Definition
The ground-based surveillance radar transmitter that sends a coded signal to an aircraft's transponder, prompting the transponder to reply with identification and altitude information. Together with the airborne transponder, the interrogator forms the secondary surveillance radar (SSR) system that controllers use to track aircraft.
Plain English
The ground radar equipment that asks each aircraft 'who are you and what altitude are you at?' Your transponder answers automatically, and the controller sees the result on the radar screen.
Context Anchor
Seen in radar, transponder, Mode A, Mode C, and Mode S discussions, especially when describing how air traffic control receives aircraft information.
Derivation
From the Latin 'interrogare,' meaning 'to question.' The name fits exactly what the equipment does: it asks the aircraft a question and waits for the transponder to reply.
Why Pilots Care
The interrogator enables ATC to positively identify and track aircraft on radar, supporting separation, traffic advisories, and instrument flight services.
Intuition Check
Do not read interrogator as a person asking questions. In this aviation use, it is electronic equipment that sends a radio request to an aircraft transponder.
Example Sentence 1
The ground interrogator sends a pulse, and the aircraft's transponder replies with the assigned squawk code and altitude.
Example Sentence 2
A Mode S interrogator selectively addresses each aircraft to reduce reply frequency congestion.