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Definition: transponder


A receiver/transmitter that transmits after receiving. For example, the transponder in an RFID system transmits identification data upon receiving a signal that energizes it. An airplane transponder sends back a signal after receiving signals from air traffic control (see ADS-B). A communications satellite receives a signal from earth (uplink), amplifies it and retransmits it back to earth (downlink). A satellite has several transponders.