Electricity: measuring and testing – Of geophysical surface or subsurface in situ – By magnetic means
Patent
1994-01-14
1996-10-29
O'Shea, Sandra L.
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
By magnetic means
324244, 324326, 361149, B63G 906, H01F 1300, G01V 308, G01R 3302
Patent
active
055700233
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a portable station for measuring and adjusting the magnetic signature of a naval ship making it possible to determine and/or modify the characteristics of the currents flowing in the magnetic immunization loops with which this naval ship is furnished.
DISCUSSION OF BACKGROUND
It is known that the presence of ferromagnetic materials in a naval ship renders the latter detectable by means for detecting its "magnetic signature". These means of detection can for example be incorporated within mines or be carried by aircraft. In the latter case, the risk associated with the detection of the magnetic signature of the naval ship is called the M.A.D. (Magnetic Anomaly Detection) risk.
The magnetic signature of a ship consists of its permanent magnetization and its induced magnetization. The permanent magnetization is due to the ferromagnetic materials involved in the construction of the ship and is substantially constant. On the other hand, the induced magnetization is essentially variable and, in the case of a vessel, depends on its orientation in the earth's magnetic field, its heading and its inclination due to roll and pitch.
The magnetic signature of the ship therefore makes it possible to locate and track it, and possibly to guide or fire missiles intended to destroy it. It is therefore very important to minimize or even cancel this magnetic signature in order to prevent its detection by a magnetic method.
This magnetic immunization operation is performed by creating in the volume of the ship a magnetic field which compensates that of the ship, so as to cancel its magnetic signature. To do this, the ship is equipped with a set of circuits termed immunization loops, round which an electric current circulates.
The dimensions and arrangement of the loops and the currents flowing in them are determined so as to minimize, as far as possible, the "magnetic signature" of the ship, regardless of its orientation in the earth's magnetic field, that is to say regardless of its heading and inclination due to roll and pitch. These immunization loops are distributed in three directions corresponding to the axes of roll, yaw and pitch.
The currents in the immunization loops are adjusted with the help of a measurement station. Conventionally, the measurement stations consist of two linear arrays of magnetic sensors arranged on the bottom of the sea, one installed in a North-South direction and the other in an East-West direction, for example. These arrays of magnetic sensors are connected by cables to a station installed on:land which analyses the measurements made so as to determine the characteristics of the currents to be made to flow in the various loops.
The total magnetization (permanent+induced) can be written:
I is the induced magnetization,
L, V and T are the three axes along which the immunization loops are arranged,
.alpha. represents the magnetic heading of the vessel.
These various components of the permanent magnetization and of the induced magnetization of a naval ship are produced with the help of such a measurement station by making the naval ship travel the same track twice above the arrays of magnetic sensors, following two opposite headings.
The permanent magnetization, ship-related, changes with the latter's motion, whereas the induced magnetization does not change. In order to ascertain the induced magnetization it suffices to subtract the measurement results in the two opposite directions. Then, knowing the induced magnetization and the total magnetization, the permanent magnetization is obtained directly. Thus, a North-South and then South-North pass enables IL to be determined and an East-West and then West-East pass enables IT to be calculated.
However, this method of measurement does not enable IV to be determined.
Indeed, the vertical induced magnetization cannot be determined from measurements made at a single latitude. This component is in fact dependent on the variation in the vertical component of the earth's magnetic field. Since statio
REFERENCES:
patent: 4292590 (1981-09-01), Wilson
patent: 4808923 (1989-02-01), Posseme
patent: 4812759 (1989-03-01), Duthoit
"Thomson-CSF"
O'Shea Sandra L.
Patidar Jay M.
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