global positioning system

A satellite-based radio-positioning, navigation, and time-transfer system operated by the U.S. Department of Defense. The space segment of the system consists of a constellation of twenty-four satellites in six orbits inclined at 55° to the equator. Twenty-one of these satellites are active while the others remain in standby mode. The system provides highly accurate position and velocity information and precise time, on a continuous global basis, to an unlimited number of properly equipped users. The system is unaffected by weather and provides a worldwide common grid reference system. The GPS concept is predicated upon accurate and continuous knowledge of the spatial position of each satellite in the system with respect to time and distance from a transmitting satellite to the user. The GPS receiver automatically selects appropriate signals from the satellites in view and translates these into a three-dimensional position, velocity, and time. The system accuracy for civil users is normally 300 ft (100 m) horizontally. The constellation of satellites for a similar system operated by the Russian Federation is called GLONASS. See global navigation satellite system.