Definition
Radio pulses transmitted by ground-based secondary surveillance radar (SSR) that prompt an aircraft's transponder to respond with a coded reply. The interrogation is sent on 1030 MHz, and the transponder replies on 1090 MHz with information such as the assigned squawk code and, in Mode C or Mode S, altitude data.
Plain English
Signals sent up from a radar station on the ground that ask each aircraft's transponder, 'Who are you, and how high are you?' The transponder hears the question and sends back an answer.
Context Anchor
Seen in radar and transponder discussions, especially when explaining how ground radar equipment identifies an aircraft and receives its altitude reply.
Derivation
From Latin interrogare, meaning 'to question.' The radar is literally questioning the aircraft, and the transponder answers — which is why the system is called secondary surveillance radar rather than primary radar (primary radar just bounces signals off the airframe with no question-and-answer involved).
Why Pilots Care
Pilots must keep the transponder powered and correctly set so ATC can receive reliable position and altitude replies triggered by these signals.
Analogy
It is like a roll call. The ground system asks, “Which aircraft are you?” and the transponder answers electronically.
Intuition Check
Interrogation does not mean a human interview here. It means an electronic request that asks a transponder to reply.
Example Sentence 1
When the aircraft entered radar coverage, the transponder began responding to interrogation signals from the ATC facility.
Example Sentence 2
When flying in radar coverage, the aircraft receives interrogation signals and automatically transmits its altitude in response.