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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 satellitess.
The concept of the communications satellite was first proposed by Arthur C. Clarke, based on Herman Potočnik's previous 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 communcations satellite.
Telstar was the first active communications satellite. Belonging to AT&T as part of a a multi-national agreements between AT&T, Bell Telephone Laboratories, NASA, the British 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 degree angle above the equator.
The first geosynchronous communications satellite was 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.
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 it passes over, large numbers of these satellites are needed to ensure coverage. A group of satellites working in concert is known as a satellite constellation.
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.