The project addresses the areas of climate change and adaptation as well as building strategic partnerships to protect critical infrastructure and human safety. Earth’s time variable gravity field is a key quantity in Earth-, climate and geophysical research, as it reflects mass change in the Earth system. Observing gravity field changes gives insights into the state and evolution of a variety of geophysical systems, including the terrestrial water cycle, the global oceans, and the cryosphere. To enable a comprehensive analysis of these climate-relevant geophysical processes, a long, consistent, and temporally stable time series of observations is necessary. With the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On two dedicated gravity field satellite missions have been in orbit since 2002. While they have fundamentally changed our understanding of mass transport in the Earth system, their measurement principle and instrumentation has certain weaknesses. Thermal effects of the on-board accelerometers and the degraded accelerometer performance of GRACE-FO cause systematic effects in the zonal spherical harmonic coefficients. Additionally, even if both missions rely on low-low satellite tracking and are therefore very sensitive to the long wavelengths of the gravity field, the estimation of the lowest degrees still poses problems. These low-degree harmonics are crucial for global Earth science applications due to their significant contribution to ocean-, ice mass and water storage balance estimates. In this context, a combination with satellite laser ranging (SLR) can offer substantial benefits. Due to their spherical design geodetic satellites are predestined for the determination of the long-wavelength part of Earth’s gravity field and consequently a perfectly complement to GRACE/GRACE-FO. For this reason, also the GRACE/GRACE-FO Science Data System recommends a substitution of the zonal coefficients of degree 2 and 3 by SLR. However, a simple substitution of these two coefficients, discards all the GRACE/GRACE-FO information and only utilizes minimal SLR information.

In the project COVER, a consistent combination of SLR and GRACE/GRACE-FO for improved mass change estimates. For this purpose, a methodology to allow for a combination of both observation techniques in consideration of random and systematic errors is developed. The foundation of these enhanced methodology will be the applicants long-term experience in the fields of GRACE gravity field recovery and the orbit determination of GNSS based satellites. Thus, the envisaged implementation of the SLR processing chain can already rely on reliable and well approved methods like for instance the modeling of non-gravitational forces, the rotation of the Earth or station displacements including non-tidal loading effects. This guarantees a consistent processing of SLR and GRACE/GRACE-FO, which is mandatory to achieve the proposed objectives.

A major advantage in the realization will be by the close cooperation with the local SLR station Graz-Lustbühel operated by Austrian Academy of Sciences. By means of regular on-site measurement campaigns it can be ensured that the evolving SLR processing chain will represent the best possible solution under real life working conditions.