LONG-WAVELENGTH GLOBAL GRAVITY-FIELD MODELS - GRIM4-S4, GRIM4-C4

GFZ Potsdam and GRGS Toulouse/Grasse jointly developed a new pair of g lobal models of the Earth's gravity field to satisfy the requirements of the recent and future geodetic and altimeter satellite missions. A precise gravity model is a prerequisite for precise satellite orbit re stitution, tracking station positioning and altimeter data reduction. According to different applications envisaged, the new model exists in two parallel versions: the first one being derived exclusively from s atellite tracking data acquired on 34 satellites, the second one furth er incorporating satellite altimeter data over the oceans and terrestr ial gravity data. The most recent ''satellite-only'' gravity model is labelled GRIM4-S4 and the ''combined'' gravity model GRIM4-C4. The mod els are solutions in spherical harmonics and have a resolution up to d egree and order 60 plus a few resonance terms in the case of GRIM4-S4, and up to degree/order 72 in the case of GRIM4-C4, corresponding to a spatial resolution of 555 km at the Earth's surface. The gravitationa l coefficients were estimated in a rigorous least squares adjustment s imultaneously with ocean tidal terms and tracking station position par ameters, so that each gravity model is associated with a consistent oc ean tide model and a terrestrial reference frame built up by over 300 optical, laser and Doppler tracking stations. Comprehensive quality te sts with external data and models, and test are computations over a wi de range of satellites have demonstrated the state-of-the-art capabili ties of both solutions in long-wavelength geoid representation and in precise orbit computation.