GPS (Global Positioning System) is a program that was originally developed by the United States Department of Defense. Its objective is to allow users equipped with the necessary equipment to determine their position at any moment in a global reference system. Mainly designed for maritime or aerial navigation, this system is composed of three segments:  the space segment, the user segment and the terrestrial segment.
The thirty or so satellites comprising the GPS space segment emit two radiofrequencies. They travel between six orbital planes at an altitude of approximately 20 200 kilometres.
GPS receivers, comprising the user segment, are devices capable of using these two signals to make a real-time calculation of the position of an observer, anywhere, anytime on Earth, whatever the atmospheric conditions, with a precision of 5 to 20 metres.
The terrestrial segment is comprised of five stations that calculate the orbit of satellites, check that they are working properly as well as verifying the quality of their clock.