## 4 PPP-B2b ZTD Estimation and Evaluation of the Results
### Comparison between ZTD Derived with PPP-B2b/CNES and the Post-Processed Results  
As shown in Figure 2, 32 stations were selected for this study to further investigate the performance of the ZTD derived from the PPP-B2b orbit/clock products at different locations. Since the receivers at these stations are not capable of decoding PPP-B2b corrections, the ZTDs at these stations were calculated using the stored PPP-B2b orbit/clock products in SP3 format at station BJ01. As shown in Table 6, the heights of the stations vary from \(-\)78.5 m (SGOC, Sri Lanka) to 3646.3 m (LHAZ, Tibetan Plateau). Among these stations, there are four stations with only GPS observations, four stations with GPS observations and some of the BDS-3 satellites, and 24 stations with both GPS and BDS-3 observations. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**GNSS**} & \multicolumn{3}{c}{**Co-Located Radiosonde**} \\ \hline
**Station** & **Latitude** & **Longitude** & **Height** & **Observation** & **Station** & **Horizontal** & **Height** \\
**Name** & **(\({}^{\circ}\))** & **(\({}^{\circ}\))** & **(m)** & **System** & **Name** & **Distance** & **Difference** \\ \hline ULAB & 47.9 & 107.1 & 1575.6 & G + C & MGM00044292 & 14.98 & \(-\)117.4 \\ URUM & 43.8 & 87.6 & 858.9 & G + C & CHM00051463 & 3.09 & 2.4 \\ CHAN & 43.8 & 125.4 & 271.5 & GPS only & CHM00054161 & 21.89 & 20.1 \\ BIK0 & 42.9 & 74.5 & 744.9 & GPS only & N/A & N/A & N/A \\ TASH & 41.3 & 69.3 & 439.7 & G + C & N/A & N/A & N/A \\ BJ01 & 40.1 & 116.3 & 85.5 & G + C & N/A & N/A & N/A \\ BJFS & 39.6 & 115.9 & 87.4 & GPS only & CHM00054511 & 49.19 & 64.6 \\ USUD & 36.1 & 138.4 & 1508.6 & G + C & N/A & N/A & N/A \\ GAMG & 35.6 & 127.9 & 925.9 & G + C & N/A & N/A & N/A \\ JFNG & 30.5 & 114.5 & 71.3 & G + C & N/A & N/A & N/A \\ LHAZ & 29.7 & 91.1 & 3646.3 & GPS only & CHM00055591 & 3.00 & 1.0 \\ LCK4 & 26.9 & 81.0 & 64.2 & G + C & INM00042369 & 19.43 & 7.2 \\ SHLG & 25.7 & 91.9 & 1007.2 & G + C & N/A & N/A & N/A \\ TWTF & 25.0 & 121.2 & 201.5 & G + C (C19-37) & N/A & N/A & N/A \\ NCKU & 23.0 & 120.2 & 98.2 & G + C (C19-30) & N/A & N/A & N/A \\ HKFN & 22.5 & 114.1 & 41.2 & G + C (C19-30) & N/A & N/A & N/A \\ HKQT & 22.3 & 114.2 & 5.2 & G + C (C19-30) & HKM00045004 & 6.10 & \(-\)17.2 \\ PTGG & 14.5 & 121.0 & 84.9 & G + C & N/A & N/A & N/A \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Information on the GNSS stations and co-located radiosonde stations. Figure 5: ZTD error distributions for PPP-B2b solutions. The errors were calculated using the GBM solution as a reference. The results show that the errors are mostly within \(-\)30 to 30 mm. To simulate the real-time experiment, the GNSS observations and PPP-B2b and CNES orbit/clock products were read and processed epoch by epoch, and the ZTDs were estimated using the strategy listed in Table 1. Using the ZTDs computed with the GBM final products (a combination of GPS and BDS-3) and the ZTD provided by IGS as a reference, the ZTDs computed with the PPP-B2b products were then evaluated over these 32 stations from DOY 069 in 2022 to DOY 054 in 2023. The results show that the CNES solution outperforms the PPP-B2b solution for all stations, regardless of whether the results are compared with those of the GBM or IGS solution. Using the GBM solution as a reference, the RMSEs of the CNES results range from 5.6 mm to 11.3 mm, with a mean value of 8.0 mm. For the PPP-B2b solution, the RMSEs range from 11.1 mm to 35.3 mm, with a mean value of 21.7 mm. In addition, the ZTDs calculated with the CNES products agree better with the GBM solutions than with the IGS solution in terms of the RMSE at all stations. This could be because both the CNES and GBM solutions were calculated using a combination of GPS and BDS-3 observations, while the IGS solutions were calculated using only GPS observations [81]. The statistical results presented in Table 7 indicate the noticeable dependence of the RMSE of the PPP-B2b ZTD on latitude.