A communications satellite's transponder is the series of interconnected units that 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:
an input band-limiting device (an input band-pass filter), an input low-noise amplifier (LNA), designed to amplify the signals received from the Earth station(normally very weak, because of the large distances involved.) a frequency translator (normally composed of an oscillator and a frequency mixer) used to convert the frequency of the received signal to the frequency required for the transmitted signal, an output band-pass filter, a power amplifier (this can be a traveling-wave tube or a solid-state amplifier).
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 (i.e., u-bend)
principle, sending back to Earth 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.
With data compression and multiplexing, several video (including
digital video) and audio channels may travel through a single
transponder on a single wideband carrier.
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 shift and thus
infer range and speed from a communication signal without allocating
power to a separate ranging signal.
A transponder equivalent (TPE) is a normalized way to refer to
transponder bandwidth. It simply means how many transponders would be
used if the same total bandwidths used only 36 MHz
transponders. So, for example, the
^ Roddi, Dennis (2001). Satellite Communications (3rd ed.). New York:
McGraw Hill. p. 587. ISBN 0-07-138285-2.
^ Space Network Users Guide.
^ "Commercial Geostationary Satellite Transponder Markets for Latin
America". Frost & Sullivan Research Service. 2003-07-03. Retrieved
^ Jacbo Gullish, Futron (February 2011). "The North American Satellite
Market New Methodology Required..." www.satmagazine.com. SatMagazine.
^ "Transponder". JSat International. Retrieved 2015-08-31.
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