Abstract
Satellite synthetic aperture radar (SAR) is a unique tool to collect measurements over sea surface but the physical interpretation of such data is not always straightforward. Among the different sea targets of interest, low-backscattering areas are often associated to marine oil pollution even if several physical phenomena may also result in low-backscattering patches at sea. In this study, the effects of low-backscattering areas of anthropogenic and natural origin on the azimuth autocorrelation function (AACF) are analyzed using VV-polarized SAR measurements. Two objective metrics are introduced to quantify the deviation of the AACF evaluated over low-backscattering areas with reference to slick-free sea surface. Experiments, undertaken on six Sentinel-1 SAR scenes, collected in Interferometric Wide Swath VV+VH imaging mode over large low-backscattering areas of different origin under low-to-moderate wind conditions (speed ≤ 7 m/s), spanning a wide range of incidence angles (from about 30° up to 46°), demonstrated that the AACF evaluated within low-backscattering sea areas remarkably deviates from the slick-free sea surface one and the largest deviation is observed over oil slicks.
Highlights
Continuous and effective monitoring of the oceans is of paramount importance to improving global marine awareness and the understanding of ocean dynamics, including man-made target surveillance, pollution monitoring, and the impact on climate change [1].Satellite Earth observation represents a valuable tool that provides extensive data collection over the oceans
Here the analysis is focused on the azimuth autocorrelation function (AACF) that is obtained as the inverse Fourier transform of the power spectral density (PSD) following a guideline similar to the one proposed in [39] to estimate λc
Considering the lowbackscattering sea areas, i.e., both oil slicks and look-alikes, the average signal-to-noise ratio (SNR) estimated over the corresponding regions of interest (ROIs) significantly varies along the synthetic aperture radar (SAR) dataset depending on the damping properties and incidence angle
Summary
Satellite Earth observation represents a valuable tool that provides extensive data collection over the oceans. An important sensor for ocean observation is the synthetic aperture radar (SAR), an active, band-limited, and coherent microwave imaging sensor that provides day and night imagery in almost all-weather conditions [2]. The exploitation of SAR imagery for marine and maritime applications is well-established [3,4,5,6] and has been further boosted when the European Space Agency (ESA) started providing Sentinel-1 (S1) SAR satellite measurements free of charge in 2014 [7,8,9,10]. In the context of marine pollution, low-backscattering areas, whose normalized radar cross section (NRCS) is lower than the surrounding sea one, represent a broad class of targets of interest since they can be often associated to natural or anthropogenic oil slicks. Even though low-backscattering sea areas may have different origins, e.g., organic films, low-wind areas, etc., most analyses on low-backscattering areas have been focused on oil spill detection and classification [11,12,13,14,15,16,17], with important advancements that have
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