# Practical Performance Assessment of Water Vapor Monitoring Using BDS PPP-B2b Service
## 4 Results and Discussion
### Verification of Near Real-Time (NRT) Processing Mode Through Comparison with Post-Processing Mode
Figure 4: Time series of ZTD estimated from NRT processing model using GPS-only ((**a**), blue), BDS-only ((**b**), red), and GPS and BDS ((**c**), green) satellite combinations during the entire research period. The black line in each subfigure represents reference ZTD estimated from post-processing mode. Performance Evaluation of NRT ZTD Estimation from PPP-B2b Service Through Comparison with Post-Processing Mode  
Subsequently, the accuracy of ZTD estimated through the NRT processing mode from the monitoring system is evaluated to analyze the performance of the PPP-B2b service. For the same observations, the real-time ephemeris provided by CAS through IGS RTS and that from the PPP-B2b service were, respectively, adopted in the monitoring system based on the NRT processing mode. Three satellite combinations including G/C/GC are applied for ZTD estimation. Figure 5 presents the time series of NRT ZTD estimated using different RT ephemeris (i.e., CAS and B2b) with different satellite combinations during the research period. Overall, ZTD results estimated from the CAS and B2b product are relatively consistent with the reference values. However, compared with the reference obtained by the post-processing mode, ZTD estimated using two RT ephemeris (CAS and B2b) has more significant fluctuations and some outlier. This feature is more obvious for B2b results, which might be due to the relatively lower accuracy of B2b ephemeris compared with CAS products [44]. The corresponding bias and RMS of ZTD error evaluated by ZTD from the post-processing mode are summarized in Figure 6, including the NRT processing results with WUM final ephemeris. Statistics results of accuracy indicators show that the bias of ZTD estimation for the CAS ephemeris with the satellite combination of GPS-only, BDS-only, and  
Figure 5: Time series of ZTD estimated from NRT processing model using GPS-only (blue), BDS-only (red), and GPS and BDS (green) satellite combinations based on CAS (**a**,**c**,**e**) and B2b ephemeris (**b**,**d**,**f**) during the entire research period. The black line in each subfigure represents reference ZTD estimated from post-processing mode. GPS and BDS combinations are 0.95, 6.98, and 1.71 mm, respectively, and the corresponding RMS are 14.18, 22.85, and 16.16 mm, respectively, while these values for B2b ephemeris are 1.64, \(-\)6.54, 1.35 mm for bias, and 24.32, 23.34, 21.49 mm for RMS. These numerical results are basically similar to previous studies [31], that is, ZTD accuracy based on PPP-B2b service is approximately 20 mm. Although the performance of B2b is slightly worse than that of CAS products, according to the theory of error propagation [45], the corresponding PWV accuracy can achieve around 4 mm with PPP-B2b service. This can basically meet the requirements of real-time water vapor monitoring for meteorological applications [24]. Performance Evaluation of NRT PWV Monitoring from PPP-B2b Service Through Comparison with ERA5 Reanalysis  
Finally, the accuracy of PWV monitored through NRT processing mode from the monitoring system are evaluated to verify the practical effectiveness of the PPP-B2b service for water vapor monitoring. PWV is converted using ZTD and in situ meteorological observations from the monitoring system with the method described in Section 3. Different from ZTD, the assessment of PWV was conducted using that from the radiosonde observations from the adjacent meteorological stations (with a horizontal distance of about 15 km) and ERA5 reanalysis products, which has been proven to be a valid external reference for evaluating water vapor [35; 46]. The radiosonde-derived PWV data for the research period can be directly retrieved from the GNSS meteorological ensemble tools (SMET) [35]. Figure 7 exhibits the scatter plots of PWV retrieved from radiosonde observations (Radio-PWV) and monitored from two RT ephemeris (i.e., CAS and B2b) with different satellite combinations during the research period.