# 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  
This subsection first verifies the effectiveness of the NRT processing mode adopted by the monitoring system. To achieve the verification requirements, ZTD estimated from the post-processing mode using WUM final ephemeris products with GPS-only observations is adopted as the reference. Afterwards, the original GPS and BDS observations for a total of 25 days from DOY 076 to DOY 100 in 2025 are consistently processed via the PPP technique on a daily batch. The processing strategy are exhibited in Table 1. Based on the consistent strategies for parameter and input data (including GNSS observations and satellite ephemeris), ZTD time series in 1 h temporal resolution can be obtained for the entire research period. This guarantees that the comparison between ZTD from NRT and post-processing mode are generally in accordance, despite the satellite utilization and processing mode. For satellite utilization, three combinations of satellite systems are selected, i.e., the GPS-only (G), BDS-only (C), and the combination of GPS and BDS (GC). The time series of ZTD estimated from different satellite combinations during the entire research period are shown in Figure 4. In all subplots of Figure 4, the black lines represent the referenced ZTD time series estimated using WUM final ephemeris in post-processing mode, while ZTD time series in NRT processing mode for G/C/GC combinations are illustrated in blue, red, and green, respectively. It can be observed from the figures that no matter which satellite combination is adopted, ZTD estimated by the NRT processing mode can be well consistent with the results from the post-processing mode. However, three time series of ZTD estimated from NRT processing mode are all less smooth than that from post-processing mode, exhibiting some distinguished outliers. This may be attributed to the fact that the accuracy of RT ephemeris (i.e., CAS and B2b) is generally worse than the final ephemeris for some epochs, which influences the accuracy of data processing [8]. Afterwards, two accuracy indicators, i.e., the bias and RMS of ZTD error, are utilized for quantification. It shows that the bias of ZTD using GPS-only observation is close to 0.01 mm, and the corresponding RMS is 6.91 mm. The phenomenon that almost no bias exists is possibly attributed to the fact that the ZTD reference from the post-processing mode was also estimated using the GPS-only observation, thus guaranteeing the consistency. This also partly proves the consistency between the NRT processing mode and the post-processing mode. In contrast, the estimation result of BDS-only manifests a relatively large bias with the value of \(-3.39\) mm, manifesting an evident bias between the processing results. This might be because of the poorly calibrated antenna-related error for the BDS satellites. According to previous research (e.g., [42; 43]), the authors have also discovered that the significant systemic biases exist for ZTD derived from BDS and GPS constellations. They attributed this to the poorly calibrated PCO/PCV errors for BDS satellites, which in turn influences the performance of data processing, while for the GPS and BDS combination, the bias of ZTD is closer to 0 than that of BDS-only result with the value of \(-2.15\) mm. This should be because the joint processing after introducing GPS observations can improve the robustness and accuracy of parameter estimation [8]. ZTD RMS from the combined GPS and BDS is the smallest with the value of 6.88 mm, revealing a tiny improvement of about 0.03 mm. This accuracy is basically consistent with previous studies [40]. In general, the NRT processing mode can obtain comparable ZTD estimation to those from the post-processing mode. Therefore, the NRT mode implanted on the monitoring system can be used for water vapor monitoring.