Definition
An airborne electronic device that receives an interrogation signal from a ground-based secondary surveillance radar and automatically transmits a coded reply identifying the aircraft and, when equipped, reporting its pressure altitude. The reply produces a stronger, clearer return on the controller's radar display than a primary radar echo alone, and allows the aircraft to be tagged with its assigned identification code and altitude.
Plain English
A small radio in the aircraft that listens for radar signals from the ground and answers back with a code that tells the controller which aircraft this is and how high it is flying.
Context Anchor
You encounter this term when setting a transponder code, using altitude reporting, receiving radar services, or reading about air traffic control radar equipment.
Derivation
Transponder combines 'transmitter' and 'responder' -- a device that responds by transmitting. The name describes exactly what it does: it answers radar interrogations by sending a signal back.
Why Pilots Care
It gives controllers positive identification and altitude data, allowing safe separation of traffic in controlled airspace.
Analogy
It is like a name tag that can answer when called. The radar sends a request, and the transponder replies with information that tells the controller which aircraft you are.
Intuition Check
Do not think of it as the aircraft’s own radar scanner. A radar beacon transponder does not search for other aircraft; it replies to signals sent by air traffic control radar.
Example Sentence 1
Before entering Class C airspace, the pilot set the radar beacon transponder to the assigned squawk code and turned it to ALT.
Example Sentence 2
During the preflight check the pilot verified that the radar beacon transponder was powered on and replying to test interrogations.