The future EUMETSAT Polar System-Second Generation (EPS-SG) satellites, with orbit height of about 820 km, are primarily targeted at neutral atmospheric observation. In addition, EPS-SG will also provide ionospheric radio occultation (RO) data, but only below impact heights of 500 km, in order to guarantee a full data gathering of the neutral part. In the previous study, the Vary-Chap Extrapolation Technique (VCET) has been proven as a good tool to extrapolate the electron density to the blind area, from a previously determined accurate electron density profile below 500 km (Manuel Hernández-Pajares et al. 2017). In order to fully solve this problem, how to get accurate estimation below the blind area with the lack of observation above 500 km becomes an important and challenging point. To overcome the difficulty caused by the interdependence of electron density estimation and vary-Chap extrapolation, two approaches are being considered:
(1) The Simple Estimation of Electron density profiles from topside Incomplete Radio Occultation data (SEEIRO), considering an exponential approximation of the linear Vary-Chap electron density model, which does not depend on an initial guess of hmF2 and NmF2.
(2) A hybrid of Abel inversion and Vary-Chap model is proposed to retrieve the full electron density profile in a fully consistent way. In order to validate these methods, over 3700 radio occultation measurements of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) in the United States and the Formosa Satellite Mission 3 (FORMOSAT-3) in Taiwan during four representative weeks, including solar maximum, solar minimum and geomagnetic storm, are chosen. In the assessment, the Abel inversion results with full observations are considered as reference and the measurements above 500 km are discarded to simulate the EPS-SG scenario. Additionally first results on the potential usage of ionospheric climatic models such as NeQuick to solve this problem will be also discussed.