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In telecommunication, the term transponder has the following meanings:

1 . An automatic device that receives, amplifies, and retransmits a signal on a different frequency.

2. An automatic device that transmits a predetermined message in response to a predefined received signal.

Note: An example of transponders is in identification-friend-or-foe systems and air-traffic-control secondary radar (beacon radar) systems.

3. A receiver-transmitter that will generate a reply signal upon proper interrogation.

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