## 4 Results
### Combined GPS/BDS2 Prod  
Figure 10 is the GPS-based and combined position dilution of precision (PDOP) of TH-2A on DOY 307 in 2019. For GPS-based PDOP, the total ration of less than 4 in an epoch was 96.8%, and it was 99.5% for combined PDOP. Figure 11 shows the daily average of the GPS-based and combined PDOP. Adding BDS2 to GPS, the PDOP improved from 2.42 to 1.94, an improvement of 19.8%. Figure 12 displays the visible navigation satellites from TH-2A and TH-2B based on GPS-only and GPS + BDS2. Adding BDS2 to GPS, the average number of visible satellites increased from 7.5 to 9.5, and the maximum could reach 18. All of these are beneficial to further enhance the accuracy of PROD. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Weight of GEO Satellites** & **1** & **0.5** & **0.2** \\ \hline \hline  & R & 2.01 & 2.25 & 3.23 \\ Global/mm & T & 5.85 & 6.61 & 9.26 \\ N & 3.44 & 3.90 & 5.80 \\  & 3D & 7.08 & 8.00 & 11.40 \\  & R & 2.16 & 2.42 & 3.41 \\ The non-Asia-Pacific & T & 6.62 & 7.49 & 10.43 \\ region/mm & N & 3.87 & 4.37 & 6.73 \\  & 3D & 7.96 & 9.00 & 12.69 \\  & R & 1.44 & 1.63 & 2.61 \\ The Asia-Pacific & T & 2.04 & 2.32 & 3.73 \\ region/mm & N & 1.45 & 1.74 & 3.40 \\  & 3D & 2.89 & 3.32 & 5.69 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Difference between the GPS-based and BDS2-based PROD with different weights of GEOs (excluding maneuver periods). Figure 9: Global distribution of difference between the GPS-based and BDS2-based PROD in 3D components (excluding maneuver periods). Here, GPS and BDS2 each had a reference satellite to construct the DD observations, as the frequencies of GPS and BDS2 were different. The ambiguity per system was solved separately. Figure 13 displays the difference between the GPS-based and combined PROD. The averages of the daily RMS of orbit differences in the R, T, and N directions were 0.48 mm, 1.01 mm, and 0.35 mm, respectively. The GPS-based and combined PROD results were consistent, which indicated that adding BDS2 to GPS did not cause pollution of the PROD result. Figure 14 shows the DD ionospheric-free phase residuals on DOY 305 in 2019. The RMS of the DD ionospheric-free observation minus computation (O-C) phase residual of GPS and BDS2 was 4.2 mm and 8.5 mm, respectively. There were no large deviations in the vicinity of the maneuver, which shows that maneuver accommodated orbits fit the GPS and BDS2 observations well. An orbit overlap comparison was adopted to reflect the orbit consistency. The result shows the average of daily RMS values of the residuals of 6 h relative orbit overlapping for combined PROD in the R, T, and N directions were 0.53 mm, 0.97 mm, and 0.33 mm, respectively. Figure 11: GPS-based and combined PDOP of TH-2A. Figure 10: GPS-based and combined PDOP of TH-2A on DOY 307 in 2019. Figure 12: Visible navigation satellites from TH-2A and TH-2B. Figure 13: Difference between the GPS-based and combined PROD results. The success rate of the integer ambiguity has a great influence on the PROD's accuracy, and it depends largely on the accuracy of the float ambiguity; the lower the accuracy of the float ambiguity, the longer the search time. The ambiguity dilution of precision (ADOP) measures the average precision of the ambiguities and is invariant for the class of admissible ambiguity transformations [41; 42]. It is based on the determinant of the ambiguity variance matrix. Here, ADOP can be used to evaluate the accuracy of the float ambiguity, which depends on the number of visible satellites and the quality of their phase and code observations [43]. Figure 15 displays the GPS ADOP on DOY 311 in 2019 and Figure 16 shows the daily value of GPS ADOP. When BDS2 co-worked with GPS, the average of GPS ADOP reduced from 0.160 cycle to 0.153 cycle. This showed that the accuracy of the float ambiguity was effectively improved when BDS2 co-worked with GPS. The ambiguity fixed rate of GPS and BDS2 were both 100.0%. Although the ambiguity of GPS and BDS were resolved separately, the orbital and dynamic parameters were the same.