Detection and Characterization of Low-Grazing-Angle GNSS-R Observations from PlanetiQ Satellites

In this paper, low-grazing-angle reflections observed by the PlanetiQ satellites are analyzed. The study focuses on detecting reflection events, assessing the coherency of the detected reflection, and evaluating the performance of surface information derived from PlanetiQ reflection observations. To detect reflections in PlanetiQ radio occultation (RO) measurements, we developed a radio-hologram-based detection methodology capable of identifying reflections at very low grazing angles. We analyzed the coherency of the detected reflections and surface characteristics using interferometric phase. Altimetry information was retrieved from the low-grazing-angle reflections by implementing and applying a ray-tracing approach. An examination of data availability and geographic coverage shows that PlanetiQ provides a high volume of globally distributed observations with high signal-to-noise ratio (SNR). Preliminary results on reflection detection rates suggest that PlanetiQ satellites can capture a large number of reflection events, with detection rates reaching approximately 85% across all ocean latitudes. The coherency analysis using interferometric phase shows that these values can serve as indicators of surface roughness / surface slope, particularly over Greenland and Antarctica. Elevated standard deviations of differential interferometric phase over low-latitude ocean regions may be associated with strong fluctuation in RO signals driven by tropical weather systems and ionospheric variability. Preliminary altimetry results validate the feasibility of retrievals based on raytracing. Overall, the PlanetiQ dataset demonstrates potential for advancing scientific knowledge about the Earth system through the use of low-grazing-angle reflections, supported by its high SNR and global coverage. Further refinement of the altimetry methodology is needed to achieve higher accuracy in the retrieved surface heights.