Improvement in global gravity field recovery using GFZ-1 satellite laser tracking data

The passive satellite GFZ-1 has been orbiting the Earth since April 1995. T he purpose of this mission is to improve the current knowledge of the Earth 's gravity field by analysing gravitational orbit perturbations observed at unique low altitudes, below 400 km. GFZ-1 is one target of the internation al satellite laser ranging ground network-tin evaluation of the first 30 mo nths of GFZ-1 laser tracking data led to a new version of the global GRIM4- S4 satellite-only gravity field model: GRIM4-S4G. Information was obtained from GFZ-1 data for spherical harmonic coefficients up to degree 100, which was not possible in any earlier satellite-only gravity held solution. GFZ- 1's contribution to a global 5 x 5 degrees geoid and gravity held represent ations is moderate but visible with a 1 cm and 0.1 mGal gain in accuracy on a level of 75 cm and 5 mGal, respectively.