Constellation-Dependent GNSS PWV Retrieval Under Humid Conditions in Beijing

Noncollocation can obscure small constellation-dependent differences in Global Navigation Satellite System (GNSS) precipitable water vapor (PWV). We assessed five constellation configurations processed with a common precise point positioning strategy at the same four physical station groups surrounding the Beijing radiosonde site during June–September 2023–2025. In the original year-specific matched samples, BeiDou Navigation Satellite System (BDS)-only Root Mean Square Error (RMSE) was 1.3%, 5.4%, and 7.9% lower than GPS-only RMSE in 2023, 2024, and 2025, respectively. Because these percentages were descriptive, we added a direct paired analysis on strict-common samples (n = 225, 231, and 216). Paired BDS-minus-GPS RMSE differences were −0.19 mm (95% CI: −0.41 to −0.03), −0.39 mm (−0.61 to −0.16), and −0.44 mm (−0.65 to −0.23), respectively. Fixed-geometry nearest-station, alternative inverse-distance weighting, and elevation-sensitive tests showed that the small constellation difference was not invariant to spatial transfer. Residuals became progressively more negative with increasing radiosonde PWV, and high-moisture (>50 mm) biases were −10.75 mm for BDS and −11.36 mm for GPS. Full-season leave-one-year-out PWV percentile achieved Area Under Curve (AUC) = 0.846, but its transferred threshold yielded False Alarm Ratio (FAR) = 92.2% and Critical Success Index (CSI) = 7.6%. The common five-configuration processing provides a controlled comparison; the rainfall result supports retrospective regional moisture monitoring, not stand-alone operational prediction.