Global Positioning System

Written by Tadashi Moody
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The global positioning system (GPS for short) is a network of satellites, signals, ground-based hardware and software, and people which make possible the identification of one's precise location on the Earth's surface. It is composed of a constellation of 24 satellites originally put into orbit by the U.S. Department of Defense. A GPS receiver on the ground can use signals from these satellites to calculate its precise location.

Global Positioning System Accuracy

The accuracy of a location measurement using the global positioning system depends on the technology and features of the GPS receiver. High-end, survey-grade GPS receivers can calculate locations accurate to within centimeters. But even the low-end, commercial handheld receivers, available at reasonable prices to the public, may be accurate to within fifteen meters.

Originally, commercial global positioning system units were accurate to only about 100 meters because the military scrambled the signal--a program called Selective Availability. In the year 2000, the U.S. government removed Selective Availability, improving the accuracy of even basic handheld units to within about fifteen meters. Today, with additional information from ground based correction services such as WAAS or U.S. Coast Guard differential GPS beacons, accuracy can be improved dramatically.

The WAAS (Wide Area Augmentation Service) is a system of ground based reference stations located across the United States. These stations receive global positioning system signals and correct them for inaccuracies caused by clock drift and by the atmosphere, and then broadcast this corrected signal from a stationary satellite over the equator. GPS receivers equipped with WAAS technology, including many offered by Garmin, can receive and interpret this corrective signal.

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