GRACE Monthly Solutions

The monthly gravity field solutions in terms of normalized spherical harmonic coefficients up to degree and order n = 80 are computed at IfE and provided here. The solutions are obtained using the GRACE-SIGMA software which is designed and developed at the Institute of Geodesy.

The GRACE-SIGMA is a completely MATLAB-based programme and estimates the orbit and gravity field parameters using the direct method or the classical variational approach. More details of the processing chain can be found in our EGU 2018 presentation.

The LUH-GRACE solutions are unconstrained monthly gravity field models and are based on the analysis of the K-band range rate observations on board GRACE and span the time period between January 2003 until December 2009 (The solution set will be updated soon for the whole GRACE mission time). The models are individually validated and compared and show full agreement with the solutions provided by the three official centers, CSR, JPL and the GFZ.

1. First, the initial state vector as well as the accelerometer bias parameters are adjusted. For every 3 hourly arc 9 parameters are estimated. In this step reduced-dynamic GNV1B orbits serve as observations. The iterative estimation converges usually after two to three iterations.
2. During the second step the gravity field parameters up to degree and order 80 are added as unknown quantities. Altogether the gravity field is modelled by 6557 normalized spherical harmonic coefficients. In addition, 8 geometric-empirical KBRR parameters per arc and satellite pair are solved. During this step KBRR measurements and reduced-dynamic positions are combined with a relative weight of 1×10¹⁰. For the determination of the gravity field coefficients no regularization is applied.

The LUH-GRACE2018 solutions agree well with the solutions of other analysis centres (see figure above). For dynamic satellite orbit modelling the following force models were used: