global positioning system

US satellite-based navigation system, a network of more than 24 satellites in six orbits, each circling the Earth twice every 24 hours. Each satellite sends out a continuous time signal, plus an identifying signal. To fix position, a user needs to be within range of four satellites, one to provide a reference signal and three to provide directional bearings. The user's receiver can then calculate the position from the difference in time between receiving the signals from each satellite.

The position of the receiver can be calculated to within 0.5 m/1.6 ft, although only the US military can tap the full potential of the system. Other users can obtain a position to within 15 m/50 ft. This is accurate enough to be of use to boats, walkers, and motorists, and suitable receivers are on the market.