## 2 SLAR Image  
Oil spill detection is usually carried out by governments, principally using two types of sensors: the Synthetic Aperture Radar (SAR), which is installed on satellites, and the Side-Looking Airborne Radar (SLAR), which is installed on aircraft (Figure 1). Unlike other sensors, such as visible spectrum and infrared sensors, both the SAR and the SLAR can monitor a wide area of the sea, even if there is low visibility owing to adverse weather conditions such as clouds or rain. In addition, they can work well at night. The SAR is able to acquire image segments from between 200 and 1400 km, whereas the SLAR image represents image segments with a width and height of 45 km, depending on the number of minutes that the mission lasts. Consequently, SLAR provides images occupying fewer bytes than SAR images. Due to this fact, the use of SLAR enables the aircraft to supply real-time image transmission using a satellite communication link with an operator on earth. Moreover, the acquisition process works on-demand controlled by the flight operator, who commands the aircraft sensors according to flight planning and the type of surveillance mission. SLAR images are a visual representation of the interaction of the radar pulse emitted and the surface (sea, land, ship, oil slick, etc.) onto which it is projected. Each pixel in a SLAR image, therefore, represents the backscattering coefficient of that scanned area, and the stronger the backscatter (return signal), the higher the brightness value. When both the SAR and the SLAR acquire an intensity image, the oil slicks in it are represented as dark spots (i.e., pixels with low grayscale). This is because dark pixels indicate areas with low backscatter whereas light pixels indicate areas with high backscatter. Nevertheless, it is usual for SLAR images to display other targets, such as ocean phenomena (i.e., the sea surface with an absence of waves owing to little wind) or natural activities (i.e., algae banks and phytoplankton blooms), as dark spots. More specifically, the wind must have a velocity of more than 5 km. for sea to be sufficiently agitated to produce waves so that the sensor works well. The absence of waves does not allow the SLAR to distinguish between oil slicks and sea zones because the backscatter is low and very similar in both cases. Moreover, SLAR images always show other phenomena which do not appear in SAR images and which are represented by dark spots that cause confusion in the oil-slick detection task. The problem is that the location of SLAR antennas, under the aircraft wings (Figure 1b), causes noisy areas in image that may lead to confusion in the detection task. Aircraft maneuvers (turns, slips, etc.) can, similarly, also cause other noisy areas (Figure 2). Another important difference of SLAR versus SAR is the type of noise. SAR image usually presents granular appearance because of speckle noise, while SLAR presents artifacts. Unlike SAR, SLAR image is actually a mosaic image composed of simultaneous scans performed by each antenna. Therefore, SLAR image can include noise caused by small differences in alignment between both antennas. The position and inclination of each antenna determine the difference between the look angles. This issue influences both the shape and the size of noisy area, named as hidden area in Figure 2. Additionally, this hidden area is modified depending on both the flight altitude and ascent/descent speed. SAR image does not suffer this problem because it does not have any hidden area since it is not a mosaic image. Figure 1: (**a**) Aircraft belonging to Spanish government used to acquire SAR images; (**b**) Detail of SLAR Terma-sensor antenna, which are 12 ft in dimension. The width and intensity of noisy areas depend mainly on motion in the longitudinal axis parallel to the fuselage reference line. Due to this rotational movement, the antennas change the look angles. One antenna receives bad measures while the other receives backscatter loss because it looks towards the sky.