14-01-2013, 04:23 PM
Sattelite Network Using Dama Mac
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Introduction:
Satellite networks are the most widely deployed networks. Though the initial installation cost is more, the benefit of reaching users spread over very large areas makes it a viable and very effective option for sending information in which some loss is tolerable. They provide large channel bandwidth with long transmission delays. The cost of providing a user with access to a satellite resource is independent of location. This is in contrast to terrestrial links where the installation costs are proportional to the distance from the service provider. The unique location of satellites enables direct communications access to and from a large potential user population, ideal for broadcast or multicast applications as many users can listen to a common signal on a common channel without replication of the information for each individual user. There are three primary classifications of satellite orbit dependent on the orbit altitude:
Geostationary Earth Orbit (GEO)
A geostationary orbit, or Geostationary Earth Orbit (GEO), is a circular orbit 35,786 kilometres (22,236 mi) above the Earth's equatorand following the direction of the Earth's rotation. An object in such an orbit has an orbital period equal to the Earth's rotational period (one sidereal day), and thus appears motionless, at a fixed position in the sky, to ground observers. Communications satellites andweather satellites are often given geostationary orbits, so that the satellite antennas that communicate with them do not have to move to track them, but can be pointed permanently at the position in the sky where they stay. A geostationary orbit is a particular type ofgeosynchronous orbit.
The notion of a geosynchronous satellite for communication purposes was first published in 1928 (but not widely so) by Herman Potočnik.[1] The first appearance of a geostationary orbit in popular literature was in the first Venus Equilateral story by George O. Smith,[2] but Smith did not go into details. British Science Fiction author Arthur C. Clarke disseminated the idea widely, with more details on how it would work, in a 1945 paper entitled "Extra-Terrestrial Relays — Can Rocket Stations Give Worldwide Radio Coverage?", published in Wireless World magazine. Clarke acknowledged the connection in his introduction to The Complete Venus Equilateral.[3]The orbit, which Clarke first described as useful for broadcast and relay communications satellites,[4] is sometimes called the Clarke Orbit.[5] Similarly, the Clarke Belt is the part of space about 35,786 km (22,236 mi) above sea level, in the plane of the Equator, where near-geostationary orbits may be implemented. The Clarke Orbit is about 265,000 km (165,000 mi) long.
Practical uses
See also: Geosynchronous satellite
Most commercial communications satellites, broadcast satellites and SBAS satellites operate in geostationary orbits. A geostationary transfer orbit is used to move a satellite from low Earth orbit (LEO) into a geostationary orbit. (Russian television satellites have usedelliptical Molniya and Tundra orbits due to the high latitudes of the receiving audience.) The first satellite placed into a geostationary orbit was the Syncom-3, launched by a Delta-D rocket in 1964.
A worldwide network of operational geostationary meteorological satellites is used to provide visible and infrared images of Earth's surface and atmosphere. These satellite systems include:
the United States GOES
Meteosat, launched by the European Space Agency and operated by the European Weather Satellite Organization, EUMETSAT
the Japanese MTSAT
India's INSAT series
A statite, a hypothetical satellite that uses a solar sail to modify its orbit, could theoretically hold itself in a geostationary "orbit" with different altitude and/or inclination from the "traditional" equatorial geostationary orbit.
Orbital stability
A geostationary orbit can only be achieved at an altitude very close to 35,786 km (22,236 mi), and directly above the Equator. This equates to an orbital velocity of 3.07 km/s (1.91 mi/s) or a period of 1,436 minutes, which equates to almost exactly one sidereal day or 23.934461223 hours. This ensures that the satellite is locked to the Earth's rotational period and has a stationary footprint on the ground. All geostationary satellites have to be located on this ring.