Mitigation of platform generated magnetic noise impressed on a magnetic sensor mounted in an autonomous underwater vehicle

TLDR: In this paper, the authors describe the procedures required to successfully place a magnetic gradient sensor array on an AUV and demonstrate the magnetic degradation caused by the presence of the powered AUV's electrical systems.

Abstract: The detection and localization of targets of interest in the very shallow water areas and especially in the surf zone are much more difficult problems than in the deep ocean. To overcome these problems, it is necessary to augment existing fleet sensor capability with additional technology. Furthermore in keeping with Navy policy, it is desirable to remove the diver from harms way. A potential solution is to utilize a small autonomous underwater vehicle (AUV) instrumented with both a buried target sonar and the real time tracking gradiometer (RTG). This paper describes the procedures required to successfully place a magnetic gradient sensor array on an AUV. The first step was to magnetically characterize the AUV. This was accomplished by passing the AUV by a stationary RTG and then by externally mounting that RTG at several locations on the AUV which was then placed in simulated at-sea motion on a nonmagnetic three-axis motion table. Analysis of the data revealed that the favored location of a magnetic sensor is near the nose of the vessel and that the initial degradation, in this location, caused by the presence of the powered AUV was 30 dB above sensor noise. The degradation was caused primarily by the vehicle's electrical systems. Initial measurements also revealed the presence of several ferromagnetic components that should be replaced with nonmagnetic equivalents when practical. A detailed plan of magnetic noise mitigation is also presented. It involves several steps for implementation, including the substitution of nonferrous components for ferrous, maximizing the separation between the sensor and magnetic field sources, minimizing current loops and using auxiliary current and magnetic field sensors capable of generating noise-canceling signals.