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. |
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