*Article* **Assessment of the Water Vapor Tomography Based on Four Navigation Satellite Systems and Their Various Combinations**

**Fei Yang 1,2,3,4,\*, Jinyang Wang 1, Hongsen Wang 1, Xu Gong 1, Lei Wang <sup>5</sup> and Bin Huang <sup>2</sup>**


**Abstract:** With the gradual improvement of Galileo and the opening of BDS-3 services, water vapor tomography based on multi-GNSS can be effectively carried out to reconstruct three-dimensional water vapor distribution. In this paper, experiments in Hong Kong were conducted to analyze and assess the performances of GPS, BDS, GLONASS, and Galileo and their combinations in water vapor tomography. Numerical results show that the number of available signal rays varies widely in the four satellite systems, and the value can be increased by the combination of satellite systems; the combinations also increase the number of voxels crossed by signal rays, but this value is not directly related to the number of available signal rays; the number and distribution of the voxels with sufficient signal rays, which most closely related to the structure of the tomographic model, show no obvious differences in the four satellite systems and their combinations. Comparative results of slant water vapor (SWV) estimated by GNSS data and water vapor density derived from radiosonde data reveal that the differences in the water vapor tomography of the four satellite systems are small, and their combinations have limited improvement in the tomographic results.

**Keywords:** GNSS meteorology; water vapor; tomography; multi-GNSS
