Advances and challenges in developing the ground-based GNSS/MET water vapor observation network in China

The ground-based global navigation satellite system meteorology (GNSS/MET) water vapor observation network has become an important part of China's integrated meteorological observation system. With the promulgation of the national policy promoting the large-scale application of the Beidou navigation satellite system (BDS) in key sectors such as meteorology, this paper discusses how to construct, deploy, and develop this network, and how to maximize its value by leveraging the strengths and characteristics of the meteorological departments. To address these issues, this paper reviews the development of the domestic ground-based GNSS/MET water vapor observation network over the past 20 years and elaborates on the challenges it currently faces. The ground-based GNSS/MET water vapor observation network in China has developed rapidly, with the number of stations increasing from 87 in 2008 to more than 1,000 by the end of 2024. The total number of stations is expected to exceed 2,300 by the end of 2025. All stations will be equipped with BDS signal receiving technology and achieve integrated monitoring of atmospheric parameters in both the troposphere and ionosphere. In eastern China, the average distance between stations will be 50-80 km, while in the western regions, it will be about 100 km. Currently, a relatively complete observation quality control procedure and the indices for the observation data have been established, with the uncertainty of precipitable water vapor (PWV) operational products being less than 2 mm, meeting the requirements of the optimal cost-effectiveness indicators proposed by World Meteorological Organization (WMO). These products have played a significant role in operational numerical weather prediction, weather diagnosis, and other meteorological services. However, there is still a considerable gap between China's network layout and those of developed regions or countries such as Europe, the United States, and Japan. And, it has not yet met the WMO's recommended coverage for meso- and micro-scale weather systems. Furthermore, the performance of observation instruments needs to be further improved, as well as the anti-electromagnetic interference ability and the signal capture and tracking capabilities. Finally, this paper gives a perspective on the upgrade, optimization, and product applications of the network. This study can provide a scientific reference for the development of the ground-based GNSS/MET water vapor observation network in China.