LOW EARTH ORBIT SATELLITES " Mr Watson , come here I want you " screamed Alexander Graham Bell as he spilled some acid on himself and the first telephone message in the history of man was heard across the hall way , on March ,10 , 1876 .(4). Now more than a hundred years later people in different parts of the globe can communicate through cellular phones . And in less than 6 months people will be able to go anywhere in the world and carry one mobile phone with one phone number . They will be able to be reached in the middle of Los Angels or at the top of Mount Everest . A new system of low orbiting satellites that will allow cellular phones to work in any spot on the globe is now being launched .Geo-synchronous earth orbiting satellites Until recently, geo-synchronous earth orbiting satellites (GEOS) have dominated the skies. Big and expensive, they orbit the earth at an altitude of 22,300 miles and at the same speed as the earth's rotation. That enables them to remain over the same spot on earth all the time. Hence, to a person standing on earth they would appear stationary . The big advantage of GEOS is that one satellite can cover a huge region, thanks to its great distance from the earth . Three satellites , for example , placed in geo-synchronous orbit and at equal distances from each other around the equator can cover the entire globe, with the exception of the poles . (1 , p 23 ) Disadvantages of GEOSHowever, that great distance is also a disadvantage when it comes to relaying voice communications. It takes a signal travelling at the speed of light about half a second to reach a GEO satellite and return to earth. That quarter-second delay makes telephone conversations remarkably annoying. Further more this remote distance from earth makes it impossible to establish a direct link between a GEO satellite and a hand held cellular like phone . It is for this reason that cellular phones rely on ground stations to transmit ca...
