Contribution of SLR to combined hlSST+SLR solutions for bridging the gap between GRACE and GRACE-Follow On

GRACE and GRACE-Follow On gravity field solutions form an unprecedented time series of time-variable gravity field which is indispensable for geosciences, water and climate monitoring. 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. The most promising method for the long-wavelength part of the gravity field is a combination of high-low satellite-to-satellite tracking (hlSST) of low-Earth orbiting satellites by GNSS in combination with satellite laser ranging (SLR), where SLR is known to provide the 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. In this contribution, we discuss the importance of the SLR contribution to the combined solution showing that a hlSST solution is underperforming and a considerable improvement and stabilization for the low degrees can be achieved by the inclusion of SLR for the very low degrees. We also shed light on different combination techniques on the normal equation level with and without variance component estimation and discuss their advantages and difficulties in the implementation.