Investigation of the Anisotropic Patterns in the Altimeter Backscatter Measurements Over Ocean Wave Surfaces

Xi-Yu Xu,Bingxu Geng,Ke Xu,Maofei Jiang,Lingwei Shi

doi:10.3389/feart.2021.731610

Abstract

This article attempts to analyze the influence of the anisotropic effects of the ocean wave surface on SAR altimetry backscatter coefficient (Sigma-0) measurements, which has not been intensively addressed in publications. Data of Sentinel-3A, Cryosat-2, and Jason-3 altimeters allocated by the WW3 numeric wave model were analyzed, and the patterns of Sigma-0 with respect to the wave direction were acquired under ∼2 m significant wave height. The ocean waves were classified into six categories, among which the moderate swell and short win-wave cases were analyzed intensively. Swell-dominated ocean surface shows less randomness than the wind-wave-dominated ocean surface. Clear and significant sinusoid trends are found in the Sigma-0 and SSB patterns of both operational modes (SAR mode and PLRM mode) of the Sentinel-3A altimeter for the moderate swell case, indicating the sensitivity of Sigma-0 and SSB measurements to the anisotropic features of the altimeter measurements. The anisotropic pattern in the Sentinel-3A PLRM Sigma-0 is somewhat counterintuitive, but the analysis of Jason-3 altimeter data would show similar results. Additionally, by comparing the anisotropic patterns of two orthogonally polarized SAR altimeters (Sentinel-3A and Cryosat-2), we could draw the conclusion that the Sigma-0 measurements are not sensitive to the polarization mode. As for the SSHA patterns, no clear sinusoid could be identified for the moderate swell. A possible explanation is that the SSB pattern may be overwhelmed in the complicated factors that can influence the SSHA pattern.

Highlights

Satellite altimetry is one of the most intriguing ocean remote sensing techniques, which has provided operational products of the sea level and ocean wave and wind, for decades, and has made solid contribution in the global ocean and climate study (Chelton et al, 2001; Stammer and Cazenave, 2017).In the late 1990s, the concept of the delay-Doppler altimeter ( called the “Synthesis Aperture Radar Altimeter (SARA)” in the altimetry community later) was proposed by Raney (Raney, 1998)
Ocean Anisotropic Effects on Altimetry resolution is improved by an order of magnitude, which brings advantages in sea ice and coastal and hydrologic application; the instrument height noise is reduced by roughly a half, which improves the precision of the sea level measurement over open ocean surfaces
The ocean wave characteristics collocated to the altimeter measurements were analyzed, the waves showed a distinct anisotropic feature, and the east-west propagating wave dominated in the histogram (Figure 2A)

Summary

Introduction

Satellite altimetry is one of the most intriguing ocean remote sensing techniques, which has provided operational products of the sea level and ocean wave and wind, for decades, and has made solid contribution in the global ocean and climate study (Chelton et al, 2001; Stammer and Cazenave, 2017). In the late 1990s, the concept of the delay-Doppler altimeter ( called the “Synthesis Aperture Radar Altimeter (SARA)” in the altimetry community later) was proposed by Raney (Raney, 1998). SARA outperforms Conventional Radar Altimetry (CRA) in many aspects: the along-track. Ocean Anisotropic Effects on Altimetry resolution is improved by an order of magnitude, which brings advantages in sea ice and coastal and hydrologic application; the instrument height noise is reduced by roughly a half, which improves the precision of the sea level measurement over open ocean surfaces. SSB can be derived from the three-dimension backscattering field of the radar (Elfouhaily et al, 2000), SSB

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Conclusion