Filling the Gap between GRACE and GRACE Follow-on: the Potential of SWARM and SLR to Detect Time-variable Gravity
Baur, Oliver¹; Reubelt, Tilo²; Weigelt, Matthias^3
1Austrian Academy of Sciences, AUSTRIA; ²University of Stuttgart, GERMANY; ³University of Luxembourg, LUXEMBOURG

Owing to the unpredictable life-time of the GRACE project geo-scientific communities are seriously concerned about the continuation of gravity field time-series, and hence the opportunity to directly quantify surface mass variation. As the GRACE follow-on mission will not become operational before 2017, a gap between GRACE and GRACE-FO has to be expected. The ESA satellite mission SWARM is a suitable candidate to fill this gap. The SWARM spacecraft are equipped with GPS receivers and accelerometers. These observations allow inference of (time-variable) gravity from high-low satellite-to-satellite tracking (hl-SST). Furthermore, SLR has been shown to be sensitive to (time-variable) gravity, although the technique is limited to the very low-degree harmonics. The combination of SWARM and SLR, thus, might be considered as the ultima ratio to derive surface mass variation in the absence of GRACE.
We demonstrate the potential of hl-SST (CHAMP/GOCE orbit analysis) and SLR (analysis of ranges to geodetic satellites such as LAGEOS) by the comparison with GRACE results. In this context, due to superior sensor accuracy, the findings from GRACE can be considered as benchmark. In order to assess the potential of SWARM for the detection of surface mass variation, we present detailed closed-loop simulation studies [preliminary results based on real data, if available, are envisaged]. In a first step, models for mass variation in the hydrosphere, cryosphere and solid Earth are used to simulate the orbits of the satellites. In a second step, we recover time-variable gravity by orbit analysis. Finally, in a third step, we convert time-variable gravity to mass change and compare the results with the input models.