CoAuthors

Samira Amraoui (Collecte Localisation Satellites (CLS), France); Marcel Kleinherenbrink (Geoscience and Remote Sensing, Delft University of Technology, Netherlands); Marc Naeije (Astrodynamics and Space Missions, Delft University of Technology, Netherlands); Thomas Moreau (Collecte Localisation Satellites (CLS), France); Claire Maraldi (CNES, France)

The ability of Synthetic Aperture Radar (SAR) altimeters to image long waves, i.e., swells, has been recently demonstrated by Altiparmaki et al. (2022). The intensity modulations, observed in the trailing edge of the waveform, can be exploited to compute intensity modulation spectra, similarly to the process followed with Sentinel-1 images. One of the main constructive intensity modulation mechanisms, known as velocity bunching, stems from the vertical motion of the waves. However, random surface motions lead to a destructive behavior, reducing the azimuthal resolution. This work is based on previous studies in SAR imaging (e.g., Stopa et al., 2015) and exploits this signal behavior to estimate the so-called azimuth cutoff in SAR altimetry. In particular, the azimuth cutoff describes the minimum wavelength that can be adequately resolved by the SAR instrument and is largely determined by the vertical velocity variance of the ocean surface under moderate conditions. Considering this, we here demonstrate the ability of Sentinel-6A to provide these two measures using fully-focused SAR data and discuss potential limitations. To this end, we use L1A high resolution data from one repeat cycle (10-days duration) of Sentinel-6A globally and assess our results with respect to ECMWF Reanalysis ERA5 data.



References

Altiparmaki, O., Kleinherenbrink, M., Naeije, M., S. C., and Visser, P.
(2022). SAR altimetry data as a new source for swell monitoring. Geophysical Research Letters, 49,
e2021GL096224

Stopa, J. E., Ardhuin, F., Chapron, B., and Collard, F. (2015). Estimating wave-
orbital velocity through the azimuth cutoff from space-borne satellites. J. Geophys. Res. Oceans,
120, 7616–763