Satellite Networks

In an article called “Extra-Terrestrial Relays,” which appeared in the October 1945 issue of the magazine Wireless World, journalist and science-fiction author Arthur C. Clarke theorized that three satellites orbiting the globe at equal distances from each other could provide communication between almost any two points in the world. Nearly 20 years later, in July 1962, the American Telephone and Telegraph Company (AT&T) launched Telstar, the first real-time telecommunications satellite. Most people have some knowledge of satellites—we see large satellite dishes in our neighbor's yards, or small dishes on rooftops—but few people realize the global impact that these orbiting clusters of electronics have had, or the impact that they are likely to have in the future of communications.

Satellite communication systems are made up of two parts: the satellite itself and the ground station that sends and/or receives the signal. In a simple use of a satellite system, a signal at a ground station is amplified and transmitted to a satellite. There, the signal is amplified once again and transmitted back down to other receiving ground stations. True to Clarke's prediction, it takes only three satellites spaced around the globe to cover all but the polar regions of the planet. At least, that is the case with a signal that is meant primarily as a one-way, or broadcast, transmission. Because of the altitude of the satellites in orbit, the time it takes for the signal to make a complete trip may be over half a second, which is fine for a broadcast signal, but not very useful for a telephone or Internet signal. To reduce transmission time, satellites must be put into lower orbit.

High, Geostationary Earth Orbit (GEO) and lower, Low Earth Orbit (LEO) satellites are differentiated by their positions relative to Earth's surface. GEO satellites are positioned at about 35,400 kilometers (about 22,000 miles) above Earth, and their position is constant; they revolve along with Earth, and are always over the same spot. LEO satellites are positioned much closer to the surface, orbiting at altitudes between 645 and 1,610 kilometers (400 to 1,000 miles). Because of gravitational pull, they must be in motion, and their area of signal coverage is much smaller than that of a GEO satellite. For example, a GEO can cover about one-third of the planet, but a LEO can cover an area no larger than a small country, or a state in the United States.

The advantage of LEO satellites is that signal transmission takes much less time, and can be used effectively for telephone and Internet communication. LEO satellites may also be used to cover the polar regions that GEO satellites cannot cover. The more complex problem is that, since LEO satellites do not remain in a constant position relative to ground stations, they must be “tracked” by antenna arrays as they pass overhead. Also, since they move faster relative to Earth than GEOs, there must be more of them to create an effective network. LEO satellites pass transmissions off to each other, creating wireless networks that ring the planet; these are called “constellation” systems, and they work well for global voice and data networks.

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