A feasibility study of the radio occultation electron density retrieval aided by a global ionospheric data assimilation model

Radio Occultation (RO) has been proven a powerful technique on profiling ionospheric electron density profile. The currently used Abel inversion in RO electron density profile retrieval has degraded performance in regions with large horizontal gradients because of an assumption of spherical symmetry as indicated by many studies. Due to the success of COSMIC/FORMOSAT-3, the Joint U.S.-Taiwan Executive Steering Committee has decided to move forward with a follow-on RO mission (called COSMIC-2/FORMOSAT-7), which implies that the number of RO observations will increase rapidly in the near future. It is thus worthwhile continuing retrieval method improvements. In this study, a global ionospheric data assimilation model is constructed. A series of simulation tasks are conducted to study the feasibility of RO electron density profile retrieval aided by a global ionospheric data assimilation model based on the planned COSMIC-2/FORMOSAT-7 orbits and existing GPS, GLONASS and ground-based Global Navigation Satellite System (GNSS) stations configuration. We find that the data assimilation aided retrieval can significantly improve upon the Abel inversion and offers an optimal electron density profile retrieval approach if sufficient RO and ground GNSS observations are available. The retrieval error is affected by the data quantity, ionospheric variability, and accuracy of the data assimilation background. It is found that ∼700 globally distributed occultations per hour are sufficient to get better performance than the Abel inversion with the resolution of 2.5° in latitude, 5° in longitude, and 20 km in altitude in the data assimilation model.