Geostationary-orbit satellites

For any satellite in the circular orbit around the earth, revolution period gets longer as altitude increases. At an altitude of around 22,300 miles, a satellite in a circular orbit takes one day to complete each revolution. If a satellite is placed in such an orbit over equator, and if it revolves in same direction as earth rotates, it is called as geostationary-orbit (GEO) satellite or simply a geostationary satellite. From viewpoint of someone on earth, a GEO satellite stays in same spot in the sky all the time.

One GEO satellite can cover around 40 % of the earth’s surface. A satellite over Ecuador, for instance, can link most cities in North America and South America. Three satellites in geostationary orbits spaced 120 % apart (1/3 of a circle) provide coverage over the entire civilized world. Geostationary satellites are used in television (TV) broadcasting, telephone and data communication, for the weather and environmental data, and for radiolocation.

In GEO-satellite networks, earth-based stations can communicate via a single bird only when the stations are both on a line of sight with the satellite. If the two stations are nearly on opposite sides of the planet, say in Australia and Wisconsin, they should operate through two satellites to obtain a link. In this situation, signals are relayed between the two satellites, as well as between both satellite and its respective earth-based station.The main problem with two-way GEO-satellite communication is the fact that the signal path is long: at least 22,300 miles up to the satellite, and at least 22,300 miles back down to the earth. If two satellites are used in the circuit, the path is substa

                                                Figure--A communications link that employs 2 GEO satellites.
 


longer. This does not cause problems in television broadcasting or in one way data transfers, but it slows things down when the computers are linked with the intention of combining their processing power. It is noticeable in telephone conversations.

Problem:What is the minimum round-trip signal delay when a GEO satellite is used? Suppose that the satellite retransmits signals at same instant they are received.

Solution:-
Radio waves travel at speed of light. The minimum path length to and from a geostationary satellite is 22,300 miles, for the total round trip distance of 44,600 miles. Thus, the delay is at least 44,600/186,282 second, or around 0.239 seconds, or 239 milliseconds.The delay will be a longer if the transmitting and receiving stations are located distance from each other as measured over earth’s surface. Practically a slight additional delay may be caused by conditions in ionosphere.