Accuracy Evaluation And Climatological Characteristics Of Fengyun-3 Series Ionospheric Radio Occultation Products

GNSS（Global Navigation Satellite System）radio occultation observation,as a new detection technique,can obtain the three-dimensional structure of the global atmosphere and ionosphere in a long-term stable and economical way,with technical characteristics such as low cost,self-calibration,global coverage,all-day,all-weather,high accuracy,and high vertical resolution.The atmospheric products（temperature,humidity,and pressure profiles）and ionospheric products（electron density profiles,ionospheric scintillation）are crucial for numerical weather prediction,space weather monitoring,atmospheric physics research and ionospheric research.China launched the C（FY3C）and D（FY3D）satellites of the Fengyun-3（FY3）series in 2013 and 2017,respectively,forming a dual satellite observation network.In addition,four satellites（E,F,G,R）are planned for the follow-up of the FY3 series,which are intended to form a continuous observation network of at least three satellites in the next 10 years.Compared with other international occultation projects that can only receive GPS（Global Positioning System）signals,FY3 series satellites carry the world’s first new payload,GNSS occultation sounder（GNOS）,which can simultaneously receive Bei Dou Navigation Satellite System（BDS）and GPS signals.With the long-term stable operation of FY3 series satellites and the successive observation network of different satellites,it is urgent to carry out more extensive and in-depth research on the accumulated massive ionospheric occultation data,so as to bring the ionospheric occultation data into greater scientific and application values.This thesis introduces the development status of ionospheric radio occultation observation and the basic ionospheric occultation inversion principle.Based on the current research and application of ionospheric occultation data observed by FY3,a series of work,such as error evaluation,scientific research and application of ionospheric products are carried out,1.The architecture of the radio occultation inversion system based on FY3C-GNOS and the inversion process of each level of occultation data products are described,the basic algorithm of ionospheric occultation inversion and the calculation method of ionospheric scintillation are also introduced.2.The data accuracy of the electron density profile（EDP）products and ionospheric scintillation products of FY3C and FY3D are systematically evaluated.The results show that,compared with ionosondes,the correlation coefficient,bias and standard deviation（std）of the F2-layer maximum electron density（Nm F2）observed by FY3C GPS and FY3C BDS are higher than 0.95 and lower than 10%and 20%,respectively,which proves the consistency of the EDP retrieval accuracy under different GNSS systems of FY3C-GNOS.The FY3D GPS Nm F2 also has the same accuracy performance with the ionosondes,which proves the reliability of FY3D EDP inversion accuracy and the continuity of EDP accuracy between FY3C and FY3D.Compared with COSMIC（Constellation Observing System for Meteorology Ionosphere and Climate）,the F-layer maximum amplitude scintillation index(S4_max)observed by FY3C GPS and FY3D BDS have the bias and std of less than 0.03 and 0.1,which both show reliable detection accuracy of ionospheric scintillation.3.The completeness of the ionospheric climatological features of FY3C is systematically studied and its consistency with the climatological features of COSMIC is analyzed.In this process,the corresponding data quality control and screening methods are assembled to select the occultation data,and a parameter gridding method applied to the ionospheric climate research is developed to visualize the global climatological characteristics of FY3C-GNOS.The results show that both FY3C and COSMIC exhibit typical climatological features in Nm F2,such as equatorial ionospheric anomaly（EIA）,semi-annual anomaly,annual anomaly,and Weddell Sea anomaly（WSA）.Both of them also show the same hemispheric asymmetry and WSA features in hm F2,which proves the feasibility and reliability of FY3C in the study of ionospheric climatology.4.The IRI（International Reference Ionosphere）-2016 model during the recent low solar activity（LSA）period is evaluated and compared using the ionospheric EDP data,so as to provide scientific reference for the improvement of the model.The results show that in statistical analysis,there is a systematic deviation of less than 10%in Nm F2between IRI-2016 URSI（International Union of Radio Science）option and ionospheric radio occultation（IRO）observation,and the std of IRI-2016 SHU（SHUbin）hm F2 is about 5km lower than BSE（Bilitza-Sheikh-Eyfrig）hm F2 and AMTB（Altadill-Magdaleno-Torta-Blanch）hm F2.In the climatological analysis,IRI-2016 URSI Nm F2is generally identical to IRO Nm F2 in terms of ionospheric climatological characteristics,the slight inconsistencies mostly appear in the oceanic region.Compared with the BSE and AMTB options,the IRI-2016 SHU option reproduces a more consistent ionospheric climatology with the IRO hm F2,which demonstrates the effectiveness of this option to fuse the ionosonde and IRO hm F2 data.