A
communications satelliteA communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
’s
transponder, is the series of interconnected units which form a communications channel between the receiving and the transmitting antennas .
A transponder is typically composed of:
- An input band limiting device (a band pass filter)
- An Input low-noise amplifier
Low-noise amplifier is an electronic amplifier used to amplify possibly very weak signals . It is usually located very close to the detection device to reduce losses in the feedline. This active antenna arrangement is frequently used in microwave systems like GPS, because coaxial cable feedline is...
(LNA), designed to amplify the (normally very weak, because of the large distances involved) signals received from the earth station
- A frequency translator (normally composed of an oscillator and a frequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
) 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 relayRadio stations that cannot communicate directly due to distance, terrain or other difficulties sometimes use an intermediate radio relay station to relay the signals. Examples include airborne radio relay, microwave radio relay, and communications satellite...
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, and then re-encoded and modulated on board the satellite. This type of transponder , a 'regenerative' transponder, has many advantages but it is much more complex.
With
data compressionIn computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
and
multiplexingThe multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
, several
videoVideo is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
(including
digital videoDigital video is a type of digital recording system that works by using a digital rather than an analog video signal.The terms camera, video camera, and camcorder are used interchangeably in this article.- History :...
) and audio channels may travel through a single transponder on a single
widebandIn communications, wideband is a relative term used to describe a wide range of frequencies in a spectrum. A system is typically described as wideband if the message bandwidth significantly exceeds the channel's coherence bandwidth....
carrierIn telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
.
Original
analog videoAnalog video is a video signal transferred by an analog signal. An analog color video signal contains luminance, brightness and chrominance of an analog television image...
only had one channel per transponder, with
subcarrierA subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...
s for audio and automatic transmission identification service
ATISThe Automatic Transmitter Identification System is a communications protocol used for the station identification of television channels carried on satellite TV....
. Non-multiplexed
radio stationRadio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
s can also travel in
single channel per carrierSingle channel per carrier refers to using a single signal at a given frequency and bandwidth. Most often, this is used on broadcast satellites to indicate that radio stations are not multiplexed as subcarriers onto a single video carrier, but instead independently share a transponder...
(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
landlineA landline was originally an overland telegraph wire, as opposed to an undersea cable. Currently, landline refers to a telephone line which travels through a solid medium, either metal wire or optical fibre, as distinguished from a mobile cellular line, where transmission is via radio waves...
s to send it to an earth station for
multiplexingThe multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
with other stations.