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Abstract
Satellite communication systems consist of Earth-orbiting communications platforms that receive and retransmit signals from earth-based stations. A typical television satellite receives a signal from a base station and broadcasts it to a large number of terrestrial receivers. Signals to satellites are called "uplinks," and signals from satellites are called "downlinks." Uplinks have also been called "shooting the bird." The downlink covers an area called the "footprint," which may be very large or cover a focused area. Satellites use microwave frequencies. Since they are overhead, the transmissions are line of sight to the receiver.

Communications satellite
A communications satellite (sometimes abbreviated to COMSAT) is an artificial satellite stationed in space for the purpose of telecommunications. Modern communications satellites use a variety of orbits including geostationary orbits, Molniya orbits, other elliptical orbits and low (polar and non-polar) Earth orbits. For fixed (point-to-point) services, communications satellites provide a microwave radio relay technology complementary to that of submarine communication cables. They are also used for mobile applications such as communications to ships, vehicles, planes and hand-held terminals, and for TV and radio broadcasting, for which application of other technologies, such as cable, is impractical or impossible.


References:
http://en.wikipediawiki/Communications_satellite
http://linktionarys/satellite.html

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