Communications Satellite

A communications satellite relays and amplifies radio signals through a transponder, creating a channel between transmitters and receivers separated by the curve of the Earth, which otherwise blocks the line-of-sight radio waves used for telecommunications. Arthur C. Clarke set out the concept in his October 1945 article Extraterrestrial Relays in the British magazine Wireless World, describing artificial satellites placed in geostationary orbit to relay signals, which is why that orbit is still sometimes called the Clarke Belt. Early experiments used passive relays: the United States Naval Research Laboratory's Communication Moon Relay bounced signals off the Moon from 1956, and NASA's Echo 1, an aluminized balloon satellite launched on 12 August 1960, reflected microwave signals between ground stations. The first satellite built to actively relay communications was the Advanced Research Projects Agency's Project SCORE, launched 18 December 1958, which carried a taped Christmas message from President Dwight D. Eisenhower before its batteries failed after eight hours. Telstar, launched by NASA for AT&T on 10 July 1962, was the first active satellite to carry commercial traffic and made the first transatlantic television transmission, a first sometimes credited instead to the passive relay experiments that preceded it. Hughes Aircraft's Syncom 3, launched 19 July 1964, became the first satellite in true geostationary orbit, and Intelsat 1, known as Early Bird, launched 6 April 1965, became the first commercial satellite in that orbit, carrying transatlantic telephone and television traffic for the Intelsat consortium.