Definition
An airborne radio transmitter-receiver that automatically replies to interrogation signals from ground-based secondary surveillance radar. When triggered, the transponder sends back a coded reply containing the aircraft's assigned four-digit identification code and, when Mode C or Mode S is enabled, the aircraft's pressure altitude. This reply allows air traffic control to identify, track, and separate the aircraft on radar.
Plain English
A small radio device in the aircraft that answers radar signals from the ground. When ATC's radar pings it, the transponder sends back a code that tells controllers which aircraft this is and how high it is flying.
Context Anchor
You use the transponder when following air traffic control instructions, operating in airspace where it is required, and setting the assigned code before or during flight.
Derivation
The word is a blend of 'transmitter' and 'responder.' That captures exactly what it does: it transmits a response when interrogated, rather than broadcasting on its own.
Why Pilots Care
It allows controllers to identify and separate aircraft in the sky and is required for flight in most controlled airspace.
Intuition Check
A transponder is not the same as the voice radio. The voice radio carries what the pilot says; the transponder automatically sends aircraft identification and altitude-related information to surveillance systems.
Example Sentence 1
Before takeoff, the pilot set the transponder to the assigned squawk code of 4271 and selected Mode C so altitude would be reported.
Example Sentence 2
ATC instructed the pilot to verify the transponder was operating in Mode C for altitude reporting.