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Reduction of Magnetic Field Signature of Military Vehicles and Ships

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.

 

Corrosion damage is a major factor in ship maintenance and availability. Paints and shipboard impressed current cathodic protection (ICCP) systems are important established tools in the reduction of corrosion damage to ships.

ICCP systems are designed to take advantage of the electrochemical corrosion phenomenon. By applying an external source of current to anodes on the ship hull current passes through the surrounding sea water to the parts of the hull to be protected. The ICCP system is designed to ensure that the current flowing from the anodes is sufficient to maintain the potential on the hull below a certain value of potential which inhibits the electrochemical
reaction which causes corrosion.
Electromagnetic signatures are playing an important role in the detection of naval vessels and in the fusing of intelligent mines. The static electric signature is the electric field associated with the DC corrosion or cathodic protection current which flows through the sea water around a vessel. This is sometimes referred to as the Underwater Electrical Potential or UEP. The corrosion-related magnetic (CRM) field is the coupled magnetic field caused by the corrosion-related electric currents flowing in the seawater between the anodes and the ship hull.

It is important to note that UEP and CRM signatures exist even in the absence of a cathodic protection system. They are caused by the galvanic potential differences between the metallic structures in contact with the sea water. For example, the relative position in the electrochemical table of steel and bronze provides a sufficient driving potential to create an electric field.

Vehicle detection

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.

Ships and submarines can be detected by enemy defense systems due to their magnetic signatures. Therefore, numerical analysis of the magnetic signature is of great importance in the design and operation of such vessels.
Once you have built the geometry of the submarine and you have retrieved its material properties, you can work toward predicting its magnetic signature. Submarines are essentially long steel tubes. Although the ship hull is built to withstand enormous pressure, it is still relatively thin compared to the size of the vessel. Simulation of such a structure would be computationally demanding using standard finite element analysis because volume meshes of thin, long structures tend to contain either a very large number of elements or elements with a large aspect ratio. This results in a lot of degrees of freedom (high memory requirements) or a mesh that has issues resolving the geometry and the resulting fields.

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.

In order to prevent detection, engineers use degaussing techniques; i.e., methods to suppress the ship’s signature to safe levels. One such technique involves dragging cables that carry strong currents along the ship every now and then, which gives the hull a slight magnetic bias. Other techniques use coils incorporated in the ship’s design, generating a counteracting magnetic field with the appropriate strength and direction to compensate for the disturbance. In order to effectively use these methods, one must be able to analyze the signature based on a ship’s design and magnetic properties.
Australian Army itself team from the recently re-raised 10th Light Horse Regiment in Perth demonstrated a deployable deperm facility during  QTC 2022 which acts as a passive countermeasure against magnetic detection. Using the technology, metal objects can be made to appear much smaller than they actually are to a magnetic sensor and vice versa; for example making a small metallic unmanned ground vehicle (UGV) appear like a main battle tank.

References and Resources also include:

https://www.researchgate.net/publication/229047369_Predicting_corrosion_related_electrical_and_magnetic_fields_using_BEM

https://www.comsol.com/blogs/reducing-magnetic-signature-submarine/

About Rajesh Uppal

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