Low - degre load harmonic coefficients from combining GRACE, GPS time series and a-priori dynamics

Abstract

One of the primary aims of the Gravity Recovery and Climate Experiment (GRACE) mission is to monitor large-scale surface mass changes by measuring their associated variations in gravity. We proposed in earlier work to combine the GRACE measurements with GPS time series provided by e.g. the International GNSS service (IGS) to strengthen the long wavelengths of the solution. This appears necessary because GRACE monthly gravity solutions not yet match the targeted baseline accuracies at the lower degrees, and they do not provide degree- 1 coefficients. The method of GPS inversion has been proposed by Blewitt et al. (2001), and sensitivity studies based on formal error propagation have indicated that GPS could contribute up to 60% to degrees 2-4, and up to 30% for selected higher degrees in a combination. Characteristic in our approach is a) the combination of GRACE and GPS in a weighted least-squares sense, and b) the introduction of a dynamic constraint on the mass change in time. Degree-1 load coefficients, derived from GPS through loading inversion, benefit indirectly from the combined determination of higher-order coefficients from GRACE and GPS. We find geocenter estimates which are consistent with previous studies. Selected degree-2 coefficients are provided in this paper.