Tide model errors and GRACE gravimetry: towards a more realistic assessment

The manner in which errors in tide models affect monthly gravity solutions from GRACE satellite data is re-examined. We perform a complete simulation of GRACE gravity estimation, including allowance for accelerometer calibrations and satellite state-vector adjustment. Errors in monthly geoid estimates are for most regions of a order of 0.5 mm, or roughly 1 cm of equivalent water height when smoothed over radii of 600 km. In polar regions where ocean tide models are suspect, errors can grow significantly larger: over 2 mm in geoid height, although over very localized regions. Errors from diurnal tides tend to cancel, but they are not completely eliminated, inducing geoid errors of the order of 0.3 mm. The currently accepted GRACE total error budget is roughly an order of magnitude larger than the component arising from tide modelling errors.