## 3 Results
### Comparing Polar-Orbit Constellation Detection Capability with the Spire Constellation  
In order to explore the RO detection capabilities of the constellations with polar-orbit satellites and the Spire/GPS RO system including 100 Spire weather LEO satellites quantitatively, Figures 5 and 6 show the global coverage and the number trend beside the GCF trend of the RO events for 100 RS/GPS, 6 RS/multi-GNSS, and 12 RS/multi-GNSS constellations in 6 h, 12 h, and 24 h.  
As shown in Figure 5a-c, the GCF value of the 100 RS/GPS RO events in 6 h reached 94% and formed a uniform global coverage, which could basically meet the requirements of the RO number and timeliness for the GNWP for which products are released in every 6 h. The 12 h and 24 h 100 RS/GPS RO events' GCF values could reach 100%, and the number of RO event revisits for a single-grid cell in the mid-latitude zones could reach more than 40. As shown in Figure 5d-f, the GCF values of 6 RS/multi-GNSS RO events in 6 h, 12 h, and 24 h were 75%, 95%, and 99%, respectively, which were smaller than those of the 100 RS/GPS system, and their global coverage uniformity was also poor compared with the 100 RS/GPS system. Figure 5g-i show that the GCF values of the 12 RS/multi-GNSS RO events in 6 h, 12 h, and 24 h were 90%, 99%, and 100%, respectively. Therefore, the 24 h GCF value of the 12 RS/multi-GNSS RO events was comparable to the 100 RS/GPS RO system. Regarding the number of the RO event revisits, the 12 RS/multi-GNSS constellation performed higher in polar regions but lower in middle- and low-latitude zones than the 100 RS/GPS RO system. Figure 6 illustrates the occultation number trend beside the GCF trend of the RO events for 100 RS/GPS, 3 RS/multi-GNSS, 6 RS/multi-GNSS, 9 RS/multi-GNSS, and 12 RS/multi-GNSS RO systems within 24 h. As shown in Figure 6, with the increase in the number of polar-orbit RS satellites, the slope of the occultation number and GCF curves increased significantly; particularly, the curves of 12 RS/multi-GNSS and 100 RS/GPS RO systems were similar to each other, and in some time intervals, the corresponding occultation number lines and GCF curves overlapped. This indicates that in the multi-GNSS combination scenario, the RO detection capability of RS constellation composed of 12 polar-orbit LEO satellites was basically comparable to that of the 100 RS/GPS RO system. Specifically, both  
Figure 5: The global coverage of GNSS RO events for 100 RS/GPS, 6 RS/multi–GNSS and 12 RS/multi –GNSS RO systems, illustrating the number of RO event revisits using a \(4^{\circ}\times 4^{\circ}\) latitude and longitude grid coordinate. Figure 6: Occultation number and global coverage fraction trend of 100 RS/GPS and polar-orbit RS constellation/multi –GNSS RO events. the 12 RS/multi-GNSS and 100 RS/GPS RO systems achieved a global coverage fraction of 100% in about 12 h, and the daily RO event numbers reached more than 60,000, which could meet the basic requirements of global numerical weather prediction for occultation data volume and timeliness. Table 3 presents the RO events' global coverage fraction UCI(1) and global coverage uniformity UCI(m) data for the seven RO detection systems in 6, 12, and 24 h. From Table 3, one can see that in the single RS satellite scenario compared with the MET/BDS-3 constellation, the multi-GNSS combination significantly improved the RO event global coverage and uniformity, but the performance was still poor. Even in the multi-GNSS combination scenario, the limitation of the single RS satellite in terms of the RO event spatial and temporal distribution was obvious, which could not meet the requirements of the global numerical weather prediction and climate research. In the multi-GNSS combination scenario, when the number of the RS satellites increased from one to three, the UCI(1) and UCI(m) indices increased significantly. With the RS satellite number increase, the UCI(1) parameter increased monotonously, but the UCI(m) parameter showed a gradual decreasing trend with fluctuations.