## 4 PPP-B2b ZTD Estimation and Evaluation of the Results
### Comparison between ZTD Derived with PPP-B2b/CNES and the Post-Processed Results
Using the GBM ZTD data as a reference, the average RMSE of the PPP-B2b ZTD within the latitudinal band from 30\({}^{\circ}\) N to 48\({}^{\circ}\) N is 15.7 mm, whereas it is 23.9 mm within the latitudinal band from 0\({}^{\circ}\) to 30\({}^{\circ}\) N and 25.0 mm between 0\({}^{\circ}\) and 38\({}^{\circ}\) S. In contrast, the accuracy of the CNES-derived ZTDs does not exhibit a pronounced dependence on latitude. The mean biases of the GNES-derived ZTDs are \(-0.8\) mm and \(-0.1\) mm for the IGS and GBM solutions, respectively, and 0.7 mm and 2.1 mm for the PPP-B2b solution compared to the IGS and GBM solutions, respectively. The results in Figures 6 and 7 also indicate that the accuracy of the ZTDs derived with PPP-B2b is strongly dependent on the location of the stations. For nine stations (i.e., ULAB, URUM, BIK0, CHAN, TASH, BJ01, BIFS, GAMG, and JFNG) between 30\({}^{\circ}\) N and 48\({}^{\circ}\) N, the RMSEs of the ZTD range from 11.1 mm to 22.8 mm, with a mean value of 15.7 mm. Among these nine stations, three (CHAN, BIK0, and BIFS) have only GPS observations, and their ZTD results are worse than those of the other stations when a combination of GPS and BDS-3 observations is used. The RMSE of the ZTD results obtained with PPP-B2b at the LHAZ station on the Tibetan Plateau reaches 27.4 mm, which might be due to the lack of BDS-3 observations at this station. For the other stations between 30\({}^{\circ}\) N and 38\({}^{\circ}\) S, the RMSE ranges from 17.8 mm to 35.3 mm, with a mean value of 24.4 mm, which is obviously larger than that at the stations located between 30\({}^{\circ}\) N and 48\({}^{\circ}\) N. The results also reveal that the PPP-B2b service for the MOBS station was rarely applicable during most \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**GNSS**} & \multicolumn{4}{c}{**Co-Located Radiosonde**} \\ \hline \multirow{2}{*}{\begin{tabular}{c} **Station** \\ **Name** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Latitude** \\ **(\({}^{\circ}\))** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Longitude** \\ **(\({}^{\circ}\))** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Height** \\ **(m)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Observation** \\ **System** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Station** \\ **Name** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Horizontal** \\ **Distance** \\ **(km)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Height** \\ **Difference** \\ **(m)** \\ \end{tabular} } \\ \hline CUSV & 13.7 & 100.5 & 76.1 & G + C & N/A & N/A & N/A \\ GUAM & 13.6 & 144.9 & 201.9 & G + C & GQM00091212 & 14.86 & 70.3 \\ IISC & 13.0 & 77.6 & 843.7 & G + C & N/A & N/A & N/A \\ SGOC & 6.9 & 79.9 & \(-\)78.5 & G + C & N/A & N/A & N/A \\ SEYG & \(-\)4.7 & 55.5 & \(-\)37.6 & G + C & SEM00063985 & 1.62 & \(-\)1.0 \\ CIBG & \(-\)6.5 & 106.8 & 169.1 & G + C & N/A & N/A & N/A \\ DGAR & \(-\)7.3 & 72.4 & \(-\)64.9 & G + C & N/A & N/A & N/A \\ COCO & \(-\)12.2 & 96.8 & \(-\)35.3 & G + C & CKM00096996 & 0.10 & \(-\)0.2 \\ KATI & \(-\)14.4 & 132.2 & 184.3 & G + C & N/A & N/A & N/A \\ PTVL & \(-\)17.7 & 168.3 & 86.4 & G + C & N/A & N/A & N/A \\ VACS & \(-\)20.3 & 57.5 & 421.2 & G + C & N/A & N/A & N/A \\ MCHL & \(-\)26.4 & 148.1 & 534.6 & G + C & N/A & N/A & N/A \\ YARR & \(-\)29.0 & 115.3 & 241.4 & G + C & ASM00094403 & 68.58 & 229.45 \\ MOBS & \(-\)37.8 & 145.0 & 40.6 & G + C & N/A & N/A & N/A \\ \hline \hline \end{tabular}
\end{table} 
Table 6: _Cont._epochs, resulting in no resolution obtained for over 70% of the epochs. This limitation arises from the fact that the PPP-B2b correction information is derived from tracking stations deployed within the territory of China, and its service is primarily designed for China and its surrounding areas.