Transponder (satellite)

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A transponder on a satellite receives data and transmits it again. For example, an earth station can send television data signals to a geostationary satellite , which the satellite sends back to earth. Any suitable satellite antenna with a line of sight to the satellite can receive the signal.

construction

Simplified block diagram of a transponder.

In the greatly simplified block diagram, the antenna on the left works both as a receiver ( Rx : receive ) and as a transmitter ( Tx : transmit ). The orthomode transducer (OMT) separates or combines the two polarizations of the microwave radiation, while the diplexer (DX) combines the received and transmitted radiation to form the antenna. The bandpass filters limit the signal. The receiver ( RD , Eng. Receiver ) with mixers or digital down converters shifts the signals to lower frequencies, in order to simplify further processing. From the input demultiplexer (IMUX) they go to the switching matrix (SWM), which distributes the signals to the power amplifiers (AMP). Power amplifiers are designed to be highly redundant as the most interference-prone element of the transponder (for example 19:16, i.e. 19 amplifiers for 16 signals). The output multiplexer (OMUX) combines the power signals and forwards them to the diplexer for transmission.

Data rate

In the Ku band , the bandwidth of a transponder is typically 36 MHz. This allows an analog television channel with a frequency range of approx. 5 MHz to be transmitted. The estimate follows from the Carson formula with a frequency deviation of 10 MHz.

A noise-loaded digital channel with a bandwidth of 36 MHz and a signal-to-noise ratio of 10 dB could ideally transmit approx. 100 Mbps. Real effective values ​​are 40 Mbps, corresponding to the capacity for 16 simple digital TV channels (approx. 2 Mbps per channel) or 4 HDTV channels (5–8 Mbps per channel).

A large satellite with an electrical output of 10 kW can operate around 50 transponders and broadcast around 500 television programs. Satellite providers rent transponder capacities for around EUR 4,000 per MHz per month, corresponding to around EUR 2 million per year for a transponder with a 40 MHz bandwidth.

For transmissions such as B. Live broadcasts in news broadcasts are offered by third-party providers slots on transponders from somewhat older or less popular satellites. The costs for these feeds are around 3 € (4.5 MHz slot) per minute, depending on the bandwidth.

Working method

Most transponders work transparently . They receive data and immediately re-emit it at a different frequency. For this, the term Bent Pipe has become common in English usage . As if through a curved pipe, the data stream is passed on without any qualitative change.

The regenerative mode is different . Satellites receive the data, demodulate, decode and correct it in order to recode it and forward it to selected recipients.

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