## 2 Brief Review of Previous Occultation Mission and Orbit Design Studies  
The occultation concept was first proven by the Mariner V spacecraft during its Venus exploration in 1967 [24]. The first Earth observation occultation experiment using geostationary and LEO satellites was performed in 1974 [25]; however, at that time, the occultation technique developed very slowly because this type of constellation provided limited occultation data and many satellites were required for global coverage of the Earth, leading to unaffordable costs. Decades later, with the emergence of the Global Positioning System (GPS) constellation, GPS RO was proposed to probe the Earth's atmosphere using GPS and LEO satellites (Tx and Rx, respectively). The GPS RO was demonstrated by the proof-of-concept GPS/Meteorology (GPS/MET) mission [3; 26]. Subsequently, many GPS RO missions have been launched, including Challenging Minisatellite Payload (CHAMP) [27; 28; 29], Gravity Recovery and Climate Experiment-A (GRACE-A) [30; 31], and Meteorological Operational Satellite-A/-B (MetOp-A/-B) [32], which are single Rx constellations, and the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) [33; 34; 35], which is the first Rx constellation with six Rxs. In recent years, with the development of Global Navigation Satellite Systems (GNSSs), such as the Chinese BeiDou navigation satellite system (BDS), the European Galileo navigation satellite system (GALILEO), and the Russian GLObal NAvigation Satellite System (GLONASS), the new GNSS RO missions are commonly equipped with multi-GNSS RO receivers, such as the FengYun-3C/-3D/-3E GNSS Occultation Sounder (FY-3C/-3D/-3E GNOS) [36; 37], the COSMIC II, and the spire weather constellation [38]. In the GNSS-LEO RO constellation design and exploration capability evaluation, the orbital parameters of the GNSS satellites are fixed and known, so only the orbital parameters of the LEO satellites need to be designed and analyzed for the observation of a particular geographic area or the whole Earth by a certain number and distribution of RO events. To obtain more GNSS RO events and a better spatial-temporal distribution for particular areas, several researches have been carried out and published. The impacts of LEO orbital and constellation parameters on the GNSS RO event number and global distribution have been investigated [14; 39; 40]. Practical LEO orbital parameters and constellations were designed and optimized for GNSS RO observations in particular regions, such as a tropical zone [41], the Asia Pacific region [42], and an area in Egypt [43; 44]. Moreover, with the development of global and regional navigation satellite systems, the optimal design of LEO constellations for multi-GNSS RO missions has also been conducted [14; 39]. Given that the importance of occultation and its data was well-proven by the GNSS RO, a number of LEO-LEO missions, such as the Atmospheric Moisture and Ocean Reflection Experiment (AMORE) [4], the Water vapor and Temperature in the troposphere and Stratosphere (WATS) mission [45], the Active Temperature, Ozone, and Moisture Microwave Spectrometer (ATOMMS) [15], the Atmosphere and Climate Explorer Plus (ACE+) [5; 46; 47; 48], the Atmospheric Climate and Chemistry in the UTLS Region And climate Trends Explorer (ACCURATE) mission [20; 49; 50], and the Climate and Atmospheric Composition Exploring Satellites (CACES) mission, have been proposed [19]. The LMO concept was also translated to Mars in a mission concept called the Mars Atmospheric Climate Observatory (MACO), whose development began with seed funding from National Aeronautics and Space Administration (NASA) in 2000 [51]. An author group including the lead authors of this study has also conducted a detailed review of low Earth orbit occultation using microwave and infrared laser signals for monitoring the atmosphere and climate [18].