## 3 Results  
### Detection Capability Improvement of Multi-GNSS for a Single Polar-Orbit Satellite  
In order to analyze the RO detection capability enhancement of a single polar-orbit satellite by the multi-GNSS integration, the GNSS RO events in five days from 21 to 25 June 2021 were simulated using MET and multi-GNSS satellites' orbital data. The global distribution of the MET/multi-GNSS RO events on 21 June 2021 is shown in Figure 2, in which the red triangles indicate MET/BDS-3 RO events, and the colorful dots indicate the RO events of different GNSS systems marked by the legends. From Figure 2, one can see that on 21 June 2021, there were 568 MET/BDS-3 RO events sparsely distributed on the Earth while there were 3783 MET/multi-GNSS RO events that occurred, and those RO events formed a more uniform and dense coverage of the Earth. Figure 2: Distribution of the MET/multi\(-\)GNSS RO events on 21 June 2021. To quantitatively analyze the spatial and temporal distribution of each GNSS system RO event, the time series of GCF and UCI of the GNSS RO events were calculated by using the methods introduced in Section 2.2. However, the equal-area projection distorts the world map by angle and length, and the most serious distortion is in the polar regions. The world map distortion makes it difficult to plot the RO events' global coverage using latitude and longitude coordinates. To show the RO events' distribution in a common way and avoid the RO events being counted many times, Figure 3 shows the number of RO event revisits using a 4\({}^{\circ}\times 4^{\circ}\) latitude and longitude grid coordinate. To further explore the RO detection capability improvement of the integration of multiple GNSS for a single polar-orbit LEO satellite quantitatively, Figures 3 and 4 show the global coverage and the number trend beside the GCF trend of the occultation events for MET/BDS-3 and MET/multi-GNSS RO systems. Figure 4: GNSS RO number and global coverage fraction trend for MET/BDS\(-\)3 and MET/multi\(-\)GNSS RO systems. Figure 3: The global coverage of GNSS RO events for MET/BDS\(-\)3 and MET/multi\(-\)GNSS RO systems, illustrating the number of RO event revisits using a 4\({}^{\circ}\times 4^{\circ}\) latitude and longitude grid coordinate. Figure 3a-f describe the global coverage situations of the MET/BDS-3 RO events that occurred in 6 h, 12 h, 24 h, 48 h, 72 h, and 96 h, respectively, while Figure 3g-i describe the global coverage situations of MET/multi-GNSS RO events that occurred in 6 h, 12 h, and 24 h, respectively. As shown in Figure 3, the GCF of RO events gradually increased with time accumulation, and the GCF values of MET/BDS-3 RO events that occurred in 6 h, 12 h, 24 h, 48 h, 72 h, and 96 h were 4%, 7%, 14%, 26%, 35%, and 44%, respectively, while the GCF values of the MET/multi-GNSS RO events that occurred in 6 h, 12 h, and 24 h were 20%, 38%, and 61%, respectively. Comparing the performance of the two RO systems, one can see that the GCF of MET/multi-GNSS RO events in 24 h was about four times that of the MET/BDS-3. Furthermore, the GCF of the MET/multi-GNSS RO events in 24 h was larger than that of MET/BDS-3 RO events in 96 h, and its maximum number of RO event revisits was also higher. However, as shown in Figure 3g, the GCF of 6 h MET/multi-GNSS RO events was only 20%, which could not meet the requirements of GNWP for RO data volume and timeliness. There were two blank regions in the 6 h GNSS RO Figure 3a,g. These distinct performances of the GNSS RO event distribution and uniformity were caused by the limitations of the short time interval and LEO satellite number. The single meteorology LEO satellite's orbital period was about 1.69 h, which means it can only run about 3.55 circles around the Earth in 6 h; therefore, its track could not cover the entire Earth's surface in such a short time.