**2.5 New proposed solution: Wide-band radar on drone platforms**

Based on the previous criteria, visible and ultraviolet cannot work at night. Moreover, infrared sensors cannot provide estimations of thicknesses. Therefore, we select radar sensors for the proposed monitoring system because they operate during the day and night and under all weather conditions. Whenever mounted on satellites, radar is a very useful active sensor to detect oil over a large area. However, the synthetic aperture radar technique is limited to a narrow range of wind speeds. The ocean's slight surface roughness due to very low wind speeds (below 4 m/s) leads the backscattering to be dominated by the specular component, challenging SAR systems for oil spill detection [38]. Therefore, it would be advantageous to study the radar observations from nadir-looking systems (transmit and receive at zero angles from the normal to the ocean surface) since they cover scenarios that cannot be studied by SAR systems. Being largely independent of surface roughness, the returns from nadir (or near-nadir) systems will benefit from the dominance of the specular scattering and enable detection in very low wind conditions. Therefore, drone platforms are designated suitable for the proposed solution. The drone-based radar solution allows quick

assessment of the area where the flag of possible spills is raised by witnesses. As a drawback, operating these platforms as nadir-looking systems decreases the surface of the scanned area viewed by the radar compared with that scanned by "side-looking" platforms. Also, using drones instead of satellites does not allow a synoptic view of the spill. But this can be compensated by using multiple drones at a time instead of scanning with a single drone. The parallelization in scanning can cover a large area at a critical time. Once a possible oil spill is announced, the drones can be directly used as tactical-response systems to scan the scene and report results. Afterward, whenever the spill is confirmed, the satellites can be used for strategic planning by providing a synoptic view of the spill area. Furthermore, scanning with drones provides a high spatial resolution compared with satellites and with a principal advantage of a relatively low cost compared with dedicated airborne detection systems.

Hence, we propose a new approach for oil spill monitoring with the following features, which are discussed in more detail in Section 3:

	- a. functionality #1 by detecting thick oil slicks
	- b. functionality #2 by estimating the thicknesses of detected slicks in the mm range
	- c. functionality #3 by classifying the oil type

The proposed solution is not intended to replace state-of-the-art techniques but rather complements them by providing a complementary system to satellite SAR systems. During the early stages of a possible oil spill, drone systems act as small-scale tactical-response systems improving the large-scale surveillance obtained by satellite systems. Over the spill duration and based on satellite scans, the drones can track the spill using the high spatial resolution feature provided by the mounted wide-band radars.
