# Practical Performance Assessment of Water Vapor Monitoring Using BDS PPP-B2b Service
## 5 Conclusions  
The PPP-B2b service has significant potential for application in meteorological fields, such as standalone water vapor monitoring in depopulated areas without Internet. While most current studies are generally based on the simulated real-time PPP mode, analyzing its practical performance should be conducted. For this purpose, a continuously operated water vapor monitoring system using the PPP-B2b service was established in Wuhan, China. The effectiveness of the NRT processing mode adopted by the monitoring system is firstly verified through comparison of ZTD estimated from the post-processing mode. Subsequently, the accuracy of ZTD estimated through the NRT processing mode using two RT ephemeris is evaluated. Accuracy results show that the RMS of ZTD error for the B2b ephemeris are 24.32, 23.34, 21.49 mm, respectively. These values were generally comparable with those from CAS ephemeris, which can basically meet the requirements for the following conversion to water vapor. Finally, the accuracy of PWV monitored through the NRT processing mode are evaluated to verify the practical performance of PPP-B2b service. Taking PWV from radiosonde observations as the reference, RMS of PWV based on B2b ephemeris are 3.71, 4.66, and 3.73 mm for G, C, and GC combinations, respectively, while those values are 4.55, 4.40, 3.95 mm for G, C, and GC combinations, respectively. These accuracies are generally comparable with those from CAS ephemeris and can generally meet the demand for NRT water vapor monitoring. To sum up, water vapor monitoring based on the BDS PPP-B2b service is feasible to be promoted to operational meteorology. Future work lies in two directions. First, the assessment in the presented study generally adopts GPS estimation as the reference, while further analysis with more satellite systems (e.g., GLONASS and Galileo) can provide a more comprehensive perspective on the performance of water vapor monitoring using BDS PPP-B2b service. Second, this \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ephemeris** & **Satellite Combinations** & **Bias (mm)** & **RMS (mm)** \\ \hline \multirow{3}{*}{CAS} & GPS-only (G) & \(-\)0.34 & 2.94 \\  & BDS-only (C) & \(-\)1.32 & 4.36 \\  & GPS and BDS (GC) & \(-\)0.29 & 3.46 \\ \hline \multirow{3}{*}{B2b} & GPS-only (G) & \(-\)0.23 & 4.55 \\  & BDS-only (C) & 0.96 & 4.40 \\ \cline{1-1}  & GPS and BDS (GC) & \(-\)0.29 & 3.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Statistical results of PWV accuracy monitored from NRT processing mode using CAS and B2b ephemeris with different satellite combinations. study only contains results of 25 days in a single place, while a further evaluation with a longer time span and more stations should be conducted. The characteristics of accuracy in different seasons (including rainy and non-rainy periods) and areas can be explored. This will provide a more robust reference for its promotion in the near future. Conceptualization, Y.C. ; methodology, Y.C. and L.Z. ; software, L.Z. validation, L.Z. and Y.C. ; formal analysis, L.Z. ; investigation, Y.C. ; resources, Y.C. and H.L. ; data curation, L.Z., E.Z., Z.H. and Y.C. ; writing--original draft preparation, L.Z. ; writing--review and editing, Y.C. and H.L. ; visualization, L.Z., X.L. and M.Q. ; supervision, Y.C. and H.L. ; project administration, Y.C. and H.L. ; funding acquisition, Y.C. and H.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (Grant No. U2442214), the Observational Experiment Project of Meteorological Observation Center of China Meteorological Administration (Grant No. GCSYJH24-21, GCSYJH24-02), and the Innovation and development project of China Meteorological Administration (Grant No. CXFZ2024J061). Not applicable. All data that support the findings of this study are included within the article. The authors are grateful to the ECMWF for providing the ERA5 reanalysis products. The authors declare no conflicts of interest.