## 1 Introduction  
Traditional ionospheric observation methods, such as sounding rocket, ionosonde/digisonde and ISR (Incoherent Scatter Radar), are susceptible to natural conditions, thus unable to provide long-term stable ionospheric observations at a global scale [1]. Radio occultation (RO) technology has been applied to ionosphere and atmosphere observation of the planet since the 1960s and is still an important technique of planetary exploration [2]. After being compatible with the Global Navigation Satellite System (GNSS) led by Global Positioning System (GPS), Global Navigation Satellite System (GLONASS), GALILEO and Beidou Navigation Satellite System (BDS), RO technique has become one of the most promising space exploration technologies in the 21 st century. It has many advantages over the traditional ionospheric observation method, such as low cost, self-calibration, wide coverage, all-time, all-weather, high precision, and high vertical resolution [3; 4]. The ionospheric radio occultation (IRO) products provided by GNSS RO technique have great scientific significance in ionospheric climatology research, ionospheric model and data assimilation, ionospheric scintillation research, space environmental monitoring, numerical weather prediction, etc., thus have broad application values and development prospects in fields of meteorology, communication, astronomy, and national defense [2; 5; 6]. In the 1990s, the Global Positioning System/Meteorology (GPS/MET) experiment was kicked off with launch of the MicroLab-1 low-orbit satellite equipped with a GPS receiver [7]. It is found that the mean relative error of F2-layer maximum electron density (NmF2) between GPS/MET and ionosondes is 1% and the standard deviation (std) is 20% [8]. Therefore, GPS/MET theoretically and practically confirmed the feasibility of the radio occultation technique of the earth's ionosphere [9; 10; 11]. Following the success of the GPS/MET, countries are racing to build their domestic RO projects. In 2001, Challenging Mini-Satellite Payload (CHAMP) satellite equipped with a GPS receiver was launched by Germany [12; 13; 14; 15], which produced the first unique long-term GPS RO dataset. Existing studies comparing foF2 and hmF2 between CHAMP and ionosondes have found that their foF2 discrepancy is less than 1 MHz [15], the std of foF2 and hmF2 are 18% and 13%, respectively [13]. In 2006, the United States and Taiwan jointly launched the first six satellite occultation constellation, namely, the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), to observe the earth's atmosphere/ionosphere [16; 17; 18; 19; 20]. In 2013, China launched the FengYun-3C(FY3C) satellite, which was equipped with the GNSS occultation sounder (GNOS) compatible with both GPS and BDS for the first time [21; 22; 23]. Then in 2017, FengYun-3D(FY3D) satellite was launched, with the increase in number of occultation channels and the enhancement of antenna coverage, the GNOS can perform atmospheric/ionospheric network observation with FY3C and greatly increases the observed number of occultation events [23; 24; 25]. Furthermore, with the success of the COSMIC-I project, the United States and Taiwan actively planned another six-satellite occultation project COSMIC-II and successfully launched it in June 2019, which is expected to revolutionize the atmospheric/ionospheric occultation observation [26; 27]. Among these occultation projects, COSMIC is most eye-catching. Launched in 2006, COSMIC is a constellation of six microsatellites that are situated in a low earth orbit with an altitude of about 800 km and an orbital inclination of 72\({}^{\circ}\), the longitude interval between each satellite is 30\({}^{\circ}\). It can observe 2500 occultation events per day (up to 3000 at most) [16; 17; 18; 19; 20; 28].