Global Positioning System, or GPS, is a worldwide navigation system that uses radio signals broadcast by satellites. A computerized radio receiver in an airplane, ship, or other vehicle uses the satellite signals to calculate its own location. Hikers and other people on foot may use small, portable receivers. The United States Air Force operates the satellites, but the system has both military and civilian users.

The GPS has 24 satellites, called Navstars, in six orbits with a height of about 12,500 miles (20,200 kilometers). As many as eight satellites may be above the horizon when viewed from any point on earth.

A GPS receiver uses signals from at least four, and often more, satellites. Each signal indicates the location of the satellite that sent the signal, and the broadcast time. The receiver can determine its latitude and longitude using only three satellites if its altitude is known.

GPS users can normally determine their location within 10 meters (33 feet). A technique called carrier phase GPS can be accurate to within 1 centimeter (0.4 inch). The U.S. armed forces began developing the GPS in the early 1970's. The system became fully operational in 1995.

See also Navigation.

• Alison K. Brown, Ph.D., CEO/President, NAVSYS Corporation, Colorado Springs.

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