Part of the Data transmission glossary:

 

When a satellite follows a non-circular orbit around the earth, the satellite's path is an ellipse with the center of the earth at one focus.Such a satellite has variable altitude and variable orbital speed. The point of highest altitude is called apogee.The term also applies to the maximum distance in kilometers or miles between the satellite and the center of the earth.(Apogee can be measured between the satellite and the earth's surface, although this is a less precise specification because the earth is not a perfect sphere.The difference is approximately 4,000 miles or 6,400 kilometers.)

At apogee, a satellite travels more slowly than at any other point in its orbit. When viewed from the earth's surface, a satellite at or near apogee takes along time to traverse the sky. In communications, apogee is the best time to access a satellite. Although its distance means that the signal path is long, the fact that the satellite is slowly moving means that it is accessible for a comparatively longtime. In addition, if a directional antenna is used at a ground-based station, it is relatively easy to track the satellite because the position of the antenna (azimuth and elevation) need not be adjusted very often or rapidly.

One of the principal advantages of a geostationary satellite is the fact that it follows a circular orbit, so the orbital speed is constant. In addition, the satellite's synchronization with the earth's orbit means that the antenna of an earth-based station can be pointed at a fixed spot in the sky, and no further adjustment of antenna orientation is needed.

Compare perigee .

This was last updated in June 2010
Posted by: Margaret Rouse

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