## 5 Discussion  
Traditionally, real-time estimation of ZTD or PWV requires stable internet access to obtain orbit and clock corrections. The PPP-B2b service, transmitted via BDS-3 satellites, introduces a significant advancement by enabling real-time atmospheric sensing without internet dependence. This capability is particularly beneficial for remote and low-income regions, where access to real-time atmospheric data can mitigate the disastrous impacts of severe weather events.  
Our study demonstrates that while the PPP-B2b service achieves acceptable ZTD accuracy in certain regions, such as northern China, with an RMS of approximately 15 mm, comparable to results from previous studies, its performance is notably latitude-dependent. Accuracy decreases to around 25 mm between 30\({}^{\circ}\) N and 38\({}^{\circ}\) S, and the service becomes nearly unavailable beyond 38\({}^{\circ}\) S due to insufficient satellite visibility and poor geometric conditions. This reduction in accuracy significantly limits the global applicability of PPP-B2b, especially for users in the southern hemisphere who require reliable atmospheric data for applications such as real-time weather monitoring, disaster preparedness, and climate research. In regions where accurate weather data are critical for agricultural planning or emergency response, these limitations could lead to less informed decision-making. Previous studies were mostly conducted over very short periods (such as one week). In contrast, our findings, based on a one-year period, reveal that the RMSE of PPP-B2b-derived PWV generally increases with higher PWV values, particularly in regions with significant seasonal variation, when compared with radiosonde-derived PWV.  
In contrast, ZTD/PWV accuracy derived from CNES data consistently outperforms PPP-B2b across all stations when compared to post-processed data from ERA-5 and radiosonde observations. According to the World Meteorological Organization (WMO), the  
Figure 14: PWV variations and the monthly RMSs of the differences between GNSS-derived PWV and radiosonde-derived PWV at 11 stations (**a–k**).  
threshold and breakthrough accuracy requirements for IWV in nowcasting and real-time monitoring are 5 mm and 2 mm, respectively. While IWV accuracy derived from CNES approaches the \"breakthrough\" level with a mean RMS of 2.4 mm, the PPP-B2b service currently meets only the \"threshold\" in China and its surrounding areas. This underscores the need for further refinement of the PPP-B2b service to achieve \"breakthrough\" accuracy, which is crucial for its operational use in high-accuracy IWV products. Achieving \"breakthrough\" accuracy is essential because it represents a level of precision that significantly enhances the reliability and utility of atmospheric data in critical applications. While \"threshold\" accuracy ensures that the data are adequate for basic monitoring purposes, \"breakthrough\" accuracy enables more precise predictions and timely responses, particularly in nowcasting and very short-range forecasting, where the accuracy and timeliness of data are paramount. Attaining this level of accuracy would make PPP-B2b a more viable tool for operational use, enabling it to compete with or even surpass more established systems in regions where internet access is limited but precise atmospheric data are crucial.  
Despite these limitations, satellite-based PPP services like PPP-B2b and Galileo's High Accuracy Service offer valuable alternatives for real-time positioning and related applications by providing high-accuracy satellite orbit and clock corrections without requiring internet access. To support further research and application development, we have established a server to broadcast these corrections in real-time at stream.champ.org.cn via port 2101. Researchers interested in accessing these corrections can request access by contacting the authors.