"Blind range" in naval navigation and collision avoidance systems

Navigation and Collision Avoidance naval systems rely mainly on electromagnetic radiation, reflected back from obstacles and targets. The principle is that of radar systems, that is, radiating an electromagnetic signal and extracting navigational information by means of processing the returned signal. The concept, which is simple in free space conditions, becomes complicated in the naval theatre. Due to reflections from ocean surface, there are two signals observed by the navigating sensor. These two are originated from the direct and reflected waves, the source of both being the observed target. For specular reflection from the ocean surface, these two waves will be coherent, that is, with the same frequency and a deterministic phase difference. The phase difference depends on the difference in path length between the two waves and phase change at reflection point on the water surface. Since the difference in path length depends on the obstacle range and the two waves are summed coherently as phasors, there will be peaks and dips in the received power as the range changes. This effect is known as "multipath." As a result, blind ranges may occur - where obstacles are practically invisible. The paper investigates these blind ranges for the commonly employed navigation radar systems and improvements to overcome the problem are suggested.