## 5 Conclusions  
The improvement of the number and global coverage fraction of RO events for a single polar-orbit RS satellite by the multi-GNSS integration, as well as the RO detection capability of the polar-orbit satellite constellations under the conditions of the existing multi-GNSS were quantitatively analyzed. The main conclusions were as follows.  
For a polar-orbit RS satellite, compared with the BDS-3, the multi-GNSS integration increased the number of RO events and their growth rate by 6.8 times while increasing the 24 h and 5 day RO events' GCF values by 3 times and 1 time, respectively.  
In the multi-GNSS scenario, the RS constellation of 12 polar-orbit satellites could reach the detection capability of the Spire/GPS RO system, and the global coverage fraction of 24 h RO events could reach 100% while the global coverage uniformity could reach 44%, which could meet the basic requirements of global numerical weather prediction in terms of the RO event number, timeliness, and global coverage fraction.  
The RO events of polar-orbit RS satellites and constellations with GEO, IGSO, and LEO navigation satellites as TS systems contributed more to the coverage in the middle- and high-latitude regions; thus, they were beneficial to the atmospheric sounding in middle-latitude and polar regions.  
According to the requirements of global numerical weather prediction and climate monitoring, a simulation study on the effect of multiple GNSS integration on the number, global coverage fraction, and uniformity of RO events was conducted. In this study, polar-orbit satellites and constellations were designed, and 100 Spire weather satellites were selected to form the receiving satellite systems, in addition to the GNSS, regional navigation satellite system, and Iridium NEXT constellations that were selected to form the multi-GNSS transmitting satellite systems. The assessment approaches and quantitative results were significant to provide references for the design of GNSS RO constellations, receivers, and satellite platforms in the multiple GNSS scenario.  
Conceptualization, C.L., Y.S. and W.B.; methodology, C.L., Q.D. and W.B.; software, C.L.; validation, C.L.; formal analysis, C.L.; investigation, C.L. and W.B.; resources, Y.S. and C.L.; writing--original draft preparation, C.L.; writing--review and editing, X.W. and P.L.; reviewing X.W. and P.L.; visualization, C.L. and W.L.; supervision, Y.S. and Q.D.; project administration, C.L., W.L., W.B., Q.D. and Y.S. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded by the Youth Innovation Promotion Association of the Chinese Academy of Sciences (grant no. 2019151), the Strategic Priority Research Program of Chinese Academy of Sciences (grant no. XDA15021002), the National Natural Science Foundation of China (grant nos. 42074042, 41775034, 41405039 and 41606206), the Youth Talents Program Foundation of Beijing Organization Department (grant no. 2018000097607G380), the Youth Cross Team Scientific Research Project of the Chinese Academy of Sciences (grant no. JCTD-2021-10) and the FengYun-3 (FY-3) Global Navigation Satellite System Occulation Sounder (GNOS) development and manufacture project led by NSSC, CAS.  
Not applicable.  
The authors thank [PERSON], [PERSON], and [PERSON] (WEGC/University of Graz, Austria) for valuable discussions and support in the occultation event simulation approaches used in this study, as well as the website [[https://www.celestrak.com/](https://www.celestrak.com/)]([https://www.celestrak.com/](https://www.celestrak.com/)) (accessed on 19 May 2021) for providing TLE track data.  
The authors declare no conflict of interest.