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Communications satellite

From Academic Kids

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Milstar.jpg
U.S. military MILSTAR communications satellite

A communications satellite (sometimes abbreviated to comsat) is an artificial satellite stationed in space for the purposes of telecommunications or reconnaisance 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 location on the ground with satellite dishes used to transmit to or receive from these is called an earth station or teleport.

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 for highly interactive applications such as video phones, VOIP or real-time online video games.

Contents

History

The concept of the geosynchonous communications satellite was first proposed by Arthur C. Clarke, building on work by Konstantin Tsiolkovsky and on the 1929 work by Herman Potočnik (writing as Herman Noordung) "Das Problem der Befahrung des Weltraums - der Raketen-motor". In October 1945 Clarke published an article titled "[Extra-terrestrial Relays (http://www.lsi.usp.br/~rbianchi/clarke/ACC.ETRelaysFull.html)]" in the British magazine Wireless World. The article described the fundamentals behind the deployment of artificial satellites in geostationary orbits for the purpose of relaying radio signals. Thus Arthur C. Clarke is often quoted as being 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 regardless of the position of its solar-cell arrays. The United States launched their own geosynchronous communication satellites afterward, with Western Union launching their Westar 1 satellite in 1974, and RCA Americom launching Satcom 1 in 1975.

It was Satcom 1 that was instrumental in helping early cable TV channels such as WTBS (now TBS Superstation), HBO, CBN (now ABC Family Channel), and The Weather Channel become successful, because these channels distributed their programming to all of the local cable TV headends using the satellite. Additionally, it was the first satellite used by broadcast TV networks in the USA, like ABC, NBC, and CBS, to distribute their programming to all of their local affiliate stations. The reason that Satcom 1 was so widely used is because it had twice the communications capacity of Westar 1 (24 transponders as opposed to Westar 1's 12), which resulted in less transponder usage costs.

By 2000 Hughes Space Systems, now part of Boeing, had built nearly 40 percent of the satellites in service worldwide.

LEOs

A low Earth orbiting satellite is a satellite with a low orbit and an orbital period significantly less 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 generally operate in the upper portion of the Ku band. Other frequency bands include the original C band, Ku band, and eventually Ka band. See broadcast satellites for further information.

Amateur Radio

Amateur Radio operators have access to the OSCAR satellites that have been designed specifically to carry amateur radio traffic. Most such satellites operate as spaceborne repeaters, and are generally accessed by amateurs equipped with UHF or VHF radio equipment and highly directional antennas such as Yagis or dish antennas. Due to the limitations of ground-based amateur equipment, most amateur satellites are launched into fairly low earth orbits, and are designed to deal with only a limited number of brief contacts at any given time. Some satellites also provide data-forwarding services using the AX.25 or similar protocols.

See also

External links

es:Satlite de comunicaciones ja:通信衛星

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