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According to Kepler's Third Law, the orbital period of a satellite in a circular orbit increases with increasing altitude. Space stations and shuttles in Low Earth Orbit (LEO), typically two or four hundred miles above the Earth's surface make between fifteen and sixteen revolutions per day. The Moon, at an altitude of about 240,000 miles (385,000km), takes thirty days to make a complete rotation. Between those extremes lies the "magic" altitude of 22,300 miles (35,786km) at which a satellite's orbital speed exactly matches the rate at which the earth rotates: once every sidereal day (23 hours 56 minutes). In that case, the satellite is said to be geosynchronous.
If a geosynchronous satellite's orbit is not exactly aligned with the equator, it will appear (when viewed by someone on the ground) to oscillate daily around a fixed point in the sky. As the angle between the orbit and the equator decreases, the magnitude of this oscillation becomes smaller; when the orbit lies entirely over the equator, the satellite remains stationary relative to the Earth's surface – it is said to be geostationary.
Geostationary satellites appear to hover over one spot above the equator. Receiving and transmitting antennae on the earth do not need to track such a satellite. These antennae can be fixed in place and are much less expensive than tracking antennae. These satellites have revolutionized global communications, television broadcasting and weather forecasting, and have a number of important defense and intelligence applications.
One disadvantage of geosynchronous satellites is a result of their high altitude: radio signals take a fraction (approximately 1/4) of a second to reach and return from the satellite, resulting in a small but significant signal delay. This delay increases the difficulty of telephone conversation and reduces the performance of common network protocols such as TCP/IP, but does not present a problem with non-interactive systems such as television broadcasts. There are a number of proprietary satellite data protocols that are designed to proxy TCP/IP connections over long-delay satellite links -- these are marketed as being a partial solution to the poor performance of native TCP over satellite links.
The concept was first proposed by the science fiction author Arthur C. Clarke around 1945, based on Herman Potočnik's previous work. Working prior to the advent of solid-state electronics, Clarke envisioned a trio of large, manned space stations arranged in a triangle around the planet. Modern satellites are numerous, unmanned, and
often no larger than an automobile.
The first geosynchronous satellite was Syncom 2, launched on a Delta rocket B booster from Cape Canaveral 26 July, 1963. It was used a few months later for the world's first satellite relayed telephone call, between U.S President John F. Kennedy and Nigerian Prime minister Abubaker Balewa.
See also: Satellite televisionDefinition
Application
History