Evaluating the effect of the incidence angle of ALOS-2 PALSAR-2 on detecting aquaculture facilities for sustainable use of coastal space and resources

Abstract

Background: Driven by the growing world population, aquaculture plays a key role in meeting the increasing demand for food. However, aquaculture facilities in Japan are widely installed in coastal waters where natural disasters, such as typhoons and tsunamis, might wash these facilities away, thereby interfering with maritime navigation safety. Therefore, it is imperative to efficiently monitor the state of aquaculture facilities daily, particularly after a disaster in real time. To this end, several new space-borne L-band synthetic aperture radars (SARs) continue to be launched now and in the future, whose utilizations are expected to increase nationally and internationally. An example is the Japan Aerospace Exploration Agency, currently operating a SAR that can be operated day and night, and even under cloudy conditions, called ALOS-2 PALSAR-2. Methods: Based on the above facts, this study evaluated the effect of the incidence angle of ALOS-2 PALSAR-2 HH single-polarization data, using 3 m spatial resolution, on aquaculture raft detection. As the study site, we selected Ago Bay, located on the Pacific coast of Mie Prefecture in central Japan since the Nankai Trough Megathrust Earthquake and tsunamis have been predicted to occur in the future around this area. Then, we analyzed the sigma zero (backscattering coefficient) of aquaculture rafts and their surrounding sea surfaces, including the relationships between satellite orbits and aquaculture raft directions. Results: Investigations revealed that the optimum incidence angle for detecting aquaculture rafts in this study was 33.8–45.1. Differences in the sigma zero values existed between the ascending and descending orbits. However, the incidence angles differed on the orbits. Then, differences in the median sigma zero values across a range of incidence angles were evaluated under the descending orbit. In addition, when the directions of the aquaculture rafts were closely perpendicular to the satellite orbit, aquaculture rafts tended to show the highest values of sigma zero due to Bragg resonance scattering. Hence, this knowledge may allow for the rapid detection of aquaculture rafts during an emergency without going on-site.