Global Positioning System (GPS)
The GPS system is a special radio navigation system supported by 24 satellites and corresponding ground stations. With the assistance of the GPS system, it is possible to determine the exact 3-dimensional position, the speed and the time at any point of the world and under all weather conditions.
Satellites for GPS are equipped with very accurate atomic chronometers and emit the time information in the form of codes, so that the receiver can continuously detect the time of emission of the signal. The signal contains data that are used for processing the exact position. So the time difference between the time emission and the time of reception is used to determine the distance to the satellite. The receiver takes in account the delaying effect of ionosphere and troposphere. The distances to 3 satellites and their position at the emission time allows the receiver to calculate his position in the geographical length and latitude. Therefor the receiver has to be equipped with an atomic chronometer as well. If the GPS receiver is able to process the signals of 4 satellites, a normal quartz chronometer is sufficient.