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Communications satellite

A communications satellite (sometimes abbreviated to comsat) is an artificial satellite stationed in space for the purposes of telecommunications using radio at microwave frequencies. Most communications satellites use geosynchronous orbits or near-geostationary orbits, although some recent systems use low Earth-orbiting satellites. A place on the ground with satellite dishes used to transmit to or receive from these is called an earth station.

Communications satellites provide a technology that is complementary to that of fiber optic submarine communication cables. Unlike fiber optic communication, satellite communication has a propagation delay (also called a path delay) of at least 270 milliseconds, which is the time it takes the radio signal to travel 35,800 km from earth to a satellite and then back to earth. Satellite Internet connections average a 600 to 800 millisecond delay, about ten times that of a terrestrial Internet link. This delay is a challenge to deployment of virtual private networks over satellite Internet connections.

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History

The concept of the communications satellite was first proposed by Arthur C. Clarke, based on Herman Potočnik's pseudonymous work from 1929. In 1945 Clarke published an article titled "Extra-terrestrial Relays" in the magazine Wireless World. The article described the fundamentals behind the deployment artificial satellites in geostationary orbits for the purpose of relaying radio signals. Thus Arthur C. Clarke is often quoted as the inventor of the communications satellite.

The first satellite to relay communications was Project SCORE in 1958, which used a tape recorder to store and forward voice messages. It was used to send a Christmas greeting to the world from President Eisenhower. NASA launched an Echo satellite in 1960. This 100 foot aluminized Mylar balloon served as a passive reflector for radio communications.

Telstar was the first active, direct relay communications satellite. Belonging to AT&T as part of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British General Post Office, and the French National PTT (Post Office.) to develop satellite communication, it was launched by NASA from Cape Canaveral on July 10, 1962, the first privately sponsored space launch. Telstar was placed in an elliptical orbit (completed once every 2 hours and 37 minutes), rotating at a 45 angle above the equator.

The first geosynchronous communications satellite was Hughes' Syncom 2, launched on July 26 1963. However, Syncom 2 was positioned in an inclined orbit, so special tracking equipment was needed to see it. The first geosynchronous communications satellite that could be seen from a fixed satellite antenna (over North America) was Anik 1, a Canadian satellite launched in 1973. This class of satellite was spin-scan stabilized, meaning that the satellite's antennas could receive/transmit useful information from/to Earth irregardless of the position of its solar-cell arrays. By 2000 Hughes had built nearly 40 percent of the satellites in service worldwide.

LEOs

A low Earth orbiting satellite is a satellite with a low orbit with an orbital period much shorter than a day. As these satellites can only be seen from any given part of the Earth for a short time as they pass over, large numbers of these satellites are needed to ensure continuous coverage. A group of satellites working in concert thus is known as a satellite constellation. Examples of satellite constellations are the GPS and the Iridium and Globalstar satellite telephony services.

It is possible to offer discontinuous coverage using a low Earth orbit satellite capable of storing data received while passing over one part of Earth and transmitting it later while passing over another part. This will be the case with the CASCADE system of Canada's CASSIOPE communications satellite.


Television

A direct broadcast satellite is a special high-powered communications satellite that transmits to small DBS satellite dishes. Direct broadcast satellites always operate in the upper portion of the Ku band. Other frequency bands include the original C band, and later Ku band. See broadcast satellites for further information.

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Last updated: 10-24-2004 05:10:45