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Satellite Communication Systems
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 communication systems consist of Earth-orbiting communications platforms that receive and retransmit signals from earth-based stations. A typical television satellite receives a signal from a base station and broadcasts it to a large number of terrestrial receivers. Signals to satellites are called "uplinks," and signals from satellites are called "downlinks." Uplinks have also been called "shooting the bird." The downlink covers an area called the "footprint," which may be very large or cover a focused area. Satellites use microwave frequencies. Since they are overhead, the transmissions are line of sight to the receiver.
As pictured in Figure S-2, there are "high-orbit" GEO (geosynchronous satellites), "low-orbit" LEO (low earth orbit) satellites, and satellites in a variety of mid-orbits and elliptical orbits (some spy satellites use these orbits so they can drop in for a close look).
Geosynchronous satellites are placed in high stationary orbits 22,300 miles (42,162 kilometers) above the earth. The satellites are typically used for video transmissions. The speed and height of these satellites allow them to stay synchronized above a specific location on the earth at all times. One problem with high-orbit geosynchronous satellites is that a typical back-and-forth transmission has a delay of about a half second, which causes problems in time-critical computer data transmissions, as discussed in a moment. Satellites in LEO orbit are low enough to minimize this problem.
LEOs are close to the earth, usually within a few hundred kilometers, and inclined to the equatorial plane. Since the satellites are near the earth, earth-based devices don't require as much power to communicate with the satellites. Thus, they are ideal for phones and hand-held devices. However, LEOs are in fast orbits and do not stay stationary above a point on the earth. Therefore, a country-wide or global communication system requires a constellation of satellites that basically project moving footprints above the earth. As one satellite moves out of position, another takes over coverage. Calls and other transmissions are handed off from one satellite to another in this process. This is just the opposite of cellular phone systems where people move in and out of cells.
There is debate about which system is better for data communications: GEO or LEO. While LEOs are ideal for mobile wireless devices, the current trend is to enable GEOs with more bandwidth. Still, the delay of GEOs is a problem for time-critical applications.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.