perigeeWhen a satellite follows a non-circular orbit around the earth, thesatellite's path is an ellipse with the center of the earth at one focus. Such asatellite has variable altitude and variable orbital speed. The point of lowestaltitude is called perigee. The term also applies to the minimum distance inkilometers or miles between the satellite and the center of the earth. (Perigee canbe measured between the satellite and the earth's surface, although this is a less precisespecification because the earth is not a perfect sphere. The difference isapproximately 4,000 miles or 6,400 kilometers.) 
At perigee, a satellite travels faster than at any other point in itsorbit. When viewed from the earth's surface, a satellite at or near perigeetraverses the sky at a rapid pace. In communications, perigee is the least desirabletime to access a satellite. Although its proximity means that the signal path isshort, the fact that the satellite is rapidly moving means that it is accessible for only a brief time. In addition, if a directional antenna is used at a ground-basedstation, it is difficult to track the satellite because the position of the antenna(azimuth and elevation) must be constantly and rapidly adjusted. 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 anearth-based station can be pointed at a fixed spot in the sky, and no further adjustmentof antenna orientation is needed. Compare apogee .
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| Last updated on:
Mar 17, 2008 |
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