## Processing Setup and Description
### Orbit Solutions  
In order to enable internal quality assessment and uncertainty estimation for the LEO RO receiver orbit, independent from orbit products of external providers, three POD runs are conducted routinely and in parallel at WEGC (see also Figure 1):  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Bernese GNSS Software v5.2** & **NAPEOS v3.3.1** \\ \hline
**Reference system** & & \\ Polar motion and UT1 & IERS C04 08 & IERS finals2000A\_data \\ Polar model & IERS 2010 Conventions [80] & IERS 2003 Conventions [81] \\ Precision/Nutation & IERS 2010 Conventions & IERS 2003 Conventions \\ Reference frame & ITRF 2008 [82] & ITRF 2008 \\ \hline
**GPS measurement characteristics** & & \\ Observations & undifferenced ionosphere corrected & undifferenced ionosphere corrected \\  & code and phase & code and phase \\ Sampling & 30 s & 30 s \\ Elevation angle cut-off & 5 degrees & 5 degrees \\ GPS antenna PCO/PCV & igs08.tx [83] & igs08.tx \\ Antenna phase wind-up & applied [61] & applied \\ Relativity & applied & applied \\ \hline
**GPS parameters** & & \\ GPS orbits & CODE repro2015 (15 min) / IGS repro2 (15 min) & CODE repro2 (15 min) \\ GPS clocks & CODE repro2015 (5 s/30 s) / IGS repro2 (30 s) & CODE repro2 (5 s/30 s) \\ \hline
**Gravitational forces** & & \\ Gravity field model (static) & EGM2008 (120 \(\times\) 120) [84] & EIGEN-682 (120 \(\times\) 120) [85] \\ Solid Earth tides & applied (IERS2010) & applied (IERS2003) \\ Ocean tide model & FES2004 (50 \(\times\) 50) [86] & FES2004 (50 \(\times\) 50) \\ Planetary ephemeris & DE-405 [87] & DE-405 \\ Earth pole tide & IERS2010 & IERS2003 \\ Ocean pole tide & IERS2010 & IERS2003 \\ \hline
**Non-gravitational forces** & & \\ Atmospheric density model & n/a & MSISE-90 [88] \\ Atmospheric drag & n/a & constant area (10/24 h) \\ Radiation pressure model & n/a & constant area \\ Earth Radiation & n/a & NAPEOS model for Albedo and IR \\ 1/rev empiricals & n/a & 1/12 h (along- and cross-track) \\ Other empiricals & piece-wise constant accelerations in RTN & n/a \\  & (every 6 min); constraints: 0.5 nm/s\({}^{2}\) & \\ \hline
**Arc cut** & & \\ Arc lengths & 24 h & 24 h \\ Maneuvers & n/a & n/a \\ Handle of data gaps & yes & yes \\ \hline
**Estimation** & & \\ Estimation & batch least squares & batch least squares \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of orbit processing models, parameters, and data. For more detailed descriptions see the references cited herein and the software documentations [30; 31].  
* WEGC-BC: The orbit solution selected for the following RO data processing. The calculation is based on Bernese and the GNSS orbit and clock data from the CODE reprocessing in 2015; referred to as _primary_ solution hereafter.
* WEGC-BI: Same Bernese setup as for WEGC-BC above but employing GNSS orbit and clocks from JPL (as part of IGS repro2).
* WEGC-NC: This solution is calculated using NAPEOS, introducing the GNSS orbit and clock products from CODE as for WEGC-BC.  
Among these three orbit solutions the latter two only serve for the quality assessment of the primary solution and the uncertainty estimation through inter-comparison and are not used in the further RO processing.  
Additionally, for this study external orbit data from other leading processing centers are used for the validation of the internal primary solution. This comprises orbits from UCAR ([89; 90]), EUMETSAT [28; 91], and AIUB [92; 93]. First inter-agency comparison at WEGC was carried out within a prototype study of several test days [94]. The extensive UCAR/CDAAC RO data archive holds standard re-processed orbits from their reprocessing campaign 2016 for CHAMP and Metop and post-processed orbits for GRACE. EUMETSAT provided orbit products used for comparison for Metop-A/B and CHAMP. AIUB, as a leading institute for orbit determination provides reliable orbit solutions for the CHAMP and GRACE missions. Table 2 summarizes the different orbit solutions used in this study.