Definition
Onboard radio devices that automatically reply to interrogations from secondary surveillance radar with a coded signal, allowing air traffic control to identify the aircraft, display its position, and (in Mode C or Mode S) read its altitude and other data on the controller's screen.
Plain English
A small radio in the aircraft that answers when ground radar 'pings' it, sending back a code so controllers can see who you are, where you are, and how high you are flying.
Context Anchor
Seen in airport surface movement systems, radar surveillance, and procedures that tell pilots when to operate the transponder on the ground or in flight.
Derivation
Transponder' comes from combining 'transmitter' and 'responder.' The name describes exactly what it does: it transmits a response when something asks for one.
Why Pilots Care
Transponders let controllers identify and track each aircraft, which is essential for safe separation on busy airport surfaces.
Analogy
An aircraft transponder is like an electronic name tag that answers when a security scanner asks, “Who are you?” It does not replace seeing the aircraft, but it helps the system label and track it correctly.
Intuition Check
Do not think of aircraft transponders as normal voice radios. A pilot does not talk through them; they automatically send coded replies to surveillance systems.
Example Sentence 1
Before takeoff, the pilot set the transponder to the squawk code assigned by ground control.
Example Sentence 2
Controllers rely on aircraft transponders to distinguish one plane from another during low-visibility surface operations.