Bridging the gap between GRACE and GRACE-Follow On by the combination of HLSST and SLR

GRACE has been undoubtedly one of the most important sources to observe mass transport at global and regional scales. Within the COST-G project, GRACE and GRACE-Follow On gravity field solutions from various processing centers are being combined to further increase the spatial and temporal resolution. However, the GRACE and GRACE-Follow On time series suffer from a data gap of about one year. Thus, there is a need for an intermediate technique that will bridge the gap between the two missions and will allow 1) for a continued and uninterrupted time series of mass observations and 2) to compare, cross-validate and link the two time series. Here, we present a complete series that covers the gap period between the end of the GRACE mission in 2017 and the first available solutions of GRACE-Follow On in 2018. We will focus on the combination of high-low satellite-to-satellite tracking (hlSST) of low-Earth orbiting satellites by GNSS in combination with SLR. SLR is known to provide highest quality time-variable gravity for the very low degrees (2-5) and hlSST is able to provide a higher spatial resolution at a lower precision in the very low degrees. We discuss also the achievable spatial and temporal resolutions and possible applications in GIA and in interseasonal variation analysis.