Anais XVI Simpósio Brasileiro de Sensoriamento Remoto - SBSR, Foz do Iguaçu, PR, Brasil, 13 a 18 de abril de 2013, INPE Maritime environmental surveillance with RADARSAT-2 Gordon Staples1 Douglas Fraga Rodrigues2 1MDA 13800 Commerce Parkway – Richmond – B.C – Canada – V6V 2J3 [email protected] 2Threetek Soluções em Geomática México Str., 41 – Downtown – 20031–144 – Rio de Janeiro – RJ, Brazil [email protected] Abstract: Two key maritime surveillance applications for RADARSAT-2 are ship detection and oil slick detection. Ship detection applications primarily focuses on illegal bilge dumping and oil slick detection focuses on the detection of natural oil seeps and the accidental discharge of oil from offshore drilling platforms. Ship detection depends on three main parameters: wind, ship properties, and radar characteristics. Ship detection improves with decreasing wind speed and increasing ship length. The use of cross-polarized data in the near- range and co-polarized data in the far-range provide good ship detection performance across a wide range of incidence angles. Oil slick detection depends primarily on wind speed. Radar polarization also plays a role in oil slick detection. Co-polarized (HH or VV) images provide similar results, but cross-polarized return is generally not used due to weak scattering from oil. Case studies that outline the application of RADARSAT-2 data for oil spill monitoring, oil seep mapping, and illegal bilge dumping are discussed. End user needs are discussed as either a requirement for data or information products. To be of the greatest value both data and information products are usually needed in a timely manner, and must be interpretable and interoperable, allowing for end users to use the information in any manner. Key Words: MDA, Threetek, RADARSAT-2, bilge dumping, AIS, seeps, ship detection, SAR 1. Introduction Since the launch of RADARSAT-1, maritime surveillance has been a key application of the data from the satellite, and this trend has continued with RADARSAT-2. Large areas of interest coupled with the limited accessibility of the maritime environment lend itself to the use of spaceborne radar. The two key maritime surveillance applications for RADARSAT-2 are ship detection and oil slick detection. Ship detection is primarily focused on illegal bilge dumping and illegal fishing, and oil slick detection is primarily focused on the detection of natural oil seeps, illegal dumping of bilge oil or accidental discharge of oil from offshore drilling platforms. With the continuing demand for oil, there is a trend to move offshore exploration into deeper water, which has resulted in increased interest in the use of RADARSAT-2 for natural oil seep detection. A secondary oil-slick application is oil spill tracking, as was demonstrated during the Macondo spill in the Gulf of Mexico. Due to RADARSAT-1’s single polarization capability, there was a dichotomy between the choice of incidence angles for ship detection versus oil spill detection. In general, a large incidence angle is preferred for ship detection and a small incidence angle for oil spill detection. In addition, HH polarization is the preferred choice for ship detection and VV polarization for oil spill detection. Given these conflicting demands, selection of the best imaging geometry was problematic for end users that were interested in both applications for a single area of interest (AOI). RADARSAT-2 introduced additional polarization modes. The dual-polarized modes, which balance spatial coverage and higher resolution for detection, 8445 Anais XVI Simpósio Brasileiro de Sensoriamento Remoto - SBSR, Foz do Iguaçu, PR, Brasil, 13 a 18 de abril de 2013, INPE have effectively eliminated the constraints for ship and oil spill detection inherent with RADARSAT-1. Ship detection and oil spill detection can be readily done by using the cross polarized mode (e.g. VH) for ship detection and co-polarized mode (e.g. VV) for oil spill detection. This paper discusses the capabilities of RADARSAT-2 in support of maritime environmental surveillance. Section 2 describes the radar capability for ship detection and oil slick detection from a perspective of radar, target, and environmental parameters. Section 3 discusses ship detection with a focus on the use of dual polarized data for illegal bilge dumping and how the Automatic Identification System (AIS) data can be integrated with the RADARSAT-2 data. Section 4 outlines the use of RADARSAT-2 data for oil slick detection, with a focus on oil spill mapping and oil seep detection. The paper concludes with a description of end-user needs. 2. Radar Capabilities for Maritime Environmental Surveillance 2.1 Ship Detection Ship detection using RADARSAT-2 depends on three main parameters: wind, ship properties, and radar characteristics (MDA 2011, 2007, 2005; Vachon and Tunaley, 2007; Pichel et al., 2004; Crips, 2004; Greidanus et al., 2004). For a given ship, detection improves as the wind speed decreases. The improved detection for lower wind speeds is due to the reduction of ocean-surface roughness and the corresponding reduction in the radar backscatter, leading to higher signal-to-clutter ratio (SCR) and better detection. As the wind speed increases, the ocean-surface backscatter increases which reduces the signal-to-clutter ration and impedes detection. The detection also depends on the wind speed relative to the radar look-direction. Detection is better when the wind is blowing perpendicular to the radar look-direction and worse when the wind is blowing toward the radar look-direction. With respect to ship properties, ship detection depends on the ship length, the ship’s primary construction material and the ship’s orientation relative to the radar look-direction. The radar return is proportional to the ship’s length, so the longer the ship, the higher the probability of detection. Detection also depends on the ship’s primary construction material. Metal produces a stronger radar return than wood or fiberglass. As a guideline, the minimum detectable ship length is roughly one-half the radar resolution for metal ships and approximately equal to the ship length for a non-metallic ships. Studies by MDA (2011) have verified the relationship between radar resolution and detection of metallic ships. The three radar characteristics that affect detection are resolution, incidence angle, and polarization. As mentioned, ship detection depends on ship length, so for a constant ship length, the detection improves as the radar resolution improves. The minimum detectable ship length decreases as the radar resolution improves. Ship detection also improves with increasing incidence angle (MDA 2005; Crisp 2004). The radar backscatter is largely invariant with incidence angle, but the backscatter from the ocean decreases significantly with incidence angle. Therefore, at a small incidence angle, the SCR is smaller than the SCR at a larger incidence angle. Detection also depends on the radar polarization. In general, the use of co-polarized HH delivers better results than VV since the ocean-surface backscatter from HH is less than VV. The use of HH provides good performance at large incidence angle and for low wind speeds, but at small incidence angles and high wind speeds, the use of HH is problematic. To mitigate these issues, cross-polarization (e.g. HV) is used. The HV ocean- surface backscatter from the ocean’s surface is largely invariant with incidence angle and is considerably weaker than the co-polarized backscatter (MDA, 2011). 8446 Anais XVI Simpósio Brasileiro de Sensoriamento Remoto - SBSR, Foz do Iguaçu, PR, Brasil, 13 a 18 de abril de 2013, INPE 2.2 Oil Slick Detection The detection of oil slicks on the ocean’s surface requires discrimination between the ocean-surface backscatter and the backscatter from the oil slick. Scattering from the ocean’s surface is predominantly due to scattering from capillary waves and short gravity waves, and one of the effects of a slick is to attenuate these waves, and reduce the backscatter. Oil spill detection using C-band radar has been demonstrated and validated using SAR data (Migliaccio et al., 2012; Brekke and Solberg, 2004, Staples and Hodgins, 1998). These studies suggest a range of incidence and wind speeds that are optimal for oil slick detection. Two of the three key-parameters that define ocean-surface backscatter are wind speed and SAR incidence angle: the backscatter increases with increasing wind speed, and decreases with increasing incidence angle. The detection of oil is enhanced at small incidence angles and for wind speeds roughly between 3 m/s and 12 m/s (Staples and Hodgins, 1998). When wind speeds are low, the ocean surface appears smooth relative to the SAR wavelength, resulting in the backscatter being similar to the backscatter from the oil. When wind speeds are high, oil-induced attenuation is dominated by wind-induced surface roughness. As a result, oil detection is optimal at moderate wind speeds, but can be problematic at low and high wind speeds. A third parameter that dictates SAR oil slick detection is the polarization state of the radar. It is generally accepted that the co-polarization state, VV, provides stronger backscatter from the ocean surface than HH polarization (MDA, 2011). Cross-polarized radar returns (HV or VH) from the ocean surface are inherently low relative to returns from terrestrial targets or targets on the ocean surface such as ships. Figure 1 shows a RADARSAT-2 Wide mode dual-polarized (VV + HV) image. The VV image clearly shows ocean-surface features such as oil slicks and ship wakes. The bright return on the right-hand side of the image is due to increased wind speed associated with the presence of an atmospheric front. In contrast to the VV image, it is easier to identify the ships in the VH image. RADARSAT-2 Data and Products © MacDonald, Dettwiler and Associates Ltd (2010). All Rights Reserved.