Multidipoles model of ship's magnetic field

Abstract

Ships made of ferromagnetic metals interfere with Earth's magnetic field in their surrounding. The disturbance of the magnetic field makes possible localization and even identification of the ship, which could determine a threat to the ship. The measurement of the magnetic field around the ship enables to determine its magnetic signature. The paper presents a multidipoles model of the ship magnetic field, which allows to determine parameters of the ship magnetic signature. The ship built of ferromagnetic elements disrupts Earth's magneticfield in its surrounding. This disor- der is defined as the ship's own magnetic field. This field contains constant and induced components(2). Thecharacteristicfeaturesofthe ship'sownmagneticfield are called magnetic signature(2,4). Magnetic signature allows to specify the distribution of the magnetic field around the ship. Magnetic signature can be determined using numerical modeling or the measurement method (3). The numerical modeling of the ship's magnetic field with commercial software has limited capacity, because the thickness of ship's metal sheets is very small in comparison with its size (3). That makes it necessary to use a dense grid of a huge number of finite elements. Additionally, the existence of the ship's permanent magnetism complicates the numerical modeling. The paper proposes a new computational method for measuring anddeterminingthe magneticsignatureofa ship, whichis basedon measurementof themagneticfieldin selected points in space around the ship, and then on creating a model of the multidipoles magnetic field of the ship. In the multidipoles model of magneticfield, a certain numberof magnetic dipoles distributed in space inside the ship are assumed. Subsequently, the spatial distribution, the values and directions of the magnetic dipole moments are found using the optimization method. The multidipoles model allows to reconstruct the ship's multidipoles magnetic field distribution in the ship's surrounding (2,4). 2. The own ship's magnetic field Measurements of the ship magnetic flux density are carried out on the measuring range. In practice, the magnetic field distribution is measured along a certain trajectory around the ship at one depth. Due to the passive protection of the ship, knowledge of the distribution of the magnetic field across the lower half-spaceinthevicinity oftheshipisrequired. Thereforeoneneedsaneffectivemethodfordetermining