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Earth stations
In the early days of satellite communications the dominating service was long distance communications between nodes of the global telecommunications network. The earth stations were similar in construction and had many of the same functions.
 
Such terminals are also important nowadays, but there is also a wide variety of other earth stations, or user terminals, for a wide range of different applications. We will therefore limit the description of what we could call a generic earth station.

A block diagram is shown below. Starting with the antenna, an earth station in the INTELSAT system could today have antenna diameter up to 15 meter. The first INTELSAT Standard A earth stations had antenna diameters up to 30 meter. Such antennas have beam widths as small as 0,2 degrees and must be equipped with tracking systems to follow the motion of nominally geostationary satellites.

The same antenna is used for both transmission and reception, and must therefore be equipped with a diplexer which directs the transmitted signal from the power amplifier to the antenna, and the received signal from the satellite to the low noise amplifier. Power leakage from the high power amplifier might destroy the low noise amplifier, even when they work at different frequencies. A C-band INTELSAT station would transmit at frequencies around 6 GHz and receive at around 4 GHz. The frequency difference between carriers of a two way link would normally have a frequency difference of 2225 MHz.
 
 
The received signals are converted to a lower frequency band where demodulation is performed. It is convenient to perform the demodulation at a fixed Intermediate Frequency (IF), which is normally at 70 or 140 MHz. These are standard frequencies for Commercial Off The Shelf (COTS) equipment. This means that the frequency shift of the down conversion must be controlled to select the wanted carrier. The carriers are demodulated and the baseband signals may be de-multiplexed into individual channels, for example telephone channels, before they are connected to the terrestrial network.
 
Umm Haraz earth station, Khartoum.
Umm Haraz earth station, Khartoum.
On the transmit side individual telephone channels may be multiplexed into a single baseband signal and modulated on carriers at the IF. The carriers are then up-converted to the correct transmit frequency. The total transmitted signal from an earth station will normally consist of several carriers.

The power amplifier must therefore be operated in the quasi-linear region, well below saturation. Otherwise so-called intermodulation (IM) products will be emitted that may interfere with other signals in the satellite. The satellite system operators impose strict limits on the levels of unwanted signals from an earth station. The nominal rating of a high power amplifier could for example be 1,5 kW at saturation, while the average transmitted power would be limited to 300 Watt.

In addition to the equipment in the signal path of the earth station it must also be equipped with a number of other functions, such as system control and power supply, accommodation for the staff, stores and workshops. The location of an earth station must also be appropriately chosen to avoid interference to or from other telecommunications systems in the area, which may have a radius of more than 100 km. An earth station in the INMARSAT system with 30 meter antenna diameter, Khartoum, Sudan, is shown in the picture.
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This theme includes:
Introduction, satcom systems, functions, history
The space segment: Satellite orbits
The space segment: The satellite platform
The space segment: Transparent, "bent pipe" transponder
The space segment: Communications satellite description
Radio transmission: Antennas
Free space wave propagation
Polarization
Travelling wave amplifiers
Mobile satellite communications
Satellite broadcasting
Satellite broadcasting: DVB (Digital Video Broadcasting)
Satellite broadcasting: Interactive broadcasting, DVB-RCS
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in co-operation with the Norwegian Space Centre, www.spacecentre.no.
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