The Global Positioning System (GPS) consists of three sections or segments: the space segment, the control segment, and the user segment. The space segment--a constellation, or group, of satellites--sends out radio signals that are monitored and controlled by the control segment stations on Earth. The user segment--people with GPS receivers--works with the data provided by these signals.

24 satellites orbit 10,900 nautical miles above the earth. Because of their high altitude, the satellites can broadcast to a much greater area of the world than any station on the ground. In addition, tall obstructions such as mountains and buildings are less likely to block signals originating from that height. Four satellites in each of six orbits circle earth every 12 hours for world-wide coverage. At any one time, in any one place, there are enough satellites available for an accurate GPS position fix.

The control segment consists of four monitoring stations located in Hawaii, the Ascension Islands, Diego Garcia, and Kwajalein and a master station at Falcon Air Force Base in Colorado Springs. The monitoring stations check on the position of the satellites and assure that the information they are broadcasting is correct.

Raw GPS data for the typical user is only accurate to about 100 meters (325 ft.). Civilian users enjoy accuracy down to about 16 meters (50 ft.), but the U.S. Department of Defense slightly degrades GPS accuracy for security purposes.

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