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A transmitter-receiver capable of accepting the challenge of an interrogator and automatically transmitting an appropriate reply.
A receiver-transmitter, such as on satellites, which receives a transmission and retransmits it at another radio frequency.
A receiver/transmitter that will generate a reply signal upon proper interrogation; the interrogation and reply being on different frequencies (ICAO). The transponder may be ground-based or be in an aircraft. It also could be the airborne radar beacon receiver/transmitter portion of the air traffic control radar beacon system (ATCRBS), which automatically receives radio signals from interrogators on the ground and selectively replies with a specific reply pulse, or pulse group, only to those interrogations received on the mode to which it is set to respond.