Samenvatting

The present chapter is about coastal border control for threats entering the coastal waters under the water surface. This threat consists mainly of covertly operating submarines. Acoustic sensors are commonly used for the detection of these submarines, as illustrated by the recent (renewed) deployment of networks of acoustic sensors by several Asian nations. However, acoustic conditions in shallow (littoral) water are relatively poor and this increases the importance of other sensors to detect the entering submarine. We review technology for detecting submarines by exploiting their magnetic field signature. Detection of submarines by their magnetic field signature is not new. For example during World War I the first inductive loops were installed on the seafloor in front of English harbours to detect German submarines. A line of mines laid near or in these loops could be triggered after detection of an enemy submarine. In World War II induction loops were used by the Allies to protect about 50 of their harbours worldwide, but all loops were dismantled after the war. Magnetic Anomaly Detection (MAD) from maritime patrol aircraft was introduced in World War II, it was commonly used during the Cold War, and remains an important sensor in modern anti-submarine warfare. Nowadays portable underwater magnetic barriers and swarms of long-endurance UAVs equipped with MAD are a realistic scenario. Is it not time to reconsider induction loops in the control of the underwater coast line, and in particular the entrance to harbours? What is the order of magnitude for the detection thresholds of these systems? Does an optimal geometry exist for the induction loop for harbour control? Which sources on board the submarine are related to the magnetic signature, and which level of variation of the recorded signature can be expected? These questions will be addressed in the present chapter by using open sources and some basic calculations."