## 4 Results and Discussion
### Satellite Laser Ranging Validation
The results comprise the internal and external POD solutions derived from GPS measurements. Figure 5 illustrates SLR residuals that were calculated based on observations from the selected high-quality stations only and the WEGC-BC primary orbit. In general, the mean and standard deviation for the validated orbit solutions (Table 4) decreases with the limitation to high-quality (HQ) stations. For GRACE-A, however, the WEGC-BC, WEGC-BI, and UCAR solutions, which are all solutions based on unscreened L1A GPS navigation tracking data, show a larger mean for the HQ station selection than for all stations. Furthermore, regardless of the station selection, those solutions exhibit standard deviations more than two times larger than the WEGC-NC and AIUB solutions, which are based on the screened L1B GPS navigation tracking data (see Section 3.1). The SLR residuals in Figure 5 are based on high-quality station subset after applying a 20 cm outlier rejection and an elevation mask of 10 degrees. The high quality stations contribute from 70% (GRACE-A) to 71% (CHAMP) to all normal tracking points, a number generally high since the GRACE and CHAMP missions were top-ranked according to the ILRS mission priority list ([[https://ilrs.cddis.eosdis.nasa.gov/missions/mission_operations/priorities/priorities_20080926.html](https://ilrs.cddis.eosdis.nasa.gov/missions/mission_operations/priorities/priorities_20080926.html)]([https://ilrs.cddis.eosdis.nasa.gov/missions/mission_operations/priorities/priorities_20080926.html](https://ilrs.cddis.eosdis.nasa.gov/missions/mission_operations/priorities/priorities_20080926.html))). Overall, the SLR intercomparison results do not exhibit notable systematic variations within the time periods considered. With a similar number of observations (CHAMP: 8271, GRACE-A: 8533) both data series show a small mean. However, the larger scattering which is observed for GRACE-A (Figure 5b) manifests in an increased standard deviation of about 1 cm compared to CHAMP. Furthermore, it was found that using screened L1B RINEX data for the GRACE-A POD \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Solution**} & \multicolumn{2}{c}{**CHAMP**} & \multicolumn{2}{c}{**GRACE-A**} \\ \cline{2-5}  & **All Stations** & **HQ Stations** & **All Stations** & **HQ Stations** \\ \hline WEGC-BC & \(-0.49\pm 2.35\) & \(-0.15\pm 1.83\) & \(-0.02\pm 3.08\) & \(0.17\pm 2.88\) \\ WEGC-BI & \(-0.49\pm 2.49\) & \(-0.13\pm 1.98\) & \(0.01\pm 3.19\) & \(0.21\pm 2.95\) \\ WEGC-NC & \(-1.04\pm 2.61\) & \(-0.75\pm 2.10\) & \(-0.70\pm 1.92\) & \(-0.44\pm 1.39\) \\ UCAR & \(-0.19\pm 2.43\) & \(0.15\pm 2.01\) & \(0.03\pm 4.26\) & \(0.44\pm 4.30\) \\ EUHTSAT & \(-0.40\pm 2.48\) & \(-0.12\pm 2.08\) & n/a & n/a \\ AIUB & \(-0.84\pm 2.32\) & \(-0.56\pm 1.77\) & \(-0.43\pm 1.77\) & \(-0.15\pm 1.12\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mean and standard deviation (in cm) of SLR residuals for the full set of SLR stations and selected high-quality (HQ) stations for CHAMP and GRACE-A in Jun/Jul/Aug 2008. Figure 5: SLR residuals of (**a**) CHAMP and (**b**) GRACE-A orbits for July to September 2008, calculated with Bernese using GNSS input from CODE for a set of 12 high-quality SLR stations (legends at bottom). Average mean, standard deviation, and 3D-root-mean-square (RMS) estimates over the period are noted in the upper-left legend (units cm). For outlier rejection a 20 cm threshold and a station’s elevation mask of 10 degrees was applied. reduces the standard deviation (comparable to CHAMP), although this is not applicable in the case of RO (Section 3.1).