Communications Satellites

It is difficult to go through a day without using a communications satellite at least once. Do you know when you used a communications satellite today? Did you watch T.V.? Did you make a long distance phone call, use a cellular phone, a fax machine, a pager, or even listen to the radio? Well, if you did, you probably used a communications satellite, either directly or indirectly.

Communications satellites allow radio, television, and telephone transmissions to be sent live anywhere in the world. Before satellites, transmissions were difficult or impossible at long distances. The signals, which travel in straight lines, could not bend around the round Earth to reach a destination far away. Because satellites are in orbit, the signals can be sent instantaneously into space and then redirected to another satellite or directly to their destination.

The satellite can have a passive role in communications like bouncing signals from the Earth back to another location on the Earth; on the other hand, some satellites carry electronic devices called transponders for receiving, amplifying, and re-broadcasting signals to the Earth.

Communications satellites are often in geostationary orbit. At the high orbital altitude of 35,800 kilometers, a geostationary satellite orbits the Earth in the same amount of time it takes the Earth to revolve once. From Earth, therefore, the satellite appears to be stationary, always above the same area of the Earth. The area to which it can transmit is called a satellite's footprint. For example, many Canadian communications satellites have a footprint which covers most of Canada.

Communications satellties can also be in highly elliptical orbits. This type of orbit is roughly egg-shaped, with the Earth near the top of the egg. In a highly elliptical orbit, the satellite's velocity changes depending on where it is in its orbital path. When the satellite is in the part of its orbit that's close to the Earth, it moves faster because the Earth's gravitational pull is stronger. This means that a communications satellite can be over the region of the Earth that it is communicating with for the long part of its orbit. It will only be out of contact with that region when it quickly zips close by the Earth.

Click here to learn about the Canadian communications satellite Anik.

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Produced by Galactics.
Last updated on: 8 August 1997.