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A communications satellite's transponder, is the series of interconnected units which form a communications channel between the receiving and the transmitting antennas. It is mainly used in satellite communication to transfer the received signals.
A transponder is typically composed of:
Most communication satellites are radio relay stations in orbit, and carry dozens of transponders, each with a bandwidth of tens of megahertz. Most transponders operate on a "bent pipe" principle, sending back to earth of what goes into the conduit with only amplification and a shift from uplink to downlink frequency. However, some modern satellites use on-board processing, where the signal is demodulated, decoded, re-encoded and modulated aboard the satellite. This type, called a "regenerative" transponder, has many advantages, but is much more complex.
Original analog video only had 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.
NASA distinguishes between a "transponder" and a "transceiver", where the latter is simply an independent transmitter and receiver packaged in the same unit, and the former derives the transmit carrier frequency from the received signal. This linkage allows an interrogating ground station to recover the Doppler and thus infer range and speed from a communication signal without allocating power to a separate ranging signal.