Communication satellites
The launching of the first satellite by the Russians in 1957 began what has become known as the `space race`, the first stage of which culminated with the Americans landing on the moon twelve years later. A whole range of satellites now orbit the Earth and are used for a variety of purposes. Low orbit satellites, the typical height of which varies from 150 to 450 kilometers, are of little use for telecommunications for they are only in line of sight of each earth station for about 15 minutes. Their rotation period around the Earth is about one and a half hours and their main use is for remote sensing, a field in which digital processing techniques are proving especially valuable. A low orbit satellite, equipped with a multispectral scanner system (MSS), can observe the Earth in great detail providing us with extremely accurate information about agriculture, forestry, water resources and pollution patterns. It also has a multitude of applications in such fields as weather forecasting, environmental monitoring, geology, oceanography, cartography. There are important defence implication too, since they can be used to `spy` on the activities of a potential enemy. Medium altitude satellites are used for telecommunications, especially in countries which cover a vast geographical area like the USSR. They `fly` at a typical height of 9000 to 18000 kilometres, orbiting the Earth in a period of five to twelve hours. They are in line of sight of each earth station for between two and four hours. The most important type of satellite for telecommunications is the geosynchronous, or geostationary, satellite positioned over the Equator at a height of 35800 kilometres. Its rotation period is 24 hours, the same as the Earth’s, and consequently, seen from the Earth, this type of satellites appears to remain motionless in the sky. It is within line of sight of an earth station for its entire life. A communication satellite is, in essence, a microwave relay station which receives signals in a given frequency band and retransmits them at a different frequency to avoid problems of interference between the weak incoming signal and the powerful retransmitted signal. The equipment which receives a signal, amplifies it, changes its frequency and then retransmits it, is called a transponder. A satellite can handle large amounts of traffic which it can send over vast areas of the Earth. It therefore represents a relatively cheap way of transmitting information over long distances. For countries which do not already have sophisticated cable or microwave networks the use of a satellite can be extremely beneficial as it can be used in their place. The first satellites were seen as a way of communicating with people who lived in isolated areas of the world. As a result, earth stations began to appear in the remotest parts of the globe. The cost of satellite communication began to fall steadily and, consequently, satellites have to complete with submarine cables as a way of linking continents cheaply. With the arrival of optical fibre undersea cables, however, a more balanced division of intercontinental circuits between the two is likely. Satellites were soon used to broadcast TV programmes `live` from one side of the Earth to the other, and then to link up computer terminals in different parts of the world. The use of digital transmission and multiplexing techniques has led to an enormous increase in the capacity of satellites.
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