Satellite missions which explore the earth gravity field detect the instantaneous distribution of mass in the Earth, including all solid, liquid, and atmospheric particles. Due to the fluctuation of those masses at various spatio-temporal scales a long observation time does not guarantee that the introduced variations in the gravity field are cancelled out. This fact makes the analysis of the time variable gravity field rather complex. A common practice to avoid aliasing effects is to account for the known part of the mass variations via models and introduce corrections with respect to the mean state.
Sources for gravity field variations include:
For GRACE (Gravity Recovery and Climate Experiment) only the short term variations are of immediate importance, because monthly gravity field solutions are used to construct seasonal variations. GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) measures the second derivatives of the gravity potential and thus the influence of long wavelengths is much smaller. Nevertheless, GOCE products will also contain seasonal effects, which have to be reduced using GRACE gravity solutions.
|Geoid height variation based on ECMWF operational surface pressure data for January 1, 2008, 0 h UTC. Units: [mm].|
For the determination of atmospheric gravity field coefficients (AGF, in terms of spherical harmonics) it is common practice to use high resolution numerical weather model data that echo the three-dimensional distribution of air masses. By subtracting the spherical harmonic coefficients of the instantaneous atmosphere from the ones of the mean field, residual series are obtained. These deviations from a mean state are usually divided by a constant gravity acceleration and expressed as geoid height. An example is given in the figure on the right.
Within GGOS Atmosphere different approaches to determine AGF were evaluated and new interesting applications were explored. Thse key issues included:
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