Positioning in space requires three independent measurements. Artificial celestial bodies that circle around the Earth at fixed orbits serve as known points in satellite navigation - in place of marked objects in classical navigation.

In doing so, they transmit their exact astronomical position and on-board time regularly. To this end, they have atomic clocks on board (figure). All satellites of a system are controlled and synchronized from surface stations.

The distance between satellite and receiver is determined by the running time of the signal. In addition to the three measurements used to fix the position, the signals of a fourth satellite are needed to serve the precise time measurement.