The Computer Revolution/Communication/satellites

< The Computer Revolution‎ | Communication

Communications satellites are stationed in space for the purposes of telecommunications.

The first satellite was Soviet Sputnik I which was launched in 1957. In 1958, Project SCORE was the first American satellite used to relay communications. It used a tape recorder to store and forward voice messages. President Eisenhower used it to send a Christmas greeting to the world. In 1960, NASA launched an Echo satellite that was used for radio communications.

NASA launched the first privately sponsored space launch of Telstar in July, 1962. Telstar was the first active, direct relay communications satellite, and it belonged to AT&T.

Modern communications satellites use three different types of orbits. One of these is geostationary. From earth, the satellite appears to be in a fixed position. It revolves around the earth at a constant speed once per day over the equator. It is a useful application for communications purposes. In 1972, the first geostationary communications satellite, Anik I (a Canadian satellite) was launched.

The United States launched Satcom 1 in 1975, which was used by broadcast TV networks such as ABC, NBC and CBS to distribute their programming to all of their local affiliate stations.

Another type of orbit used by communications satellites is the low earth orbit. It is typically a circular orbit about 150 kilometers above the earth's surface and takes about 90 minutes to revolve around the earth. Because low earth orbiting satellites are stationed closer to the ground than geostationary orbiting satellites, they are less expensive to position in space. A low earth orbiting satellite is also able to store data received while passing over one part of Earth and transmit it later while passing over another part of Earth.

Molniya orbiting communications satellites are used for providing services at high latitudes. The Molniya orbit provides good elevation (the satellite's position above the horizon) over selected positions during the northern part of its' orbit. It takes one half day for the satellite to complete a full revolution.

Molniya orbiting satellites are more commonly used for telephone and TV services over Russia because of their high latitude capabilities. They are also used for mobile radio systems, even at lower latitudes, because cars traveling in the presence of tall buildings need access to satellites at high elevation to secure the signal.