Global Navigation Satellite System is the complete structure of a satellite-based positioning system. Various GNSS constellations have been developped by nations like GPS (USA), GLONASS (Russia) and Galileo (European Union). Setting up a satellite-based positioning system requires the implementation of three hardware segments:
Satellites have allowed us to create geocentric reference systems and to increase the level of spatial accuracy substantially. Satellite-based systems are critical tools in geodetic engineering for the maintenance of the ITRF. They also play a key role in mapping and surveying in the field, as well as in a growing number of applications requiring positioning techniques.