Chapter 3 proposes a real-time GNSS PPP processing for precipitable water vapor derivation, using the single systems as well as the combined dual-systems. The real-time GNSS meteorology approach, including the principle, data acquisition and processing strategies, and results are illustrated. The performance of real-time water vapor estimates retrieved from the single GNSS: GPS, GLONASS, and BeiDou, are evaluated by intra-technique comparison as well as independent comparison and validation with data from water