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 ship equipped with a
satellitereceiver can calculate the time taken for signals to arrive
froma 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.
A new system, known as Differential GPS, has now been developed that gives even higher accuracy - within about 5 metres. This is achieved by using a fixed receiver on land in addition to the ship's receiver. The fixed receiver continuously monitors any errors in the GPS calculations and transmits these to the ship. DGPS is a relatively new system but is already being used in over 30 countries.