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A communications satellite, Comsat, is an artificial satellite, which is stationed in space for the purposes of telecommunications. The latest design communications satellites use the geosynchronous orbits, the Molniya orbits or low polar Earth orbits. For the fixed services, the communications satellites provide a technology complementary to that of fiber optic submarine communication cables. These are also used for the mobile applications such as communications to the ships and planes.
For an earth-based observer, a satellite in a geostationary orbit seems to be in a fixed position. A geostationary satellite revolves around the earth at a constant speed once each day over the equator. The geostationary orbit is useful for communications because the ground based antennas, which must be directed toward the satellite, can operate effectively without the requirement for expensive equipment for tracking the motion of the satellite.
The applications that need ground antennas in large numbers like the direct TV distribution, involve extra cost and onboard complexity of lifting a satellite into the relatively high geostationary orbit. A low Earth orbit typically is a circular orbit about 150 km above the surface of earth and, a period of about 90 minutes. These satellites are visible from within a radius of approximately 1000 km from the sub-satellite point.
Satellites in the low earth orbit frequently change their position relative to the ground position. Low earth orbiting satellites are not so expensive to position in space than geostationary satellites. These require lower signal strength, because of their closer proximity to the ground; these need lower strength of the signal. A satellite constellation is a group of satellites that are working in concert.
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Submitted by admin on Thu, 04/19/2007 - 06:42.