Grazing-angle ionospheric delays observed during the GNSS-R PRETTY mission

Abstract Spaceborne GNSS reflectometry (GNSS-R) has emerged as a valuable technique for surface and atmospheric remote sensing, particularly under grazing-angle geometries where atmospheric effects are amplified. Single-frequency missions such as ESA passive REflecTomeTry and dosimetrY (PRETTY) rely on model-based corrections to account for ionospheric and tropospheric delays. In this study, we exploit PRETTY’s capabilities to perform observations down to 1 degree (at the specular point) to investigate ionospheric effects at very low angles. We analyze six GNSS-R events recorded over the North Polar region in July 2024, focusing on the estimation of the relative ionospheric delay using code delay observations. Comparisons with model-based ionospheric delays from NEDM2020, NeQuick, and IRI show close agreement, with NEDM2020 consistently exhibiting the lowest residual differences, ranging from 1.28 to 4.39 m across all events. This supports the ability of GNSS-R code delay observables to capture the first-order ionospheric delay with reasonable fidelity. Uncertainty analysis reveals that the observed delay fitting process dominates the overall error budget, with additional contributions from tropospheric correction and surface height uncertainty. Furthermore, inversion of the fitted delays using the Chapman layer model yields plausible F-layer parameters, with peak heights ranging from 307 to 367 km and a mean delay RMSE of approximately 1.2 m (~ 4 TECU). Comparisons with ionosonde and EISCAT measurements show differences within ± 15 km. These results demonstrate the potential of single-frequency GNSS-R missions for retrieving ionospheric structure, particularly in remote regions where conventional techniques are limited or unavailable.