To be able to determine the exact position of a receiver with satellite navigation, the GPS frames of at least three satellites are required. In this case, one speaks of a two-dimensional model that does not take into account any altitude information and treats the GPS receiver as if it were at sea level.
For positioning with altitude determination, the GPS signals from at least four satellites are required. This is a three-dimensional model.
Generally, the position is calculated from the signal travel times and the satellite positions to the GPS receiving station.
The navigation satellites continuously transmit signals that indicate the satellite position, the satellite orbit data and, most importantly, the time at which the GPS signal was transmitted. The GPS receiver compares the time of emission with its own time reference and determines the satellite's distance from the time difference. The calculated deviation from the actual position is given by the accuracy of its own time reference. This means that the more accurate the time reference of the GPS receiver, the more accurate the position determination. A time error of 500 ns results in a positioning error of 150 m. However, since the time reference of the receiver cannot be increased arbitrarily for cost reasons, the location accuracy is calculated using the known distance between the satellites.
With two satellite signals received, the GPS receiver can determine its position based on the two satellite distances and the satellite positions, but related to sea level. Taking into account the altitude calculation and the inaccuracy of the GPS receiver time reference, an exact location can be determined with three satellites.
In addition to two-dimensional position determination, there are various apps for 3D GPS. These apps are especially interesting for hikers, cyclists, mountaineers and other outdoor sports enthusiasts because the display is in 3D perspective. Thus, the viewer not only recognizes the path, but also the inclines and differences in altitude.