Observing Green Tide in L- and C-band SAR Imagery

Green tides, characterized by the excessive prolif-eration of green macroalgae, have become a significant coastal issue in the Yellow Sea, adversely affecting both the marine environment and coastal economies. Monitoring these events primarily relies on optical satellite imagery; however, its effectiveness is often compromised by cloud cover. To address this limitation, Synthetic Aperture Radar (SAR) imagery serves as a valuable supplement due to its all-weather capabilities. Operational SAR satellites commonly operate in L-, C-, and X-bands, with C-band being the most extensively utilized for green tide detection. This study aims to enhance the multi-system approach for increased revisit times by comparing the imaging characteristics of green tide mats using near-synchronous L-band Advanced Land Observing Satellite-2 (ALOS-2) and C-band Sentinel-1 SAR imagery. The analysis reveals that, in C-band imagery, floating green tide mats consistently appear brighter than the surrounding algae-free sea surfaces. Conversely, in L-band imagery, these mats may appear either brighter or darker relative to the sea surface. Additionally, the normalized radar cross section (NRCS) contrast between green tide mats and seawater is stronger in co-polarized channel compared to cross-polarized one.