Prediction of the magnetic field beyond the array of sensors

The objective of this work was to develop a method to predict the magnetic field around a magnetic source. This knowledge is useful for developing a way to reduce the magnetic signature. A marine vessel is an example of a magnetic source for which magnetic silencing is desired. Because of the shape of the vessel, the work focuses on the prediction of the field due to an elongated magnetic source. The set of harmonic-expansion coefficients of magnetic scalar potential or so-called magnetic multipole image is used for characterization of a magnetic source. The multipole image of the source is restored by application of selective functions to the measured magnetic fields obtained from magnetic sensors located in the vicinity of the source. The magnetic field beyond the sensor array can then be predicted from the restored multipole image. Five standard sources were used to create the fields. Prolate spheroidal array, cylindrical array, circular planar array, and rectangular planar sensor arrays were tested. A virtual array created by a movement of a ship over a line of sensors was also tested with a simplified model of ship dynamics. Non-idealities in localization, orientation of the sensors, and noise were included in the tests in order to simulate the real measurement environment. Under a noisy environment, the selective function method worked well for all arrays, except for rectangular array which was sensitive to the noise. A Truncated Singular Value Decomposition method was developed to reduce this sensitivity and it yielded a more accurate field prediction under a noisy environment