Satellite Communications

I

Introduction

Satellite Communications, any Earth-orbiting spacecraft that provides communication over long distances by reflecting or relaying radio-frequency signals.

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.

III

Geostationary (or Geosynchronous) Orbit

A satellite in a geosynchronous orbit follows a circular orbit over the equator at an altitude of 35,800 km (22,300 mi), completing one orbit every 24 hours—the time that it takes the Earth to rotate once. Moving in the same direction as the Earth’s rotation, the satellite remains in a fixed position over a point on the equator, thereby providing uninterrupted contact between ground stations in its line of sight. The first communications satellite to be placed in this type of orbit was Syncom 2, launched by NASA in 1963. Most of those that followed were also placed in geosynchronous orbit.

IV

Commercial Communications Satellites

Deployment and operation of communications satellites on a commercial basis began with the founding of the Communications Satellite Corporation (COMSAT) in 1963. When the International Telecommunications Satellite Organization (Intelsat) was formed in 1964, COMSAT became the US member. Based in Washington, D.C., Intelsat is owned by more than 120 nations. Intelsat 1, known as Early Bird, launched in 1965, provided either 2,400 voice circuits or one two-way television channel between the United States and Europe. During the 1960s and 1970s, message capacity and transmission power of the Intelsat 2, 3, and 4 generations were progressively increased by beaming the satellite power only to the Earth and segmenting the broadcast spectrum into transponder units of a certain bandwidth.

With the launch of the Intelsat 3 satellites in 1969, full global coverage was achieved for the first time, covering each of the three main oceans. With the Intelsat 5 series (1980), introduction of multiple-beam operation resulted in additional increases in capacity. A satellite’s power could now be concentrated on small regions of the Earth, making possible smaller-aperture (coverage area), lower-cost ground stations. An Intelsat 5 satellite can typically carry 12,000 voice circuits. The Intelsat 6 satellites, which entered service in 1989, can carry 24,000 circuits and feature dynamic on-board switching of telephone capacity among six beams, using a technique called SS-TDMA (satellite-switched time division multiple access).

By the early 1990s, Intelsat had 15 satellites in orbit, providing the world’s most extensive telecommunications system. Other systems also provide international service in competition with Intelsat. In 1997 all regulatory restraints to such competition were lifted. The growth of international systems has been paralleled by domestic and regional systems, such as Europe’s Eutelsat and Telecom, and America’s Telstar, Galaxy, and Spacenet programmes.