transponder
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. [JP1]