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Free keywords:
Dynamic oblateness, Gravity field, Non-tidal mass variations, Atmosphere
and ocean de-aliasing, LAGEOS, GRACE
Abstract:
The dynamic oblateness of the Earth, in terms of the J2 or C20 coefficient of the
Earth´s geo-potential has been derived by analysis of LAGEOS Satellite Laser
Ranging (SLR) data. Although recent analyses of GRACE (Gravity Recovery and
Climate Experiment) mission data of monthly C20 values since 2002 have shown high
temporal correlations with LAGEOS results, significant differences still remain. As it is
common practice in GRACE data processing to remove a priori the short-term nontidal
atmospheric and oceanic induced variations of the gravity potential via the socalled
Atmosphere and Ocean De-aliasing Level-1B (AOD1B) products, their use for
LAGEOS data processing would allow a direct comparison of results and a rigorous
combination of the solutions. Since the consideration of short-term non-tidal
atmospheric and oceanic mass redistributions by means of AOD1B time series
reveals a significant impact on LAGEOS data processing results, the AOD1B time
series has been consistently prolongated back to the advent of LAGEOS-1 in 1976.
An analysis for the period 1993-2006 leads to the major findings that the
consideration of atmospheric and oceanic mass redistributions result in a
considerably reduced seasonal signal in the LAGEOS C20 time series. This in turn
reduces the correlation to GRACE significantly by about 30%. This is in opposite to
what one would expect, i.e., a better agreement between GRACE and LAGEOS if
common standards are applied. Thus, a LAGEOS solution corrected for short-term
non-tidal atmosphere and ocean effects is much less sensitive to primarily annual
hydrological mass variations than GRACE. In addition, significant semi-annual
signals remain. Other indicators such as the unresolved bias between LAGEOS and
GRACE in the order of 2·10-10 or LAGEOS orbits and Satellite Laser Ranging (SLR)
observation residuals are hardly affected by the AOD1B model.
