INTERGEO Agenda - english

Spaceborne Global Navigation Satellite System Reflectometry (GNSS-R) is an emerging bistatic remote sensing technique that enables continuous Earth observations using reflected GNSS signals. Beyond surface applications, GNSS-R observations from LEO satellites acquired at low elevation angles are highly sensitive to the distribution of ionospheric layers and provide new opportunities for investigating the ionospheric vertical structure. In this study, measurements from the PRETTY CubeSat mission operating in the L5/E5 band are used to investigate ionospheric effects at grazing geometries. PRETTY enables the analysis of GNSS-R code delay observables at specular point elevation angles down to approximately 1°, substantially increasing sensitivity to ionospheric path delays. The retrieved relative ionospheric delay reaches values of up to ~18 m, with a local maximum near ~3° elevation. Six GNSS-R events acquired over the Arctic region during July 2024 are analyzed. Relative ionospheric delays are retrieved from delay map observations after correcting for geometric and tropospheric contributions. The retrieved delays are compared with predictions from the Neustrelitz Electron Density Model (NEDM2020), NeQuick, and International Reference Ionosphere (IRI). Good agreement is observed across all models, with NEDM2020 yielding the smallest residuals, ranging from approximately 1.3 to 4.4 m. The retrieval uncertainty can reach up to ~2.6 m due to the quantization imposed by the delay map resolution, which still limits the tracking precision. Nevertheless, the results demonstrate that grazing-angle GNSS-R code delay observations can resolve first-order ionospheric delay variations and are sensitive to the vertical electron density structure of the ionosphere. To further investigate this sensitivity, the retrieved delays are fitted using a Chapman-layer representation of the F-region. The estimated F-layer peak heights range between approximately 307 and 367 km and agree within ±15 km with independent ionosonde and EISCAT radar measurements. In addition, a cancellation point in the relative ionospheric delay is identified, corresponding to the elevation angle at which the direct and reflected ionospheric contributions compensate each other, potentially providing a proxy for retrieving the F-layer peak height. These findings demonstrate the potential of grazing-angle GNSS-R observations as a novel ionospheric observable for future ionospheric monitoring and modeling.