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Free keywords:
time-variable gravity, GRACE satellite mission, ocean tides, aliasing, tidal models
Abstract:
The GRACE dual satellite mission (launched in March, 2002) offers the possibility of
computing monthly highly accurate mean gravity fields over an expected lifetime of
five years. Unfortunately, the quality of these monthly gravity field products does not
yet reach the pre-launch expectations. Possible error sources might be insufficient instrument data processing and parameterization or modeling of short-term atmospheric and oceanic mass variations. Another candidate is the ocean tide model. Especially, incomplete subtraction (de-aliasing) of short period tides may be partially aliased into the monthly gravity field solutions. Therefore, we analyzed the difference of two ocean tide models (FES2004 and CSR 4.0) which are used at the GRACE Science Data System level-2 processing centers at GFZ Potsdam and CSR (Center for Space Research, Austin), respectively, and which may serve as a measure of the ocean tide model error.
We have computed: a) straightforward monthly means of tidal elevation differences and
b) simulations of tidal elevation differences at footpoints of GRACE A. The results of
a) represent the differences of the monthly means of both tide models with respect
to an uniform sampling (grid). The results of b) include the influence of spatially
uneven sampling distribution (only along the orbit) and show that for the S2 and K2
tidal constituents, aliasing causes effects which cannot be neglected with respect to
the presently achievable GRACE measurement accuracy for degrees n·7 (S2) and n·8
(K2).
