Magnetic sensor development for mine countermeasures using autonomous underwater vehicles

Magnetic sensor technologies are being developed at NAVSEA, Coastal Systems Station to enhance the ability of Autonomous Underwater Vehicles (AUVs) to perform detection, localization and classification (DLC) of mines for mine countermeasures in Surf Zone/Very Shallow Water (SZ/VSW) environments. This work involves the design, development, and eventual systems integration onto AUVs of passive sensors for DLC of ferrous mines and active sensors for DLC of both non-ferrous and ferrous mines. The magnetic sensors must provide useful detection ranges and robust target localization capability for nine DLC tasks while operating aboard AUVs with challenging operational conditions of small size and power budgets, large changes in vehicle orientation in turbulent SZ/VSW areas and potentially severe electromagnetic compatibility (EMC) problems. Similar technical issues must be addressed when applying magnetic sensors to solve the mission-critical problems of AUV navigation and communications in SZ/VSW environments. This paper specifically presents results from field tests of passive, DC- magnetic sensors against small ferrous targets. The passive sensors are triaxial fluxgate magnetometer/solid state accelerometer packages mounted on a non-magnetic platform and software-configured as a first order gradiometer. Initial tests of the passive sensor indicate a target detection range of up to 4 meters for small ferrous targets. Target detection and homing algorithms based on combinations of rotationally invariant quantities constructed from magnetic field components and their gradients are being developed to allow the passive sensor to detect and provide robust target homing information on mine-like objects at distances of several meters. A brief review is presented of a short range active magnetic anomaly sensor that is being developed to induce and detect kHz frequency magnetic signatures in both ferrous and nonferrous mines. An overview is presented of EMC issues related to vehicle magnetic signature that must be addressed to ensure successful sensor integration onto AUVs.