Using DDM Asymmetry Metrics for Wind Direction Retrieval From GPS Ocean-Scattered Signals in Airborne Experiments

Reflectometry of signals of opportunity such as those emitted by a global navigation satellite system, known as GNSS-R, has been developed over the past years as a technique with great potential for ocean scatterometry, among other applications. Different approaches have been proposed to use GNSS-R for remote sensing of ocean surface roughness. One of them is based on deriving some descriptor/metric from the measured delay-Doppler map (DDM) and directly relating it to a geophysical property of the scattering surface. For instance, different descriptors have been proposed in the literature to measure the DDM spreading caused by the increase in ocean surface mean square slopes due to surface winds. In this paper, a new descriptor based on the DDM is proposed for wind direction retrieval. This descriptor, designated as the skewness angle φ_1,skew, measures the asymmetry in the DDM power distribution along the Doppler frequency axis, and it was modeled as a function of wind direction by means of a simulation study. Then, that model was validated using real GNSS-R data from an airborne experiment. After validation, the DDM skewness model was successfully used for wind direction retrieval, with a resulting rms error on the order of 20 °.