# Practical Performance Assessment of Water Vapor Monitoring Using BDS PPP-B2b Service
###### Abstract  
BeiDou navigation satellite system (BDS) precise point positioning (PPP)-B2b has significant potential for application in meteorological fields, such as standalone water vapor monitoring in depopulated area without Internet. In this study, the practical ability of water vapor monitoring using the BDS PPP-B2b service is illustrated through a continuously operated water vapor monitoring system in Wuhan, China, with a 25-day experiment in 2025. Original observations from the Global Positioning System (GPS) and BDS are collected and processed in the near real-time (NRT) mode using ephemeris from the PPP-B2b service. Precipitable water vapor PWV monitored with B2b ephemeris are evaluated with radiosonde and ERA5 reanalysis, respectively. Taking PWV from radiosonde observations as the reference, RMS of PWV based on B2b ephemeris varies from 3.71 to 4.66 mm for different satellite combinations. While those values are with a range from 3.95 to 4.55 mm when compared with ERA5 reanalysis. These values are similar to those processed with the real-time ephemeris from the China Academy of Science (CAS). In general, this study demonstrates that the practical accuracy of water vapor monitored based on the BDS PPP-B2b service can meet the basic demand for operational meteorology for the first time. This will provide a scientific reference for its wide promotion to meteorological applications in the near future. BeiDou navigation satellite system (BDS); zenith tropospheric delay (ZTD); precipitable water vapor (PWV); precise point positioning (PPP) 2020  
0000-0002-2200-22000-22000-22000-2200  
Theoretically, GNSS signals will be affected by the atmosphere in the troposphere when transmitting from satellites to receivers, resulting in the zenith tropospheric delay (ZTD) [10]. ZTD usually consists of the zenith hydrostatic delay (ZHD) caused by dry atmosphere and the zenith wet delay (ZWD) caused by water vapor. The principle of GNSS water vapor retrieval is to obtain the wet part and convert it to the precipitable water vapor (PWV) with the help of related meteorological observations. This technique has been developed since 1990s and continuously improved [11; 12]. The accuracy of GNSS-PWV has been proved to be about 2 mm for post-processing mode compared with water vapor from multiple techniques (e.g., [13; 14; 15]). At present, water vapor retrieved from the mainstream navigation systems (including the Global Positioning System (GPS), Galileo, BeiDou navigation satellite system (BDS), and GLONASS (Global Navigation Satellite System)) has greatly contributed to both meteorological operations and research (e.g., [16; 17; 18; 19; 20]). To better meet the requirements of operational meteorology for the timeliness of water vapor monitoring, real-time (RT) or near real-time (NRT) GNSS water vapor retrieval have been proposed, which utilizes the real-time products from the Real-Time Service (RTS) of the International GNSS Services (IGS). The accuracy of PWV from RT/NRT data processing mode based on the precise point positioning (PPP) technique is slightly worse than that from post-processing mode. In general, the accuracy of RT/NRT PWV estimated using different satellite ephemeris is about 3 to 5 mm compared to the reanalysis products among different regions around the globe [21; 22; 23]. This can meet the general requirement of water vapor accuracy for meteorological applications [24; 25]. The real-time products from the IGS RTS are generally provided through the real-time data streams via the Internet. This not only increases the instability and error of water vapor monitoring due to the network fluctuations but also raises the requirements of equipment for data processing. In contrast, the corrections to orbit and clock offset of BDS PPP-B2b service are broadcasted by three BDS-3 Geostationary Earth Orbit (GEO) satellites (i.e., C59-C61) and can be received and decoded by receivers [26].