GPS

A Global Positioning System, also known as a Global Position Determining System, with the acronym GPS, refers to any satellite supported navigation system. In terms of general language use, the term GPS specifically refers to the NAVSTAR GPS system of the US Ministry of Defense, used in the area of global positioning determination. The acronym GPS officially came into use on 17th July 1995.

GPS originally referred to positioning determination and navigation in the scope of military operations (weapons systems, warships, aeroplanes) etc. Today, however, it is also finding increasing usage in the civil domain - in shipping, aviation, and in vehicle navigation systems.

GPS is based on satellites which continually transmit their self-alternating positions and the exact time of day. On the basis of their signal transfer time, GPS receivers can then determine their own position and speed. Theoretically, signals from three satellites are enough to do this. In reality however, the GPS receivers don’t have a clock which is exact enough to be able to measure the runtimes correctly. That’s why the signal of a fourth satellite is needed with which the exact time can then be calculated in the receiver.

With the GPS signals, it isn’t only possible to determine the position, but also the speed of the receiver. In general terms, this takes place through the measurement of the Doppler effect or the numerical differentiation of the location according to the time.
The direction of movement by the receiver can also be determined, serving as an artificial compass or for the alignment of electronic maps.

So that a GPS receiver always has contact with at least four satellites, a minimum of 24 satellites are used which orbit the earth every sidereal day twice and at a height of 20,183 km. A satellite is above the same point above the earth every 23 hours, 55 minutes and 56.6 seconds.