As a vessel travels under or on the surface of water, it causes a detectable local disturbance in Earth’s magnetic field. This disturbance, referred to as the magnetic signature, makes the vessel vulnerable to enemy defense systems. In submarine warfare, adversaries use increasingly sophisticated magnetic sensor and signal processing equipment to detect and track these signatures.
The magnetic signature contains two main magnetization components: permanent and induced. The permanent type of magnetization depends on ship size, its “magnetic history” (production and storage of ship’s sheet metal, and ship’s building technology), ferromagnetic properties of sheets, or even mechanical strikes and temperature stresses during exploitation.
The induced magnetization is related to the reaction of ferromagnetic material placed in the Earth’s magnetic field and is dependent on the current geographical position and orientation (course) of the ship in the Earth’s magnetic field.
Marine vessels, such as submarines and ships, consist of ferromagnetic material which can disturb the Earth’s magnetic field. The significance of a marine vessel’s magnetic field has been proverbial since Germany blockaded Britain with magnetic naval mines, causing great losses to the British Navy in World War II.
Vehicle detectors based on spintronic sensors have been widely used for vehicle detection applications. The Earth provides a uniform and stable magnetic field over the planet surface. A ferrous or metal object, like a vehicle, can be considered as a model consisting of a number of bipolar magnets with N-S polarization direction. A vehicle can cause a local disturbance in the Earth’s field when it moves or stands still. The disturbance depends on the ferrous material, the size and the moving orientation of this object.
By analyzing the disturbance signal, the presence, moving speed, direction and classification of this vehicle can be determined. To obtain a smoother magnetic field signal, a digital filtering algorithm is usually used to eliminate noise, which may utilize fast Fourier transform, median filter, and Gaussian filter, and so on. Each category of vehicle signal has its own characteristics due to the different structures and sizes.
A MAD sensor responds to temporally-and spatially-varying earth’s magnetic field, magnetic noise generated by the aerial platform, ocean-induced magnetic noise, geomagnetic noise, magnetic noise caused by local geological features, and magnetic signals generated by ferromagnetic objects of interest. The desired magnetic signature is produced by the target object’s ferromagnetism, motion-induced eddy currents, and corrosion-related sources.
Magnetic Signature caluclation
Magnetic signature technology has a practical significance for naval transport, as it allows object detection and classification, as well as performing a safety analysis by predicting ships’ own magnetization and analyzing their own magnetic risk of being detected by naval mines.
While the induces component can be easily calculated, the permanent component has to be estimated based on measurements -its deterministic calculation is not possible without knowing the magnetic history of the object. In order to control the signatures and to preserve the integrity of a vessel it is essential to be able to predict the impact of the design and operation of the ICCP system on the electric fields. Computational models have been widely used to predict the electromagnetic fields associated with vessels due to on board systems and ferromagnetic aspects.
Reducing Magnetic Signature
Both passive and active techniques are employed for reducing the magnetic signatures produced by a vessel’s ferromagnetism, roll-induced eddy currents, corrosion-related sources, and stray fields.