Geosynchronous satellite

The number of times a satellite in a circular orbit goes around the earth each day decreases steadily with increasing altitude. Space stations and shuttles that are only two or four hundred miles up make between fifteen and sixteen revolutions a day. The moon, our natural satellite about 240,000 miles (385,000km) away, takes thirty days to go around. In between those extremes, there is a "magic" altitude where a satellite's orbital speed just matches the rate at which the earth rotates: 22,300 miles (35,786km). In that case, it's said to be "geosynchronous."

If a geosynchronous satellite's circular orbit is tipped with respect to the equator, it will appear to someone on the ground to bob around a fixed point in the sky each day. But as the amount that the orbit is tipped becomes less and less, the bobbing becomes smaller, and when the orbit lies entirely over the equator, the satellite doesn't bob at all: it's stationary in the sky as seen from the ground and so is called "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.

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.

One disadvantage of geosynchronous satellites is a result of their high altitude: radio signals take a fraction of a second to reach and return from the satellite, resulting in a small but significant signal delay, which increases the difficulty of telephone conversation and reduces the performance of common network protocols such as TCP/IP. This does not present a problem with non-interactive systems such as television broadcast.

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.

See also: Satellite, Satellite television, Low Earth orbit satellite[?]