## 1 Introduction
After the full deployment in early 2007, the occultation geometry of COSMIC is well optimized to complete the observations around the globe in about 100 min, and the process of COSMIC receiving a signal from the receiver to generating the IRO products can be completed within 3 h [2]. It is found that the Root-Mean-Square error (RMSE) of foF2 between COSMIC and ionosondes is around 0.5 MHz [29], the bias and std of NmF2 differences between COSMIC and ionosondes are 0.72% and 8.42%, respectively, and the bias and std of hmF2 are 2.80 and 11.46 km [30], respectively, which shows the great precision of COMSIC IRO data. Thanks to the large amount of timely and high-quality IRO products provided by COSMIC, the operationalization of occultation observation has been greatly promoted [20; 31], and the IRO products of COSMIC have been widely used in ionospheric research, such as lower atmosphere-ionosphere coupling [32; 33], ionospheric climatology study [1; 34; 35; 36; 37], model assimilation [38; 39; 26; 33], scintillation research [4; 40; 41], earthquake precursor study [42; 43; 44], and so on. It is worth mentioning that the globally distributed high-precision electron density profiles (EDPs) given by COSMIC provide unprecedented details for global ionospheric climatology study. For example, the ionospheric climatological characteristics of the Weddell Sea Anomaly (WSA) was presented in the monthly ionospheric maps of COSMIC NmF2 in 2007 and its prominence around midnight hours was found in work of [PERSON] et al. [45]. Then, [PERSON] et al. [46] showed the global NmF2/hmF2 of COSMIC measurements in a low solar activity (LSA) year 2007 in four seasons and found the typical ionospheric NmF2 characteristics like equatorial anomaly, annual anomaly, semiannual anomaly, longitudinal peaks and so on. [PERSON] et al. [47] also observed the winter anomaly in the middle-latitude F region during 2007 by variations of COSMIC NmF2 with respect to latitude, longitude, and hemisphere. Besides, [PERSON] [48] studied the seasonal variation of the electron density obtained from COSMIC measurements over Indian longitudes during the deep solar minimum year 2008 and found the equinoctial asymmetry in it. It is not hard to see that the uniqueness of the IRO data during one-year LSA period attracted a lot of interest in the field of ionospheric climatological research. Launched in 2013, FY3C/GNOS is the world's first radio occultation project with BDS and GPS occultation observation capabilities, which locates at a sun synchronous orbit of about 836 km. It can probe about 700 atmospheric/ionospheric occultation events (500 GPS + 200 BDS) every day. After 6 years, FY3C/GNOS has accumulated a large amount of ionospheric occultation data, and there is a lot of potential to be discovered in it. The accuracy of IRO products of FY3C/GNOS has been verified with ionosondes as follows, the bias and std of NmF2 relative errors between FY3C/GNOS GPS RO and ionosondes are 6.71% and 18.03%, respectively, and the bias and std of hmF2 are 4.68 and 25.96 km, respectively [3]. Besides, the bias and std of differences of maximum S4 index between FY3C/GNOS and COSMIC are 0.004 and 0.063, respectively [4]. The above results illustrate the decent precision of FY3C/GNOS GPS RO products, which offer the possibilities for its application in magnetic storm research [41; 49], indicating its significant potential in studying the physical mechanism of the event-specific magnetic storm. However, its application in ionospheric climatology study is still in an initial stage and has lagged far behind COSMIC. Therefore, this work aims to explore and verify the feasibility of the FY3C/GNOS IRO products in ionospheric climatology research by observing the consistency of climatological characteristics of NmF2/hmF2 between FY3C/GNOS and COSMIC.