transponder

A transponder is a device that receives a signal and then automatically transmits its own signal in response. This technology, coined during World War II from the words "transmitter" and "responder," is fundamental to modern aviation, communications satellites, and even everyday items like toll transponders and car key fobs.1

In telecommunications, satellites use transponders as radio repeaters. They receive signals from Earth (uplink) and retransmit them back (downlink), enabling services like long-distance calls, broadcasting, and data communication. Modern satellites can have many transponders, each handling specific frequencies and functions, allowing for simultaneous communication with numerous users.2