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The satellite Global Positioning System (GPS) distributes precise time, frequency and position worldwide. At a height of about 20,000 km satellites circle around the earth on different orbits. An atomic clock (accuracy at least 1 x 10 E-12) is running in every satellite whose time is constantly transmitted together with the orbit data. The GPS-receiver records the data of up to 6 satellites and uses this information to calculate its position.

In order to fulfill these requirements, it was necessary to position a total of 24 satellites in 6 almost circular orbits. The satellites' orbits are inclined at a 55° angle to the equator with an orbital period of about twelve hours – corresponding to an orbital altitude of 20183 km.

The transmitted orbit data are used to determine the travel time of the signals of the individual satellites. These values form the basis for a very precise GPS-time which is maintained in the system via an adjustable crsytal at an accuracy of ± 1 μsec.

These satellites are now in operation. Each satellite weighs about 430 kg and contains a transmitter, a receiver, an antenna, an atomic clock for maximum precision and a control system for the satellite's functions. The principal applications of the GPS System are determination of positions and navigation.

Further information at Wikipedia