## 4 Discussion  
From the results, one can see that the number and GCF of the 12 RS/multi-GNSS RO events basically reached the level of the Spire/GPS RO system. However, the global coverage uniformity of the 12 RS/multi-GNSS RO events was worse. As shown in Figure 5, compared with the Spire/GPS constellation, the number of the RO event revisits of the 12 RS/multi-GNSS RO events was smaller at low latitudes and larger at high latitudes.  
To investigate the RO event distribution characteristics of the polar-orbit RS constellations in the multi-GNSS combination scenario, all high-orbit TS satellites (i.e., Geosynchronous Orbit (GEO) and Inclined Geosynchronous Orbit (IGSO) satellites) from the existing GNSS satellites, the MEO (Medium Earth Orbit) satellites from the BDS-3 constellation, and the Iridium NEXT constellation were selected to represent the high-orbit, medium-orbit, and low-orbit TS satellites, respectively, for a 24 h RO event simulation analysis. The occultation events' global coverage and GCF values for eight representative  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline  & **MET/BDS-3/(\%)** & **MET/Multi/(\%)** & **3 RS/Multi/(\%)** & **6 RS/Multi/(\%)** & **9 RS/Multi/(\%)** & **12 RS/Multi/(\%)** & **100 RS/GPS/(\%)** \\ \hline \(UC16(1)\) & 4 & 20 & 54 & 75 & 87 & 90 & 94 \\ \(UC16(m)\) & 4 & 20 & 54 & 51 & 47 & 45 & 54 \\ \(UC12(1)\) & 7 & 38 & 79 & 95 & 99 & 99 & 100 \\ \(UC12(m)\) & 7 & 38 & 50 & 48 & 45 & 43 & 55 \\ \(UC124(1)\) & 14 & 61 & 94 & 99 & 100 & 100 & 100 \\ \(UC124(m)\) & 14 & 28 & 44 & 40 & 46 & 44 & 57 \\ \hline \hline \end{tabular}
\end{table}
Table 3: GCF and uniformity of radio occultation events.  
As shown in Figure 7, the polar-orbit RS/GEO RO events were mainly distributed in the polar regions, increasing the number of RO event revisits in these regions; however, there were almost no RO events located in the middle- and low-latitudes zones (i.e., the area between \(\pm 60^{\circ}\)). Therefore, the polar-orbit RS/GEO RO events were beneficial to the climate change monitoring in the polar regions. The polar-orbit RS/IGSO RO events were unevenly distributed in both latitudinal and longitudinal directions, with more events occurring at middle- and high-latitude regions. The global distribution of polar-orbit RS/MEO RO events was more uniform. The polar-orbit RS/LEO RO events could cover the Earth globally, and there were more RO events in high-latitude areas than in middle- and low-latitude areas. Therefore, a constellation with polar-orbit RS satellites could form a global RO event coverage and was more beneficial for middle- and high-latitude region atmospheric monitoring.  
In this paper, we analyzed the detection capabilities of all abovementioned GNSS RO systems for the occultation event number and their spatiotemporal distribution level, which represent the maximum detection capability of those RO systems. Practically, not all the GNSS RO events' atmospheric products can be retrieved from the raw RO observation data successfully with high precision; thus, the low quality and even failure of the RO products retrieval can somehow reduce the RO systems detection capabilities for GCM and GNWP applications. The GNSS RO retrieval and quality control were not the focus of this study, and it may be investigated in our future work.  
Figure 7: The global coverage of GNSS RO events for MET/GEO/IGSO/MEO/LEO and 12 RS/GEO/IGSO/MEO/LEO RO systems, illustrating the number of RO event revisits using a \(4^{\circ}\times 4^{\circ}\) latitude and longitude grid coordinate.