BACKGROUND 
       [0001]    1. Field of the Disclosure 
         [0002]    The disclosure generally relates to methods of, and apparatus for, assessing the magnetic signature of a vessel. Once such a signature has been assessed, it is then possible to control or modify the signature in a desired manner. 
         [0003]    2. Related Technology 
         [0004]    It is important for military vessels on voyage to have the ability to go undetected by surveillance methods which utilize the change in magnetic and electric fields produced by the presence of a vessel. Vessels which are constructed of ferromagnetic material have a magnetic field which is combination of two magnetic artifacts: the induced field and the permanent field. The induced field is proportional to the instantaneous incident field (i.e. the Earth&#39;s field). The permanent field changes in complex ways related to the magnetic history of the vessel and is dependent on the stress on the hull. 
         [0005]    The general concept of degaussing is to reduce the magnetic signature of a vessel by installing a number of direct current carrying coils on board the vessel. In principle, by applying suitable currents to these coils, a magnetic field can be generated which matches the ferromagnetic field associated with the vessel, but with opposite sign, thus reducing the signature to zero. In practice of course, this ideal zero field is not actually achievable and the degaussing problem becomes one of optimizing/minimizing the field with respect to some appropriate measure. Methods have been developed over the years to accomplish this and are generally very successful at reducing the field to the required levels for a limited period of time. 
         [0006]    The performance of a degaussing technique is typically assessed using a degaussing range, which typically consists of an array of magnetometers on the sea bed which measure the magnetic signature of a vessel as it traverses the range. The data collected from a degaussing range is also used to calculate the optimum currents for the degaussing coils. These current settings are then kept for the duration of the voyage until the vessel&#39;s next visit to a degaussing range. This is known as open loop degaussing (OLDG). 
         [0007]    It is known that, whilst at sea, and particularly in the case of submersibles whilst diving, the permanent magnetic field of the vessel can change significantly enough so that the signature is no longer within acceptable levels. An OLDG system cannot respond to this, and so the currents will be kept the same until the vessel is ranged again, which in itself is a costly and time consuming process. 
       SUMMARY 
       [0008]    In one embodiment, a magnetic signature assessment apparatus for a vehicle includes sensors for incorporation in the vehicle to measure the magnetic field normal to a closed surface at least approximately bounding the vehicle and processing means for calculating from the normal field measurements a scalar magnetic potential outside the surface. 
         [0009]    In another embodiment, a method of assessing the magnetic signature of a vehicle includes measuring the magnetic field normal to a closed surface at least approximately bounding the vehicle using sensors incorporated in the vehicle and calculating from the normal field measurements a scalar magnetic potential outside the surface. 
         [0010]    In certain embodiments, the no al field measurements are interpolated to provide finer coverage of the closed surface. 
         [0011]    In certain embodiments, the scalar magnetic potential is used to estimate the magnetic field outside the surface such that degaussing equipment can be tuned to suppress that magnetic field. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    By way of example only, certain embodiments of the invention will now be described by reference to the accompanying FIGURES, in which: 
           [0013]      FIG. 1  is a schematic diagram of a cross-section of a submarine. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  shows across section through a submarine  10 . The FIGURE shows only those parts of the submarine  10  that are necessary for describing the invention. The submarine  10  comprises a pressure hull  12  mounted within a casing  14 . The outer surface of the pressure hull  12  is studded at intervals with magnetic field sensors shown as black circles, e.g.  18 . Each of the magnetic field sensors is arranged to measure the normal component of the magnetic field that is present at the location of the sensor concerned. The submarine  10 , carries a set of degaussing coils  22  that can be energized appropriately in an attempt to minimize the magnetic signature of the submarine as would be perceived by, say, a magnetic mine in some zone, typically the sea bed, beyond the submarine  10 . The construction, arrangement and operation of degaussing coils is well known in this field and will therefore not be discussed further. 
         [0015]    The normal magnetic field measurements produced by the sensors are transmitted to a computer  20 , which controls the energization of the degaussing coils  22 . The computer  20  uses the normal magnetic field measurements to estimate the magnetic signature of the submarine  10  and then determines the output of a set of degaussing coils  22  in order to minimize the magnetic signature. Whenever desired, the computer  20  can read the outputs of the magnetic field sensors and adjust the output of the degaussing coils  22  in an effort to maintain the minimization of the submarine&#39;s magnetic signature. Therefore, the degaussing system employed by the submarine  10  is called a closed loop degaussing (CLDG) system. 
         [0016]    In general terms, the computer  20  calculates from the normal magnetic field measurements the scalar magnetic potential outside a notional closed surface Ω that envelops the pressure hull  12 . It is assumed that surface Ω encloses all or substantially all of the ferromagnetic material associated with or forming part of the submarine  10 . Vector {right arrow over (r)} shall be taken to specify a point in the region outside Ω and vector {right arrow over (r)}′ shall be taken to specify a point on surface Ω. In  FIG. 1 , the surface Ω is represented, in cross-section, by dashed line  16 . In practice, Ω is made a close fit to the pressure hull  12  with the magnetic field sensors lying substantially in the surface Ω. Then, from the scalar magnetic potential, the computer  20  calculates the magnetic field {right arrow over (B)}({right arrow over (r)}) in the space outside the surface Ω. Then, the computer  20  controls the degaussing coils  22  so that they minimize the magnetic field {right arrow over (B)}({right arrow over (r)}) that is the magnetic signature of the submarine  10 . This so-called CLDG algorithm will shortly be described in more detail. 
         [0017]    The aforementioned minimization of the magnetic field {right arrow over (B)}({right arrow over (r)}) would occur over all space outside surface Ω if the arrangement of the coils  22  were perfect. However, in practice, there is limited space to accommodate the degaussing coils so compromises must inevitably be made in terms of their number, location and orientation within the submarine  10 . This means that in practice the minimization of {right arrow over (B)}({right arrow over (r)}) will not be perfect throughout the space {right arrow over (r)}. In practical applications, then, the system will aim to minimize the magnetic signature in locations where threats are expected, e.g. at the sea bed. 
         [0018]    For the purposes of the CLDG algorithm, it is assumed that the measurements made by the magnetic field sensors on the pressure hull are measurements of the normal component of the magnetic field at locations on the surface Ω, {right arrow over (B n )}({right arrow over (r)}′). The computer  20  interpolates the {right arrow over (B n )}({right arrow over (r)}) measurements for additional points on the surface Ω using the magnetic field data provided by the sensors located on the pressure hull (it will be recalled that the algorithm assumes that the magnetic field sensors on the pressure hull are coincident with the Ω). The creation of software to increase the number of {right arrow over (B)} n ({right arrow over (r)}′) measurements by interpolation is entirely within the capabilities of the skilled person and needs no further explanation here. 
         [0019]    The magnetic field normal to the surface Ω, {right arrow over (B)} n ({right arrow over (r)}′), is related to the scalar magnetic potential φ({right arrow over (r)}) outside the surface Ω by the following equations: 
         [0000]    
       
         
           
             
               
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         [0020]    Equation           is the solution for space outside the surface Ω and excluding Ω itself and equation ℑ is the solution for points on Ω. In these equations: 
         [0000]    {right arrow over (r)}′ is a vector specifying a point on surface Ω.
 
{right arrow over (r)} is a vector specifying a point in the space beyond surface Ω.
 
dS′ indicates that the integral is to be performed over the surface Ω.
 
μ is the permeability of the medium beyond surface Ω.
 
{right arrow over (B)} n ({right arrow over (r)}′) is the normal component of the magnetic field at point {right arrow over (r)}′ on surface Ω, with positive {right arrow over (B)} n ({right arrow over (r)}′) being taken to point into Ω.
 
         [0021]    The CLDG algorithm then determines an array of φ({right arrow over (r)}) values for an array of points on and beyond Ω using equations           and ℑ respectively. The array of φ({right arrow over (r)}) values is then used to calculate {right arrow over (B)}({right arrow over (r)}) using the following equation: 
         [0000]      {right arrow over ( B )}({right arrow over ( r )})=−μ∇φ({right arrow over ( r )}).  —→ 
         [0022]    As will be apparent to the skilled person, equation           can readily be solved, given the pool of φ({right arrow over (r)}) values, using finite element analysis techniques, which techniques will be readily understood by the skilled person and hich therefore are not described further here. 
         [0023]    Once {right arrow over (B)}({right arrow over (r)}) deduced, the degaussing coils  22  can be set appropriately so as to suppress the magnetic signature. 
         [0024]    A derivation of equations ← and ↑ describing the scalar magnetic potential φ({right arrow over (r)}) is provided in an annex to the description. 
         [0025]    In practice, at least some part of the ferromagnetic material of the submarine  10  lies outside the surface Ω. The greater the amount of ferromagnetic material protruding beyond Ω, the less effective the computer&#39;s suppression of the magnetic signature will be. 
         [0026]    In the embodiment shown in  FIG. 1 , the sensors  18  are mounted on the pressure hull. In variants of this arrangement the sensors  18  can instead be located on the casing  14  or it may even be the case that some of the sensors are on the casing whilst other are mounted on the pressure hull. All that is important is that the sensors  18  provide useful magnetic field information for a surface bounding at least the majority of the submarine&#39;s ferromagnetic material. 
         [0027]    In another embodiment, all or part of one of the degaussing coils  22  lies outside the surface Ω. This means that there is a source of magnetic flux in the zone beyond surface Ω, which renders the derivation of equations ← and ↑ invalid. However, because the flux emanating from the coil in question is well known, and because the induced effect of that coil on the vessel can be measured on a degaussing range, a correction for the presence of the outlying coil can be made using the principle of superposition of magnetic fields. The contribution from the outlying coil is subtracted from the normal field measurement locations, and the appropriate keel signature due to the coil added. 
         [0028]    In other embodiments, more than one of the degaussing coils  22  lies at least partially outside the surface Ω. However a plurality of outlying degaussing coils is in principle handled in the same manner as for one such coil so therefore this embodiment will not be described in further detail. 
         [0029]    While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions or deletions may be made to the disclosed embodiments without departing from the spirit and scope of the invention.
Magnetic signature assessment apparatus for a vehicle comprising sensors for incorporation in the vehicle to measure the magnetic field normal to a closed surface at least approximately bounding the vessel and processing means for calculating from the normal field measurements a scalar magnetic potential outside the surface. Apparatus may be provided on the vehicle to generate a magnetic field to suppress the magnetic signature corresponding to the scalar potential. The invention also extends to corresponding methods and to programs for implementing those methods.