## 4 PPP-B2b ZTD Estimation and Evaluation of the Results
### Comparison of the GNSS-Derived ZTD/PWV with TWT Derived from ERA5  
In this study, GNSS-derived ZTD/PWV values are compared with those obtained with ERA5 from DOY 069, 2022 to DOY 059, 2023 at 32 stations. Table 8 shows the comparison results between the GNSS-derived ZTD/PWV (IGS, GBM, CNES, and PPP-B2b) and the ERA5-derived ZTD/PWV. The comparison results reveal that the ZTDs derived from GBM products exhibit the best agreement with the ERA5-derived ZTDs, with a mean RMSE of approximately 18.4 mm across 32 stations. The RMSEs of the IGS and CNES results are 19.0 mm and 19.6 mm, respectively. The PPP-B2b results exhibit the least agreement with the ERA5-derived ZTDs, with a mean RMSE of 27.8 mm. As shown in Table 8 and Figure 8, the RMSE variations for all GNSS solutions exhibit a latitudinal pattern, with higher RMSE values observed between \(0^{\circ}\) and \(30^{\circ}\) N than within the latitudinal bands of \(30^{\circ}\) N to \(48^{\circ}\) N and \(0^{\circ}\) to \(38^{\circ}\) S. In general, the RMSEs of the ZTDs for the IGS, GBM, and CNES solutions are approximately 22 mm between \(0^{\circ}\) and \(30^{\circ}\) N, exceeding that between \(30^{\circ}\) N and \(38^{\circ}\) S, where the mean value is approximately 17 mm. These spatial variations might be attributed to the fact that the atmospheric parameters in ERA5 are not of the same quality in all regions of the globe. Similarly, GNSS measurements and processing errors can change spatially. For the PPP-B2b ZTDs, the RMSE is 20.9 mm within the latitudinal band from \(30^{\circ}\) N to \(48^{\circ}\) N, which is slightly greater than that of the other three GNSS solutions. However, the RMSE of the PPP-B2b ZTDs reaches approximately 30 mm between \(30^{\circ}\) N and \(38^{\circ}\) S, which is significantly worse than that of the other solutions. The large errors in the PPP-B2b ZTD results are mainly dominated by the errors in the PPP-B2b orbit/clock products. The mean biases for the IGS, GBM, CNES, and PPP-B2b solutions are \(-0.9\) mm, \(-1.7\) mm, \(-2.0\) mm, and \(0.7\) mm, respectively. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Solutions**} & **Latitude** & \multicolumn{2}{c}{**IGS Results as Reference**} & \multicolumn{2}{c}{**GBM Results as Reference**} \\ \cline{3-6}  & **Range** & **RMSE** & **Bias** & **RMSE** & **Bias** \\ \hline \multirow{4}{*}{CNES} & \(30\)–\(48^{\circ}\) N & \(10.0\) & \(-1.3\) & \(8.0\) & \(-0.4\) \\  & \(0\)–\(30^{\circ}\) N & \(11.2\) & \(0.1\) & \(8.0\) & \(-0.4\) \\  & \(0\)–\(38^{\circ}\) S & \(10.3\) & \(-0.9\) & \(7.9\) & \(0.3\) \\  & Mean & \(10.4\) & \(-0.8\) & \(8.0\) & \(-0.1\) \\ \hline \multirow{4}{*}{PPP-B2b} & \(30\)–\(48^{\circ}\) N & \(16.2\) & \(1.5\) & \(15.7\) & \(2.7\) \\  & \(0\)–\(30^{\circ}\) N & \(25.7\) & \(2.5\) & \(23.9\) & \(2.6\) \\ \cline{1-1}  & \(0\)–\(38^{\circ}\) S & \(24.8\) & \(-1.0\) & \(25.0\) & \(1.1\) \\ \cline{1-1}  & Mean & \(22.2\) & \(0.7\) & \(21.7\) & \(2.1\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison between the ZTD derived with PPP-B2b/CNES corrections and the GBM/IGS final orbit/clock products (unit: mm).