Bridging the gap – linking GRACE and GRACE-Follow On by hlSST and SLR

GRACE and GRACE-Follow On are the standard tools for observing the time variable gravity field. Unfortunately, there is no overlap of the two time series of monthly gravity field solutions due to the ending of the GRACE mission in 2017 before the first monthly solutions of GRACE-Follow On became available in June 2018. 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. As a bridging technology hlSST/SLR combinations are arguably the most promising candidate. We presented earlier combinations of those based on 41 kinematic orbit products of 27 satellites and 9 SLR satellites. Here, we progress to the next step and present results where we use the combined hlSST/SLR solution within a Kalman environment to link GRACE to GRACE-Follow On via the hlSST/SLR time series. The combination is conducted on coefficient level: after reducing the climatology derived from GRACE, a modified continuous Wiener process acceleration (CWPA) model is employed as the driving dynamic model of the Kalman filter for the prediction step. Subsequently the predicted time step is updated by (residual) observations when available. The resulting time series is thus complete for all months starting from April 2002 till today. We will discuss the benefit and limitations of the approach. The research is conducted within the framework of the International Gravity Field Service (IGFS) product center (COST-G) which is dedicated to the combination of monthly global gravity field models.