CA2089312C - Synthetic aperture array dipole moment detector and localizer - Google Patents
Synthetic aperture array dipole moment detector and localizerInfo
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- CA2089312C CA2089312C CA 2089312 CA2089312A CA2089312C CA 2089312 C CA2089312 C CA 2089312C CA 2089312 CA2089312 CA 2089312 CA 2089312 A CA2089312 A CA 2089312A CA 2089312 C CA2089312 C CA 2089312C
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Abstract
A synthetic array comprising a moving magnetic sensor that provides a signal representative of an array of magnetic sensors is coupled to a digital signal processor is used to break a magnetic dipole field into its components, and a magnetic, signature of the present magnetic field is created. Predicted dipole signatures are precomputed for multiple magnetic orientations of the dipole at each of a plurality of range locations, expressed in terms of Anderson functions, which are a set of equations that decompose the magnetic field in each of the magnetic response locations, and are stored in a lookup table for reference. Input data measured by the moving synthetic array are processed and each magnetic value is predicted to process against background noise. A long term time average consistent with the relative motion of the dipole is computed using band-pass filtering of the signals from the synthetic array. The bandpass filtered data is used to update the predicted data so that anomalies and other non-dipole data is removed from the signals that are processed. The data are then expressed in terms of Anderson functions. The data are matched filtered, wherein it is correlated with the stored pre-computed predicted dipole signatures. The correlation yields a set of values that is then thresholded. If a dipole is present at any of the locations, then the correlated normalized value computed as stated above will be higher than the chosen threshold. The dipole locations that are above the threshold are then displayed on a monitor showing the relative location and dipole orientation.
Description
SY~ IC APERTURE ARRAY DIPOLE
MOMENT DETECTOR AND LOCALIZER
BACKGROUND
The present invention relates generally to systems and methods that provide for dipole moment detection and localization, and more particularly, to systems and methods that provide for dipole moment detection and localization using synthetic aperture array.
Conventional magnetic anomaly detection systems employ proximity detection of a magnetic dipole. Typical of this type of system is the well known airborne magnetic anomaly detection system that is towed by a P3 aircraft, for example.
In this system, detection is performed using a single sensor to detect a change in the total local magnetic field. The disadvantage of this approach is the limited amount data generated by the system, in that it provide for no dipole direction, no dipole orientation, gross localization and the system has relatively low processing gain.
To overcome the limitations of such conventional magnetic anomaly detection systems, an improved dipole moment detector and localized system assigned to the assignee of the present invention is disclosed in U.S. Patent No. 5,239,474, issued August 24, 1993. In this dipole moment detector and localizer system, the detection and localization is accomplished using the outputs from a fixed vector-magnetometer array. The dipole moment detector and localizer described in this patent application has yielded dramatic performance imp~uve~cnt over the current magnetic anomaly detection (MAD) systems that use a single sensor to detect a change in the total local magnetic field derived from a dipole.
Thus, it would be an improvement in the art to have a system and method that provides dipole detection and the localization and orientation of the dipole.
- 208q3 1 2 SU~RY OF THE INVENTION
Other aspects of this invention are as follows:
A method of detecting and locating a magnetic dipole comprising the steps of:
sensing the magnetic field produced by the dipole using a moving magnetic sensor that is moved relative to the location of a magnetic dipole to synthesize a plurality of output signals that are representative of a plurality of spatially distributed magnetic sensors and that are indicative of the magnetic field sensed by the moving magnetic sensor;
generating a magnetic signature produced by the magnetic field and the dipole which represents a magnetic response function of the dipole by processing the output signals from the moving magnetic sensor to decompose the magnetic field into its magnetic field components; and processing the magnetic response function to produce magnetic signature features therefrom that are indicative of the location and relative orientation of the magnetic dipole and means for displaying the location of the dipole.
A method of detecting and locating a magnetic dipole comprising the steps of:
forming a normalized estimate of the magnetic field to be sensed by a moving magnetic sensor that is moved relative to the location of a magnetic dipole using known magnetic dipole orientations located at each of a plurality of preselected locations to provide an array of estimated signals;
processing the estimated signals to produce a first set of Anderson function expansion coefficients representative of each of the estimate signals, and storing the Anderson functions;
measuring the magnetic field using the moving magnetic sensor that is moved relative to the location of the magnetic dipole ~hat is to be detected to produce measured magnetic field signals;
2a removing spatial and temporal variations in the magnetic field measured by the sensor by temporally and spatially smoothing the measured magnetic field signals:
processing the temporally and spatially smoothed measured magnetic field signals to produce a second set of Anderson function expansion coefficients representative of each of the measured magnetic field signals;
correlating the first and second sets of Anderson function expansion coefficients to produce a response function for the magnetic dipole location; and identifying the location of the dipole if one of the correlations has a significantly larger value than the others, and if it is greater than a predetermined threshold, and the location of the dipole corresponds to the location represented by the measured magnetic field signals that produced in the significantly larger value, and providing a visual display of said location.
A method of detecting and locating a magnetic dipole comprising the steps of:
precomputing a plurality of magnetic field signatures that are representative of a magnetic field using a moving magnetic sensor that is moved relative to the location of a magnetic dipole that-provides signals representative of an array of sensors, assuming that there is a magnetic dipole located at each of a plurality of selected locations relative to the moving sensor, to provide for calculated estimate values;
storing the calculated estimate values;
t~k;ng measurements of the magnetic field at each of a plurality of locations in the presence of a magnetic dipole within the field by moving the sensor relative to the magnetic dipole and synthesizing signals that would be provided by the array of sensors;
removing the spatial and temporal variations present in the measured magnetic field caused by the naturally occurring 20~93 1 2 2b background using temporal and spatial smoothing techniques;
correlating the measured values with each of the calculated estimate values for the synthesized array of sensors, by multiplying the calculated estimate values with the measured values and summing the results over the synthesized array of sensors; and if one of the resulting correlations has a significantly larger value than the other~ and if it is greater than a predetermined threshold, declaring a detection for the location correspo~; ng to the calculated values which resulted in the larger correlation value, and providing a visual display of said location.
Apparatus for detecting and locating a magnetic dipole comprising:
a moving magnetic sensor for sensing the magnetic dipole to provide a synthesized signal representative of an array of spatially distributed magnetic sensors that each provide output signals that are indicative of the magnetic field sensed thereby;
processing means for generating a magnetic signature of the magnetic field produced by the magnetic dipole which represents a magnetic response function of the dipole by processing the synthesized output signals from the magnetic sensor to decompose the magnetic field into its magnetic field components; and processing means for processing the magnetic response function to produce predetermined features therefrom that are indicative of the location and relative orientation of the magnetic dipole display means for displaying the location of the identified dipole.
Apparatus for detecting and locating a magnetic dipole comprising:
a synthetic array comprising a moving magnetic sensor that provides an output signal representative of an array of magnetic sensors;
- - 208~31 2 2c processing means coupled to the synthetic array, for storing an estimate the magnetic field signature to be detected by the moving sensor using a known set of magnetic dipole orientations at each of a plurality of preselected locations to provide an array of estimate signals that are represented by a first set of Anderson functions, and for processing magnetic field signals indicative of the magnetic field measured by the synthetic array in the presence of the magnetic dipole that is to be detected, and for removing spatial and temporal variations in the magnetic field measured by the synthetic array by temporally and spatially smoothing the measured magnetic field signals, and for processing the temporally and spatially smoothed magnetic field signals to produce a second set of Anderson functions representative of each of the magnetic field signals, and for correlating the first and second sets of Anderson functions and for gll~;ng the results over the array of sensors to produce a response function for the magnetic dipole location, and for identifying the location of the dipole if one of the correlations has a significantly larger value than the others, and if it is greater than a predetermined threshold, and the location of the dipole corresponds to the location represented by the measured magnetic field signals that produced in the significantly larger value; and display means for displaying the location of the identified dipole.
By way of added explanation, in order to provide the above improvement, the present invention provides for a dipole detection and localization system and method that combines the dipole moment detector and localizer system disclosed in the above-identified U.S. patent with a synthetic aperture array disposed on a moving platform or aircraft to achieve improved performance. Using the dipole moment detector and localizer system disclosed in the above-identified patent application to process the output of the moving sensor that synthesizes an 2d array of sensors not only provides for dipole detection, but also provides for localization and orientation of the dipole.
In the present invention, the processing algorithm of the dipole moment detector and localizer system processes the output of a synthetic aperture array generated by the forward motion of a single magnetometer (or multiple magnetometers) located on a moving platform instead of the sensor outputs from a fixed array of magnetometers. In the present synthetic aperture array dipole moment detector and localizer system and method, the array output i8 synthesized by the movement of a magnetic sensor or a small number of sensor through space. In a manner similar to the dipole moment detector and localizer disclosed in the above-identified patent application, one embodiment of the synthetic aperture array dipole moment detector and-localizer system and method of the present invention is used to detect and localize surface and subsurface ocean going vessels for airborne ASW applications.
The present synthetic aperture array dipole moment detector and localizer system and method provide for accurate localization of a dipole, in addition to the basic detection function, a capability that is currently lacking in present magnetic anomaly detection (MAD) systems.
The present dipole moment detector and localizer uses the synthetic aperture array and digital signal processing to break a magnetic field into x, y, and z components or into its total field component at each of a plurality of positions relative to the synthetic aperture array. In doing this, a magnetic signature of the magnetic field of a magnetic dipole located in the field is created. This magnetic signature provides an easily recognizable feature for an automatic pattern recognizing system.
The present invention precomputes predicted dipole signatures for multiple magnetic orientations of the dipole at each of a plurality of range locations. Thus, a plurality of predicted signatures are computed and stored in a lookup table '- 20893 1 2 2e for reference. Input data measured by the synthetic aperture array are processed against the background ambient noise using a linear model. Also a long term time average consistent with the relative motion of a dipole is computed. This amounts to bandpass filtering or long term a~eraging of the signals from the synthetic aperture array. The bandpass filtered data is used to update the predicted data 80 that anomalies and other non-2089~12 dipole data is removed from the signals that are processed. The data is then e~r~,ssed in terms of Anderson functions, which are a set of m~ l functions that decom-pose the magnetic field into its components in each of the m~gnetic res~onse locations.
Then the data expressed in terms of the Anderson functions is matched filtered, S wherein it is mathematically correlated by means of a dot pl~lucl with the stored pre-col-l~uled predicted dipole signatures. The dot product, or correlation, of these two sets of data yields a set of values including the largest value in the set of correlated data and is then norm~li7ed This norm~li7e~1 data is then thresholded, and if a dipole is present at any one of the locations, then the correlated, dot product, norm~li7ed value cc-ll~uled as stated above will be higher than the chosen threshold.
The dipole locations that are above the threshold are then displayed on a monitor allowing the relative location and dipole orientation. This display of the data is typically updated at frequent intervals. In this way, anomalies and other non-dipole data that are detected only appear at one display interval and then disappear during the next display interval. Since most ocean going vessels are magnetic dipoles, the dipole molllent detector and localizer may be used to detect and localize surface and sub-surface ocean going vessels. The dipole moment detector and localizer provides for a new approach to shallow water dipole detection. Other applications for the dipole molllenl detector and localizer include battlefield management, geological survey, and harbor protection, and ground traffic management.
The advantages of the synthetic array dipole ,-,.",~ delecl~r and loc~li7~r system and method of the present invention are that they provide detection, localization and dipole orientation with a greater system signal processing gain over existing sys-tems, because it uses the synthetic aperture array. The processing ~lrulmed by the present invention allows dipole of interest tracking to be ~lrulllled more accurately and at greater ranges than with the PrOA111~11Y detection employed in conventional systems, such as the P3 airborne magnetic anomaly detection system.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of the present invention may be more read-ily understood with reference to the following detailed des~ lion taken in conjunction with the accompanying drawings, wherein like IGr~ll;nce numerals design~te like struc-tural elements, and in which:
Fig. 1 shows a prior dipole moment detector and localizer system, a portion of which forms part of the present invention;
Fig. 2 shows a block diagram illustrating the processing steps utilized in the system of Fig. l;
Fig. 3 shows a synthetic aperture array dipole moment detector and localizer system in accordance with the principles of the present invention;
Fig. 4 shows the windowing process used in the system of Fig. 3 to synthesize a sensor array; and Fig. 5 shows the details of the processing performed in the system of Fig. 3.
DETAILED DESCRIPTION
Referring to the drawing figures, Fig. 1 shows a prior dipole moment detector and localizer system 10, a portion of which forms part of the present invention, as will be described below. This system 10 is described in aforementioned U.S. Patent No. 5,239,474. Fig. 1 depicts the system processing diagram of a fixed line array dipole moment detector and locator. This system 10 employs a fixed line array of magnetic sensors 11 to detect and localize a moving dipole 12. A complete understAn~; ng of the design and operation of this system 10 may be had from a reA~; ng of the above-identified patent application. As was mentioned above, a small disadvantage of this approach is the limited information generated by the system 10. This system 10 provides no dipole orientation information and only provides for gross localization of the location of the dipole 12.
More particularly, Fig. 1 shows a detection and location system 10 that includes an array of magnetic sensors 11 that is coupled to processing apparatus (shown in Fig. 2). The array of circular locations represent magnetic response locations 12 that are defined relative to the array of magnetic sensors 11. The magnetic response locations 12 have precomputed magnetic responses associated therewith which represent the magnetic field that would result if a magnetic dipole were present within each particular magnetic response location 12 at a plurality of different orientations. Also 208q31 2 -4a shown in Fig. 1 is a particular magnetic response location 12a (highlighted) in which is located a magnetic dipole 13, represented by the submarine. Also a plurality of arrows representing magnetic vectors 14 are shown exten~;ng from each sensor of the array of magnetic sensors 11 toward the particular magnetic response location 12a in which the magnetic dipole 13 is located. Each sensors re~;ng contributes a set of vectors indicative of the location of the magnetic dipole 13, thereby forming a sensed signature that is processed using the concepts of the present invention as described below.
Referring to Fig. 2, it shows a block diagram of processing apparatus 20 and the processing steps implemented therein utilized in the system 10 of Fig. 1. In particular, Fig. 2 shows the processing steps performed within the processing apparatus 20 of the present invention. As shown in Fig. 2, in a first processing step 31, data from the array of sensors 11 is processed using a linear model to predict each sensor's value using the other sensors. In addition, in a second proces~ing step 32, the data is time averaged to pclr~lllls long term integration thereof which ope.~tes as a low pass filter on the data. This data is used to adjust the values of the data co~ uLcd in the first pro-5 cessing step 31. Furthermore, in a third processing step 33, the predicted data, asmodified by the time averaged data, is subtracted from the measured data and the resul-tant data is exp~n-le~l in terms of Anderson's functions in step 34. Prior to operational use of the present invention, and as is in~ ate~ in process step 35, stored data is gen-erated in terms of the Anderson functions which comprise magnetic responses associ-10 ated with each of the m~gneti~ response locations 12 that l~lcscn~ the magnetic fieldthat would result if a m~gn~ti~ dipole were present within each particular m~gnetic response location 12.
The precomputed data generated in step 35 is then correlated with the resultant data gen~ldled in step 34 in procescing step 36. This correlation comprises a dot prod-15 uct of the two sets of data, and the correlation produces sharply increased dot productvalues when the two data values are substantially the same, while producing relatively low values when the data is only moderately different. A ~ imulll, or peak, corre-lated value is selected in procescing step 37. A threshold is selected in step 37, say for example, 50% of the peak correlated value, and correlated and no~n~li7~ signals 20 above the threshold are displayed by interpolating the relative positions (locations) of the magnetic response location and displaying them on a monitor, for example, as is illustrated in steps 39 and 40.
Fig. 3 shows a synthetic a~ellulc array dipole moment detector and localizer system 50 in accordance with the principles of the present invention. This system 50 is 25 comprised of a moving platform 51, such as an aircraft 51, for example, in which is disposed a synthetic apellulc array 52. The synthetic al~ellulc array 52 is coupled to processing apparatus 20 shown in Fig. 2. The synthetic a~)el~ulc array 52 is comprised of a single magl.clolllGlel~ for example, or an array of m~ .. lo~ which are moved by the aircraft 11 relative to the dipole. This motion generates a large amount of data 30 that is used to synthesize an array of sensors. This will be described in more detail below.
The array of circular locations represent the magnetic response locations 12 that are defined relative to a magnetic sensor 53, or magnelomeler 53, located on the aircraft 51 that houses the synthetic a~Gllul., array system 50. The magnetic response locations 35 12 have precolll~u~ed m~gnetic responses associated therewith which represent the magnetic field that would result if a magnetic dipole were present within each particular magnetic response location 12 at a plurality of dirrelGIlt orientations. Also shown in Fig. 3 is a particular m~gnetic response location 12a (hi~hlighte~) in which is located a magnetic dipole 13, represented by the subm~nn~. Also a plurality of aTrows repre-senting magnetic vectors 14 are shown extending from the sensor 53 on the aircraft 51 toward the particular magnetic response location 12a in which the magnetic dipole 13 is locatçd The sensor reading taken at a plurality of locations along a particular flight path of the aircraft 51 contributes a set of vectors indicative of the location of the mag-netic dipole 13, thereby forming a sensed signature that is processed using the concepts of the present invention as described below.
The synthetic a~llule processing utilized in the system 50 of the present inven-tion will be described with reference to Figs. 3 and 4. Fig. 3 shows the moving plat-form 51 having the magnelc met~ 53 disposed thereon that is moving to the left in the drawing figure. Fig. 4 shows the windowing process used in the system of Fig. 3 to synthesize a sensor array; and Referring to Fig. 4, there are a plurality of data samples 54 shown as dots along the direction of travel of the platform 51 (shown moving to the right in Fig. 4). A plurality of windows 55 are shown that each encompass a subset of N samples from the total number of samples taken by the synthetic a~llul~ radar 52.
Each subsequent widow 55 is offset from the preceding window 55 by one data sample 54.
The synthetic a~~ , is formed by sampling the vector (or resnn~nce) magne-loll~l.,r 53 on the airborne plafform 51. Once a predetermined number (N) samples 54 are collected, matched field processing is p~rc,lllled generally in accordance with the fixed array embo(lim.ont disclosed in the above-referenced patent application and Figs. 1 and 2. Initially, N s~mrles 54 are collected prior to processing the grid of interest in window 1. Thereafter, one or more new samples 54 are added (with the same numberof samples 54 being dropped off at the front end) to process a new window. At any given time after processing begins, the N samples 54 used for processing are repre-sented by the equation x = H + N = sThM3 + noise where s is the dipole matrix, h is the Anderson matrix, and 3xN is used for a vector m~gnelomelel 53 and lxN is used for a resonance m~gnetometer 53.
A detailed discussion of the processing p~rolllled in the present system 50 willbe ~ cll~se~l with reference to Fig. 5. Fig. S shows the details of the dipole moment detection and localization processing performed in the synthetic array dipole moment detector and l~ c~1i7Pr system 50. The data samples 54 from the m~ lometer 51 are stored in a memory and the Anderson functions are also stored, one for each possible response location.
Synthetic array spatial matching is achieved as follows. The two vector func-tions to which the product is applied comprise the c~lc~ te~l and measured magnetic S field vectors along the array. When the calculated field matches well with the measured field, a large value is e~cpec~ed for the inner product. The Anderson functions are defined with respect to a given position (xl,yl,zl). Each of the set of these positions is called a m~ihllulll response location. Thus each llla~illlUIII response location has its own associated Anderson function, and for a given c,lienlalion, has its own correlation 10 with the measultmellts. The maximum value for the inner products on the ma~imulll response location s (let~rmin~ where the dipole 12 is located (which maximum response location) and with what orientation.
Discrete implr.l.~nl~ion of the dipole moment ~etection and localization proces-sor 20 is achieved as follows. The dipole Illomellt detection and loc~li7~tion processor 15 20 has been implemented in the continuous domain. It is necessary however to mech-anize this processor 20 in the discrete domain since in the actual system implem~nt~-tion, only spatially sampled measulelllellts of a dipole 12, i.e. outputs from a synthe-sized array of sensors, are available for pr~cessing To model the discrete implementa-tion of the dipole moment detection and localization processor 20, a line of N magnetic 20 sensors is disposed along an x-axis with each sensor located at Xn. Let h (3xn) be the Anderson function matrix having the 1th row and nth column element defined as follows:
hln = sinl (~n)coss~ 3n) Eq. (1) where l = 0,1,3 and n = 1,2,. . . ,N, are indices of the order of the Anderson functions and the sensor number along the synthesized array, respectively. In this e~lJIt;ssion, ~n. the angle of the nth sensor relative to the dipole 12 located at (xl ,Yl), is the arctan-gent of the directional unit vector from the sensor to the dipole 12, i.e.,~0 n ( yl ) Eq. (2) Define a dipole orientation matrix, s(~l), as follows:
2o893l2 ---m l a 2m 1 ~~ -cos ~l 2sin ~1 s(~l)=M 3--ml ~~ 3--ml ~a =M 3sin~l 3cos~l Eq.(3) 2--m l ~a ---m l ~ ~ 2cos ~l -sin ~l By pre-multiplying the Anderson functions matrix by the transpose of the dipole orientation matrix, s(~l), a matrix H is produced defined in Eq. (4) below, which is the output of the array of sensors in response to a dipole 12 located at (xl ,Yl) with the 5 orientation angle ~ relative to the x-axis.
sT(~l) h - [Hx Hy3T= M -cos ~1 h ~. 3sin ~1 h + 2cos ~1 h Yl 2sin ? ~1 3cos ~1 -sin ? ~1 1 Eq. (4) 10 where ho, hl and h2 are vectors of values representing the discrete Anderson functions evaluated at the sensors' locations relative to the dipole 12 in according to Eq. (2.17) above.
-h ~ cosS~l cos5~2 cosS~N
h = hl = sin~lcos4~l sin~2cos4~2~- sin~Ncos4~N Eq. (5) - - sin2~lcos3~1 sin2~2Cos3~2Sin2lpNcos3q)N
Eq. (4) describes the field of a dipole 12 in terms of a weighted sum of the Anderson functions where the weights are dependent on the position and orientation of the dipole 12. Since the Anderson functions are not orthonormal, the expansion of an ~bill~y function requires the use of three adjoint Anderson functions which are 20 defined as follows:
ho h = hl = [hhT]-lh Eq. (6) h2 Now, for an albiLI~ function f = [fl f2 ... fN]T ~ senting the outputs of an 25 array of sensors, the expansion of this function in terms of a given set of Anderson functions is obtained as shown in Eq. (7) ai = hi f = ~ hinfn; i = 0,1,2 Eq. (7) n= 1 If Hs is the output of the array in response to a dipole 12 located at (xs=xl, Ys=Yl) with an orientation ~s= ~1, and it is exp~nded in terms of the Anderson func-5 tions defined in Eq. (5), the resulting Anderson weights are the elements of the dipoleorientation matrix shown in Eq. (3) and are given in Eq. (8) below.
~ a~,l ayl -a = [a,~ ay] = ax2 ay2 = hHT = [hHT]-lhHsT = [hhT]-lh(hTs(~S)) = s(~l) Eq. (8) a~3 ay3 The primary conceptual di~.Gnce between the system 50 of the present inven-10 tion and the prior fixed sensor array system 10 can be better understood by colllp~illgthe systems 10, 50 shown in Figs. 1 and 3. Specifically, the p~ y conceptual dif-ference lies in the manner by which the magnetic field is measulcd and converted to input data for subsequent processing. In the fixed line array system 10 of Fig. 1, mea-surements are taken continuously by all sensors 11 in the array, with each sensor 11 15 having a slightly different perspective of the dipole moment, which when processed together by the proces~in~ procedure del~lmilles the location of the dipole 12. On the other hand, in the synthetic aperture array system 50 of the present invention, the sen-sor 53 is moved physically in the aircraft 51 to synthesi~ an array of sensors in order to duplicate the required different aspect measurements of a dipole 12. The required 20 different aspect mea~ulelllGIlls are not ~imlllt~neously accomplished in the present system 50. Since it is anticipated the dipole 12 will not change position or orientation signifi~ntly within the time frame during which the synthetic apG~ G array is generat-ed, asynchronous measurements will not cause signif~nt change in the perf~n~n~e of the system 50 of the present invention. Aside from this major difference in the manner 25 of obtaining and inputting measured data, the processing procedure for detecting and delGlllli~ g the locations of dipoles 12 in the synthetic apGl~ulG array system 50 is substantially identical to the procedure in the fixed array system 10, and is shown and described with reference to Fig. 2 above.
The proces~ing perforrned using the method and apparatus of the present inven-30 tion will now be (li~cll~sed in more detail and ~ltili7ing ~ltçrnative terminology. Specif-ically, the data is IGImPO1a11Y filtered. This is accomplished by the low pass filtering discussed with reference to Fig. 2. Spatially coherent fluctuations are removed. This is also accomplished by the low pass filtering rli~cussed with reference to Fig 2. Then spatial matched filtçring is employed to dGI~lll~ihle the existence of magnetic dipoles in the detection region of the array of sensors 12. This is accomplished by the correlation step 36 discussed with reference to Fig. 2. Next the matched filtered data is pl~Jcessed using a square law detection scheme, and then this data is smoothed. The smoothed data is then thresholded and magnetic dipoles located in the data are loc~li7e~ This 5 corresponds to the threshold and interpolation steps 38, 39 ~ cusse~l with reference to Fig. 2. Finally, the position of the detected magnetic dipoles are displayed for viewing by the user of the system 30.
The following is a description that illustrates the generality of the synthetic array magnetic detection and locali_ation system S0 of the present invention. The method and 10 the exarnple describing the magnetic detection and loc~li7~tion technique of the present invention are based on a particular application of the principles disclosed; namely it has used a particular method of generating the response function, known as and described as the correlation. For the correlation, it uses a particular form~ tion of an inner prod-uct, in which the measured and the com~)u~ed m~gnsti~ ,~nses are each expanded in lS terms of Anderson functions.
The basic method used in generating the responses at the various chosen loca-tions relative to the system 50 is the following. Form an estim~te of the magnetic field on the moving sensor 53 (m~gnetc"~.ele~l, if there were a magnetic dipole located at each of the selected locations (these e~ s are the calculated values for the array 20 magnetic fields). Take measurements of the magnetic field at each location of the sensor 53 at the time desired to detect and localize a magnetic dipole within the field.
This ~llounls to collecting and storing data samples over time using a magne-lollle~ 53 located on a moving platform 51. The array of sensors is synthesi7P~1 from the collected data Remove as much of the spatial and l~lllpol~l variations caused by the 25 naturally occurring background as is possible using the leln~ol~l and spatial smoothing techniques disclosed herein.
Co~relate the Illc~ulGd values with each of the calculated values for the synthe-sized array of sensors, whereby the term correlate we mean the multiplication of the calculated array of values with the array of mea~ult;lll~nls and s~ g the results over 30 the synthe~i7Pd array of sensors (the values of these correlations for each selected loca-tion off of the synthesized array of sensors, expressed as a function of the actual dipole location is what is called the response function J for that selectç(l location). If one of the correlations resulting has a signifi~ntly larger value than the others and if it is greater than some predete~ninYl threshold, a detection is declared for the location off of 35 the synthesi7P,d array corresponding to the calculated values which resulted in the larger value of the correlation.
In the system 50, temporal gain results from matchillg the signal and noise bandwidths. Spatial coherence gain removes fluctuations common to the sensors.
Spatial ll~al~;hing gain arises from m~t~hing the spatial filter to the field of the dipole 12 using the Anderson functions. Post detection gain (track before detect gain) allows the S use of a lower recognition differential (false alarms are controlled by l~uiling a viable or realistic track).
Thus there has been described a new and improved system and method that pro-vide for dipole moment detection and localization using synthetic a~tillul~ array. It is to be understood that the above-described emb~Ylim~nt~ are merely illustrative of some of 10 the many specific embo~im~nt~ which represent applications of the principles of the present invention. Clearly, llull~l~us and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.
MOMENT DETECTOR AND LOCALIZER
BACKGROUND
The present invention relates generally to systems and methods that provide for dipole moment detection and localization, and more particularly, to systems and methods that provide for dipole moment detection and localization using synthetic aperture array.
Conventional magnetic anomaly detection systems employ proximity detection of a magnetic dipole. Typical of this type of system is the well known airborne magnetic anomaly detection system that is towed by a P3 aircraft, for example.
In this system, detection is performed using a single sensor to detect a change in the total local magnetic field. The disadvantage of this approach is the limited amount data generated by the system, in that it provide for no dipole direction, no dipole orientation, gross localization and the system has relatively low processing gain.
To overcome the limitations of such conventional magnetic anomaly detection systems, an improved dipole moment detector and localized system assigned to the assignee of the present invention is disclosed in U.S. Patent No. 5,239,474, issued August 24, 1993. In this dipole moment detector and localizer system, the detection and localization is accomplished using the outputs from a fixed vector-magnetometer array. The dipole moment detector and localizer described in this patent application has yielded dramatic performance imp~uve~cnt over the current magnetic anomaly detection (MAD) systems that use a single sensor to detect a change in the total local magnetic field derived from a dipole.
Thus, it would be an improvement in the art to have a system and method that provides dipole detection and the localization and orientation of the dipole.
- 208q3 1 2 SU~RY OF THE INVENTION
Other aspects of this invention are as follows:
A method of detecting and locating a magnetic dipole comprising the steps of:
sensing the magnetic field produced by the dipole using a moving magnetic sensor that is moved relative to the location of a magnetic dipole to synthesize a plurality of output signals that are representative of a plurality of spatially distributed magnetic sensors and that are indicative of the magnetic field sensed by the moving magnetic sensor;
generating a magnetic signature produced by the magnetic field and the dipole which represents a magnetic response function of the dipole by processing the output signals from the moving magnetic sensor to decompose the magnetic field into its magnetic field components; and processing the magnetic response function to produce magnetic signature features therefrom that are indicative of the location and relative orientation of the magnetic dipole and means for displaying the location of the dipole.
A method of detecting and locating a magnetic dipole comprising the steps of:
forming a normalized estimate of the magnetic field to be sensed by a moving magnetic sensor that is moved relative to the location of a magnetic dipole using known magnetic dipole orientations located at each of a plurality of preselected locations to provide an array of estimated signals;
processing the estimated signals to produce a first set of Anderson function expansion coefficients representative of each of the estimate signals, and storing the Anderson functions;
measuring the magnetic field using the moving magnetic sensor that is moved relative to the location of the magnetic dipole ~hat is to be detected to produce measured magnetic field signals;
2a removing spatial and temporal variations in the magnetic field measured by the sensor by temporally and spatially smoothing the measured magnetic field signals:
processing the temporally and spatially smoothed measured magnetic field signals to produce a second set of Anderson function expansion coefficients representative of each of the measured magnetic field signals;
correlating the first and second sets of Anderson function expansion coefficients to produce a response function for the magnetic dipole location; and identifying the location of the dipole if one of the correlations has a significantly larger value than the others, and if it is greater than a predetermined threshold, and the location of the dipole corresponds to the location represented by the measured magnetic field signals that produced in the significantly larger value, and providing a visual display of said location.
A method of detecting and locating a magnetic dipole comprising the steps of:
precomputing a plurality of magnetic field signatures that are representative of a magnetic field using a moving magnetic sensor that is moved relative to the location of a magnetic dipole that-provides signals representative of an array of sensors, assuming that there is a magnetic dipole located at each of a plurality of selected locations relative to the moving sensor, to provide for calculated estimate values;
storing the calculated estimate values;
t~k;ng measurements of the magnetic field at each of a plurality of locations in the presence of a magnetic dipole within the field by moving the sensor relative to the magnetic dipole and synthesizing signals that would be provided by the array of sensors;
removing the spatial and temporal variations present in the measured magnetic field caused by the naturally occurring 20~93 1 2 2b background using temporal and spatial smoothing techniques;
correlating the measured values with each of the calculated estimate values for the synthesized array of sensors, by multiplying the calculated estimate values with the measured values and summing the results over the synthesized array of sensors; and if one of the resulting correlations has a significantly larger value than the other~ and if it is greater than a predetermined threshold, declaring a detection for the location correspo~; ng to the calculated values which resulted in the larger correlation value, and providing a visual display of said location.
Apparatus for detecting and locating a magnetic dipole comprising:
a moving magnetic sensor for sensing the magnetic dipole to provide a synthesized signal representative of an array of spatially distributed magnetic sensors that each provide output signals that are indicative of the magnetic field sensed thereby;
processing means for generating a magnetic signature of the magnetic field produced by the magnetic dipole which represents a magnetic response function of the dipole by processing the synthesized output signals from the magnetic sensor to decompose the magnetic field into its magnetic field components; and processing means for processing the magnetic response function to produce predetermined features therefrom that are indicative of the location and relative orientation of the magnetic dipole display means for displaying the location of the identified dipole.
Apparatus for detecting and locating a magnetic dipole comprising:
a synthetic array comprising a moving magnetic sensor that provides an output signal representative of an array of magnetic sensors;
- - 208~31 2 2c processing means coupled to the synthetic array, for storing an estimate the magnetic field signature to be detected by the moving sensor using a known set of magnetic dipole orientations at each of a plurality of preselected locations to provide an array of estimate signals that are represented by a first set of Anderson functions, and for processing magnetic field signals indicative of the magnetic field measured by the synthetic array in the presence of the magnetic dipole that is to be detected, and for removing spatial and temporal variations in the magnetic field measured by the synthetic array by temporally and spatially smoothing the measured magnetic field signals, and for processing the temporally and spatially smoothed magnetic field signals to produce a second set of Anderson functions representative of each of the magnetic field signals, and for correlating the first and second sets of Anderson functions and for gll~;ng the results over the array of sensors to produce a response function for the magnetic dipole location, and for identifying the location of the dipole if one of the correlations has a significantly larger value than the others, and if it is greater than a predetermined threshold, and the location of the dipole corresponds to the location represented by the measured magnetic field signals that produced in the significantly larger value; and display means for displaying the location of the identified dipole.
By way of added explanation, in order to provide the above improvement, the present invention provides for a dipole detection and localization system and method that combines the dipole moment detector and localizer system disclosed in the above-identified U.S. patent with a synthetic aperture array disposed on a moving platform or aircraft to achieve improved performance. Using the dipole moment detector and localizer system disclosed in the above-identified patent application to process the output of the moving sensor that synthesizes an 2d array of sensors not only provides for dipole detection, but also provides for localization and orientation of the dipole.
In the present invention, the processing algorithm of the dipole moment detector and localizer system processes the output of a synthetic aperture array generated by the forward motion of a single magnetometer (or multiple magnetometers) located on a moving platform instead of the sensor outputs from a fixed array of magnetometers. In the present synthetic aperture array dipole moment detector and localizer system and method, the array output i8 synthesized by the movement of a magnetic sensor or a small number of sensor through space. In a manner similar to the dipole moment detector and localizer disclosed in the above-identified patent application, one embodiment of the synthetic aperture array dipole moment detector and-localizer system and method of the present invention is used to detect and localize surface and subsurface ocean going vessels for airborne ASW applications.
The present synthetic aperture array dipole moment detector and localizer system and method provide for accurate localization of a dipole, in addition to the basic detection function, a capability that is currently lacking in present magnetic anomaly detection (MAD) systems.
The present dipole moment detector and localizer uses the synthetic aperture array and digital signal processing to break a magnetic field into x, y, and z components or into its total field component at each of a plurality of positions relative to the synthetic aperture array. In doing this, a magnetic signature of the magnetic field of a magnetic dipole located in the field is created. This magnetic signature provides an easily recognizable feature for an automatic pattern recognizing system.
The present invention precomputes predicted dipole signatures for multiple magnetic orientations of the dipole at each of a plurality of range locations. Thus, a plurality of predicted signatures are computed and stored in a lookup table '- 20893 1 2 2e for reference. Input data measured by the synthetic aperture array are processed against the background ambient noise using a linear model. Also a long term time average consistent with the relative motion of a dipole is computed. This amounts to bandpass filtering or long term a~eraging of the signals from the synthetic aperture array. The bandpass filtered data is used to update the predicted data 80 that anomalies and other non-2089~12 dipole data is removed from the signals that are processed. The data is then e~r~,ssed in terms of Anderson functions, which are a set of m~ l functions that decom-pose the magnetic field into its components in each of the m~gnetic res~onse locations.
Then the data expressed in terms of the Anderson functions is matched filtered, S wherein it is mathematically correlated by means of a dot pl~lucl with the stored pre-col-l~uled predicted dipole signatures. The dot product, or correlation, of these two sets of data yields a set of values including the largest value in the set of correlated data and is then norm~li7ed This norm~li7e~1 data is then thresholded, and if a dipole is present at any one of the locations, then the correlated, dot product, norm~li7ed value cc-ll~uled as stated above will be higher than the chosen threshold.
The dipole locations that are above the threshold are then displayed on a monitor allowing the relative location and dipole orientation. This display of the data is typically updated at frequent intervals. In this way, anomalies and other non-dipole data that are detected only appear at one display interval and then disappear during the next display interval. Since most ocean going vessels are magnetic dipoles, the dipole molllent detector and localizer may be used to detect and localize surface and sub-surface ocean going vessels. The dipole moment detector and localizer provides for a new approach to shallow water dipole detection. Other applications for the dipole molllenl detector and localizer include battlefield management, geological survey, and harbor protection, and ground traffic management.
The advantages of the synthetic array dipole ,-,.",~ delecl~r and loc~li7~r system and method of the present invention are that they provide detection, localization and dipole orientation with a greater system signal processing gain over existing sys-tems, because it uses the synthetic aperture array. The processing ~lrulmed by the present invention allows dipole of interest tracking to be ~lrulllled more accurately and at greater ranges than with the PrOA111~11Y detection employed in conventional systems, such as the P3 airborne magnetic anomaly detection system.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of the present invention may be more read-ily understood with reference to the following detailed des~ lion taken in conjunction with the accompanying drawings, wherein like IGr~ll;nce numerals design~te like struc-tural elements, and in which:
Fig. 1 shows a prior dipole moment detector and localizer system, a portion of which forms part of the present invention;
Fig. 2 shows a block diagram illustrating the processing steps utilized in the system of Fig. l;
Fig. 3 shows a synthetic aperture array dipole moment detector and localizer system in accordance with the principles of the present invention;
Fig. 4 shows the windowing process used in the system of Fig. 3 to synthesize a sensor array; and Fig. 5 shows the details of the processing performed in the system of Fig. 3.
DETAILED DESCRIPTION
Referring to the drawing figures, Fig. 1 shows a prior dipole moment detector and localizer system 10, a portion of which forms part of the present invention, as will be described below. This system 10 is described in aforementioned U.S. Patent No. 5,239,474. Fig. 1 depicts the system processing diagram of a fixed line array dipole moment detector and locator. This system 10 employs a fixed line array of magnetic sensors 11 to detect and localize a moving dipole 12. A complete understAn~; ng of the design and operation of this system 10 may be had from a reA~; ng of the above-identified patent application. As was mentioned above, a small disadvantage of this approach is the limited information generated by the system 10. This system 10 provides no dipole orientation information and only provides for gross localization of the location of the dipole 12.
More particularly, Fig. 1 shows a detection and location system 10 that includes an array of magnetic sensors 11 that is coupled to processing apparatus (shown in Fig. 2). The array of circular locations represent magnetic response locations 12 that are defined relative to the array of magnetic sensors 11. The magnetic response locations 12 have precomputed magnetic responses associated therewith which represent the magnetic field that would result if a magnetic dipole were present within each particular magnetic response location 12 at a plurality of different orientations. Also 208q31 2 -4a shown in Fig. 1 is a particular magnetic response location 12a (highlighted) in which is located a magnetic dipole 13, represented by the submarine. Also a plurality of arrows representing magnetic vectors 14 are shown exten~;ng from each sensor of the array of magnetic sensors 11 toward the particular magnetic response location 12a in which the magnetic dipole 13 is located. Each sensors re~;ng contributes a set of vectors indicative of the location of the magnetic dipole 13, thereby forming a sensed signature that is processed using the concepts of the present invention as described below.
Referring to Fig. 2, it shows a block diagram of processing apparatus 20 and the processing steps implemented therein utilized in the system 10 of Fig. 1. In particular, Fig. 2 shows the processing steps performed within the processing apparatus 20 of the present invention. As shown in Fig. 2, in a first processing step 31, data from the array of sensors 11 is processed using a linear model to predict each sensor's value using the other sensors. In addition, in a second proces~ing step 32, the data is time averaged to pclr~lllls long term integration thereof which ope.~tes as a low pass filter on the data. This data is used to adjust the values of the data co~ uLcd in the first pro-5 cessing step 31. Furthermore, in a third processing step 33, the predicted data, asmodified by the time averaged data, is subtracted from the measured data and the resul-tant data is exp~n-le~l in terms of Anderson's functions in step 34. Prior to operational use of the present invention, and as is in~ ate~ in process step 35, stored data is gen-erated in terms of the Anderson functions which comprise magnetic responses associ-10 ated with each of the m~gneti~ response locations 12 that l~lcscn~ the magnetic fieldthat would result if a m~gn~ti~ dipole were present within each particular m~gnetic response location 12.
The precomputed data generated in step 35 is then correlated with the resultant data gen~ldled in step 34 in procescing step 36. This correlation comprises a dot prod-15 uct of the two sets of data, and the correlation produces sharply increased dot productvalues when the two data values are substantially the same, while producing relatively low values when the data is only moderately different. A ~ imulll, or peak, corre-lated value is selected in procescing step 37. A threshold is selected in step 37, say for example, 50% of the peak correlated value, and correlated and no~n~li7~ signals 20 above the threshold are displayed by interpolating the relative positions (locations) of the magnetic response location and displaying them on a monitor, for example, as is illustrated in steps 39 and 40.
Fig. 3 shows a synthetic a~ellulc array dipole moment detector and localizer system 50 in accordance with the principles of the present invention. This system 50 is 25 comprised of a moving platform 51, such as an aircraft 51, for example, in which is disposed a synthetic apellulc array 52. The synthetic al~ellulc array 52 is coupled to processing apparatus 20 shown in Fig. 2. The synthetic a~)el~ulc array 52 is comprised of a single magl.clolllGlel~ for example, or an array of m~ .. lo~ which are moved by the aircraft 11 relative to the dipole. This motion generates a large amount of data 30 that is used to synthesize an array of sensors. This will be described in more detail below.
The array of circular locations represent the magnetic response locations 12 that are defined relative to a magnetic sensor 53, or magnelomeler 53, located on the aircraft 51 that houses the synthetic a~Gllul., array system 50. The magnetic response locations 35 12 have precolll~u~ed m~gnetic responses associated therewith which represent the magnetic field that would result if a magnetic dipole were present within each particular magnetic response location 12 at a plurality of dirrelGIlt orientations. Also shown in Fig. 3 is a particular m~gnetic response location 12a (hi~hlighte~) in which is located a magnetic dipole 13, represented by the subm~nn~. Also a plurality of aTrows repre-senting magnetic vectors 14 are shown extending from the sensor 53 on the aircraft 51 toward the particular magnetic response location 12a in which the magnetic dipole 13 is locatçd The sensor reading taken at a plurality of locations along a particular flight path of the aircraft 51 contributes a set of vectors indicative of the location of the mag-netic dipole 13, thereby forming a sensed signature that is processed using the concepts of the present invention as described below.
The synthetic a~llule processing utilized in the system 50 of the present inven-tion will be described with reference to Figs. 3 and 4. Fig. 3 shows the moving plat-form 51 having the magnelc met~ 53 disposed thereon that is moving to the left in the drawing figure. Fig. 4 shows the windowing process used in the system of Fig. 3 to synthesize a sensor array; and Referring to Fig. 4, there are a plurality of data samples 54 shown as dots along the direction of travel of the platform 51 (shown moving to the right in Fig. 4). A plurality of windows 55 are shown that each encompass a subset of N samples from the total number of samples taken by the synthetic a~llul~ radar 52.
Each subsequent widow 55 is offset from the preceding window 55 by one data sample 54.
The synthetic a~~ , is formed by sampling the vector (or resnn~nce) magne-loll~l.,r 53 on the airborne plafform 51. Once a predetermined number (N) samples 54 are collected, matched field processing is p~rc,lllled generally in accordance with the fixed array embo(lim.ont disclosed in the above-referenced patent application and Figs. 1 and 2. Initially, N s~mrles 54 are collected prior to processing the grid of interest in window 1. Thereafter, one or more new samples 54 are added (with the same numberof samples 54 being dropped off at the front end) to process a new window. At any given time after processing begins, the N samples 54 used for processing are repre-sented by the equation x = H + N = sThM3 + noise where s is the dipole matrix, h is the Anderson matrix, and 3xN is used for a vector m~gnelomelel 53 and lxN is used for a resonance m~gnetometer 53.
A detailed discussion of the processing p~rolllled in the present system 50 willbe ~ cll~se~l with reference to Fig. 5. Fig. S shows the details of the dipole moment detection and localization processing performed in the synthetic array dipole moment detector and l~ c~1i7Pr system 50. The data samples 54 from the m~ lometer 51 are stored in a memory and the Anderson functions are also stored, one for each possible response location.
Synthetic array spatial matching is achieved as follows. The two vector func-tions to which the product is applied comprise the c~lc~ te~l and measured magnetic S field vectors along the array. When the calculated field matches well with the measured field, a large value is e~cpec~ed for the inner product. The Anderson functions are defined with respect to a given position (xl,yl,zl). Each of the set of these positions is called a m~ihllulll response location. Thus each llla~illlUIII response location has its own associated Anderson function, and for a given c,lienlalion, has its own correlation 10 with the measultmellts. The maximum value for the inner products on the ma~imulll response location s (let~rmin~ where the dipole 12 is located (which maximum response location) and with what orientation.
Discrete implr.l.~nl~ion of the dipole moment ~etection and localization proces-sor 20 is achieved as follows. The dipole Illomellt detection and loc~li7~tion processor 15 20 has been implemented in the continuous domain. It is necessary however to mech-anize this processor 20 in the discrete domain since in the actual system implem~nt~-tion, only spatially sampled measulelllellts of a dipole 12, i.e. outputs from a synthe-sized array of sensors, are available for pr~cessing To model the discrete implementa-tion of the dipole moment detection and localization processor 20, a line of N magnetic 20 sensors is disposed along an x-axis with each sensor located at Xn. Let h (3xn) be the Anderson function matrix having the 1th row and nth column element defined as follows:
hln = sinl (~n)coss~ 3n) Eq. (1) where l = 0,1,3 and n = 1,2,. . . ,N, are indices of the order of the Anderson functions and the sensor number along the synthesized array, respectively. In this e~lJIt;ssion, ~n. the angle of the nth sensor relative to the dipole 12 located at (xl ,Yl), is the arctan-gent of the directional unit vector from the sensor to the dipole 12, i.e.,~0 n ( yl ) Eq. (2) Define a dipole orientation matrix, s(~l), as follows:
2o893l2 ---m l a 2m 1 ~~ -cos ~l 2sin ~1 s(~l)=M 3--ml ~~ 3--ml ~a =M 3sin~l 3cos~l Eq.(3) 2--m l ~a ---m l ~ ~ 2cos ~l -sin ~l By pre-multiplying the Anderson functions matrix by the transpose of the dipole orientation matrix, s(~l), a matrix H is produced defined in Eq. (4) below, which is the output of the array of sensors in response to a dipole 12 located at (xl ,Yl) with the 5 orientation angle ~ relative to the x-axis.
sT(~l) h - [Hx Hy3T= M -cos ~1 h ~. 3sin ~1 h + 2cos ~1 h Yl 2sin ? ~1 3cos ~1 -sin ? ~1 1 Eq. (4) 10 where ho, hl and h2 are vectors of values representing the discrete Anderson functions evaluated at the sensors' locations relative to the dipole 12 in according to Eq. (2.17) above.
-h ~ cosS~l cos5~2 cosS~N
h = hl = sin~lcos4~l sin~2cos4~2~- sin~Ncos4~N Eq. (5) - - sin2~lcos3~1 sin2~2Cos3~2Sin2lpNcos3q)N
Eq. (4) describes the field of a dipole 12 in terms of a weighted sum of the Anderson functions where the weights are dependent on the position and orientation of the dipole 12. Since the Anderson functions are not orthonormal, the expansion of an ~bill~y function requires the use of three adjoint Anderson functions which are 20 defined as follows:
ho h = hl = [hhT]-lh Eq. (6) h2 Now, for an albiLI~ function f = [fl f2 ... fN]T ~ senting the outputs of an 25 array of sensors, the expansion of this function in terms of a given set of Anderson functions is obtained as shown in Eq. (7) ai = hi f = ~ hinfn; i = 0,1,2 Eq. (7) n= 1 If Hs is the output of the array in response to a dipole 12 located at (xs=xl, Ys=Yl) with an orientation ~s= ~1, and it is exp~nded in terms of the Anderson func-5 tions defined in Eq. (5), the resulting Anderson weights are the elements of the dipoleorientation matrix shown in Eq. (3) and are given in Eq. (8) below.
~ a~,l ayl -a = [a,~ ay] = ax2 ay2 = hHT = [hHT]-lhHsT = [hhT]-lh(hTs(~S)) = s(~l) Eq. (8) a~3 ay3 The primary conceptual di~.Gnce between the system 50 of the present inven-10 tion and the prior fixed sensor array system 10 can be better understood by colllp~illgthe systems 10, 50 shown in Figs. 1 and 3. Specifically, the p~ y conceptual dif-ference lies in the manner by which the magnetic field is measulcd and converted to input data for subsequent processing. In the fixed line array system 10 of Fig. 1, mea-surements are taken continuously by all sensors 11 in the array, with each sensor 11 15 having a slightly different perspective of the dipole moment, which when processed together by the proces~in~ procedure del~lmilles the location of the dipole 12. On the other hand, in the synthetic aperture array system 50 of the present invention, the sen-sor 53 is moved physically in the aircraft 51 to synthesi~ an array of sensors in order to duplicate the required different aspect measurements of a dipole 12. The required 20 different aspect mea~ulelllGIlls are not ~imlllt~neously accomplished in the present system 50. Since it is anticipated the dipole 12 will not change position or orientation signifi~ntly within the time frame during which the synthetic apG~ G array is generat-ed, asynchronous measurements will not cause signif~nt change in the perf~n~n~e of the system 50 of the present invention. Aside from this major difference in the manner 25 of obtaining and inputting measured data, the processing procedure for detecting and delGlllli~ g the locations of dipoles 12 in the synthetic apGl~ulG array system 50 is substantially identical to the procedure in the fixed array system 10, and is shown and described with reference to Fig. 2 above.
The proces~ing perforrned using the method and apparatus of the present inven-30 tion will now be (li~cll~sed in more detail and ~ltili7ing ~ltçrnative terminology. Specif-ically, the data is IGImPO1a11Y filtered. This is accomplished by the low pass filtering discussed with reference to Fig. 2. Spatially coherent fluctuations are removed. This is also accomplished by the low pass filtering rli~cussed with reference to Fig 2. Then spatial matched filtçring is employed to dGI~lll~ihle the existence of magnetic dipoles in the detection region of the array of sensors 12. This is accomplished by the correlation step 36 discussed with reference to Fig. 2. Next the matched filtered data is pl~Jcessed using a square law detection scheme, and then this data is smoothed. The smoothed data is then thresholded and magnetic dipoles located in the data are loc~li7e~ This 5 corresponds to the threshold and interpolation steps 38, 39 ~ cusse~l with reference to Fig. 2. Finally, the position of the detected magnetic dipoles are displayed for viewing by the user of the system 30.
The following is a description that illustrates the generality of the synthetic array magnetic detection and locali_ation system S0 of the present invention. The method and 10 the exarnple describing the magnetic detection and loc~li7~tion technique of the present invention are based on a particular application of the principles disclosed; namely it has used a particular method of generating the response function, known as and described as the correlation. For the correlation, it uses a particular form~ tion of an inner prod-uct, in which the measured and the com~)u~ed m~gnsti~ ,~nses are each expanded in lS terms of Anderson functions.
The basic method used in generating the responses at the various chosen loca-tions relative to the system 50 is the following. Form an estim~te of the magnetic field on the moving sensor 53 (m~gnetc"~.ele~l, if there were a magnetic dipole located at each of the selected locations (these e~ s are the calculated values for the array 20 magnetic fields). Take measurements of the magnetic field at each location of the sensor 53 at the time desired to detect and localize a magnetic dipole within the field.
This ~llounls to collecting and storing data samples over time using a magne-lollle~ 53 located on a moving platform 51. The array of sensors is synthesi7P~1 from the collected data Remove as much of the spatial and l~lllpol~l variations caused by the 25 naturally occurring background as is possible using the leln~ol~l and spatial smoothing techniques disclosed herein.
Co~relate the Illc~ulGd values with each of the calculated values for the synthe-sized array of sensors, whereby the term correlate we mean the multiplication of the calculated array of values with the array of mea~ult;lll~nls and s~ g the results over 30 the synthe~i7Pd array of sensors (the values of these correlations for each selected loca-tion off of the synthesized array of sensors, expressed as a function of the actual dipole location is what is called the response function J for that selectç(l location). If one of the correlations resulting has a signifi~ntly larger value than the others and if it is greater than some predete~ninYl threshold, a detection is declared for the location off of 35 the synthesi7P,d array corresponding to the calculated values which resulted in the larger value of the correlation.
In the system 50, temporal gain results from matchillg the signal and noise bandwidths. Spatial coherence gain removes fluctuations common to the sensors.
Spatial ll~al~;hing gain arises from m~t~hing the spatial filter to the field of the dipole 12 using the Anderson functions. Post detection gain (track before detect gain) allows the S use of a lower recognition differential (false alarms are controlled by l~uiling a viable or realistic track).
Thus there has been described a new and improved system and method that pro-vide for dipole moment detection and localization using synthetic a~tillul~ array. It is to be understood that the above-described emb~Ylim~nt~ are merely illustrative of some of 10 the many specific embo~im~nt~ which represent applications of the principles of the present invention. Clearly, llull~l~us and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.
Claims (5)
1. A method of detecting and locating a magnetic dipole comprising the steps of:
sensing the magnetic field produced by the dipole using a moving magnetic sensor that is moved relative to the location of a magnetic dipole to synthesize a plurality of output signals that are representative of a plurality of spatially distributed magnetic sensors and that are indicative of the magnetic field sensed by the moving magnetic sensor;
generating a magnetic signature produced by the magnetic field and the dipole which represents a magnetic response function of the dipole by processing the output signals from the moving magnetic sensor to decompose the magnetic field into its magnetic field components; and processing the magnetic response function to produce magnetic signature features therefrom that are indicative of the location and relative orientation of the magnetic dipole and means for displaying the location of the dipole.
sensing the magnetic field produced by the dipole using a moving magnetic sensor that is moved relative to the location of a magnetic dipole to synthesize a plurality of output signals that are representative of a plurality of spatially distributed magnetic sensors and that are indicative of the magnetic field sensed by the moving magnetic sensor;
generating a magnetic signature produced by the magnetic field and the dipole which represents a magnetic response function of the dipole by processing the output signals from the moving magnetic sensor to decompose the magnetic field into its magnetic field components; and processing the magnetic response function to produce magnetic signature features therefrom that are indicative of the location and relative orientation of the magnetic dipole and means for displaying the location of the dipole.
2. A method of detecting and locating a magnetic dipole comprising the steps of:
forming a normalized estimate of the magnetic field to be sensed by a moving magnetic sensor that is moved relative to the location of a magnetic dipole using known magnetic dipole orientations located at each of a plurality of preselected locations to provide an array of estimated signals;
processing the estimated signals to produce a first set of Anderson function expansion coefficients representative of each of the estimate signals, and storing the Anderson functions;
measuring the magnetic field using the moving magnetic sensor that is moved relative to the location of the magnetic dipole that is to be detected to produce measured magnetic field signals;
removing spatial and temporal variations in the magnetic field measured by the sensor by temporally and spatially smoothing the measured magnetic field signals;
processing the temporally and spatially smoothed measured magnetic field signals to produce a second set of Anderson function expansion coefficients representative of each of the measured magnetic field signals;
correlating the first and second sets of Anderson function expansion coefficients to produce a response function for the magnetic dipole location; and identifying the location of the dipole if one of the correlations has a significantly larger value than the others, and if it is greater than a predetermined threshold, and the location of the dipole corresponds to the location represented by the measured magnetic field signals that produced in the significantly larger value, and providing a visual display of said location.
forming a normalized estimate of the magnetic field to be sensed by a moving magnetic sensor that is moved relative to the location of a magnetic dipole using known magnetic dipole orientations located at each of a plurality of preselected locations to provide an array of estimated signals;
processing the estimated signals to produce a first set of Anderson function expansion coefficients representative of each of the estimate signals, and storing the Anderson functions;
measuring the magnetic field using the moving magnetic sensor that is moved relative to the location of the magnetic dipole that is to be detected to produce measured magnetic field signals;
removing spatial and temporal variations in the magnetic field measured by the sensor by temporally and spatially smoothing the measured magnetic field signals;
processing the temporally and spatially smoothed measured magnetic field signals to produce a second set of Anderson function expansion coefficients representative of each of the measured magnetic field signals;
correlating the first and second sets of Anderson function expansion coefficients to produce a response function for the magnetic dipole location; and identifying the location of the dipole if one of the correlations has a significantly larger value than the others, and if it is greater than a predetermined threshold, and the location of the dipole corresponds to the location represented by the measured magnetic field signals that produced in the significantly larger value, and providing a visual display of said location.
3. A method of detecting and locating a magnetic dipole comprising the steps of:
precomputing a plurality of magnetic field signatures that are representative of a magnetic field using a moving magnetic sensor that is moved relative to the location of a magnetic dipole that provides signals representative of an array of sensors, assuming that there is a magnetic dipole located at each of a plurality of selected locations relative to the moving sensor, to provide for calculated estimate values;
storing the calculated estimate values;
taking measurements of the magnetic field at each of a plurality of locations in the presence of a magnetic dipole within the field by moving the sensor relative to the magnetic dipole and synthesizing signals that would be provided by the array of sensors;
removing the spatial and temporal variations present in the measured magnetic field caused by the naturally occurring background using temporal and spatial smoothing techniques;
correlating the measured values with each of the calculated estimate values for the synthesized array of sensors, by multiplying the calculated estimate values with the measured values and summing the results over the synthesized array of sensors; and if one of the resulting correlations has a significantly larger value than the others and if it is greater than a predetermined threshold, declaring a detection for the location corresponding to the calculated values which resulted in the larger correlation value, and providing a visual display of said location.
precomputing a plurality of magnetic field signatures that are representative of a magnetic field using a moving magnetic sensor that is moved relative to the location of a magnetic dipole that provides signals representative of an array of sensors, assuming that there is a magnetic dipole located at each of a plurality of selected locations relative to the moving sensor, to provide for calculated estimate values;
storing the calculated estimate values;
taking measurements of the magnetic field at each of a plurality of locations in the presence of a magnetic dipole within the field by moving the sensor relative to the magnetic dipole and synthesizing signals that would be provided by the array of sensors;
removing the spatial and temporal variations present in the measured magnetic field caused by the naturally occurring background using temporal and spatial smoothing techniques;
correlating the measured values with each of the calculated estimate values for the synthesized array of sensors, by multiplying the calculated estimate values with the measured values and summing the results over the synthesized array of sensors; and if one of the resulting correlations has a significantly larger value than the others and if it is greater than a predetermined threshold, declaring a detection for the location corresponding to the calculated values which resulted in the larger correlation value, and providing a visual display of said location.
4. Apparatus for detecting and locating a magnetic dipole comprising:
a moving magnetic sensor for sensing the magnetic dipole to provide a synthesized signal representative of an array of spatially distributed magnetic sensors that each provide output signals that are indicative of the magnetic field sensed thereby;
processing means for generating a magnetic signature of the magnetic field produced by the magnetic dipole which represents a magnetic response function of the dipole by processing the synthesized output signals from the magnetic sensor to decompose the magnetic field into its magnetic field components; and processing means for processing the magnetic response function to produce predetermined features therefrom that are indicative of the location and relative orientation of the magnetic dipole display means for displaying the location of the identified dipole.
a moving magnetic sensor for sensing the magnetic dipole to provide a synthesized signal representative of an array of spatially distributed magnetic sensors that each provide output signals that are indicative of the magnetic field sensed thereby;
processing means for generating a magnetic signature of the magnetic field produced by the magnetic dipole which represents a magnetic response function of the dipole by processing the synthesized output signals from the magnetic sensor to decompose the magnetic field into its magnetic field components; and processing means for processing the magnetic response function to produce predetermined features therefrom that are indicative of the location and relative orientation of the magnetic dipole display means for displaying the location of the identified dipole.
5. Apparatus for detecting and locating a magnetic dipole comprising:
a synthetic array comprising a moving magnetic sensor that provides an output signal representative of an array of magnetic sensors;
processing means coupled to the synthetic array, for storing an estimate the magnetic field signature to be detected by the moving sensor using a known set of magnetic dipole orientations at each of a plurality of preselected locations to provide an array of estimate signals that are represented by a first set of Anderson functions, and for processing magnetic field signals indicative of the magnetic field measured by the synthetic array in the presence of the magnetic dipole that is to be detected, and for removing spatial and temporal variations in the magnetic field measured by the synthetic array by temporally and spatially smoothing the measured magnetic field signals, and for processing the temporally and spatially smoothed magnetic field signals to produce a second set of Anderson functions representative of each of the magnetic field signals, and for correlating the first and second sets of Anderson functions and for summing the results over the array of sensors to produce a response function for the magnetic dipole location, and for identifying the location of the dipole if one of the correlations has a significantly larger value than the others, and if it is greater than a predetermined threshold, and the location of the dipole corresponds to the location represented by the measured magnetic field signals that produced in the significantly larger value; and display means for displaying the location of the identified dipole.
a synthetic array comprising a moving magnetic sensor that provides an output signal representative of an array of magnetic sensors;
processing means coupled to the synthetic array, for storing an estimate the magnetic field signature to be detected by the moving sensor using a known set of magnetic dipole orientations at each of a plurality of preselected locations to provide an array of estimate signals that are represented by a first set of Anderson functions, and for processing magnetic field signals indicative of the magnetic field measured by the synthetic array in the presence of the magnetic dipole that is to be detected, and for removing spatial and temporal variations in the magnetic field measured by the synthetic array by temporally and spatially smoothing the measured magnetic field signals, and for processing the temporally and spatially smoothed magnetic field signals to produce a second set of Anderson functions representative of each of the magnetic field signals, and for correlating the first and second sets of Anderson functions and for summing the results over the array of sensors to produce a response function for the magnetic dipole location, and for identifying the location of the dipole if one of the correlations has a significantly larger value than the others, and if it is greater than a predetermined threshold, and the location of the dipole corresponds to the location represented by the measured magnetic field signals that produced in the significantly larger value; and display means for displaying the location of the identified dipole.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US872,764 | 1992-04-14 | ||
| US07/872,764 US5387863A (en) | 1992-04-14 | 1992-04-14 | Synthetic aperture array dipole moment detector and localizer |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2089312A1 CA2089312A1 (en) | 1993-10-15 |
| CA2089312C true CA2089312C (en) | 1998-09-29 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2089312 Expired - Lifetime CA2089312C (en) | 1992-04-14 | 1993-04-07 | Synthetic aperture array dipole moment detector and localizer |
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| Country | Link |
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| CA (1) | CA2089312C (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN114217350A (en) * | 2021-06-29 | 2022-03-22 | 中国电子科技集团公司电子科学研究院 | Magnetic anomaly detection method and system and storage medium |
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1993
- 1993-04-07 CA CA 2089312 patent/CA2089312C/en not_active Expired - Lifetime
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| Publication number | Publication date |
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| CA2089312A1 (en) | 1993-10-15 |
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