Introducing the Azimuth Cutoff as an Independent Measure for Characterizing Sea-State Dynamics in SAR Altimetry

Introducing the Azimuth Cutoff as an Independent Measure for Characterizing Sea-State Dynamics in SAR Altimetry

Altiparmaki, O. (TU Delft Astrodynamics & Space Missions)
Amraoui, Samira (Collecte Localisation Satellites)
Kleinherenbrink, M. (TU Delft Mathematical Geodesy and Positioning)
Moreau, Thomas (Collecte Localisation Satellites)
Maraldi, Claire (Centre National pour l’Etudes Spatiales)
Visser, P.N.A.M. (TU Delft Space Engineering)
Naeije, M.C. (TU Delft Astrodynamics & Space Missions)

Space Engineering

2024

This study presents the first azimuth cutoff analysis in Synthetic Aperture Radar (SAR) altimetry, aiming to assess its applicability in characterizing sea-state dynamics. In SAR imaging, the azimuth cutoff serves as a proxy for the shortest waves, in terms of wavelength, that can be detected by the satellite under certain wind and wave conditions. The magnitude of this parameter is closely related to the wave orbital velocity variance, a key parameter for characterizing wind-wave systems. We exploit wave modulations exhibited in the tail of fully-focused SAR waveforms and extract the azimuth cutoff from the radar signal through the analysis of its along-track autocorrelation function. We showcase the capability of Sentinel-6A in deriving these two parameters based on analyses in the spatial and wavenumber domains, accompanied by a discussion of the limitations. We use Level-1A high-resolution Sentinel-6A data from one repeat cycle (10 days) globally to verify our findings against wave modeled data. In the spatial domain analysis, the estimation of azimuth cutoff involves fitting a Gaussian function to the along-track autocorrelation function. Results reveal pronounced dependencies on wind speed and significant wave height, factors primarily determining the magnitude of the velocity variance. In extreme sea states, the parameters are underestimated by the altimeter, while in relatively calm sea states and in the presence of swells, a substantial overestimation trend is observed. We introduce an alternative approach to extract the azimuth cutoff by identifying the fall-off wavenumber in the wavenumber domain. Results indicate effective mitigation of swell-induced errors, with some additional sensitivity to extreme sea states compared to the spatial domain approach.

SAR altimetry
azimuth cutoff
wave orbital velocity
fully-focused SAR waveforms
sea state

http://resolver.tudelft.nl/uuid:2a66587b-f9a5-412a-8173-2b760244cdf8

https://doi.org/10.3390/rs16071292

2072-4292

Remote Sensing, 16 (7)

Institutional Repository

journal article

© 2024 O. Altiparmaki, Samira Amraoui, M. Kleinherenbrink, Thomas Moreau, Claire Maraldi, P.N.A.M. Visser, M.C. Naeije