GEOSTATIONARY ORBIT SATELLITES BASIC INFORMATION

For any satellite in a circular orbit around the earth, the revolution period gets longer as the altitude increases. At an altitude of about 22,300 miles, a satellite in a circular orbit takes precisely one day to complete each revolution.

If a satellite is placed in such an orbit over the equator, and if it revolves in the same direction as the earth rotates, it is called a geostationary-orbit (GEO) satellite or simply a geostationary satellite. From the viewpoint of someone on the earth, a GEO satellite stays in the same spot in the sky all the time.

One GEO satellite can cover about 40 percent of the earth’s surface. A satellite over Ecuador, for example, can link most cities in North America and South America.

Three satellites in geostationary orbits spaced 120 degrees apart (one-third of a circle) provide coverage over the entire civilized world. Geostationary satellites are used in television (TV) broadcasting, telephone and data communication, for gathering weather and environmental data, and for radiolocation.

In GEO-satellite networks, earth-based stations can communicate via a single “bird” only when the stations are both on a line of sight with the satellite. If two stations are nearly on opposite sides of the planet, say in Australia and Wisconsin, they must operate through two satellites to obtain a link.

In this situation, signals are relayed between the two satellites, as well as between either satellite and its respective earth-based station. The main problem with two-way GEO-satellite communication is the fact that the signal path is long: at least 22,300 miles up to the satellite, and at least 22,300 miles back down to the earth.

If two satellites are used in the circuit, the path is substantially longer. This doesn’t cause problems in television broadcasting or in one-way data transfers, but it slows things down when computers are linked with the intention of combining their processing power. It is also noticeable in telephone conversations.

What is the minimum round-trip signal delay when a GEO satellite is used? Assume that the satellite retransmits signals at the same instant they are received. Radio waves travel at the speed of light (186,282 miles per second). The minimum path length to and from a geostationary satellite is 22,300 miles, for a total round-trip distance of 44,600 miles. Therefore, the delay is at least 44,600/186,282 second, or about 0.239 seconds, or 239 milliseconds.

The delay will be a little longer if the transmitting and receiving stations are located a great distance from each other as measured over the earth’s surface. In practice, a slight additional delay might also be caused by conditions in the ionosphere.