## 3 Assessment of Precise MGEX Orbit and Clock Products
### Orbit Comparisons
#### 3.1.2 Bds  
Due to the different characteristics among the GEO, IGSO and MEO satellites, BDS orbit quality was assessed according to individual satellites rather than by averaging all satellites. In addition, BDS-2 and BDS-3 are discussed separately. WUM was taken as the reference for comparison because it was the only AC providing orbit and clock products for all the available BDS satellites. Although BDS-2 GEO satellites were included in TUM products since DOY281 2018, C02 and C03 products were still absent and C01 products were only available for about two months. In addition, COD and ESM products excluded BDS-2 GEOs from their POD solutions. As a result, only BDS-2 GEO orbits from GFZ, SHA and TUM products were compared with WUM.  
Figure 3 shows the RMS of BDS-2 orbit differences with respect to WUM in the along-track, cross-track and radial components, and the average 3D RMS values of IGSO and MEO comparison results are summarized in Table 4. As can be seen, GEO satellites exhibited large orbit differences among ACs, which were up to a few meters in the along-track component and about 1.0 and 0.5 m in the cross-track and radial components, respectively. The orbits of IGSO/MEO satellites agreed well with each other among different ACs. For IGSOs, most of the RMS orbit values were within 0.1 m in the three components. Note that the newly launched IGSO satellite C16 showed larger RMS values than the other IGSO satellites. For MEOs, the COD, ESM, GFZ, SHA and TUM solutions showed an agreement of 6-8 cm in the along-track, 4-8 cm in the cross-track and 2-4 cm in the radial component with respect to WUM, respectively.  
Figure 4 shows the RMS values of BDS-3 orbit differences between ESM and WUM in along-track, cross-track and radial components. One may notice that the orbit differences of satellites manufactured by the Shanghai Engineering Center for Microsatellites (SECM) were obviously larger than those of satellites manufactured by the China Academy of Space Technology (CAST) in the along-track and cross-track components. The mean RMS values of CAST satellites were 12.3, 9.2 and 4.1 cm, while the SECM satellites had larger mean RMS values of 25.0, 15.2 and 5.7 cm. This indicates that the orbit consistency of CAST satellites was better than that of SECM satellites. The different consistencies of CAST and SECM satellites may have been caused by the different PCO values of BDS-3 satellites used in ESM and WUM solutions. For the ESM solution, PCO values based on inhouse estimation were used, while for the WUM solution, conventional MGEX PCO values were used. The better orbit consistencies of CAST satellites may have been attributable to a better consistency between the two sets of PCO values used for CAST satellites.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & **COD** & **ESM** & **GFZ** & **SHA** & **TUM** & **WUM** \\ \hline \multirow{2}{*}{COD} & IGSO & - & 21.4 & 20.6 & 17.9 & 24.0 & 18.0 \\  & MEO & - & 10.6 & 12.7 & 11.2 & 13.4 & 11.1 \\ \hline \multirow{2}{*}{ESM} & IGSO & - & 11.8 & 15.8 & 23.1 & 17.4 \\  & MEO & - & 8.1 & 8.8 & 11.2 & 8.4 \\ \hline \multirow{2}{*}{GFZ} & IGSO & & - & 11.8 & 22.4 & 14.5 \\  & MEO & & - & 7.9 & 12.5 & 8.3 \\ \hline \multirow{2}{*}{SHA} & IGSO & & & - & 24.4 & 13.8 \\  & MEO & & & - & 12.0 & 8.7 \\ \hline \multirow{2}{*}{TUM} & IGSO & & & - & 24.2 \\  & MEO & & & - & 12.1 \\ \hline \hline \end{tabular}
\end{table}
Table 4: 3D RMS of BDS-2 IGSO/MEO orbit comparisons (units: cm).  
Figure 4: Daily RMS of BDS-3 orbit differences in along-track, cross-track and radial components between ESM and WUM. Note that CAST stands for BDS-3 satellites manufactured by the China Academy of Space Technology and SECM stands for BDS-3 satellites manufactured by the Shanghai Engineering Center for Microsatellites.