→ Main article: GPS technology
The general principle of GPS satellite positioning is described in the article Global Navigation Satellite System.
GPS is based on satellites that constantly broadcast their current position and the exact time using coded radio signals. Special receivers (GNSS) can calculate their own position and speed from the signal propagation times. Theoretically, the signals from three satellites, which must be above their cut-off angle, are sufficient to determine the exact position and altitude. In practice, GPS receivers do not have a sufficiently accurate clock to correctly measure travel times against GPS time. Therefore the signal of a fourth satellite is needed, which can be used to determine the reference time in the receiver. For the number of satellites required, see also: GPS technology
The GPS signals can be used to determine not only the position, but also the speed and direction of movement of the receiver, which can then be displayed on a digital map or as a compass. Since this is generally done by measuring the Doppler effect or differentiating the location according to time (measure and direction of the detected local change), the compass measurement is only possible if the receiver has moved.
The satellite constellation was set so that it is usually possible for a GPS receiver to have contact with at least four satellites. Six orbital planes are inclined 55° to the equator and cover almost the entire world. GPS devices cannot be used in the polar regions, but other satellite navigation systems whose satellites move in other orbits can.
According to the basic GPS configuration, at least four satellites in each of the six orbital planes should orbit the Earth twice at an altitude of 20,200 km on each sidereal day. A satellite of the IIR version, for example, is designed for an operational lifetime of 7.5 years. In order to avoid failures due to technical defects, additional satellites are kept ready. Some of them are placed in extended slots of the constellation and play an active role there. Further inactive spare satellites are waiting in orbit for their deployment. A resulting gap in the constellation does not lead to a reduction in signal availability if an immediately adjacent extended slot is occupied. To fill a gap, a new satellite can be launched, a dormant satellite already in orbit can be activated, or an active satellite can be maneuvered to another position. All of these actions are time consuming. It takes months to move a satellite to the position required for deployment. Within an orbital plane, repositioning can be accomplished by a sequence of braking and acceleration maneuvers as long as the fuel supply is sufficient to do so, which is usually only used to maintain the exact position. Because of its weak engine, a satellite cannot propel itself to a higher orbital plane.
