Dual-Polarimetric Sentinel-1 SAR Backscattering Features From Green Macroalgae Floating in the Coastal Ocean

Green macroalgae (GMAs) are prominently visible as distinctive bright mats on synthetic aperture radar (SAR) imagery, which are frequently used to complement conventional optical observations for extracting GMA distribution at sea. However, the mechanisms ruling the interaction between the microwaves and the GMA are not well understood. This study aims to address this gap by systematically analyzing C-band backscattering from GMA-covered sea surfaces from a physical perspective using dual-polarized Sentinel-1 SAR imagery combined with a three-layer description of the GMA-covered sea surface. First, the dual-polarized normalized radar cross section (NRCS) is analyzed showing that the co-polarized backscatter is always above the system noise, while this is not the case for the cross-polarized backscatter that is frequently noisy. Then, polarimetric descriptors are adopted to shed light on the GMA backscatter mechanisms. The degree of polarization (DoP) is evaluated to demonstrate that the electromagnetic (EM) wave backscattered from the GMA is almost fully polarized, indicating a negligible depolarized component. Finally, the polarization ellipse associated with the EM wave backscattered from the GMA is analyzed, showing that the GMAs call for a linearly polarized (almost vertically oriented) backscattered EM wave. This finding suggests a scattering mechanism dominated by single-reflection and residual multiple-reflection components within the wet-GMA layer of the three-layer schematic model. The analysis is extended to several imageries collected under different green tide stages, showing that the abovementioned findings always apply.