Wide-Swath Global Ionospheric VTEC Mapping Using Interferometric GNSS-R Altimetry

Vertical Total Electron Content (VTEC) is a key parameter characterizing the vertical integral of ionospheric electron density, playing a significant role in space weather monitoring, satellite navigation, remote sensing, and communications. Currently, VTEC monitoring relies primarily on ground-based Global Navigation Satellite System (GNSS) dual-frequency stations. However, a severe shortage of observation stations over ocean areas, which constitute approximately 71% of the Earth's surface, leads to a notable lack of real-time observational data in these regions.Interferometric GNSS-R (iGNSS-R) altimetry technology, which employs direct cross-correlation between GNSS direct and reflected signals and exploits the broadband characteristics of GNSS satellite non-public ranging codes, enables sea surface altimetry with centimeter to decimeter-level precision. This technology has been successfully validated in Chinese ocean altimetry satellite missions.This study proposes a novel wide-swath ionospheric VTEC detection method based on Interferometric GNSS-R altimetry. The method first retrieves the slant total electron content (TEC) along the reflected and direct signal paths using dual-frequency observations of the propagation delay difference obtained from altimetry. Building upon this, a "one-satellite three-point" ionospheric retrieval model is developed by leveraging the bistatic geometry of Interferometric GNSS-R. This model integrates a two-dimensional Global Ionospheric Map (GIM), the three-dimensional NeQuick ionospheric model, and the measured difference in slant TEC between the reflected and direct paths from a single GNSS satellite to simultaneously retrieve the VTEC values corresponding to three key locations: the incident path ionospheric pierce point, the sea surface specular reflection point, and the reflected path ionospheric pierce point.The proposed method was validated using spaceborne Interferometric GNSS-R altimetry data from July 2023 to June 2024, with cross-comparisons made against ionospheric products from the Jason-3 satellite altimeter. The results demonstrate that, at a 900 km Low Earth Orbit altitude, this method enables simultaneous VTEC detection with a swath width exceeding 2000 km (including gaps). The coverage encompasses global oceans, islands, and parts of inland coastal regions. Compared to the Jason-3 altimeter-derived ionospheric TEC, the VTEC obtained by our method shows a bias of 0.044 TECU and a root mean square error (RMSE) of 1.963 TECU.This study confirms the feasibility of utilizing Interferometric GNSS-R altimetry for high-temporal-resolution and high-precision VTEC detection over oceanic and inland coastal regions. Compared to traditional altimeters limited to nadir-only observations, this technique offers a significant wide-swath advantage. Furthermore, relative to GNSS radio occultation observations, it provides more concentrated regional coverage, making it particularly suitable for ionospheric correction for instruments on the same platform. Future work plans to extend this method to polar and land regions using spaceborne Interferometric GNSS-R altimetry data to further evaluate its potential for VTEC detection in these areas.