Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Satellite systems are employed for telephone and data communications. There are geostationary satellites flying in high orbit (22,000 miles) where they can maintain the same position above the earth's surface at all times. The only problem, with such high-flying satellites is that there is a noticeable delay in real-time communications, and the power requirements to communicate with the satellites is too high for portable devices.

LEOs are more practical for mobile communication devices like mobile phones, PDAs, and automobile communication systems. An LEO satellite orbits in a relatively low earth orbit of a few hundred miles. In this orbit, the round-trip time for transmission is minimal, as are the power requirements for earth-bound communication devices. The downside of LEO satellites is that a fleet of them is required. Because of their low orbit, they move faster relative to a point on the surface, so a fleet of LEO satellites is required to maintain communications over a single point. As one LEO moves out of position, the other moves in. Each satellite covers an area that could be compared to a cell in a cellular system, except that the cell moves as the satellite orbits.

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