In telecommunication, the term transponder (sometimes abbreviated to XPDR or TPDR) has the following meanings:

• An automatic device that receives, amplifies, and retransmits a signal on a different frequency.
• An automatic device that transmits a predetermined message in response to a predefined received signal.
• A receiver-transmitter that will generate a reply signal upon proper electronic interrogation.

In particular, a communications satellite's channels are called transponders, because each is a separate transceiver or repeater. Older television satellites in the C band have 24 transponders, while newer Ku band ones have 32. With digital video data compression and multiplexing, several video and audio channels may travel through a single transponder on a single wideband carrier. Original analog video only has one channel per transponder, with subcarriers for audio and ATIS. Non-multiplexed radio stations can also travel in single channel per carrier (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder, or using landlines to send it to an earth station for multiplexing with other stations.

Another example of transponders is in identification friend or foe systems and air traffic control secondary radar (beacon radar) systems. For general aviation and Commercial aviation transponders used on planes to amplify radar signals making the plane more visible. Most transponders are also able to transmit altitude information and a four digit code known as a transponder code or a "squawk code" to help air traffic controllers in separation.

Sonar transponders operate under water are used to measure distance through water and form the basis of underwater location marking, position tracking and navigation.

See also Transceiver

Source: partly from Federal Standard 1037C and from MIL-STD-188 and from the Department of Defense Dictionary of Military and Associated Terms