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How Geostationary Orbit is Achieved
The higher an object is in orbit over the Earth, the longer it takes the object to complete 1 full orbit. For instance, the period of time the moon takes to orbit the Earth is 27.3 days. While the object orbits the Earth, the Earth is rotating meanwhile on its axis. If an object over the equator revolved around the Earth in the same period of time it takes the Earth to rotate once, the object would never get ahead or behind the point at which it first went into orbit. It would be in synch--geosynchronous--as if it were tethered to the Earth.
Geosynchronous Versus Geostationary
All geostationary orbits are geosynchronous, but not all geosynchronous orbits are geostationary. An object in orbit over the equator has a latitude of zero degrees, since that is the latitude of the equator. For its entire period of orbit, the object will stay at the zero-degree latitude. Let's say, though, that an object is orbiting at an angle to the equator, say 45 degrees. The object will cross the equator as it orbits the Earth. Thus, it's not geostationary.
History
Isaac Newton came up with the law of universal gravitation, which can predict the orbits of satellites. In the early 20th century, thinkers--Konstantin Tsiolkovsky, Hermann Oberth, Herman Potocnik (also known as Herman Noordung) began imagining space travel that included satellites in a geostationary orbit. In 1945, writer Arthur C. Clarke published an article proposing that geostationary orbits could be used for a worldwide satellite communications network. NASA began the Synchronous Communications Satellite program in 1963, successfully launching the first geostationary communications satellite in 1964.
Uses of Geostationary Orbit
As Clarke suggested, a geostationary orbit is useful for communications. The fact that a satellite is in a reliable place means that signals can be reliably sent to the satellite, which then, in turn, can send signals to the area of its coverage. One satellite can see 42 percent of the Earth's surface from its geostationary orbit. A network around the equator can see all of the Earth between the latitudes of 81 degrees south and 81 degrees north. Satellites are commonly used to watch weather, relay TV and radio signals and allow the use of mobile phones.
Limits of Available Orbits
Since geostationary orbits can only occur above the equator at an altitude of about 36,000 kilometers, there is, in effect, a ring all geostationary satellites must share. This area around the Earth is called the Clarke Belt. Not only is there limited space for geostationary satellites, each needs a certain amount of room to avoid radio-frequency interference. Countries beneath the equator feel they have a claim to the space above them. Meanwhile, countries in the same longitude but at different latitude also desire slots in the same equatorial space. The International Telecommunication Union handles all disputes.