# GNSS Real-Time ZTD/PWV Retrieval Based on PPP with Broadcast Ephemerides
## 3 Results and Discussion
### Evaluation of the PWVs Derived from BE-PPP  
1. Validation with ERA5: For the validation of the PWV derived from BE-PPP, the PWV series retrieved from RT-PPP and BE-PPP processing were compared with the series from ERA5. The temporal resolution of the RT-PPP and BE-PPP PWV solutions was 30 s, while the PWV derived from ERA5 was 1 h. To avoid additional interpolation, only PWV retrieval at common epochs of each PWV series was taken into account for the comparison. Figure 7 shows the PWV comparisons of BE-PPP, RT-PPP, and ERA5 for 720 consecutive hours at the EPN station SAS2. Figure 6: The boxplot of mean bias and STD for RT-ZTD and BE-ZTD. Figure 7: The time series of PWV derived from BE-PPP, RT-PPP, and ERA5 at station SAS2 during DOY 335-364 of 2022. The PWV retrieved with RT-PPP (blue) and BE-PPP (red) both exhibit excellent agreement with the ERA5 PWV (black), while the RT-PWV outperforms the BE-PWV. The mean bias is \(-\)1.41 mm for the RT-PWV and \(-\)2.41 mm for the BE-PWV. The STD values of the RT-PWV and BE-PWV are 1.02 mm and 2.53 mm, respectively. The difference in the STD values between the RT-PWV and BE-PWV is at the millimeter level. To further compare the differences between BE-PWV and RT-PWV, data from 20 EPN stations were used for a statistical analysis in this paper. As shown in Figure 8, the mean bias of the BE-PWV is \(-\)2.14-6.01 mm, while the mean bias of the RT-PWV is \(-\)3.98-3.11 mm. The majority of the stations exhibit a higher mean bias for the BE-PWV compared to that of the RT-ZTD, although there are a few stations where the BE-PWV outperforms the RT-PWV. Subgraph (b) shows the variation in the STD among the stations. For the BE-PWV, the STD ranges from 1.39 to 2.53 mm, while for the RT-PWV, it ranges from 0.85 to 1.25 mm. The stability of the RT-PWV is observed to be higher compared to that of the BE-PWV. The mean biases for the RT-PWV and BE-PWV from the 20 stations are \(-\)0.14 mm and 1.84 mm, respectively. Additionally, the STDs for the RT-PWV and BE-PWV are 0.96 mm and 1.69 mm, respectively. These findings further support the aforementioned conclusion that the disparity between RT-PWV and BE-PWV is negligible. Both RT-PPP and BE-PPP meet the accuracy requirements of a 1 mm target and 5 mm threshold value for real-time PWV products applied to NWP forecasting [40]. The results indicate that the difference in PWV derived from the BE-PPP and RT-PPP solutions is small, while BE-PPP demonstrates an accuracy and stability sufficient enough to serve as a viable alternative solution to RT-PPP. 2. Validation with radiosondes: The PWV values derived from the BE-PPP and RT-PPP analysis compared to the RDS PWV were also validated. Figure 9 presents the PWV results obtained from the BE-PPP, RT-PPP, and RDS solutions at station SAS2. Since the RDS provides PWV data every 12 or 24 h, the comparison of the BE-PWV, RT-PWV, and RDS-PWV is performed at common time intervals. It can be observed that both BE-PPP and RT-PPP accurately estimate the PWV, with differences within a few millimeters compared to the RDS-PWV. Additionally, the STD of the PWV differences between these two methods is approximately 0.56 mm. The mean value and STD of the PWV differences for the BE-PPP and RT-PPP solutions with respect to the RDS solution from 20 EPN stations in Europe with nearby RDS launch sites are presented in Figure 10. In subgraph (a), it can be observed that the mean bias of the BE-PWV ranges from \(-\)1.61 mm to 6.57 mm, while the mean bias of the RT-PWV ranges from \(-\)1.17 mm to 3.77 mm. At most stations, the mean bias of the RT-PWV is smaller than that of the BE-PWV. Subgraph (b) illustrates the variation in the STD among the stations, with a range of 2.15 mm to 4.96 mm for the BE-PWV and 0.93 mm to 2.45 mm for the RT-PWV.