A primary radar pulse travels to an aircraft, bounces off it, and returns to the radar. This gives it two pieces of information:

  • The direction (from the position of the antenna)
  • The range (from the time the pulse took to return)

Secondary radar, on the other hand, effectively "asks the aircraft a question". The returning pulse is not the original one bouncing off, but an entirely new pulse transmitted by the aircraft containing, potentially, a lot more data.

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Why can SSRSSR —Secondary Surveillance Radar provide more info than primary radars?