Transponder (Satellite communications)

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A communications satellite’s channels are called transponders, because each is a separate transceiver or repeater. Most comsats are microwave radio relay stations in orbit, and carry dozens of transponders, each with a bandwidth of tens of Megahertz. With digital video data compression and multiplexing, several video and audio channels may travel through a single transponder on a single wideband carrier. Original analog video only has one channel per transponder, with subcarriers for audio and automatic transmission identification service ATIS. Non-multiplexed radio stations can also travel in single channel per carrier (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder, or using landlines to send it to an earth station for multiplexing with other stations.