A primary surveillance radar simply bounces a pulse off an aircraft, which will means that a PSRPSR —Primary Surveillance Radar will show distance (from the time taken) and bearing (from the antenna position). This system does not require any equipment on board the plane, so works without a transponder.

With SSRSSR —Secondary Surveillance Radar, the pulse does not just "bounce off", but the aircraft replies to the interrogator with other pieces of information:

  • For Mode A, add aircraft identification codes ("squawk")
  • For Mode C, add (pressure) altitude readout
  • For Mode S, add a whole load of other possibilities including a two-way data link

Note than none of these include aircraft height, which would require knowing the elevation of the ground beneath the aircraft.

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What data can be determined and displayed by Primary Surveillance Radar?
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