Gravity Field Mapping from GRACE: Different Approaches—Same Results?

Abstract

GFZ as part of the GRACE Science Data System (SDS) is routinely processing time-variable global gravity field models on monthly and weekly basis throughout the whole GRACE mission period. These operational products consist of spherical harmonic coefficients which are calculated based on the so-called dynamic method, i.e. integration of variational equations. As a matter of fact, these coefficients are imperfect due to different error sources such as inaccurate background models, instrument noise and inhomogeneous sampling and thus have to be filtered during post-processing in an appropriate way. Nevertheless, the current release named GFZ RL05 shows significant improvements compared to its precursors with an average error level of only about a factor of 6 above the pre-launch estimated baseline accuracy.

Additionally, an alternative approach using radial basis functions is developed at GFZ. This approach is based on the inversion of integral equations using gradient differences as in-situ observations. The resulting gravity field products can be directly derived as gridded data making this approach also suitable for regional applications. No post-filtering is necessary, as regularization is already applied during system inversion. Additionally applying a Kalman filter, higher temporal resolution can be achieved.

This paper gives a brief overview of the methodology of both approaches and their particular strengths and weaknesses are discussed. Results from GFZ RL05 and the latest results of the radial basis function approach are compared and also validated against independent data sources.