# SLR, GRACE and Swarm Gravity Field Determination and Combination
## 3 Results and Discussion
Comparable results are achieved for the area of significant ice mass loss along the coast of West Antarctica (Figure 14), where, due to the bad localization of the mass loss in the degree 6 solutions, the five year trends derived from the degree 10 SLR gravity fields 2000-2004: \(-\)5.4 \(\mathrm{Gt}\)/year, 2005-2009: \(-\)15.1 \(\mathrm{Gt}\)/year, and 2010-2014: \(-\)50.7 \(\mathrm{Gt}\)/year are quoted here together with the degree 60 GRACE solutions 2003-2004: \(-\)85.5 \(\mathrm{Gt}\)/year, 2005-2009: \(-\)121.1 \(\mathrm{Gt}\)/year, and 2010-2014: \(-\)201.0 \(\mathrm{Gt}\)/year. Again, the trend estimates at the given low degree of SLR are underestimates due to leakage, but the increase in ice melt is captured very impressively. The individual and the combined monthly mass estimates within Greenland for the two extreme cases of 100:1 and 1:1 relative weighting of Swarm and SLR are shown in Figure 5.6 The combination starts with January 2015 because, in 2014, the GRACE data are still quite complete while the Swarm data is impaired by strong ionosphere activity and non-optimal GPS receiver settings (receiver settings were adapted in May 2015). During 2015 and in the first half of 2016, a direct comparison to GRACE results is possible. After August 2016, the accelerometer on GRACE B was completely switched off and the processing of GRACE data at AIUB stopped. From the direct comparison, the impression that Swarm generally slightly overestimates the amplitude of the mass variations while SLR overestimates the secular mass loss is confirmed. Figure 5.7 Monthly mass estimates at the coast of West Antarctica from GRACE and SLR, and best fitting trends for 5-year periods. Each 5-year period is centered around zero. In case of relative weighting 100:1, the combined mass estimates closely follow the Swarm-only results. This can be expected in view of the results of the contribution analysis (Figure 2), even more so because C\({}_{20}\) is excluded from the analysis. In the case of equal weights, the combined solution follows more closely the GRACE mass estimates, but still with higher inter-annual variability. The monthly differences between GRACE and the individual or combined alternative mass estimates are shown in Figure 5.8 During 2015, the combination of SLR and Swarm with equal weighting has the smallest differences with respect to GRACE; in the first half of 2016, the combination 0.4*SLR+Swarm is slightly closer. The RMS of the differences with respect to GRACE over all months is 166 Gt for the SLR-only solutions, 110 Gt in case of Swarm-only, 110 Gt for the combination 0.01*SLR+Swarm, 83 Gt for 0.1*SLR+Swarm, 67 Gt for 0.4*SLR+Swarm and 68 Gt in case of equal weighting. These results clearly indicate that a combination with close to equal weights of Swarm (GPS) and SLR is closer to the GRACE reference than when weights are based on variance analysis of observation residuals. This can be explained by systematic errors in the kinematic orbits due to the GPS phase processing. In fact, GPS also has to be down-weighted in combination with the GRACE K-band observations (by a factor of approx. 200, see [34]). Recent experiments using one month of kinematic GRACE orbits derived with undifferenced integer-fixed GPS phase ambiguities indicate that the inconsistency between K-band and GPS is reduced. A better assessment will be possible as soon as longer time-series of kinematic orbits based on undifferenced integer-fixed ambiguities become available, but this will be the topic of a future publication.