Annual and semiannual variations of the Earth's gravitational field from satellite laser ranging and CHAMP

[1] Temporal variability in the Earth's gravity field has been estimated from 4 years of satellite laser ranging (SLR) to five geodetic satellites, Lageos 1 and 2, Stella, Starlette, and Ajisai supplemented by over 2 years of CHAMP precise orbital positioning, attitude quaternions, and accelerometry. The study considers variability at the annual and semiannual periodicities in the full field of degrees 2 - 6. A singular value decomposition has been employed in an attempt to recover variability from stand-alone CHAMP data. Temporal gravity field variability from SLR and from SLR and CHAMP has been computed and compared against geophysical models for surface mass redistribution. For a 4 x 4 field best agreement of the annual variation results in an RMS geoid difference of 0.76 mm with correlation coefficients above 0.85. On extending to a 6 x 6 field agreement decreases to about 1.5 mm RMS in geoid height with correlations of 0.6 at the annual cycle. For the extended field the ocean mass redistribution from Ocean Circulation and Climate Advanced Modeling project and TOPEX is seen to be deficient with near comparable correlations being obtained by neglecting the ocean contribution. The semiannual signal is weaker and less well determined. For the 4 x 4 field, correlations are no better than 0.6 with RMS of 1 mm between the SLR and geophysical models. Results show that CHAMP has a positive effect on the annual variation for a 4 x 4 field but marginal for the extended 6 x 6 field. CHAMP has a negative impact at the semiannual frequency.