Satellites are now commonly used not just for communication but also for monitoringthe weather, mapping and surveys, astronomical observation and,importantly, for navigation and ground location positioning. The Global Positioning System (GPS) is based on 24 satellites that orbitthe Earth and act as reference points. A car equipped with a satellitereceiver can calculate the time taken for signals to arrive from a number of satellites.

Since the signals travel at a known speed (the speed of light), the receiver can then work out its distance from each satellite, using distance = speed x time, and hence its latitude, longitude, altitude, course and speed can then be calculated. GPS systems are accurate 95% of the time to within 30 metres; the data can be updated every second