artificial satellite that functions as part of a global radio-communications network. Echo 1, the first communications satellite, launched in 1960, was an instrumented inflatable sphere that passively reflected radio signals back to earth. Later satellites carried with them electronic devices for receiving, amplifying, and rebroadcasting signals to earth. Relay 1, launched in 1962 by the National Aeronautics and Space Administration (NASA), was the basis for Telstar 1, a commercially sponsored experimental satellite. Geosynchronous orbits (in which the satellite remains over a single spot on the earth's surface) were first used by NASA's Syncom series and Early Bird (later renamed Intelsat 1), the world's first commercial communications satellite.

In 1962, the U.S. Congress passed the Communications Satellite Act, which created the Communications Satellite Corporation (Comsat). Agencies from 17 other countries joined Comsat in 1964 in forming the International Telecommunications Satellite Consortium (Intelsat) for the purpose of establishing a global commercial communications network. Renamed the International Telecommunications Satellite Organization in 1974, Intelsat now has 143 member agencies, a network of 17 satellites in geosynchronous orbits that provides instantaneous communications throughout the world, and more than 2,000 earth stations to transmit and receive signals to and from the satellites. It has orbited eight series of Intelsat satellites, beginning with Intelsat 1 (Early Bird) in 1965. The satellites in the current series, Intelsat 8, can carry 22,500 two-way telephone calls and three color television broadcasts at the same time; this could be increased to as many as 112,500 two-way telephone circuits using advanced electronic equipment.

Comsat is also the U.S. representative to the International Mobile Satellite Organization (Inmarsat). Established in 1979 to serve the maritime industry by developing satellite communications for ship management and distress and safety applications, Inmarsat was originally called the International Maritime Satellite Organization but changed its name to reflect its expansion into land, mobile, and aeronautical communications. Its users now include thousands of people who live or work in remote areas without reliable terrestrial networks. Inmarsat presently represents 86 member countries and has nine satellites in geosynchronous orbits—four of these satellites, the latest Inmarsat-3 generation, provide overlapping global coverage, and the remainder are available as spares or for leasing to other organizations.

In addition to the Intelsat and Inmarsat satellites, many others are in orbit. These are used by individual countries, organizations, and commercial ventures for internal communications or for business or military use. A new generation of satellites, called direct-broadcast satellites, transmits directly to small domestic antennas to provide such services as cablelike television programming.

See G. D. Gordon, Principles of Communications Satellites (1993); D. H. Martin, Communications Satellites, 1958–1995 (1996); B. G. Evans, ed., Satellite Communication Systems (3d ed. 1999).

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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