communications satellite

The international telecommunications system relies on microwave and satellite links for long-distance international calls. Cable links are increasingly made of optical fibres. The capacity of these links is enormous. The TDRS-C (tracking and data-relay satellite communications) satellite, the world's largest and most complex satellite, can transmit in a single second the contents of a 20-volume encyclopedia, with each volume containing 1,200 pages of 2,000 words. A bundle of optical fibres, no thicker than a finger, can carry 10,000 phone calls – more than a copper wire as thick as an arm.

An Echo passive communications satellite, used later to help measure the air density of the Earth's upper atmosphere. Here the Echo orbiter undergoes a test inflation in the airship hangar at Weeksville Naval Air Station in North Carolina.

The Chinese satellite Chinastar 1 is a vital step in the expansion of the China's telecommunications infrastructure.

Relay station in space for sending telephone, television, telex, and other messages around the world. Messages are sent to and from the satellites via ground stations. Most communications satellites are in geostationary orbit, appearing to hang fixed over one point on the Earth's surface.

The first satellite to carry TV signals across the Atlantic Ocean was Telstar, which was launched into low Earth orbit by the USA on 10 July 1962. The world is now linked by the Intelsat system of communications satellites. Other satellites are used by individual countries for internal communications, or for business or military use. A new generation of satellites, called direct-broadcast satellites, are powerful enough to transmit directly to small domestic aerials. The power for such satellites is produced by solar cells. The total energy requirement of a satellite is small; a typical communications satellite needs about 2 kW of power, the same as an electric heater.