## 1 Introduction  
Global navigation satellite system (GNSS) radio occultation (RO) is an atmospheric remote sensing technique that can deliver global coverage, all-weather capability, long-term stability, traceability to the international standard (SI) of time, high vertical resolution, and high-accuracy atmospheric profile retrievals, i.e., bending angle, refractivity, pressure, temperature, and humidity profiles [1; 2; 3; 4; 5; 6]. Numerous studies have concluded that the GNSSRO data played important roles in both global climate monitoring (GCM) [4; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21] and global numerical weather prediction (GNWP) [16; 22; 23; 24; 25; 26; 27] applications. Initially, the GNSS RO atmospheric detection technique was demonstrated by the proof-of-concept global positioning system/METeorology (GPS/MET) mission [28]. Subsequently, many GNSS RO missions have been implemented, including the Challenging Minisatellite Payload (CHAMP) [19; 29; 30; 31], Gravity Recovery and Climate Experiment-A (GRACE-A) [29; 32], and Meteorological Operational Satellite-A/B (MetOp-A/B) [33], which are single-receiving satellite (RS) constellations, and the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) [27; 34; 35], which is the first RS constellation with six low-Earth-orbit (LEO) satellites. In the last decade, with the development of GNSS systems such as the Chinese BeiDou navigation satellite system (BDS), the European Galileo navigation satellite system (GALILEO), the Russian GLObal NAvigation Satellite System (GLONASS), and the U.S. GPS, the new GNSS RO missions have been commonly equipped with multi-GNSS RO receivers such as the FengYun-3C/-3D/-3E GNSS Occultation Sounder (FY-3C/-3D/-3E GNOS) [36; 37; 38], the COSMIC II, the Spire weather constellation [23; 39], and so on. In recent years, the emerge of regional navigation systems, e.g., Indian Regional Navigation Satellite System (IRNSS), Quasi-Zenith Satellite System (QZSS), etc., as well as the development of a spaceborne GNSS augmentation system has provided richer radio signals for RO atmospheric detection [40; 41; 42; 43; 44], which create more favorable conditions to meet the requirements of the GCM and GNWP applications with respect to the number of GNSS RO events, their global coverage, and timeliness. Studies have indicated that more GNSS RO events with better spatiotemporal coverage are more beneficial for GCM and GNWP applications [7; 45; 46]. To meet the basic requirements of GNWP, about 20,000 global uniformly distributed GNSS RO events per day have been suggested, and the increase in daily GNSS RO events has obviously improved the GNWP accuracy [9; 16; 45; 47]. This is the reason why the LEO satellite constellations and multi-GNSS RO techniques emerged and are developing so quickly. However, the orbit design and performance analyses of the GNSS RO missions have become more and more challenging [41; 42]. To obtain more GNSS RO events and a better spatiotemporal distribution for particular areas, the impacts of LEO satellite orbital and constellation parameters on the GNSS RO event number and global distribution have been investigated [48; 49; 50; 51; 52]. The results showed that LEO satellite orbit inclination is a key parameter affecting the distribution of RO events along the latitude direction, and the number of GNSS RO events decreases with the increase in the LEO satellite orbit height [43; 53]. Practical LEO satellite orbital and constellation parameters were designed and optimized for GNSS RO observations in particular regions such as a tropical zone [52], the Asia Pacific region [54], and an area in Egypt [55; 56]. Recently, with the development of global and regional navigation satellite systems, the optimal design of LEO satellite constellations for multi-GNSS RO missions has become a research hotspot in the GNSS RO atmospheric detection field [47; 48; 49; 57; 58].