Transponder (aeronautics)

A transponder (short for transmitter-responder[1] and sometimes abbreviated to XPDR,[2] XPNDR,[3] TPDR[4] or TP[5]) is an electronic device that produces a response when it receives a radio-frequency interrogation.

Aircraft have transponders to assist in identifying them on air traffic control radar.

Collision avoidance systems have been developed to use transponder transmissions as a means of detecting aircraft at risk of colliding with each other.

SSR uses an active transponder (beacon) to transmit a response to an interrogation by a secondary radar.

The pilot then selects the 0363 code on their transponder and the track on the air traffic controller's radar screen will become correctly associated with their identity.

[6][7] Similarly, the traffic collision avoidance system (TCAS) installed on some aircraft needs the altitude information supplied by transponder signals.

[6][7] In order to avoid confusion over assigned squawk codes, ATC units will typically be allocated blocks of squawk codes, not overlapping with the blocks of nearby ATC units, to assign at their discretion.

As an example, London Information—the flight information service station that covers the southern half of the UK—does not have access to radar images, but does assign squawk code 1177 to all aircraft that receive a flight information service (FIS) from them.