## 1 Introduction  
Water vapor, as a fundamental component of the atmosphere, plays a key role in the hydrological cycle and the climate system. Traditional water vapor measurements are mainly provided by the meteorological sensors, such as radiosondes and water vapor radiometers [1; 2]. However, due to the high spatiotemporal variability of the atmospheric water vapor and limitations of these traditional observing techniques, efforts have been made to obtain reliable and enhanced water vapor observations. Global Positioning System (GPS) meteorology conceived as monitoring the atmospheric water vapor with ground-based GPS receivers was firstly introduced in 1990s [3]. Since then, extensive studies concerning GPS meteorology have been carried out in the past two decades. The results demonstrated the capability of GPS for providing water vapor estimates with comparable accuracy to those offered by the meteorological sensors [4; 5; 6]. Furthermore, the GPS-derived water vapor also has its advantages, such as low operational expense, high spatiotemporal resolution, and all-weather availability. Most of these previous investigations were limited to post-processing and near-RT modes, while few studies involve the RT mode, which requires an efficient RT service system that offers RT orbits, satellite clocks, and other RT products [7]. However, with the increasing innovative applications, nowcasting and short-term weather forecasting have been becoming important trends in meteorology this requires more information of the atmosphere state being provided with short or even no latency [7; 8]. To deliver the tropospheric products in RT serving for the time-critical operational meteorology has become one of the focuses within the GNSS (Global Navigation Satellite Systems) meteorology community. Attributing to the recent development of the IGS (International GNSS Service) RT pilot project (RTPP), the RT satellite orbit/clock products provided by IGS, as well as its analysis centers (ACs) such as GeoSrschungsZentrum (GFZ) and the European Space Agency (ESA), are now available for scientific researches and projects. This offers a good potential for estimating the tropospheric delays by using the RT PPP [9; 10; 11; 12]. Dependent on the IGS-combined RT satellite orbit/clock products, the European Coordination in Science and Technology (COST) has started to offer the RT zenith total delay (ZTD) products on the basis of GPS observations. Meanwhile, the RT Demonstration campaign (RT-Demo) is organized by Working Group 1 \"Advanced GNSS Tropospheric Products\" of the COST ES1206 Action GNSS4 SWEC (\"Advanced Global Navigation Satellite Systems tropospheric products for monitoring severe weather events and climate\") [13]. This campaign was designed in 2014 and officially began on 1 April 2015, and aims at fully exploiting the current GNSS capability for the RT meteorological applications. Some of the recent studies have concentrated on retrieving the RT tropospheric products or water vapor based on GPS-only observations. The results pointed out that the high accuracy (a few millimeters) of the derived RT tropospheric delays or water vapor can be achieved when compared to the post-processing products and meteorological data [7; 11; 12]. Nowadays, the world of the satellite navigation system is undergoing rapid development into a powerful multi-GNSS constellation, along with the modernization of GPS, the recovery of the Russian Global Navigation Satellite System (GLONASS), and the newly developing European Galileo and the Chinese Beidou satellite (BDS) system [13]. As of September 2017, more than 90 navigation satellites (32 GPS, 24 GLONASS, 18 BDS, 18 Galileo) are in orbit and transmitting data profiting from the multi-GNSS constellation. Moreover, the number of the satellites in total will increase to more than 120 once all four systems (GPS, GLONASS, Galileo, and BDS) are fully deployed. Many of the GPS networks have been upgraded to the multi-GNSS observation networks.