A 40-year record of the Earth's time-variable gravity field from SLR and DORIS

Since 2002, the temporal variations of the Earth's gravity field are measured by the missions GRACE and GRACE Follow-On with unprecedented resolution and accuracy. In this study, we present a series of GRACE-like solutions going back to 1984 based on tracking data to 16 satellites, observed either by satellite laser ranging (SLR) or by means of the Doppler Orbitography and Radiopositioning Integrated by Satellites (DORIS) system. The match with GRACE in spatial resolution is achieved by representing the gravity field by empirical orthogonal functions (EOFs) obtained by a principal component analysis of the GRACE/GRACE-Follow-On solutions. To make the modelling more adaptive, the EOFs are supplemented by low-degree spherical harmonics. The presented time series is intended to replace a previous SLR-only solution applying the same parametrization. It is shown that the combined SLR/DORIS solution is clearly superior, reducing the average difference to the GRACE/GRACE-Follow-On fields by 10.6 percent; on a local scale, the error is reduced by up to 52 percent. As another enhancement, the beginning of the time series was advanced by eight years by extending the analysis to historical SLR data, albeit the noise in these early gravity solutions is higher, as expected. The time series now also includes a degree-1 solution based on an own inversion approach and using the surface mass distribution from the SLR/DORIS fields.