Definition
The ground-based radar component that transmits coded pulse signals to an aircraft's transponder, prompting it to reply with identification and altitude information. The interrogating unit is the ground side of the secondary surveillance radar (SSR) system; the transponder is the airborne side that answers it.
Plain English
The piece of ground radar equipment that sends a signal asking each aircraft, "Who are you and how high are you?" The aircraft's transponder hears the question and sends back the answer.
Context Anchor
Seen in ground-based radar navigation and distance-measuring equipment discussions, especially when describing how an aircraft gets distance information from a navigation station.
Derivation
From the Latin interrogare, meaning "to question" or "ask." The name fits exactly what the unit does: it asks the aircraft a question and waits for a reply.
Why Pilots Care
If your transponder is off, failed, or set incorrectly, the interrogating unit gets no useful reply, and ATC loses your altitude readout and identification on their scope. That changes the kind of service they can provide and may affect your clearance.
Grounding Statement
The aircraft sends a signal, the ground station answers, and the aircraft uses the time between them to calculate distance.
Intuition Check
Do not think of “interrogating” as a person questioning someone. Here it means an electronic unit sending a signal that requests an automatic reply.
Example Sentence 1
The interrogating unit on the ground sends out a coded pulse, and the aircraft's transponder replies with its assigned squawk code and altitude.
Example Sentence 2
Each interrogating unit is tuned to avoid overlapping with nearby radar sites.