II.

History and Development

The idea of artificial satellites circling the Earth and relaying communications signals dates from October 1945, when the British technology writer (and science-fiction author) Arthur C. Clarke published a magazine article under the title “Extra-Terrestrial Relays” proposing the revolutionary idea. It was another 12 years before the world’s first satellite was launched, by the Union of Soviet Socialist Republics in 1957. Named Sputnik 1, the radio transmitter it carried could qualify it as a communications satellite, although the sole purpose of this transmitter was to act as a beacon.

Dedicated communications satellites, on the other hand, act as relay stations spanning distances that could not be covered by terrestrial transmitters. Over the years their function has expanded, and whereas the first satellites acted merely as relay stations for point-to-point messages, many modern satellites carry point-to-multipoint transmissions for applications as varied as direct-to-home broadcasting, data networking between different branches of large enterprises, and wide-area mobile phone systems (see also Cellular Radio).

Some of the first communications satellites had no radio equipment aboard and were designed to operate in a passive mode. Instead of actively transmitting radio signals, they served merely to reflect signals that were beamed up to them by transmitting stations on the ground. Signals were reflected in all directions, so they could be picked up by receiving stations around the world, although the utility of such systems was severely limited by the need for powerful transmitters and large ground antennas at the Earth stations.

This early period of development coincided with the Cold War, and the majority of early satellite communication experiments were strategic in nature. In one of these experiments, in 1958, the United States launched Score, occasioning the first transmission of a human voice from space. Score was equipped with a tape recorder that stored messages received while passing over a transmitting ground station. These messages were retransmitted when the satellite passed over a receiving station.

This did not, however, provide communication in “real time”; telephone and broadcasting organizations (as well as the military) were looking towards the development of a satellite or “bird” that carried equipment for receiving and retransmitting messages on demand. Such developments form the basis of satellite communications today. Telstar 1, launched on behalf of the American Telephone and Telegraph Company in 1962, provided direct television transmission between the United States, Europe, and Japan, and could also relay several hundred telephone calls simultaneously. Launched into an elliptical orbit inclined 45° to the equatorial plane, Telstar could only relay signals between two ground stations for a short period (45 minutes at most) during each revolution, when both stations were in its line of sight.

Hundreds of active communications satellites are now in orbit, put there by many different countries. They receive signals from one ground station, amplify them, and then retransmit them at a different frequency to another station. One frequency band used, 500 MHz wide, is divided into repeater channels of various bandwidths (located at 6 GHz for upward, or “uplink”, transmission and 4 GHz for downward, or “downlink”, transmission). A band at 14 GHz (uplink) and 11 or 12 GHz (downlink) is also much in use, mostly with fixed (non-mobile) ground stations. An 80 MHz-wide band at about 1.5 GHz (uplink and downlink) is used with small, mobile ground stations (ships, land vehicles, and aircraft). Solar energy cells mounted on large panels attached to the satellite provide power for reception and transmission.