## 6 Conclusions  
From the results and discussion we can draw the following conclusions: 1 the inclination and RAAN are the main orbital parameters impacting the occultation event number and coverage, followed by the orbital height; in particular, the RAAN precession is an important orbital perturbation that cannot be neglected in the LEO-LEO occultation constellation design; 2 co-planar and counter-rotating Rx and Tx orbits were confirmed to provide an ideal two-satellite LEO-LEO constellation; 3 polar orbit and near-polar orbit (NPO) constellations make it easy to achieve global coverage of occultation events, and hence are the best choices for an unconstrained LEO-LEO occultation constellation design; 4 to co-utilize existing or planned satellites as platforms to achieve the demonstration of the LEO-LEO technique, the near-equatorial orbit (NEO) constellation can also be used; and 5 the number of LEO-LEO occultation events increases rapidly with the increase in the number of satellites and orbits; constellations with 48 and 72 satellites deliver more than 22,500 and 38,000 occultation events per day, respectively, and can achieve 100% global coverage within 18 h and 12 h, respectively. The spatio-temporal coverage of occultation events can in this way fully achieve global numerical weather prediction and climate monitoring requirements.  
In this paper, our work mainly focused on the impacts of orbital parameters of the Rx and Tx satellites on the LEO-LEO occultation number and global coverage fraction. Regarding the LEO-LEO constellation design and quantitative analysis, we took six two-orbit and three multiple-orbit constellations as representative examples to assess the performance of LEO-LEO occultation constellations with different satellite numbers and orbital planes. In our future work, the impacts of orbital parameters on the LEO-LEO occultation geometry and then on the atmospheric products, as well as the constellation optimization approaches, will be further investigated in more detail.  
Conceptualization, C.L., G.K., Y.S. and X.W.; methodology, C.L., G.K., V.P., L.T., Q.D. and W.B.; software, C.L., P.H. and G.T.; validation, C.L., C.W. and L.T.; formal analysis, C.L.; investigation, C.L., G.K. and L.T.; resources, Y.S., G.K. and X.W.; writing--original draft preparation, C.L.; writing--review and editing, G.K.; reviewing V.P.; visualization, C.L. and G.K.; supervision, G.K. and Y.S.; project administration, C.L., X.W. and Y.S. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded by the National Natural Science Foundation of China (grant nos. 41775034, 41405039, and 41606206), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (grant no. 2019151), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDA15021002), the Youth Talents Program Foundation of the Beijing Organization Department (grant no. 2018000097607G380), and the FengYun-3 (FY-3) Global Navigation Satellite System Occultation Sounder (GNOS) development and manufacture project led by NSSC, CAS. The research at the WEGC/University of Graz was partially supported by the Austrian Research Promotion Agency (FFG) project ATROMSAF2 (ASAP-15 project no. 873696) based on Austrian Space Applications Programme (ASAP) funds provided by the Ministry for Climate, Environment, Energy, Mobility, Innovation, and Technology (BMK).  
The authors thank [PERSON] (WEGC/University of Graz, Austria) for the valuable support with the development of the EGOPS software that was relevant to this study.  
The authors declare no conflict of interest.