geostationary satellite definition

Contributor(s): Gene Saunders

A geostationary satellite is an earth-orbiting satellite, placed at an altitude of approximately 35,800 kilometers (22,300 miles) directly over the equator, that revolves in the same direction the earth rotates (west to east). At this altitude, one orbit takes 24 hours, the same length of time as the earth requires to rotate once on its axis. The term geostationary comes from the fact that such a satellite appears nearly stationary in the sky as seen by a ground-based observer. BGAN, the new global mobile communications network, uses geostationary satellites.

A single geostationary satellite is on a line of sight with about 40 percent of the earth's surface. Three such satellites, each separated by 120 degrees of longitude, can provide coverage of the entire planet, with the exception of small circular regions centered at the north and south geographic poles. A geostationary satellite can be accessed using a directional antenna, usually a small dish, aimed at the spot in the sky where the satellite appears to hover. The principal advantage of this type of satellite is the fact that an earthbound directional antenna can be aimed and then left in position without further adjustment. Another advantage is the fact that because highly directional antennas can be used, interference from surface-based sources, and from other satellites, is minimized.

Geostationary satellites have two major limitations. First, because the orbital zone is an extremely narrow ring in the plane of the equator, the number of satellites that can be maintained in geostationary orbits without mutual conflict (or even collision) is limited. Second, the distance that an electromagnetic (EM) signal must travel to and from a geostationary satellite is a minimum of 71,600 kilometers or 44,600 miles. Thus, a latency of at least 240 milliseconds is introduced when an EM signal, traveling at 300,000 kilometers per second (186,000 miles per second), makes a round trip from the surface to the satellite and back.

There are two other, less serious, problems with geostationary satellites. First, the exact position of a geostationary satellite, relative to the surface, varies slightly over the course of each 24-hour period because of gravitational interaction among the satellite, the earth, the sun, the moon, and the non-terrestrial planets. As observed from the surface, the satellite wanders within a rectangular region in the sky called the box. The box is small, but it limits the sharpness of the directional pattern, and therefore the power gain, that earth-based antennas can be designed to have. Second, there is a dramatic increase in background EM noise when the satellite comes near the sun as observed from a receiving station on the surface, because the sun is a powerful source of EM energy. This effect, known as solar fade, is a problem only within a few days of the equinoxes in late March and late September. Even then, episodes last for only a few minutes and take place only once a day.

In recent years, low earth orbit (LEO) satellite systems have become popular. This type of system employs a fleet or swarm of satellites, each in a polar orbit at an altitude of a few hundred kilometers. Each revolution takes between 90 minutes and a few hours. Over the course of a day, such a satellite comes within range of every point on the earth's surface for a certain period of time. The satellites in a LEO swarm are strategically spaced so that, from any point on the surface, at least one satellite is always on a line of sight. The satellites thus act as moving repeaters in a global cellular network. A LEO satellite system allows the use of simple, non-directional antennas, offers reduced latency, and does not suffer from solar fade. These facts are touted as advantages of LEO systems over geostationary satellites.

This was first published in October 2008

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