# Seeking Optimal GNSS Radio Occultation Constellations Using Evolutionary Algorithms
## 1 Introduction  
Satellite constellation design is an essential sector of designing spacecraft missions such as global navigation, communication, remote sensing, and Earth and space observation. These satellite missions may consist of multiple spacecrafts which operate simultaneously in order to meet the optimal performance of the system and reduce the mission costs [1]. The radio occultation (RO) technique was originally used to study the atmosphere and ionosphere of Mars [2]. The idea of using GPS RO technology to study the Earth's atmosphere was pioneered by theGPS/Meteorology (GPS/MET) experiment launched in 1995 [3; 4]. Since then, large amounts of vertical profiles of atmospheric and ionospheric parameters have been obtained by following satellite missions, including the CHAllenging Minisatellite Payload (CHAMP) [5], the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) [6], the Meteorological operational satellite-A (MetOp-A) [7], MetOp-B [8], and FengYun-3C [9]. For RO missions with single LEO satellite such as GPS/MET and CHAMP, the number of daily RO events (ROEs) and the corresponding spatial and temporal resolution of RO data were limited. COSMIC, the first operational GPS RO constellation with six satellites, has observed 1000-2500 ROEs per day at the early stages of its operation [10; 11]. The resultant data contribute significantly to weather forecasting [12], the global climate change [13], space weather monitoring [6], and ionospheric climate studies [14]. Due to the great success of the COSMIC mission, the follow-on mission COSMIC-2 is initially designed with six low-inclination-angle and six high-inclination-angle orbit satellites. Although the launch of six high-inclination-angle orbit satellites of COSMIC-2 was finally canceled due to budget constraints, it is likely that an alternative source can be obtained from high latitude constellations provided by commercial companies [15]. On the other hand, the number of navigation satellites whose signal can be used for the RO observation has increased continuously in recent years. Over the past decade, navigation satellite systems are being developed and emerging, such as the Chinese BeiDou navigation system (BDS), Galileo and Japanese Quasi-Zenith Satellite System (QZSS) and will bring unique opportunities and challenges for the Global Navigation Satellite System (GNSS) RO technique, in addition to the fully operational GPS and GLONASS. With the increased number of available navigation satellites, the use of multiple GNSS signals is an opportunity for better sounding of the atmosphere [16]. The new generation of GNSS RO missions is normally equipped with multi-GNSS RO receivers. Each of the six COSMIC-2 satellites is equipped with a GNSS RO receiver, which will receive navigation signals from both GPS and GLONASS [17]. The GNSS occultation sounder (GNOS) instrument onboard FY-3C and FY-3D satellites can receive signals from Beidou Navigation Satellite System (BDS) and GPS [18; 19]. The new instrument, named GNOS-II, onboard FY-3E satellite is capable of tracking GNSS signals from GPS, BDS, and Galileo [20]. Better temporal and spatial distribution of ROEs, important components of designing a GNSS RO system, is a motivation for extensive researches. [PERSON] et al. [21] used the variable separation approach to seek the best practical orbital parameters of the LEO satellite for a tropical GPS RO mission, and the performance of the orbit design is evaluated with the number of ROEs and their longitudinal and latitudinal distributions. [PERSON] et al. [22] analyzed the impact of LEO orbit parameters on the number and global distribution of RO events. Moreover, the impacts of the orbit inclination angles of LEO satellites on the performance of the COSMIC and COSMIC-2 was investigated by [PERSON] et al. [23]. For the first time, [PERSON] et al. [24] used a genetic algorithm (GA) for the optimal design of a walker constellation composed of 12 satellites in a multi-GNSS RO mission.