An Improved Method for Ionospheric TEC Estimation Using the Spaceborne GNSS-R Observations

Ionospheric monitoring and modeling have been difficult for a long time over the data-void or data-sparse oceans. As an emerging remote sensing technique, global navigation satellite system reflectometry (GNSS-R) has presented great potential in ionosphere sounding over these regions. However, the conventional approach to generate delay-Doppler-map (DDM) involved in the GNSS-R total electron content (TEC) retrieval process ignores the effects of tropospheric delay and the topside ionospheric delay above the GNSS-R receiver. This would cause certain errors in retrieved TEC results. In this contribution, an improved method to estimate ionospheric TEC over oceans using the GNSS-R technique is proposed, which considers the influence of the tropospheric delays and the topside ionospheric delays above the spaceborne GNSS-R receiver. To achieve the best matching between measured and simulated DDM, this article employs the least squares (LS) fitting method for elastic matching. Additionally, the assessment was performed in May 2015 and 2017 at different solar activities, by comparing the ionospheric TEC derived from our proposed method with that from two ionospheric empirical models (NeQuick2 and IRI-2016), the global ionospheric maps (GIMs) final products, as well as the measured GNSS TEC. The results show good consistency between these models. Meanwhile, when considering the topside ionospheric delay and tropospheric delay in DDM, the TEC accuracy has significantly improved. In particular, the improvements of root mean square (rms) error can reach 5.3% and 23.5% during high and low solar activities, respectively, versus GNSS TEC. It is expected to benefit the application of GNSS-R in ionospheric modeling and application over the ocean area.