GNSS Ionospheric Occultation Inversion And Its Application

Originated in 1960's, radio occultation technique was first to be used to study atmosphere and ionosphere of the planetary in the solar system by Stanford University. Since success of the GPS/MET program in 1995, this technology has been widely used in the Earth's neutral atmosphere and ionospheric sounding. With advantages of all-weather, global coverage, high vertical resolution, near real-time, self-calibration, etc, GNSS ionospheric occultation inversion technique could make up for lack of ionospheric data in special areas (especially in Oceans and polar region), so as to provide more information and more choices in ionospheric studies. And the global distribution of ionospheric electron density data obtained by this technique if of extremely important scientific significance, which would greatly promote the development of space environment monitoring, data assimilation as well as space weather effects etc.This dissertation paper focuses on the GNSS ionospheric occultation inversion technique and its application. The main works and results can be summarized as follows:(1) Ionospheric occultation inversion methods are studied systematically. Comparison and analysis are made between different inversion results. Results show an agreement of the electron density profiles obtained by Abel inversion and the "onion peeling" inversion under the altitude of the F2 layer. However, at the upper ionosphere above 500km, deviation would be caused with different inversion methods. Because of the equivalence of Least squares inversion and "onion peeling" inversion method, electron density profiles obtained from these two methods are consistent.(2) Spatial and temporal resolution of the COSMIC occultation data is analyzed. Results show that there is a high vertical resolution of the occultation data, while the horizontal resolution and the temporal resolution are low. Therefore occultation data at the present stage can not do research independently but could only be used as a complement of other ionospheric observation techniques for applications with high temporal resolution demands (such as:Earthquake-ionosphere anomaly detection).(3) Inversion result from COSMIC ionospheric occultation data is compared with IRI model and ionosondes data. It shows that the trend and structure of electron density profiles of Ionospheric occultation are more consistent with ionosondes results, and the foF2 of Ionospheric occultation is consistent with both results of ionosonde and IRI model, with correlation coefficients as 0.9 and 0.88 respectively. However, there is deviation in the comparison of hmF2, with correlation coefficients as 0.339 and 0.42 respectively.(4) Several methods of dealing with integral singularity existed in Abel inversion are discussed comprehensively. "Singularity separation method" is used in handling of the integral singularity for the first time, which is more straightforward and easier programming while comparing with Hcpeg's method and the profiles are consistent with results published by UCAR.(5) Calibrated TEC method is used to eliminate the effect of upper TEC above the LEO satellite orbit altitude on inversion Result by using observation data from non-occultation period to correct for data from occultation period. Result shows that it is an effective method for low altitude LEOs, which could eliminate the effect of upper TEC above the LEO satellite orbit altitude.(6) Modified Chapman-layer function with varying scale height is used to reconstruct the vertical distribution of electron density above the CHAMP orbit altitude. Result shows that the reconstructed electron density profile by this method includes the main part of the ionosphere, and there is an agreement between the reconstructed result and the Abel inversion result from hmF2 to the CHAMP orbit altitude.(7) Effect of the asymmetry of electron density in the inversion of IRO is studied. Asymmetry factor is used for the correction of inversion result of the measured COSMIC occultation data. Study shows that there is significant improvement in inversion result when the asymmetry factor is applied. However, its effects need more actual measuring data to verify. (8) Statistical analysis is made of the relation between the appearance of negative results in E layer and the horizontal drift of the occultation tangent point. Reason of the appearance of negative results in E layer is studied, and variations of four exceptions in E layer with respect to latitude are generalized. On this basis, methods of nonlinear least squares and non-negative least squares are used for correction of the negative results, and the Chapman function is proposed for the first time to deal with the reconstruction of abnormal E layer inversion results. Result shows that the nonlinear least squares method and the non-negative least squares method cannot give physical interpretation, while reconstruction through Chapman function could obtain the electron density profiles in E layer, which greatly improved the availability of occultation data.(9) COSMIC ionospheric occultation data is applied to analyze ionospheric effects associated with total solar eclipse,earthquake and solar storm, as well as changes of the global ionosphere. Study shows that the changes of global ionosphere and ionospheric effects associated with various kinds of phenomena could be observed through COSMIC ionospheric occultation inversion, which provides a new observational approach for monitoring of ionospheric disturbances.