# 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
It can be observed from Figure 7 that PWV from all combinations reveals consistency with the radiosonde observations. The correlation coefficients for all combinations are higher than 0.85. For the accuracy indicators, results show that the bias of PWV monitored based on the B2b ephemeris using the combination of G, C, and GC satellite combinations are \(-\)0.14, \(-\)2.21, and 0.14 mm, respectively, and the corresponding RMS are 3.71, 4.66, and 3.73 mm, respectively, while those values for the results from B2b ephemeris are 0.20, 1.82, and 0.42 mm for bias, and 2.05, 4.31, and 2.41 mm for RMS. Compared to the B2b results, PWV monitored with CAS ephemeris generally exhibit better accuracy with about 45%, 8%, and 35% for G, C, and GC satellite combinations. The obviously better accuracy for the G and GC combinations indicates that the quality of orbit and clock offset for GPS satellite from B2b services are generally lower than those  
Figure 6: The bias (**a**) and RMS (**b**) of ZTD estimation from NRT processing mode using different ephemeris (WUM, CAS, and B2b) with different satellite combinations (G, C, and GC). from CAS ephemeris. While the general tiny improvements for the BDS-only combinations reveals that the qualities for BDS satellites from two RT ephemeris are generally consistent. Despite that the performance of water vapor monitored with B2b ephemeris being worse, the accuracy is about 3 to 4 mm compared to radiosonde observations, which can be reliably applied for meteorological research and operations [46]. Figure 8 exhibits the time series of PWV monitored through the NRT processing mode using two RT ephemeris (i.e., CAS and B2b) with different satellite combinations during the research period. For clearer illustration, the bias and RMS for PWV monitored with two RT ephemeris are summarized in Table 2. In general, the variation in PWV in the temporal scale monitored by CAS and B2b products are generally consistent with the reference values from the ERA5 reanalysis products. Correlating analysis indicates that the correlations between PWV monitored from different combinations all exhibit high value greater than 0.95. In addition, PWV time series obtained by the NRT processing mode also has relatively obvious fluctuations and outliers, which is similar to the results of ZTD above. This is  
Figure 7: Scatter plots of PWV retrieved from radiosonde observations and monitored 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. potentially due to the fact that the stability and accuracy of the real-time product (CAS and B2b) are lower compared to the final product (WUM). For accuracy statistical results, it shows that the bias of PWV monitored based on the CAS ephemeris using the combination of G, C, and GC satellite combinations are \(-\)0.34, \(-\)1.32, and \(-\)0.29 mm, respectively, and the corresponding RMS are 2.94, 4.36, and 3.46 mm, respectively, while those values for the results from B2b ephemeris are \(-\)0.23, 0.96, \(-\)0.29 mm for bias, and 4.55, 4.40, 3.95 mm for RMS. These values are generally about 1/6 of the ZTD error, which is consistent with the theory of error propagation mentioned above. Compared with B2b results, PWV monitored from CAS ephemeris exhibits better accuracy of about 35%, 1%, and 12% for G, C, and GC satellite combinations. Similarly to the results compared with radiosonde observations, the G and GC combinations reveals more obviously better accuracy, which might be due to the lower quality of orbit and clock offset for the GPS satellite from B2b services, while the general consistent accuracy for the BDS-only combinations (\(\sim\)1%) manifests the similar qualities for BDS satellites from two RT ephemeris. Basically, these numerical results are consistent with previous studies [29; 30], that is, the accuracy that can be achieved by about 4 mm using the PPP-B2b service.