulses is converted to a range readout. The aeroplane must distinguish between its own pulses returning and those of other aircraft,so the pulse trains are made unique to each aeroplane by using a random or "jittered" PRF.

Therefore the time interval between the paired pulses is random and the chance of two pulse trains being identical is effectively nil. The Reply Signal, on being received at the aircraft, is checked by the Echo Protection Circuit (EPC) to ensure the PRF received is the same as that transmitted, to eliminate the possibility that it is a reply to another interrogating aircraft.

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