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The Global Positioning System consists of a constellation of 24 satellites. Each satellite orbits Earth twice a day at an altitude of about 20,000 km and continuously transmits information on specific radio frequencies to ground-based receivers. GPS was developed by the U.S. Department of Defense as a worldwide navigation system and has been adopted by civilians for many other uses, including surveying, mapping, and scientific applications. Relatively inexpensive GPS receivers like those used by pilots, boaters, and outdoor enthusiasts can determine its position on the Earth’s surface to within a few meters. With more sophisticated receivers and data-analysis techniques, we can determine receiver positions to less than a centimeter.
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