ABSTRACT
Global Navigation Satellite Systems (GNSS) include constellations of Earth-orbiting satellites that broadcast their locations in space and time, of networks of ground control stations, and of receivers that calculate ground positions by trilateration. Satellite Navigation Systems use satellites as time-signal transmitters. Satellite Navigation Systems employ satellites orbiting high above the Earth and which are spread out in such a way that there are line-of-sight connections to at least four satellites from any point on the ground. Each one of these satellites is equipped with onboard atomic clocks. A significant problem to overcome when using a GNSS is the fact that there are great numbers of different coordinate systems worldwide. The geoid represents the true shape of the earth, defined as the surface where the mean sea level is zero. A geoid is often used as a reference level for measuring height. National or international map reference systems based on certain types of ellipsoids are called datums.
