Characterizing ionospheric effect on GNSS radio occultation atmospheric bending angle

Global Navigation Satellite System (GNSS) atmospheric radio occultation (RO) has been an active remote sensing method to explore the Earth's atmosphere recently. The RO products especially atmospheric bending angles have been widely used for climate study. However, the residual ionospheric error (RIE) still exists to a certain extent after the widely used linear combination in bending angle calculation. In this study, we analyzed both CHAMP and COSMIC data and did full 3-D ray tracing to characterize RIE. It is found that the order of RIE is around +/- 0.1 mu rad and has an obvious dependency on a variety of factors including altitude, latitude, local time, season, and solar activity level. The amplitude and complexity of RIE generally increase with altitude. It is closely related to the strength of theElayer and the asymmetry between LEO side and GNSS side. Along magnetic latitude direction, the RIE shows a pronounced feature of two peaks and three troughs. From both the simulated and observed RIE, we can identify clearly the ionospheric seasonal variation features, such as the semiannual variation, the equinox asymmetry and the annual anomaly. A spherical asymmetry factor was also defined to quantify the correlation between ionospheric spherical asymmetry degree and RIE. The study provides a comprehensive understanding of RIE, which could contribute to the correction of RIE in the future.