On the determination of low degree harmonics by combining SLR with GRACE / GRACE-FO

Today's state-of-the-art gravity missions GRACE and GRACE-FO monitor the Earth's gravitational field in high temporal and spatial resolution. The resulting time series of gravitational fields serves various geophysical applications. It is however recommended to replace the C(2,0) coefficients, which describe the change of the Earth's oblateness, by those determined by Satellite Laser Ranging (SLR) to geodetic satellites. There are also discussions ongoing on the C(2,1), S(2,1) and C(3,0) coefficients. Current research shows that a combination of GRACE and GRACE-FO with SLR can lead to an improvement of the determination of the low degree coefficients in view of certain geophysical applications. This contribution gives an insight into the recent research at the Helmholtz Center Potsdam - German Research Center for Geosciences (GFZ) on various methods for the multi-technique combination on normal equation level and discusses the effects on the low degree spherical harmonics.