GPS positioning is based on the pseudo range, ephemeris, satellite launch time and other measurements of a group of satellites, and the user clock error must also be known. The global navigation satellite system (GPS) is an air based radio navigation and positioning system capable of providing users with all weather 3 dimensional coordinates, speed and time information at any location on the earth's surface or near the earth's space. Therefore, it is common to say that if you want to know the height of latitude and longitude, you have to receive 4 satellites to locate accurately.