New Global Gravity Field Models from Selected CHAMP Data Sets

Abstract

Several sets of data were carefully selected out of the first 12 months of CHAMP GPS and STAR micro-accelerometer observations to derive a new model of the Earth's static gravity field. These data have been dynamically processed as 63 orbital arcs, each of one to one and a half day duration. The approach consists of two steps: the GPS satellite orbits and clocks are first determined, then the CHAMP orbital arcs are adjusted using undifferenced phase and pseudo-range SST observables, and the residuals of those are used for retrieving the gravity harmonic coefficients. Micro-accelerometer, attitude and manoeuver information are taken into account to determine the surface accelerations as well as the residual thrusts acting on the spacecraft. Temporal variations of the gravity field are introduced in the observation equations for further usage in subsequent models, but are not here solved for. Several solutions have been derived, with or without additional information coming from the previous GRIM 5 satellite model, or from other laser satellite observations over a time period encompassing the CHAMP first year mission, with regularization (based on Kaula's rule) and with different weighting factors. The solution which is presented is complete to degree and order 120, although a lack of power in the coefficients power spectrum is obvious above degree 40 approximately, as may be expected from a satellite solution, due to the orbit decreasing sensitivity to gravity at the present altitude. The quality of the field is assessed mainly through orbit determinations of several geodetic satellites, of CHAMP itself (especially with the residuals of the laser range measurements not included in the gravity modelling process), and through comparisons with the geoidal surface derived from a mean altimetric sea surface-corrected for the ocean circulation, at a comparable resolution. A significant improvement in the long to medium wavelength harmonics of the model can already be seen as compared to previous satellite solutions.