The Transponder reference article from the English Wikipedia on 24-Jul-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Transponder

In telecommunication, the term transponder (sometimes abbreviated to XPDR) 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.

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

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