(44K) This is an artist's concept of the Advanced Communications Technology Satellite (ACTS) as it would appear on-orbit.

Satellite Communication

Radio communication is the transmission of a signal placed on a radio wave from a transmitter to a receiver. On Earth, a radio wave propagates in three ways: 1) a direct wave that travels along a line of sight, 2) a ground wave (for frequencies less than a few megahertz) that travels a bit farther along the ground, and 3) sky waves that are reflected off the ionosphere. The direct wave provides the most reliable and useful communication capability, but are limited to receiving stations that are within sight of the transmitting station and cannot reach over the horizon. The higher the transmitting antenna, the farther the line of sight which is why antenna towers are so tall.

A communications satellite acts as a relay. At an altitude of 300 km (a nominal Shuttle altitude), about 2.25% of the Earth's surface is visible to the spacecraft at any one time. One ground station can transmit a signal to the spacecraft which can relay that signal back to the ground to a station thousands of kilometers away from the sending station.

A problem with these Low Earth Orbit (LEO) spacecraft is that they orbit the Earth in an hour or two, so they are only over a particular ground station for a few minutes. The higher the altitude of a spacecraft, the farther it has to travel to circle the Earth and the longer it takes for one orbit. At an altitude of 35,784 km it takes a spacecraft one day to go one orbit. This is called a synchronous orbit or, more specifically, Geosynchronous Earth Orbit (GEO). At this altitude, a satellite can see 42.4% of the Earth's surface at one time. A special geosynchronous orbit is the geostationary orbit in which a spacecraft remains above the same point on the surface. Even though radio waves travel at the speed of light, there is a delay of almost a quarter of a second for the signal to travel up to the satellite and back down to Earth.

The application to communications of a spacecraft that remains stationary with respect to the surface of the Earth was invented by Arthur C. Clarke and presented in an article in Wireless World in 1945. Today, there are around 150 operating commercial communications satellites in GEO.

Satellite communications have lost some of their advantages over wire with the advent of fiber optics. In situations where optical fibers are available, they offer the cheapest communications solution because of their wide bandwidth. Satellites will maintain a niche in providing communications where it is too expensive to run a fiber and in mobile applications (such as ships at sea).

Recently, direct broadcast television delivery by satellite has made inroads into the home market. Providing fiber to individual homes is still an expensive proposition. Long distance fiber lines are used between cities, but delivery of the telephone signal to the home still requires conversion to the analog copper wire system, which is the limiting factor. Satellite broadcast delivery can work if the cost of the ground station can be kept lower than the cost of running and renting a line (along with its associated electronics) to the user.

The first version of this web site was put on line in October 1995.