Since 1972, the Royal Observatory of Belgium is involved in a space geodetic programme aiming to determine the Earth rotation parameters and the geocentric coordinates of terrestrial sites. With respect to the center of mass of the Earth, the cartesian coordinates are currently determined with a precision close to 10 to 20 cm. Since 1983, new satellites are progressively launched and will form ... [Show full abstract] the Global Positioning System (GPS); in its final issue, expected for 1991 - 1992, it will be composed of at least 18 satellites. Since several years, experiences demonstrate the capability of this new system to obtain precision close to the centimeter level over distances extended till 30 km. For larger distances, the precision decreases as a consequence of the perturbations of the ionosphere on the radio signal propagation and the lack of accuracy of the orbit determination; however recent technological improvements of the ground receivers let believe that a precision of few units of 10-8 are obtainable; it means one centimeter over 1000 km.