1. Field of the invention
The present invention concerns processes that allow measuring the induced magnetization in a nautical vessel in order to be able to thereafter carry out a process for the magnetic immunization of said vessels. It also concerns devices that allow this process to be carried out.
2. Summary of the prior art
At the present time, most nautical vessels, boats and submarines are built from steel or iron and thus have a high magnetization. This magnetization comprises, on the one hand, a permanent proportion that is specific to vessels and, on the other hand, a variable proportion induced by the magnetic field of the earth and which thus depends upon the position of the vessel with respect to the magnetic-field of the earth.
This magnetization of the vessel is superimposed upon a magnetic field of the earth and thus generates a disturbance called the "magnetic signature" of the vessel. This disturbance allows for the location of the vessel to be and determined thereby making it possible to guide or fire missiles intended to destroy it. It is therefore extremely important to minimize, or even suppress, this disturbance in order to avoid the vessel detected by a magnetic method.
This operation, called "magnetic immunization", is performed in a manner known per se by creating in the volume of the vessel a magnetic field that is opposed to that of the vessel in order to cancel the magnetic signature.
To do this,the vessel is fitted with an assembly of circuits known as "immunization loops" through which pass electric currents. The sizes and the disposition of the loops, and the intensity of the currents that pass through them, are determined so as to fully minimize the magnetic signature of the vessel, whatever its orientation in the magnetic field of the earth, i.e. whatever its head and its list due to rolling and to pitching. These immunization loops are distributed throughout three assemblies corresponding to the axes of roll, pitch and rocking, and conventionally called "L, M, A".
To determine the intensities of the currents that will pass through the immunization loops, it is necessary to measure the magnetic signature of the vessel and for this purpose a magnetic measuring station is used.
A magnetic measuring station advantageously comprises two linear networks of magnetic sensors, called bases, placed on the sea-bed and each aligned according to a cardinal direction, N/S for one, and E/W for the other. These sensors are connected to a land-based processing unit which receives the signals produced by the passage of the vessel to be immunized above the bases according to trajectories which are preferably themselves cardinal. These signals are processed in the station to determine the intensity and the polarity of the currents in the immunization loops in such a way as to obtain satisfactory immunization, whatever the orientation of the vessel in the magnetic field of the earth.
Since the permanent magnetization is steady with respect to the vessel in direction and amplitude and only evolves very slowly as a function of time, it is possible to determine a continuous component of the current in each loop, the value of which will be set and possibly reset during a subsequent immunization operation.
On the other hand, the induced magnetization is variable and it is necessary to superimpose upon this continuous component a variable component determined according to the heading and the orientation of the vessel which are known by measuring means, such as gyroscopic or optical means, for example.
To determine respectively these continuous components and variable components, it is necessary, when measuring the magnetic signature to separate the influences of the permanent magnetization from those of the induced magnetization in the total magnetization, and this separation must be made in the three directions corresponding to the three axes of the vessel.
To do this, the vessel is usually required to travel along the same route twice above the bases according to opposite headings. The vessel thus passes in a N/S direction above the E/W base and then S/N above this same base. It thereafter passes in E/W direction above the N/S base and returns in a W/E direction above said same base. The orientation of the bases and the routes is not compulsory but it facilitates the interpretions measurements and calculations.
During these opposite passages, the permanent magnetization, which is associated to the vessel, turns with the vessel while the induced magnetization does not turn. In order to determine this induced magnetization, it is thus sufficient to subtract the two measurements corresponding to two passages in opposite directions. Once the induced magnetization and the total magnetization are known, the permanent magnetization is directly obtained.
The fact of having to complete two successive passages according to opposite heads is in itself a drawback, especially since it lengthens the duration of operations. Furthermore, it is necessary to set back the measurements, for example by interpolation, so as to perform subtraction on homologue points for both passages. Indeed, an inevitable variation exists between the routes in one direction and in the other. Since, furthermore, the trajectory measurements themselves are inaccurate, it can be seen that sources of errors accumulate.
Similarly, this method is limited to longitudinal and transversal magnetization, since it is obviously not possible to set the vessel vertical in order to separate the induced magnetization from the permanent magnetization according to the vertical direction. In this latter case, the two magnetizations are separated according to empirical methods based on the experience of the operators and measurements on models that are roughly representative of the vessel.
In order to overcome these drawbacks, the invention proposes to create in the vessel a supplementary known magnetic field that allows to generate a supplementary induced magnetization, the resulting magnetic field of which can be easily separated from that due to the magnetization of the magnetic field of the earth. This supplementary magnetic field is calculated so as to be geometrically as close as possible to that of the magnetic field of the earth in the vessel. It is obtained by means of alternating currents injected into the immunization loops. The magnetic field produced by the loops along with respect to the sensors can be calculated, thereby allowing to obtain by subtraction from the alternating magnetic field measured by the sensors of the immunization base the magnetic field due to the induced field in the vessel.