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cable in the long term. Here are some examples from the civilian sphere:
" National, regional and international telephone and data traffic in areas with a low volume
of communications, i.e. in those places where the low rate of use would make a cable
connection unprofitable;
" Temporary communications systems used in the context of rescue operations following
natural disasters, major events, large-scale building sites, etc.;
" UN missions in regions with an underdeveloped communications infrastructure.
" Flexible/mobile business communications using very small earth stations (VSATs, see
below).
This wide range of uses to which satellites are put in the communications sphere can be
explained by the following characteristics: the footprint of a single geostationary satellite can
cover almost 50% of the earth s surface; impassable regions no longer pose a barrier to
communication. In the area concerned, 100% of users are covered, whether on land, at sea or
in the air. Satellites can be made operational within a few months, irrespective of the
infrastructure available on the spot, they are more reliable than cable and can be replaced
more easily.
1
See the justification for the amendment to the G10 Law in Germany.
2
Deutsche Telekom homepage: www.detesat.com/deutsch/
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The following characteristics of satellite communications must be regarded as drawbacks: the
relatively long delay times, the path attenuation, the shorter useful life, by comparison with
cable, of 12 to 15 years, the greater vulnerability to damage and the ease of interception.
4.2. How a satellite link operates
As already mentioned (see Chapter 3), by using appropriate antennae microwaves can be very
effectively focused, allowing cables to be replaced by microwave radio links. If the
transmitting and the receiving antenna are not in line of sight, but rather, as they are on the
earth, on the surface of a sphere, then from a given distance onwards the receiving antenna
disappears below the horizon owing to the curvature of the earth. The two antennae are thus
no longer in line of sight. This would apply, for example, to an intercontinental microwave
radio link between Europe and the USA. The antennae would have to be fitted to masts 1.8
km high in order for a link to be established. For this reason, an intercontinental microwave
radio link of this kind is simply not feasible, setting aside the issue of the attenuation of the
signal by air and water vapour. However, if a kind of mirror for the microwave radio link can
be set up in a fixed position high above the earth in space, large distances can be overcome,
despite the curvature of the earth, just as a person can see round corners using a traffic mirror.
The principle described above is made workable through the use of geostationary satellites.
4.2.1. Geostationary satellites
If a satellite is placed into a circular orbit parallel to the equator in which it circles the earth
once every 24 hours, it will follow the rotation of the earth exactly. Looking up from the
earth s surface, it seems to stand still at a height of 36 000 km it has a geostationary
position. Most communications and television satellites are satellites of this type.
4.2.2. The route followed by signals sent via a satellite communication link
The transmission of signals via satellite can be described as follows:
The signal coming from a cable is transmitted by an earth station equipped with a parabolic
antenna to the satellite via an upward microwave radio link, the uplink. The satellite receives
the signal, regenerates it and transmits it back to another Earth station via a downwards
microwave radio link, the downlink. From there, the signal is transferred back to a cable
network.
In the case of mobile communications, the signal is transmitted directly from the mobile
communications unit to the satellite, from where it can be fed into a cable link, via an Earth
station, or directly transmitted to a different mobile unit.
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4.2.3. The most important satellite communication systems
If necessary, communications coming from public cable networks (not necessarily state
networks) are transmitted between fixed earth stations, via satellite systems of differing scope,
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