Satellites have now become an integral part of the worldwide communication systems. Although long–range and long distance communication took place much before the introduction of satellite systems, they had a lot of disadvantages. Point – to – point communication systems are very difficult in the case of remote & isolated locations, which are surrounded by oceans, mountains and other obstacles created by nature.
The satellite is nothing more than a radio-relay station But, they have one potential advantage- The capability of a direct line of sight path to 98% (excluding the polar caps, which are in accessible to satellites) of the earth's surface.
One of the most important event in the history of satellite communication took place when COMSAT or communication satellite corporation, launched four satellites within 6 years that is between 1965 to 1979. The first of these series was the ‘Early Bird' , which was launched in 1965. This was the first communication station to handle worldwide commercial telephone traffic from a fixed position in space. The next series INTELSAT was a group of satellites that served 150 stations in 80 countries.
The Satellite Orbit
The communications satellites are placed in orbits called equatorial geosynchronous orbit. The satellite placed in this orbit will appear stationery over a selected location on the earth' s surface. So, communications satellites are placed in an orbit that is directly over the equator, moving in a west to east direction at an altitude of 22,282 miles above sea level (36,000 km appor. as explained earlier) and with a forward velocity of 6874mph to complete one orbit in 24 hours. This orbit is called the Clarke orbit.
Up-link and Down-link
All of the ground equipment along with the transmission path and receiving antenna at the satellite are included in the up-link system. Basically, this includes everything before the input terminals of the satellite receiver. The down-link is described in terms of satellite transmitted output power, down link antenna gain and beam width and the ground area that the transmitted signal will cover the foot print.
At the attitude of the Clarke-orbit, one satellite could command a footprint area of 42.2% of the earth's surface. The beam-width from the satellite for such coverage is 17.2o Since such a satellite is not sufficient for global coverage, we need more than one to be specific 3 satellites.
These three satellites are placed 120 degrees apart in the Clarke orbit and would cover the earth's entire surface except for the polar caps. This makes it possible for one earth station to transmit to another station on the opposite side of the globe.