Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors Underwater communication cables transport large amounts of sensitive information between countries. This fact converts these cables into a critical infrastructure that must be protected. Monitoring the underwater cable environment is rare and any intervention is usually driven by cable faults. In the last few years, several reports raised issues about possible future malicious attacks on such cables. The main objective of this operational research and analysis (ORA) paper is to present an overview of different commercial and already available marine sensor technologies (acoustic, optic, magnetic and oceanographic) that could be used for autonomous monitoring of the underwater cable environment. These sensors could be mounted on different autonomous platforms, such as unmanned surface vehicles (USVs) or autonomous underwater vehicles (AUVs). This paper analyses a multithreat sabotage scenario where surveying a transatlantic cable of 13,000 km, (reaching water depths up to 4000 m) is necessary. The potential underwater threats identified for such a scenario are: divers, anchors, fishing trawls, submarines, remotely operated vehicles (ROVs) and AUVs. The paper discusses the capabilities of the identified sensors to detect such identified threats for the scenario under study. It also presents ideas on the construction of periodic and permanent surveillance networks. Research study and results are focused on providing useful information to decisionmakers in charge of designing surveillance capabilities to secure underwater communication cables.

Fusion of fluxgate sensors with navigational data for the improvement of the detection of underwater metalcontaining objects With the underwater human activity actively developing and the rise of the autonomous unmanned underwater vehicles (AUV) industry, researchers and engineers face the task of maintenance and repair of communications. For this purpose AUVs can be used, with e.g. the task of inspecting pipelines. The article describes the processing algorithm for the signal from passive ferromagnetic (fluxgate) sensors mounted on an AUV carrier used to search for metalcontaining objects at the sea bottom. A scheme for such a measurement is proposed  the installation of two sensors at the opposite ends of the carrier. This allows to measure the gradient of magnetic field between the sensors. The characteristic form of such a signal and the dependence of the signal on the motion parameters of the vehicle and external factors are determined. To eliminate false positives, filters are used based on the readings of the position, speed and orientation sensors of the navigation system. Using data on the motion parameters of the device allows to generate a reference signal, which is used to validate the detection of an object using the crosscorrelation method. The use of data on orientation angles makes it possible to compensate for the influence of the orientation of the device in the Earths magnetic field.