GB2469433A - A passive method and system for detecting, locating and identifying objects having magnetic properties - Google Patents

Abstract

A passive method is provided for detecting, localizing and identifying magnetic objects moving with respect to one or more magnetometric sensors M1, M2, wherein the signals) from the sensors after filtering are subjected to analytic projection using basis signals (figure 2B) representative of the dipolar or quadripolar signals. Computation of an energy criterion with respect to total energy or the energy of the noise present makes it possible to provide a detection signal, the maximum value of the criterion giving localization of the object, the relative speed of the object and of the sensors. Use of two sensors allows complete localization of the object and computation of its magnetic moments. The basis signals are numerical values at discrete points of a coordinate system D, E defined by the sensors, the object and their relative motions (figure 3). The object may be minerals or a ferromagnetic body for example.

Description

DESCRIPTION

A PASSIVE METHOD AND SYSTEM FOR DETECTING, LOCATING AND

IDENTIFYING OBJECTS HAVING MAGNETIC PROPERTIES

The present invention relates to a passive method and system for detecting, locating and identifying objects having magnetic properties.

In particular the present invention relates to a method and system wherein the magnetic field created by such objects is measured and the signal or signals in time from one or two magnetometric sensors is processed. The present and future applications of this device are very numerous.

All objects having magnetic characteristics are

detectable; e.g. :

15. manufactured or natural objects of any size, comprising ferrous substances, * mineral deposits which are magnetic or comprise magnetic impurities, * baked clay materials 20. objects comprising DC electric circuits or subjected to DC currents.

The fields of application of this device are also very numerous and varied, e.g. * archaeological and mining research, * monitoring the passage of vehicles, ships The gates of airports, * searching for wrecks, submarines, objects lost or buried in the ground or at sea, ducts, drilling heads.

* positioning of robots or ships...

The present invention relates to the detection of objects in relative movement with respect to measurement magnetometer(s) and behaving magnetically in a substantially dipolar or quadripolar way, which is the case with sufficient accuracy in all instances where the distance between the object to be detected and the sensor(s) is greater than 2 or 3 times the largest dimension of the object.

Numerous systems are used for detecting maqnetic objects. For passive systems, i.e. not Using known

electromagnetic field emissIOn and li6f the

response by the object to this emission, the systems for measuring and analysing the static magnetic field (natural or not) generated by the sought for objects appears well placed.

Two large categories of such systems exist -those using directional sensor information, -those using magnetic field modulus sensor information.

The invention described in the American patent No. 4 309 659 of 5th January 1982 is very representative of the first category of systems.

The author uses a set of several directional magnetometers, i.e. sensitive to the components of the magnetic field (parasite fields + field to be measured) along each of the measurement axes of said magnetometers and thus obtains measured values, on the one hand, of the components in a reference frame XYZ of the magnetic field

of the object and values of the directional field

gradients along the same axes. In the case where the object is magnetically dipolar, the unknowns for detecting and identifying it are six in number, namely: The three coordinates XYZ of the object with respect to the measurement sensors and the three components Mx, My, Mz of the dipolar moment M of the object.

The author then carries out six measurements: * three Mx, My, Mz field measurements with one of the magnetometers, -ii La c'tlx JIly O/7.

three field gradient measurements J jnj

With three or four magnetometers with reIative known spacings')x, ay, . From conventional equations of the field of a dipole, the six unknowns are calculated from the six measurements made. This device is suitable for applications in which the field of the object is very much greater than the parasite fields of various origins. For some applications in which the search for objects takes place in the superimposed presence of the Earth's natural magnetic field, this device suffers from very important limitations due to the use of directional magnetometers, to noises, i.e. to the fluctuations in time and space of the Earth's io magnetic field and to inevitable relative positioning faults of the magnetometers. For example, the gradient of the Earth's natural field due to geological structures is generally very much greater than the value of the gradient of a dipolar object even at a short distance.

In this case, the three gradients measured have no relationship with those created by the sought for object and therefore lead to erroneous solutions in solving the equations. In this first category of systems it is not assumed a priori that the object to be detected is moved with a speed relative to the sensors and the effect of such a speed is secondary for detection and thus, although the calculation principle is based on the equations of the fields of the dipolar objects, there is no possibility of checking that the signals are really those from a dipolar object and not parasite signals which may be of various origins and which are therefore used erroneously.

In the second category, the detection method is based on modulus field measurement (so insensitive to its orientation) of the field of the object superimposed on

the different parasite fields.

In this case, although a set of three directional magnetometers oriented along the axes of a trihedron may theoretically make possible the calculation of the modulus of the magnetic field, resonance magnetometers are very often used since they are not directional.

In this second category of systems, the magnetometer for measuring the modulus value of the field is driven with a relative movement with respect to the object and the detection system analyses the evolution of the measured signal as a function of time. This time dependent signal is the sum of the signal due to the object and of different magnetic noises.

Very often, detection is carried out when the signal to be measured overshoots by a certain value the signals due to noise. In some cases, the duration of this overshoot is taken into account, as also a certain number of thresholds.

By way of example, the French patent No. 2 106 657 describes a detection method based on the above principle.

In this method, the measured signal is filtered in a battery of jointing bandpass filters carrying out a

summary spectral analysis.

In each of these frequency bands the envelope of the signals is detected and the peak amplitude measured. The comparison of the peak amplitudes between the envelopes of the filtered signals coming from adjacent filters makes it possible to form a detection signal through a decision logic.

Although its performances are much better than the methods of the first category, this method is only efficient when the useful target signal is much greater than the signals created by parasite magnetic noise and thus has a limited detection range.

As before, this method does not distinguish the signals of a dipolar object from the parasite signals exceeding the decision threshold.

The object of the invention is to considerably improve the performances of prior systems in the detection of magnetic objects at distances such that their magnetic aspect is dipolar.

We will see that the invention also applies to objects of more complex magnetic form and having in particular a quadripolar behavior under practical conditions of detection.

By performance is meant in the first place the detection range. This range depends on the signal (of the object) to noise (various magnetic noises) ratio. In the prior art methods, detection is only possible when the signal is much greater than the noise in a ratio at least equal to 2 or 3..

According to the present invention thereis provided: a method for detecting, locating arid identifying.

magnetic objects by'means of at least one magnetometric sensor delivering an electric signal SM whose variation in time is generated by the magnetic moment(s) of th sought for object in its relative movement with respect to the magnetometer and is characterized by the fact that -an observation plane W is defined such that this plane contains M, the position of the magnetometric sensor, N the position of the sought for object, v the relative speed vector at a time t between said sensor and said object, -in this plane W a set of current coordinate points EiDj is defined, -the signal SM received is projected mathematically, for each coordinate point EiDj, on bases -the identified energy &.) of the signal SM projectable on the bases of each of the coordinate points EiDj is calculated, -a detection criterion Cij is calculated corresponding to each point EiDj and defined as being the ratio of the energy of the point considered over the total energy cEt, -the maximum value C2 of the criterion Cij is sought, which is such that k and 1 represent the coordinates Ek and Dl of the point E�Dj corresponding to the position of the sought for object.

The exact analytic representation is advantageously a set of orthonormed and orthogonal bases.

When the sought for object has a dipolar behavior, the bases are preferably expressed by the expressions /1 7-- with = ,4p -in which the coefficients A0A1A2 are non linear functions

of the Earth's magnetic field vector Ht and of the

magnetic moment of the sought for object and in which the parameter u is defined as the ratio of the distance travelled along the relative trajectory and of the * shortest passage distance between the sought for object and the corresponding magnetometric sensor.

When the sought for object has a quadripolar behavior, the bases are preferably expressed by relationships: (1,_z1T * 7t1 r-p777-

_______

. ________ f3' 3sir (2 ir 3.i-3' T " 1( .cir * ) 7/ withr#d134,, in which the coefficients A A1A2A3A4 are non linear functions of the Ear4's magnetic field vector lit and of the magnetic moment of the sought for object and in which the parameter u is defined as the ratio of the distance travelled along the relative trajectory and of the shortest passage distance between the sought for object and the corresponding magnetometric sensor.

Advantageously, the detection criterion Cij is calculated for each point EiDj by the expressions: r 20.

where: A is the transposed matrix of the matrix Aij of the cefficients Aij obtained after mathematical projection and so resolution of the equation t5J=1pl.lA1 is the transposed matrix of the matrix of the values of the bases, -p is the number of samples used in the measurement horizon T. The first advantage of the invention is that of being 3 able to detect dipolar signals having levels very much less than the noise and so considerably increasing the

detection range with respect to prior art systems.

A second object of the invention is to make possible the detection of magnetic objects by using only the measurement of the signal(s) delivered by one or more magnetometric sensors measuring the modulus of the field of the object and of the parasite magnetic fields and thus greatly facilitates use thereof with respect to methods employing directional sensors in which the stability of the relative orientations of the sensitivity axes is particularly difficult to provide and to maintain in time.

This is obtained, among other things, by a new analytic formulation of the signals to be detected (dipolar or quadripolar) and having as advantages, on the one hand, of making detection possible and, on the other, of leading to calculation of the location of the magnetic object without ambiguity of multiple solutions and in a mathematically accurate way.

Other than a new adapted analytic formulation, the invention also provides a new method of effective resolution of the equations leading to detection and location. In fact, these equations are not able to be solved directly considering the number of unknowns and their non linearity.

In particular, in the case of the use of two measurement magnetometric sensors, one of the advantages of the invention, besides the detection and location of t1e magnetic object, is to make possible the accurate calculation of the magnetic moments of the object, thus making possible identification thereof in its magnetic aspect, which offers the advantage of only detecting objects whose magnetic aspect corresponds to the sought for objects and largely contributes to the reliability of detection by increasing the performances and minimizing false alarms.

Another advantage of the invention is to provide a method fcr carrying out these operations. in real time, i.e. without loss of time or delay.

The present invention will now be further described byway of example, with reference to the accompanying drawings in which: -Figure 1A is a representation of the magnetic field created by a dipolar object in the space which surrounds it, Figure lB shows the three typical forms of the signals observed when a magnetic field modulus sensor moves with respect to the magnetic object1 Figure 1C shows one of the preceding signals sampled, i.e. when the measurement is made at time intervals t.

Figure 2A is the representation of the base functions of the prior art fO, fi, f 2, for representing any dipolar signal, Figure 2B is the representation of the functions of bases 92/ (/3 of the invention for not only representilg any. dipolar signal but also for detecting the object and locating it, Figure 3 is the representation in accordance with the invention of the choice of the geometric parameters chosen, defined by the object to be detected N, and the relative speed vector between the magnetometric sensor and said object to be detected, Figure 4, similar to the preceding one, shows the geometric parameters chosen in the case of using two magnetometric sensors and M2, Figure 5 is the diagram explaining the method used in accordance with the invention for obtaining the parameters for detecting and locating the magnetic object from the signal SM of a single magnetometric sensor, Figure 6, similar to the preceding one, shows the d.3gram e<:laining one of:.e me:cds fr cbtaning the parameters for de cting and ccaznc the magnetic cbeco from son3lS SM1 and 5 of:ic ge:ome:r:c sensors, Figure shows hica.: the form cf the detection criteria obtained in ac:o:fance w:th the methcd at different times before r'.d after assace through the plane of the object, Figure SA is a representation of a simplified embcdiznent using only a restricted number (8 for example) of observation points in the observation plane, Figure 8B shows, at two times befra and after passage through the plane of the object, the values of the detection criteria.

Conventionally, the field Hd of a magnetic dipole of monfent placed at 0 created at point 11 where the measurement is made is given by H 3Z( 1T where (see figure 1A) u is the unitary vector carried by the vector and R is the di3tance OM.

Because of the use for measurement at M of a magnetometer measuring the mcdulus of the total magnetic field and because the field Hd in practice is always very small, with respect to Ht, the signal present at the output of the magnetometer is the image of H, projection

of Hd on the Earth's magnetic field Ht. El) V/el

-

When the magnetic object and the magnetometric sensor are driven with a relative movement, the output information from the sensor representing H is variable in time and is a function of the variations in time of the parameters and relations expressed by the equation (1) in which the coefficient k is afunction of the chosen units.

Ecplicitly, the signal H measured at a given time is a function of the following parameters

-Orientation of the Earth's field Ht (2 angles)

-Orientation of the unitary vector(2 angles) -Orientation of the magnetic moment...t (2 angles) -Magnetic moment rnodulu (1 parameter) -Distance from the object to the sensor R (1 parameter) namely a total of 8 parameters.

The a priori knowledge of these 8 parameters makes is possible to calculate the signal 4 H, but conversely measurement of H from a single equation (1) in no case makes it possible to calculate the S unknown parameters, nor even the six in the case where the orientation of the Earth's field t is assumed to be known by means for example of auxiliary directional magnetometers.

In addition, the evolution as a function of time of 4 H involves two additional parameters which are the relative trajectory of the object and of the sensor and of their relative speed.

In current practice, the relative trajectory of the sensor and of the object may be likened to a straight line (rectilinear trajectory) and travelled over at a relative constant speed v.

This assumption is verified for substantially the whole of the applications of the invention, in addition, analysis and practical experience have shown us that the errors committed using the method of the invention for current objects and vehicles sought for have a negligible impact on the performances of the system.

In this context, experience has shown that the trend of -12 -the signals measured as a function of time by the magnetometric sensor could have three typical shapes represented by the curves 1, 2 and 3 of figure lB. Still under the same assumption, J.E. ANDERSON has shown, in 1949, that all the magnetic signals, whatever the configuration of the object and of the relative trajectory, could be expressed mathematically as a function of a linear combination of three signals or base functions. Various magnetic signals are represented by changing the contribution of each of these base functions.

The mathematical representation of these base functions is commonly called ANDERSON functions.

Using the parameter without dimension u defined as the ratio of the distance E travelled along the relative trajectory and the shortest distance D of passage between the object and the sensor, the magnetometric signal SM seen by the sensor as a function of time is expressed by: I)4?4fcL)� i4/u] (2) (3)

J

The three functions (3) are the base functions of ANDERSON, the coefficients AO, Al, A2, coefficients whose value, variable depending on the case, represents the contribution to the total signal SM of each of the base sina1s/fi f 2 -13 -Figure 2A is a graphic representation of these functions. As for the equation (1), equation (2) makes it possible to represent any magnetic signal SM whatsoever as a function of time once all the above mentioned parameters have been chosen, hut does not make it possible more than the preceding one to determine said parameters, i.e. to carry out localizing detection from the knowledge only of the magnetic signal SM as a function of time.

Analysis shows that there is in fact an infinity of possible representations of the signal SM and that as many corresponding sets of three base functions can be found able to represent SM exactly.

A first characteristic of the invention is to provide a set of three base functions making it obviously possible 15. to represent the signal SM from the a priori knowledge of the different parameters relating the object, the sensor and their respective trajectories, but in particular making it possible, from the knowledge alone of one or two signals SM, to have access to all the mentioned parameters and so to detect, locate and determine the magnetic characteristics of the sought for object.

The second characteristic of the invention is a new method for effectively calculating the different above parameters and detecting the location of the object from a signal SM (or from two signals 5M1 and SM2).

The base functions of the invention have then been determined and chosen as a function of a certain number of criteria and characteristics necessary for using the new detection method. These criteria of the invention are the following: The bases must be orthogonal and formed to 1. That is to say that no base signal is projectable on the other two bases and in particular the part of the signal SM projectable on each of the bases is unique, which leads, to the determination of unique projection coefficients (Al) for a given signal SM. Which is not the case with the -14 -ANDERSON functions for which, with the bases not orthogonal, there are an infinity of solutions for projecting the signal SM on said bases, which leads to the impossibility of determining the unknown parameters which are not separate.

The base functions must be as differentiated as possible, which in practice when the useful signal SM is drowned in various noises, makes accurate calculation of the projections possible and so of the projection coefficients Ai. So, a set of so-called "decreasing fall-off" bases was sought for, i.e. with asymptàtic behavior as different as possible from each other.

A third fundamental characteristic of the invention is the choice of the geometric parameters by which the equations are expressed so as to minimize the number of unknown parameters which made it possible to find a method for solving the problem in a first step while having only a reduced number of unknown parameters to find.

The choice of these parameters is explained in figure 3 inwhich: M and N are respectively the positions of the sensor M and of the object N at a given time, v is the relative speed vector at this time between the object and the sensor, v is the object plane such that it passes through the object N and is orthogonal to the vector v, W is the observation plane such that this plane contains M, N and v.

This choice of parameters is valid in all cases of respective movements of M and N and in particular applies not only to the detection of a mobile object N by a fixed sensor M, but also of a fixed object by a mobile sensor or of a mobile object by a mobile sensor.

In the practical case where, during the time of observation of the signals, the relative trajectory of N and M is relatively rectilinear and travelled over at a -15 -substantially constant speed v, the straight line passing through M and parallel to vis the relative trajectory of M with respect to N and intersects plane V at point 0.

o is chosen as origin of the coordinates, OX is parallel to w', OY is orthogonal to OX and passes through N, D is the shortest passage distance between M and N, E is the distance from sensor M to the object plane V. at.

a given time, Ht is any a priori Earth's magnetic field vector in the reference frame OXY.

Any signal in time S from the magnetometric sensor and created by the dipolar magnetic moment of the object N in its relative movement with respect to the sensor may be written: = 4 y namely SM = A0 + A1%q + A2'70in which expression (4) in accordance with the invention and with u -

D T-f

ç LVT. ;tLtZ*.)3/L -U *f () -s-j7-. L.)S/L (J IT 4 IL 8 The graphic representation of these new base functions, both orthonormed and with different asymptotic behavior, is given in figure 2B. It can be seen that these three new functions are very well differentiated in the whole practical range of variation of the parameter used contrary to the ANDERSON functions for which -16 - /0 and are merged for u -1 1 and are merged for u > 0 V'oP/11and/2 are merged for u 1.

In the case where the minimum passage distance D between sensor M arid object N is not very great with respect to the dimensions of the cbjecz, the dipolar model can no longer be considered as accurate.y representing the magnetic behavior of the object, these new bases may be completed by two additional bases, representing the quadripolar behavior of the object.

In this case, in accordance with the invention, the equation (4) is writ-ten: (1 where,1 are given by (5) andand by: U I &3-3)

Y

ir 45-r+ L-j 7iL These two new bases correspond to the criteria chosen for orthonormalization and for different asymptotic behavior with respect to each other and with respect to the three first ones. The set t3 describes completely and exactly the ignal coming from a quadripolar object.

The detection method of the invention applies not only to objects considered as dipolar (equations 4 and 5) but also to those with quadripolar behavior (equations 6, 5 and 7).

In the equations 4 and 6 -the coefficients Ai are non linear functions of the parameter Fit and: -the new base functions S and 7 depend on parameters E, D and Ht also non linearly. Thus, having available less equations than unknowns in the dipolar case as in the quadripolar case, the problem of detecting and. locating the object cannot be solved by conventional mathematical methods because the equations are not reversible and in addition are perfectly non linear as a function of E, D, Ht anddt'.

A first essential characteristic of the invention is the choice which we make of searching for the unknown object N in the plane W called observation plane, although this plane is a priori unknown unless it contains the relative speed vector v and the sensor M. This is made possible by the choice made of parameters E and D and of the reduced parameter u = LID for expressing the base functions solely as a function of E and D in the plane A second essential characteristic of the invention is to search for the unknown object N in certain positions chosen a priori in the plane W. For that, we define a priori in plane W a certain number of points each defined by their coordinates Ei and Dj.

Then, a table of the coordinates of these points of dimension in X n is drawn up such as the following __ED.

----E, Dj. -- E1D E D E D This table generally comprises a number m X n of current coordinate points EiDj and for each of which we know the numerical values of the base functionsf1tf2 -18 -from equations 5 in a dipolar model or of the functions , ps, P2, t3.fi+ from equations 5 and 7 in a quadripolar model..

Furthermore, in accordance with the invention, the signal SM from the magnetometric sensor is sampled at equal time intervals which gives rise to the measurement (during a given time T) called measurement horizon of p successive values of SM designated S', S, SM arid corresponding to the evolution of the signal created by the unknown object and measured by the magnetometric sensor in its relative movement with respect to the sought for object.

In accordance with the invention, for each point EiDj chosen a priori in plane W, the following equation (8) is formed from the equation (4) of the expressions of the base functions cu. of the measurement of the P samples S S of the signal SM of the magnetometer: i1 2 OLZI � the solution of this equation is H [ . fx 1) in which Iris the transposed matrix of ii and the term LIJ *M3 represents the correlation vector between the -19 -real measured signal SM and the signal which would be generated by an object N placed arbitrarily at the point of coordinates EiDj of plane W; and1Bis the matrix of the noises. It will be noted that the matrix Iis identical, except for a translation, for all points EiDj of the table corresponding to a given constant distance Dj. The equation (9) represents the solution which minimizes the error between the real measured signal and the signal of a virtual object placed at Eij, i.e. the error between the real signal and the signal from the model for.n object placed at the chosen point Eij represented by the equations (4) and (5).

In the case where the sought for object has quadripolar behavior, equations similar to (8) and (9) are established for each point Eij from the equations (6) and (5) and (7), the proposed method remaining identical in other respects.

The equation (9) is called "projection operation " since it effects the mathematical projection of the real signal SM on the bases for each of the points EiDj of plane W, i.e. identifies in the real signal SM and in the noises superimposed on this signal the part identified as coming from a dipolar (or quadripolar) magnetic source placed at the point of analysis EiDj. Another essential characteristic of the invention is to use, for the localization detection of object N, calculation of the noise energies and of the identified signal and calculation of the detection criterion defined as. the ratio of the identified energy and of the total energy equal to the sum of said identified energy and of the noise energy. It is the use of this energy criterion, which is mathematically exact, jointly with the exact analytic description of the possible signals coming from each of the points EiDj which makes the method of the invention superior to prior known methods based on amplitude measurements, because, from the signal buried in the noise, the whole of the information which it contains * I is extracted in a rigorously exact way in the' strict mathematical sense of this term.

In accordance with the invention, the detection method consists in -calculating the energy of the real signal SM projectable on the bases of each of the points EiDj, is the identified energy of the signal SM, -calculating the total energy of the real signal and of the noises E on the measurement time horizon T defined above, -calculating the detection criterion Cij corresponding to each point EiDj and defined in accordance with the invention by the ratio *; (1:1 Cij= 4 -evaluating for all the points EIDj the maximum value of the criterion Cij which is the value Ckl (k and 1 representing the coordinates at Ek and D1 of the point EiDj for which Cij is maximum) -comparing the value of Ckl with a threshold -if Ckl is greater than 0< * extracting C = C/ = detection confidence level * extracting E = Ek * DD1 which are the coordinates in the above defined axes of the object detected with respect to the magnetometric sensor M. From the notations of the equations (8) and (9), the identified energy i of the signal SM corresponding to each point EiDj is obtained by applying the following equation (10) -dfT' ] jf) -2]. -where A is the transposed matrix of matrix A of coefficients A0A1A2 for dipolar (or A0A1A2A3A4 for quadripolar).

could also be calculated by a � * ,1, 10. since the bases used are orthonormed, but with a theoretical error due to the effect of filtering and to the. fact that the noise superimposed on the signal in the measurement frequency band cannot be perfectly white for some cases of application. The total energyt, the sum of the possible energy of the real signal and of the noise energy, is obtained by (12) p (s') --.) () Each value of SM being obtained by sampling the signal from the magnetometer at the rate t on the measurement horizon T and corresponding to P successive samples with P = T/4t.

As mentioned above, the information provided by thern method of the invention when a single measurement sensor is used are: the parameter C, the detection confidence level, the parameters E and D for locating the magnetic object with respect to sensor M. E is the distance ( at a given time) between the magnetometric sensor and the object plane V which is thus completely defined, D is in this plane the distance between point 0 (known as intersection of plane V and of the relative sensor-object trajectory defined by the axis of the speed vector v) and the object detected.

The position of the object thus detected is known only by a position locus which is the circle with center 0 and radius D, or at least a fraction of a circle for all the applications. In practice, the use of a single measurement sensor does not make possible, considering the physical phenomena involved, complete localization of the objects to be detected, nor determination of their magnetic characteristics.

On the other hand, this explains that considering the particular choice of the parameters used for equating the behaviors and the base functions, which choice, by eliminating certain localization parameters, makes possible the partial resolution of the localization problem by applying the proposed method.

Despite incomplete localization of the magnetic object detected, the above described method Is sufficient particularly in applications where a position locus is a priori known of the sought for object(s). This is the case for example when searàhing for objects resting on the ground or at the bottom of the sea or slightly buried. In these cases, the search takes place by moving the magnetometer on board an Immersed fish towed by a boat, a land vehicle or an aircraft. Because the trajectory of the magnetometer is substantially horizontal, the altitude of he magnetometer is known a priori with respect to he ground or with respect to the bottom of the sea and the complete position of the detected object is determined by the intersection of the plane of the ground or the bottom of the sea and of the position circle determined by the proposed method.

In most cases, such intersection is formed of two points symmetrical with respect to the trajectory of the sensor. Raising of doubt between these two possible points of the object is, if required, obtained by effecting two -23 -successive passes for substantially orthogonal orientations of the speed vector v, Thus four possible position points are obtained (2 per pass) of which two (1 per pass) are merged thus giving the position of the detected object without ambiguity.

In other cases, it is necessary: either to be able to effect detection and complete localization of the object without knowing a priori a first position locus (horizontal plane of the ground), this is the case for example in searching for objects which may be immersed at various depths, or identifying detected objects by evaluating' their magnetic characteristics. This second case is for example that of searching for specific objects in a port, whose bottom is cluttered up with different magnetic debris which is without interest and among which it is necessary to identify a particular object through identification of particular magnetic characteristics.

In all cases, the above described method makes it possible to respond to the problems raised by using a pair of magnetometers M1 and in place of the single magnetometer M used up to now.

Figure 4 is a representation of the geometric parameters chosen for the use of two magnetometric sensors M1 and M2.

M1 and M2 are for example placed at the ends of the wings in the case of searching by aircraft. In this example, the relative speed v between the sensors and the sought for object is substantially the speed of the aircraft and the direction M1, 42 is' orthogonal to the vector v.

As before we will call the object plane (a priori unknown) the plane (V) passing through the object N and orthogonal to the vector v.

Similarly to the preceding description and in

accordance with the invention, we choose two observation planes w corresponding to each of the signals from the -24 -magnetometric sensors M1 and M2.

The plane W1, the observation plane of the signals from sensor H1, is such that it contains the point M1, the vector v and the object N. Similarly, the plane W2, the plane of observation of the signals from sensor M2, Contains the point M2, the vector v and the object N. The points H'1 and M'2 are the intersection with the object plane V of the trajectory relatively to the object of the magnetometric sensors M1 and M2.

Another essential originality of the invention consists in this case in using two sensors to apply the above described method simultaneously and independently to each of the signals from sensors M1 and M2.

The method applied to the signal from sensor M1 leads to -a detection criterion C1 -an estimation of the distance S1 -an estimation of the distance D1 similarly for the signal from M2 we obtain the parameters C2, E2, D2.

Since, theoretically, the distance M1, M2 d separating the magnetometric sensors is known, the triangle M'1H'2 situated in the object plane is completely determined by the length of its three sides D1, D2 and d, so the position of the object N in this plane with respect to M'1M'2; similarly, the distance E1 = E 2 between this plane and sensors and H2 is known at each calculation cycle.

The complete position of object N is thus obtained with respect to the magnetometric sensors.

In the case already mentioned of an airborne search for objects N, sensors H1 and H2 are placed at the ends of the wings so as to have a distance d which is not inconsiderable with respect to the detection distances and D2. In this case, the trajecTry of sensors M1 and M2 is parallel to that of the aircraft and substantially horizontal, as well as the straight line joining N1 and M2 and likewise M'1 and M'2. The object plane V -orthogonal to v, the speed of the aircraft, is a vertical plane.

We can thus use a system of cylindrical coordinates for expressing the position of the object N with respect to the center N of the aircraft (M middle of N1 M2) and to the vertical Z. x = E1 = E2 = E is the horizontal distance between N and the aircraft at a given time, D = oblique distance between N and the aircraft (shortest passage distance) = angle of elevation of N with respect to the vertical.

These cylindrical coordinates are obtained on the one hand by x = = E2, and on the other by solving the triangle M'1M'2N from the calculated parameters Dl and D2 and from the a priori knowledge of the distance d separating the magnetometers M1 and M2.

That is to say:

XE

4j yf- (13) P2J (SO, Another characteristic of the invention is that it makes possible the determination of the magnetic characterIstics of the detected object when its complete position has been determined in accordance with the above described method using two magnetometric sensors N1 and -26 -M2. In accordance with the invention, the position of the object in plane W (or planes W1 and W2) is obtained for the point of. plane W for which the detection criterion Ckl such as defined above is maximum.

For this point, we have already calculated in accordance with the equation (9) the matrix = so we know the value of the three coefficients A0A1A2 for projection of the measured signal on the known bases as corrspor(ding to the best position estimate in ___ the detected object N whose dipolar magnetic moment i.4'.

The known coefficients A,,A A, now depend solely on the I, .j 1. -parameters and or parameters Mt and depending on the chosen analytic representation. In particular, the analytic expression of the coefficients A0A1A2 is linear as a function of"t' -which makes possible the calculation of', all the other parameters now being determined.

If we call x, y, z a trirectangular trihedron called measurement trihedrori defined as follows -, . x is the direction of the relative speed vector v, yls the orthogonal direction along the axis of sensors M2, z is orthogonal to the preceding ones in the forward direction.

It will be noted that in the case of airborne search with sensors M1 M2 at the ends of the wings, the trihedron xyz is then the aircraft trihedron with x longitudinal axis y transverse axis (towards the right) normal axis (towards the "bottom").

we are able to determine the dipolar magnetic moment the object N by its three components 7'x, G'y, (d"z such as they are and with all calculations made, from the equations (1) and (B) in accordance with the equation (14) -i h = f H in which expression: * D = D1 or D2 depending on whether the coefficients A0A1A2 are used corresponding to Ckl obtained for the observation plane W1 or W2 h, hi,, h are the components following the measurement trihedrori of the local Earth's field Ht which are known a priori or are measured for example using a tn axial directional fluxgate magnetometer along said axes of the measurement trihedron.

We will now describe examples of embodiments of devices for detecting, localizing and identifying magnetic objects applying the new above described methods.

Considering the volume and complexity of the computations used in the method, any application of this method requires a more or less high speed and powerful digital computer depending on the desired objectives for each application. Thus, by way of example, for searching for objects from a moving vehicle (aircraft, boat...) a suitable computer is formed by the association of a computer of the make "DIGITAL" type PDP 11-34 and an Array Processor type MAP 200 of the make C.S.P.I.

In the case of fixed sensors detecting moving objects, a processor of the make "DIGITAL" of type VAX 8530 with floating co-processor is very well adapted.

-28 -Furthermore, each type of computer uses programs and program development tools which are proper to it and therefore lead to application algorithms which are specific to it, and which will also depend on the habits* or on the more or less great skill of the programmer.

We will then not develop here a program which would only be useful on one type of computer, but we will give all the information required making it possible for a man skilled in the art to choose the computer adapted to a particular application of the detection method, as well as to organiz and construct the different computing steps and the value of the parameters specific to the method for the different applications thus making it possible for him to carry out his practical application.

We will describe the choice of the base parameters, the organization and the sequence of the successive operations for two applications, by way of example: -searching for an object from an aircraft, -searching for an object from an underwater fish towed by a boat.

In the two preceding examples, we will show how to apply the method to the use of one or two magnetometric sensors and the influence of the choice of the number of points Eij in the observation plane on the required computing capacity and its influence on the performances in accurate localization of the detected object.

The first parameter to be defined is the time observation horizon T. Because the signal SM is a time dependent signal related to the relative sensor/object movement, the energy of this signal increases with the observation time.

With the object dipolar, an observation horizon chosen so that the ratio of the observation distance along E to the minimum passage distance between the object and the target is equal to 5 makes it possible to acquire a signal duration such that the acquired energy is about 95% of the total energy of the signal, which is largely sufficient in practice.

For an airborne application in which the optimum detection distance chosen is 500 in, the observation horizon will be 500 x 5 = 2500 m along the trajectory of the aircraft, for a typical speed of lOOmis, this horizon is then T = 25 seconds.

For this same speed of 100 rn/s. if we choose a sampling step of 20 m for the signal SM corresponding to the accuracy of the localization computation. along E, the sampling rate of SM will be 1/4 t = ___ = 5 per second, namely a sampling step in time 4t = 0.2 second.

The number of samples p on which the signal will be examined will be p = 1/At x T 5 x 25 = 125.

In the same way, from the chosen ratio E/D = 5, the detection of objects at the bottom of the water by means of a magnetometric sensor on board a fish towed by a boat at the speed of ten knots is about 5 rn/S for an optimum range of 25 m with a resolution of 1 m leads to the identical choice of these parameters, namely: = 0.2 second T =25s p = 125 samples.

By optimum range is here meant the range for which the probability of detection will be 100%, which is obviously not the limit of the range of the method, which, as we will see further on, is limited only by the above defined threshold which is usable in practice. The diagram of figure 5 explains the succession and organization of the steps for applying the proposed method in the case of using a single magnetometric sensor.

H is the clock defining the complete computing cycle and is derived from the basic clock of the computer. For the two examples envisaged, its period t is 0.2 second.

(1) is the input member of the computer for acquiring the signal SM from the magnetometric sensor. In the case where this signal is analog, (1) is a coding sampler which delivers a measurement of SM in digital form at the rate of clock H, namely every 0.2 second. This signal is transferred into the buffer memory M which contains, in our examples, p 125 successive samples of the signalS.

At each clock cycle H, a new sample is acquired and the oldest sample is eliminated.

F1 is a digital filter for improving the signal to noise ratio;-at the detector by eliminating the noises contained in frequency bands outside the band containing the desired signals. In our examples, F1 is an order 4 bandpass filter whose cut-off frequencies are respectively 0.0]. Hz for the high pass filter and 0.3 Hz for the low pass filter.

SF is the filtered digital signal SM in said frequency band. SF is computed at each clock cycle H, namely every 0.2 second. In accordance with the method, this signal SF is "compared" every 0.2 second with the theoretical signals coming from a certain number of points EiDJ of the observation plane.

For that, the function (2) generates the base functions corresponding to each of the chosen points EiDj. These signals are computed from equations 5 in the case where identification is limited to only dipolar signals, which is sufficient in the case of the chosen examples of application. In practice, this computation of the values of the three base functions is carried out in real time by the computer from u = E/Dj, with D = Dj every 0.2 second for the values E < Ei or else is provided by a pre-computed table and in this case the function (2) is reduced to a table of the digital values of the three base functions stored for each point EiDj chosen in the observation plane. The base signals are generated by reading the digital values of the functions o i 72 successively in each line Dj and for the successive vlues of Ei.

This reading from table 2 is carried out at the rate of the clock, and so in synchronism with sampling of the signal SM from the magnetometric sensor every 0.2 second.

We have then available, every 0.2 second, base functions each represented by a number p of samples (p = 125) for each of the chosen points EiDJ. As for the signal SM, each of the base functions 5"o, r 2 of each point EiDj is, in accordance with the invenLon, filtered by a digital filter F2 identical to the above described filter F1 In practice, depending on the structure of the computer used and its computing speed, the operation of filtering the base functions is carried out in series (successive passages of the different sampled base signals through a single filter) or in parallel (each base signal * is processed by a specific filter) or a combination of the two series and parallel methods.

Following these operations, we now then have two tables stored in the computer representing two of the three terms of the equation (8). We can then compute the third terni thereof which is, for each of the points EIDj, the table of the Aij, the coefficient of projection of signal SM the base functions of each point EiDj.

Function (3) is the "projection operator" which computes the Aij from the equation (9), and from stored tables of the sampled magnetometric sensor SM and from the base functions of each point EiDj.

For each of the values i and j, we carry Out the computation described by equation (9) and thus obtain a table of the coefficients Au which, like the preceding tables, is obtained every 0.2 second. The operator 5 carries out the computation, in accordance with the invention, of the identified energy, of the signal, i.e. projectable on the base functions. This operator computes the digital values of the equation (10) for each of the points EiDj, i.e. computes Eij and forms a table thereof every 0.2 second.

Concurrently with function (5), function (4) computes the total energy of the p samples of the signal SM from the magnetometric sensor by means of the equation (12) every 0.2 second. The operator (6) computes, for each of the points EIDJ, the ratio of the preceding energies and t and thus forms the' table Cij from the detection criteria, the ratio ij/.t. As before, such a table is obtained every 0.2 second.

In a variant, and depending on the nature of the noises, the detection criteria may advantageously be defined by C, r being the residual energy, namely the energy of the noise obtained in the absence of detection.

In this case, the operator (4) is blocked by the presence of the detection signal C ?ec" and keeps the vaiue4Et =Er obtained previously on detection and for the whole time during which C remains higher than or equal to L('. The operator 7 compares each of the values of the preceding criteria Cij with the threshold and only keeps the criteria greater than O('and now called Ckl table.

The operator 8 compares each of the preceding criteria Ckl two by two and only keeps the maximum criterion Ckl, i.e. whose digital value is the highest in the preceding table.

This result is obtained every 0.2 second.

The data read out from these latter tables is -data relative to the presence of at least one coefficient Ckl in the Ckl table, which is the detection signal, namely a logic signal used for example for alerting an operator of the detection, -the maximum digital value of -the corresponding values El and Dj (EkD1) which are the data concerning the confidence level of the detection and of localization of the object with respect to the magnetometric sensor.

In figure 5, the whole of the functions carried out -33 -from function 2 is designated by 11.

Figure 6 shows the sucoession and organization of the operations of one embodiment of the invention in the case of using two magnetometric sensors SM1 and SM2 leading to complete localization and to a measurement of the magnetic characteristics of the detected object. Like the preceding one, this applic3ticn of the invention relates to two embodiments for airborne detection or detection of underwater objects by a fish towed by a boat.

The digital values chosen for the different parameters are the same as before.

The clock H which defines the synchronizátion of the operations for computing the application now, drives the sampling of signals SML and SM2 by the two coding samplers 1, and transfers each of these signals into the buffer memories respectively M1 and Each of the signals Ml and M2 now discretized and' digitized is filtered as before by two filters Fl and F2 then processed by an operator 1]. which combines the whole of the above defined functions.

As before, each of the operators 11 delivers, at the 0.2 second rate, the data C E D and the (Ai)ij table for each of the signals SM1 and S21 namely respectively C1E1D1 (Akl)l arid C2E2D2 (Akl)a The function 12 groups together the data C1 and C2 concerning the confidence level of the detection delivered independently by the detection chains SM1 and 5M2* Depending on the needs, 12 is carried out by an OR gate in the case where the probability of detection is maximized or by an AND gate which minimizes the false alarms. In the first case, a single detection by SM1 or SM2 is sufficient for 12 to deliver the detection signal C. In the second case, the two detections must be present simultaneously for 12 to deliver the signal. Similarly, the digital value of the confidence level criterion C of the detection transmitted by 12 may for the same reasons -34 -be: either the greatest or the smallest of C1 or C2 Function 13 receives the digital information of estimation of the distances El and E2 of detection of the object from the two detection chains. Normally E1lE2. 13 compares El and E2 delivered at each computing cycle every 0.2 second and, if El' E2 transmits the output data E = El E2, and if not, in order to minimize the false alarms, transmits an inhibition signal preventing at 12 the generation of the detection signal. -Function 14 receives every 0.2 second the two oblique distance data Dl and D2 from the two detection chains and computes the localization parameters D and in cylindrical coordinates by the equations 13, the operator 14 carries out this computation every 0.2 second. Operator 15 estimates the detected magnetic object, namely computes its dipolar magnetic moment11 from the following data now available: _D(DlorD2) -the sets of coefficients of projection on the base functions AOA1A2 for the point E Ek, D = where the detected object is located (Akl)l or (Akl)2.

The measurements hx hy hz of the components following the measurement trihedron delivered for example by the three axis magnetometer (16)..

From this latter data and from one of the sets of parameters Dl(Akl)l or D2(Ak12), operator 15 computes digitally the equation 14, all the terms of which are known, and delivers the value of the dipolar moment of the objectMby its three components".

The device described delivers then to the user arid entirely automatically -a discrete detection alert signal, -the confidence level of the detection C, -the instantaneous distance to the object plane E, -the distance corresponding to the smallest passage -35 -distance point D when two magnetometric sensors are used, the device delivers in addition: -the complete relative coordinates of the object E, D, (S, -measurement of the dipolar (or quadripolar) magnetic moment vector of the object.

This data is delivered in real time, i.e. with a maximum delay of 0.2 second in the case of our two examples of application.

This data is available in the computing means every 0.2 second and is transmitted visually to the operator in a conventional way via a member coupling the computer to a display device by a digital data transmission bus.

Depending on the characteristics of the computer used, the parameters not described up to now may vary to a very large extent and influence the final performances obtained. By way of example, we will describe hereafter two examples of using computers of different powers usable for each of the two proposed applications: airborne detection and underwater detection by a towed fish.

In the first example, we use a very high speed computer capable of 5 000 000 arithmetic operations per second with floating point on 32 bit words (24 mantissa bits, 8 exponent bits).

This computing capacity makes it possible to choose a large number of points EiDj a priori in the observation plane W (or the two planes W1 and W2) The choice is made of 125 abscissa along E and 6 ordinates along D, namely a total of 750 uniformly distributed points EIDJ. That corresponds to a total detection horizon of 25 seconds (the computing cycle being 0.2 second) which is distributed 5 seconds before the passage in the target plane and 20 seconds after this passage. For the airborne detection example, the ordinates DO to D5 of points EiDj are chosen equal to 50, 100, 200, 400, 800, 1600 meters. --36-.

In this example, the base functions corresponding to each point EiDj are calculated as described above at each computing cycle.

The tables of the criteria Cij also contain 750 digital values. The power of this computer makes it possible to control and display these criteria tables on a cathode ray tube, as shown in figure 6.

In accordance with the method, the display is made in the plane W defined by the directions E and D. From the table of criteria Cij the computer carries out the interpolations in a conventional way for displaying the isocriteria curves.

Figure 7a shows one example of the isocriteria curves obtained in the detection of an object situated at Dj = 250m. Four isocriteria curves are obtained corresponding to four identified energy levels equal respectively to once, twice, four times and eight times the threshold 4< The figure is plotted for point Ek situated substantially 2.5 seconds before the passage of the aircraft into the object plane., i.e. at a distance of about 250m before the object plane.

Figure 7b shows the detection of the same object about 12 seconds after the passage of the aircraft into the object plane.

We can see that the identified energy level has increased by four criteria levels and that in particular the estimation of Dj is very accurate and improves with the acquisition time of the signal.

Under the above mentioned conditions, the accuracy of localization is about 20 m along E and 3%, namely less than 50 m, along D at the observation distance Dmax = 1600 m. Furthermore, the detection threshold O( in this example is of the order of 0.2, i.e. the device is capable with a substantial zero false alarm rate of detecting objects whose signal energy is 20% of the energy of the noises which results in a range gain of the order of 3 with -37 -respect to prior art methods based on comparison of the amplitude of the signal and of the noises with respect to a threshold. -In a second embodiment, we used a 32 bit floating computer of a capacity of 600 000 arithmetic operation per second. For the application of the invention, the required memory capacities are RAM about 1000 words of 32 bits Program 1000 words of 32 bits Constants 3500 words of 32 bits -In this case, we chose a number of points EiDj limited to8.

Furthermore, the base functions corresponding to each of these 8 points are not computed, but pre-computed and stored in a memory.

The points EiDj are distributed thus: = -2 seconds, i.e. 200m before object plane, E2 = � seconds, i.e. 500m after passage in object plane for the four distances Dl. to D4 equal to 100, 200, 400, 800 meters.

We thus obtain every 0.2 second a table of coefficients C11C12C13C14 corresponding to E1 and C21C22C23C24 corresponding to E2 as indicated in figure 8a in the observation plane W. Because of the small number of points along D, it is necessary, when there is detection, namely when one criterion Cii at least is greater than, to carry out an interpolation along D and Cij as shown in figure 8b. In this figure, the four values ci1C12C13C14 have been placed corresponding to the coordinates D1D2D3D4 for the abscissa E1.

By quadratic interpolation the value D = Dl corresponding to the maximum C1 max of the curve passing through the four above mentioned points is determined.

C1max is the confidence level of the detection and D11 -38 -is the shortest distance between the object and the magnetometric sensor.

Figure Bc shows the detection of the same object obtained at time t = + 5s, i.e. 5 seconds after the passage in the object plane. The total energy of the identified signal having increased, the digital values of the criteria corresponding to E2, namely C21C22C23C24, are appreciably greater than their values obtained for E1.

Similarly, the maximum of the guadric passing through the four points of coordinates D1C21, D2C22, D3C23, D4C24 gives the estimate D12 of the distance along D of the detected object and the final confidence level C2max of the detection.

In this example, the accuracies obtained are of the order of 0.4 sec, namely 40 meters along E. 15% namely at most 120 m along D for the maximum evaluated distance of 800 rn.

Although slightly degraded with respect to the preceding example, the method makes use possible with a substantially zero false alarm rate of detection thresholds of the order of 0.6, which remains appreciably greater than the prior art methods with a detection range which is substantially doubled.

Claims (21)

1. -39 -CL A I MS1. A method for detecting, localizing and identifying magnetic objects by means of at least one magnetometric sensor delivering an electric signal SM whose variation in time is generated by the magnetic moment(s) of the sought for object in its relative movement with respect to the magnetometric sensor in which method -an observation plane W is defined such that this plane contains M, the position of the magnetometric sensor, N the position of the sought for object, V the relative speed vector at a time t between said sensor and said object, -in this plane W a set of current coordinate points EiDj is defined, -the signal SM received is projected mathematically, for each coordinate point EiDj, on bases -the identified energy of the signal SM projectable on the bases of each of the coordinate points EIDj is calculated, -a detection criterion Cij is calculated corresponding to each point EiDj and defined as being the ratio of the energy of the point considered over the total energy -the maximum value C2 of the criterion Cij is sought, which is such that k and 1 represent the coordinates Ek and Dl of the point EiDj corresponding to the position of the sought for object.
2. 2. The method as claimed in claim 1, wherein a set of orthonorined and orthogonal basess considered.
3. 3. The method as claimed in claim 1, wherein the the sought for object having a dipolar behavior, the basest are expressed by the expressions Gil 7LtZ_i' fo -"7r , _,,fT cA lf -6-qT. 7L -40 -with 5M = D in which the coefficients A0A1A2 are non linear functionsof the Earths magnetic field vector Ht and of the magnetic moment tfof the sought for object and in which the parameter u is defined as the ratio of the distance travelled along the relative trajectory and of the shortest passage distance between the sought for object and the corresponding magnetometric sensor.
4. 4. The method as claimed in claim 1, wherein the sought for object having a quadripolar behavior, the basesfr/ are expressed by the expressions: iT ______ (1 /.) 1 sir -4)/&. 25. Tiij-with io 4z t in which the coefficients A A A AAA are non linear 0l2 functions of the Earth's magnetic field vector Ht and of the magnetic moment of the sought for object and in which the parameter u is defined as the ratio of the distance travelled along the relative trajectory and of the shortest passage distance between the sought for object and the corresponding magnetometric sensor. -1 1--
5. 5. The method as claimed in claim 1, wherein the detection criterion is calculated for each point ELDj by the expressions 2(/ ::é z1-

   Twhere: is the transposed matrix of the matrix Aij of the coefficients Aij obtained after mathematical projection and so resolution of the equation Jr C/) is the transposed matrix of the matrix of the values of the basest.p is the number of sarnoles used in the measurement horizon T.
6. 6. The method as claimed in claim 3, wherein the single parameter u is used as being the ratio of the distance travelled along the relative trajectory and of the shortest passage distance between the magnetometric sensor and the sought for object.
7. 7. The method as claimed in claim 1, wherein the probability of the presence of the sought for object is estimated in the plane W containing the sought for object, the magnetometric sensor and the relative speed vector between said object and said sensor, for each of the a priori chosen points of this plane.

   -42 -
8. 8. The method as claimed in claim 1, wherein the measurement signals and the base signals from the analytic model for each of the points chosen in plane W are filtered identically.
9. 9. The method as claimed in claim 1, wherein comparison of the measured signals and of the signals from the analytic model corresponding to each estimation point is carried out in real time at each time t defined by a sampling clock H and by analytic projection.
10. 10. The method as claimed in claim 1, wherein the criterion Cij is compared with a threshold"and delivers a detection indication if the value of said criterion is greater than the value of said threshold, this being carried out simultaneously for each sampling time and for each of the points of plane W.
11. ii. The method as claimed in claim 10, wherein the confidence level of detection is computed as being the ratio of the maximum value Ckl of the criteria Cij and of the threshold O('.
12. 12. The method as claimed in claim 1, wherein a reduced number of points in the observation plane W is used.
13. 13. The method as claimed in claim 1, wherein two magnetometric sensors and M2 spaced from each other by a known distance d simultaneously measure the time signals Si and S2 of the sought for object N, complete localization of this object being obtained by solving the triangle dD1D2, Di and D2 being the shortest passage distance between respectively the sensors Ml and M2 and the sought for object in the observation plane w and W2 of each of the two sensors.
14. 14. The method as claimed in claim 13, wherein complete knowledge of the localization of the sought for object is used for computing the magnetic moment of said object from values of the projection coefficients Aij following an expression of the type p -43 ---* T.in which expression * D Dl or D2 depending on whether the coefficients AOA1A2 are used corresDonding to C1 obtained for the observation plane W1 or 10. hx. hy, hz are the components along the measurement trihedrori of the local Earthts field Mt known a priori or measured for example by means of a triaxial directional fluxgate magnetometer along said axes of the measurement trihedron.
15. 15. The method as claimed in claim 10, wherein the threshold ( is computed from the energyEr of the noise measured in the absence of detection.
16. 16. A system for the detection, localization and identification of magnetic objects, comprising a digital computer with an arithmetic unit, a program memory and a data memory, input and output means, adapted for implementing the method as claimed in claim 1.
17. 17. The system as claimed in claim 16, wherein the computer has a computing capacity of 5 000 000 arithmetic operations per second with floating point on 32 bit words, the number of points chosen in each of the observation planes W, Wi' W2 being of the orderof 750.
18. 18. The system as claimed in claim 16, wherein the computer has a computing capacity of 800 000 arithmetic operations per second, the number of points chosen in each of the observation planes W, Wi' W2 being of the order of 8 to 20.
19. 19. The system as claimed in claim 16, wherein the input means are 12 bit coding samplers operating at the rate of about 5 acquisitions per second.
20. 20. A method of detecting, localising and identifying magnetic objects substantially as herein-before described with reference to the accompanying drawings.
21. 21. A system for the detection, localisation and identification of magnetic objects, constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.K (Amendments to the claims have been filed as follows 1. A method for detecting, localizing and identifying magnetic objects by means of at least one magnetometric sensor delivering an electric signal SM whose variation in time is generated by the magnetic moment(s) of the sought for object in its relative movement with respect to the magnetometric sensor, in which method: -an observation plane W is defined such that this plane contain. H, the position of the magnetometric sensor, N the position of the sought for object, ithe relative speed vector at a time t between said sensor and said object, -in this plane W a set of current coordinate points EiDj is defined, where Ei is the distance from the sensor to the point of closest approach to the object and Dj is the distance of closest approach to the object, for each of a plurality of object positions, -the signal SM received is projected mathematically, for each coordinate point EiDj, on bases -the identified energy f(j) of the signal SM projectable on the bases each of the coordinate points EiDj is calculated, -a detection criterion Cij is calculated corresponding to each point EiDj and defined as being the ratio of the energy of the point considered E (j) over a reference energy -the maximum value C2 of the criterion Cij is sought, which is such that k and 1 represent the coordinates Ek and Dl, in the locus defined by the set / of points EiDj, corresponding to the position of the sought for object.2. The method asclajmed in claim 1, wherein a set of orthonormed and orthogonal bases t'is considered.3. The method as claimed in claim 1, wherein the sought for object having a dipolar behaviour, the bases are expressed by the expressions: Ti ______ fo 7r trL).57. -o.srr I j/L with in which the coefficients A A A are non linear functions of the Earth's magnetic field vector Ht and of the magnetic moment of the sought for object computed from signal Sm and base functions t' , and in which the parameter u is defined as the ratio between (a) the distance of the sensor to the plane normal to the trajectory and containing a presumed object and (b) the shortest distance between the trajectory and said object.4. The method as claimed in claim 1, wherein the sought for object having a guadripolar behaviour, the bases are expressed by the expressions: ( t1 1!.. 13ST C, , _______ p (3s1r-f-.4)/& (T ________ with = 11O. i4i AZ in which the coefficients A0A1A2A3A4 are non linearfunctions of the Earth's magnetic field vector andof the magnetic moment of the sought for object computed from signal Sni and base functions and in which the parameter u is defined as the ratio between (a) the distance of the sensor to the plane normal to the trajectory and containing a presumed object and (b) the shortest distance between the trajectory and such object.5. The method as claimed in claim 1, wherein the detection criterion is calculated for each point EiDj by the expressions: �j fSqJA -rwhere: A1j is the transposed matrix of the matrix Aij of the coefficients Aij obtained after mathematical projection and so resolution of the equation cr is the transposed matrix of the matrix of the values of the bases P p is the number of samples used in an observation horizon P defining the total duration of measurements to be taken into consideration.6. The method as claimed in claim 1, wherein the probability of the presence of the sought for object is estimated in the plane W. 7. The method as claimed in claim 1, wherein the measurement signals and the base signals from the analytic model for each of the points chosen in plane W are filtered identically.8. The method as claimed in claim 1, wherein comparison of the measured signals and of the signals from the analytic model corresponding to each estimation point is carried out in real time at each time t defined by a sampling clock H and by analytic projection.9. The method as claimed in claim 1, wherein the criterion Cij is compared with a threshold yt. and delivers a detection indication if the value of said criterion is greater than the value of said threshold, this being carried out simultaneously for each sampling time and for each of the points of plane W. 10. The method as claimed in claim 9, wherein the confidence level of detection is computed as being the ratio of the maximum value Ckl of the criteria Cij and of the threshold c 11. The method as claimed in claim 1, wherein a reduced number of points in the observation plane W is used.12. The method as claimed in claim 1, wherein two magnetometric sensors H1 and M2 spaced from each other by a known distance d simultaneously measure the -time signals Si and S2 of the sought for object N, complete localization of this object being obtained by solving the triangle dD1D2, Dl and D2 being the shortest passage distance between respectively the sensors Ml and 1.12 and the sought for object in the observation plane Wi and W2 of each of the two sensors.13. The method as claimed in claim 12, wherein complete knowledge of the localization of the sought for object is used for computing the magnetic moment of said object from values of the projection coefficients Aij following the expression: / Lii.in which expression: * D = Dl or D2 depending on whether the coefficients A0A1A2 are used corresponding to Ckl obtained for the observation plane W1 or W2.* hx, hy, hz are the components along the measurement trihedronof the local Earth's field t known a priori or measured for example by means of a triaxial directional fluxgate magnetometer along said axes of the measurement trihedron.14. The method as claimed in claim 10, wherein the threshold is computed from the energy r of the noise measured in the absence of detection.15. A method as claimed in claim 1 wherein said reference energy t corresponds to the total energy, that is the sum of the possible energy of the realsignal and the background noise energy.16. A system for the detection, localization and identification of magnetic objects, comprising a digital computer with an arithmetic unit, a program memory and a data memory, input and output means, when programmed to implement the method as claimed in claim 1. (17. The system as claimed in claim 16, wherein the computer has a computing capacity of 5 000 000 arithmetic operations per second with floating point on 32 bit words, the number of points chosen in each of the observation planes W, Wi, W2 being of the order of 750.18. The system as claimed in claim 16, wherein the computer has a computing capacity of 600 000 arithmetic operations per second, the number of points chosen in each of the observation planes, W, Wi, W2 being of the order of 8.19. The system as claimed in claim 16, wherein the input means are coding samplers operating at the rate of about 5 acquisitions per second.20. A method of detecting, localising and identifying magnetic objects substantially as hereinbefore described with reference to the accompanying drawings.21. A system for the detection, loca].jsatjon and identification of magnetic objects, constructed and arranged substantially as hereinbef ore described with reference to and as illustrated in the accompanying drawings.