Method and apparatus for rejecting trackable subimages

Abstract: A method for rejecting inaccurate subimages is disclosed for subimages used to track an aimpoint (70). The difference between the actual location of each subimage and the location of the subimage based on an estimated aimpoint is compared to a threshold value (74). The subimage is discarded if the difference exceeds the threshold value (76). The aimpoint may then be reestimated using the revised set of subimages (78).

Description: TECHNICAL FIELD OF THE INVENTION This invention relates to computer control and more particularly to a method of rejecting subimages associated with the aimpoint of a target. BACKGROUND OF THE INVENTION In certain computer control applications, it is necessary to track and measure the image of an object passively. It is especially important in weapons delivery systems that a target be so tracked. If such a target were tracked actively (i.e., using radar or laser range finding techniques), the target might detect the presence of the tracker. Once the target has detected the presence of the tracker, it can respond in one of several ways, all of which are deleterious to the tracker. For instance, the target might "jam" the tracker by bombarding it with signals that are comparable to those which the tracker is actively using or the target might fire its own weapon at the tracker, at the source of the tracking signal, or, even at the launching site of the tracker. In this way, the target could defeat the tracker, destroy the tracker or perhaps even destroy the launch site of the tracker, including the operating personnel. Passively tracking a target, however, imposes at least one serious limitation on the tracker. A tracker cannot accurately determine the distance or "range" to a target if it cannot actively sense the object. An active tracker, for instance, could determine the distance to a target by measuring the elapsed time from the emission of a radio frequency signal to the receipt of the signal reflected off of the target. The absence of a range measurement from tracker to target limits the passive tracker's ability to compensate for the apparent change in target image as the tracker moves in relationship to the target. Without this ability, a tracker will fail to maintain a constant target. In practice, a tracker benefits by tracking several subimages of its target's image. These subimages are two dimensional representations of structures that are physically related to the exact target location or "aimpoint" in the real three-dimensional world. Multiple subimages are used for redundancy purposes and because the actual aimpoint of the target is often untrackable due to low image contrast, brightness, or other reasons. As the tracker nears the target, however, the subimages will appear to radiate outwardly with respect to each other. The position of the subimages with respect to one another may also change in other ways in certain situations. For instance, two subimages located on a target may appear to approach one another if they are located on a face of a target that is rotating away from the tracker. A tracker targeting an elongated structure such as a runway or tall building will sense complex subimage motion due to closure of the tracker on the target. These subimages will appear to move at rates that are dependent on their location within the tracker's field of view. The tracker motion can be further complicated by background subimages erroneously tracked by the tracker. A tracker will then mislocate the aimpoint and, perhaps, completely miss its target if it cannot identify and compensate for "bad" subimages. Prior attempts to passively track an object have resulted in solutions with limited flexibility and poor accuracy. Heretofore, an object once identified as an aimpoint was tracked by tracking a predetermined number of subimages in a known pattern. Typically, the pattern chosen was a square with the aimpoint at its center and four subimages located at the four corners of the square. That system would track the four subimages located at the corners of the square and infer the actual aimpoint using the simple symmetry of the predetermined square. This method faltered when the geometry of the actual target resulted in less than four suitable subimages located in the requisite pattern or when the subimage selected was not associated with the aimpoint. This system lacked the ability to identify bad subimages. Therefore, a need has arisen for a passive subimage tracker which is able to determine if a selected subimage is behaving as though it were a subimage of an object physically related to the aimpoint. US-A 4 796 187 (on which the preamble of claim 1 is based) discloses a method for selecting a target within a cluttered image scene by identifying the discrete target nearest to the centre of the scene. EP-A 0 423 984 discloses a method of tracking an object by alternating a "correlation tracker" that tracks the region of the scene surrounding the target to compensate for movement of the sensor and an "object tracker" that tracks the movement of the target relative to the scenery, partly by extrapolation from the two previous images. In accordance with the present invention, a method for rejecting a subimage associated with an aimpoint is provided which substantially eliminates or reduces disadvantages and problems associated with prior trackers. The invention provides a method for rejecting subimages associated with an aimpoint comprising the steps of: determining the location of a set of subimages associated with an aimpoint at a first time; estimating the location of the aimpoint based on the location of the set of subimages at the first time and on a geometric relationship between the positions at a second time prior to the first time of the subimages and the aimpoint; estimating a set of locations of the subimages, each estimated location based on the estimated aimpoint at the first time; setting a displacement threshold; calculating a difference between each estimated subimage location and the corresponding measured location; and rejecting each subimage whose difference exceeds the displacement threshold. The invention also provides a guidance system for tracking an aimpoint, the guidance system comprising: an aimpoint designator for establishing an aimpoint on a target; a sensor for periodically acquiring a set of subimages arbibrarily associated with the aimpoint; a normalized distance calculator responsive to the aimpoint designator and the sensor for calculating the distance between each subimage from a first set of subimages and from the established aimpoint; a processor coupled to the aimpoint designator, the sensor and the normalized distance calculator for periodically estimating and re-estimating the subsequent location of the aimpoint and for estimating the subsequent location of a set of subimages, the estimated location of the aimpoint based upon a set of periodically acquired subimages and on the calculated normalized distances, and the re-estimated aimpoint based upon a subset of the set of subimages and upon the normalized distances, and the location of the set of subimages based upon the calculated normalized distances upon the estimated aimpoint; a comparator coupled to the processor for calculating a difference between the location of the periodically acquired subimages and of the estimated subimages and for creating the subset of the subimages, the subset of subimages comprising those subimages whose difference does not exceed a threshold; and memory coupled to the processor for storing the normalized distances. It is a first technical advantage of the invention that an aimpoint tracker can reject subimages which yield poor results. Typically, these subimages yield such results because they represent image clutter which is not physically associated with the target. This improves the future confidence of aimpoint calculations. A second technical advantage of the invention is its ability to redetermine the aimpoint based on an adjusted set of subimages. The adjusted set of subimages includes only those subimages which behave as though they are connected to the aimpoint in the real world. This improves the immediate confidence of aimpoint calculations. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: FIGURE 1 is a perspective schematic view of an anti-armor "fire and forget" missile system which may advantageously incorporate the disclosed invention; FIGURE 2 is a part schematic part cross-sectional diagram of the anti-armor missile of FIGURE 1; FIGURE 3 is a half-tone replication of digitized image of the target at a certain time as seen by the "fire and forget" missile depicted in FIGURE 1; FIGURE 4 depicts trackable subimages associated with the target depicted in FIGURE 1 at the time corresponding to the time of FIGURE 3; FIGUREs 5 and 6 are schematic elevational views depicting the image and trackable subimages of the target from FIGURE 1 at a first time and at a subsequent time, respectively; FIGURE 7 depicts a flow chart of a subimage tracker according to one embodiment of the disclosed invention; FIGURE 8 depicts a target being tracked by four subimages; FIGURE 9 mathematically depicts the target of FIGURE 3 in a two-dimensional coordinate system; and FIGURE 10 depicts a flow chart of the disclosed invention. DETAILED DESCRIPTION OF THE INVENTION The preferred embodiment of the present invention and its advantages are best understood by referring to FIGUREs 1-10 of the drawings, like numerals being used for like and corresponding parts of the various drawings. FIGURE 1 depicts an anti-armor "fire and forget" missile system which incorporates the disclosed invention. A missile 10 is fired at a target 12 by an infantryman 14. Infantryman 14 selects an aimpoint on the target 12 that he believes will offer the least resistance to missile 10 upon impact. Missile 10 incorporates a passive imaging sensor 16 and a processor (not shown) for acquiring and digitizing an image. Missile 10 also has movable fins 18 which are controlled by the processor to vary the direction of the missile. Missile 10 acquires trackable subimages related to the particular aimpoint selected by infantryman 14 which are within the field of view of the sensor 16. Missile 10 computes an initial geometric relationship between the aimpoint and the initial subimages and retains this in an onboard memory (not shown). Missile 10 periodically reacquires the subimages and computes a new aimpoint as will be more fully described below. Missile 10 may then adjust its path to intercept target 12 despite subsequent movement of the target 12 relative to infantryman 14 or missile 10. The type of missile depicted in FIGURE 1 is known as a "fire and forget" missile because infantryman 14 may forget the missile after launch and seek cover or engage other targets of opportunity. Missile 10, however, will continue to track and intercept target 12. Missile 10 may select subimages based on any number of criteria including, but not limited to, the "hot spot" and "correlator" methods. In the "hot spot" method, a sensor identifies subimages according to the hottest (brightest or warmest) spot within a specified region. In the correlator method, the tracker searches for a previously identified geometric pattern. FIGURE 2 depicts "fire and forget" missile 10 which may incorporate the disclosed invention. Missile 10 delivers a warhead 20 which detonates upon impact with target 12 (FIGURE 1). The missile 10 contains an onboard sensor 16 such as a forward looking infrared camera ("FLIR"), that is sensitive to radiation emitted by the missile's target. A latch 22 temporarily saves the information received by sensor 16 so that it is more accessible by a central processing unit ("CPU") 24. CPU 24 might itself comprise various subsystems (not shown) which may be implemented by hardware or software, including an aimpoint designator for initially establishing the aimpoint on the target, a normalized distance calculator for calculating the distance between each subimage and the aimpoint, and a comparator for comparing the actual and estimated Subimage locations and for rejecting those subimages that fail a certain statistical test. CPU 24 periodically acquires images within its field of view during the operation of missile 10. CPU 24 has associated with it a memory 26. Memory 26 contains the routines which CPU 24 runs and stores data necessary to the disclosed invention. CPU 24 controls the direction of missile 10 through a fin control unit 28. Fin control unit 28 manipulates each fin 18 through, for example, a servo 30. Missile 10 is propelled by a rocket motor 32. FIGURE 3 depicts a view of target 12 by sensor 16 of missile 10. The aimpoint, as originally specified by infantryman 14, is indicated by a crosshairs. In this particular embodiment, missile 10 has an infrared sensor. The engine compartment and wheel bearings of target 12, being hottest, are therefore indicated by the color white. Cooler structures such as the background are indicated by shades of grey and by black. FIGURE 4 depicts certain trackable subimages contained in FIGURE 3. Each trackable subimage is identified as a small square. All other areas are blocked from view. A tracker on board missile 10 initially selects a number of these subimages to track the target at an initial time. The tracker also calculates a normalized distance between the aimpoint selected by the infantryman 14 and each subimage. This distance may be the actual number of pixels between the aimpoint and the subimage as sensed by the tracker. The distance may be further normalized so that at least one subimage is a certain distance, such as "1", away from the aimpoint. These dimensions are saved for future use by the tracker. FIGURE 5 depicts a view of target 12 and six trackable subimages superimposed thereon. FIGURE 6 depicts the view of target 12 and the trackable subimages as depicted in FIGURE 5 at a slightly later time. The image of target 12 appears to grow in size as the missile 10 (FIGUREs 1 and 2) closes in on target 12. As a consequence, the subimages depicted also appear to grow in size and to move radially outward from the aimpoint. Three subimages, initially at the right side of FIGURE 5, are no longer visible in the field of view. The tracker will typically adjust its course to maintain the aimpoint (crosshairs) in the center of its field of view. This apparent movement of the subimages relative to the aimpoint and the loss of some subimages must be accounted for by the tracker to insure successful guidance of missile 10 to target 12. The approach to multiple subimage tracking without range estimates is based on a generalized geometric model. This model is based on the fact that though the target and thus the subimages will be growing in the image during closure to the target, the relative dimensions of the target do not change. This assumes that the angle of attack between the tracker and target stays fairly constant, which is common during most of the terminal phase of the missile flight. In the generalized geometric approach each tracker location is related to the aimpoint location using a normalized coordinate frame. An individual subimage i at image location (x i , y i ) can be related to the aimpoint A at image location ( x A , y A ) by the following equations: x i = x A + δ x i + n x i y i = y A + δy i + n y i where (δ x i , δ y i ) represents the offset in the image plane of subimage i from the aimpoint A, and ( n x i , n y i ) are additive noise terms which corrupt measurement of the true subimage location. These equations can be combined into a single equation using vector notation: x = x A + δx + n The key to accurately modeling each subimage position is the accurate representation of the offset vector. The offset vector may be expressed as: where ( s x , s y ) are the target image magnification factors ("TIMFs") and (d x i , d y i ) are the size normalized distances between subimage i and the aimpoint A. By using two magnification factors, the tracker can mathematically compensate for the rotation of a target. Thus (d x i , d y i ) are range independent normalized distances. Although size is typically chosen as the normalization factor because it is a measurable quantity of the target, any scaling factor can be used. This offset vector leads to the matrix representation: for a single subimage i , and for N subimages. The previous expression may be more conveniently written as x = H  + n where  = [ x A s x y A s y ] T and H is a 2N x 4 matrix of "1"s, "0"s, and normalized distances as depicted above. At each successive time the tracker reacquires the subimages, all variables are known except those in  and in the noise vector n . The vector  and hence the aimpoint may be estimated by several techniques including a least squares technique where  = ( H T H ) -1 H T x where  and is an estimate . This method will minimize the effect of the noise vector n . The generalized geometric approach may be modified to use a single magnification factor s . In such a case  would be a 3 x 1 matrix and H would be 2N x 3 matrix. This modified H matrix would have the normalized dimensions in a single column. FIGURE 7 depicts a flow chart of a subimage tracker that incorporates the generalized geometric approach. An aimpoint is initially acquired on the target in block 50 by an operator. The tracker then acquires multiple subimages associated with the chosen aimpoint according to internal criteria such as image contrast or image brightness in block 52. The tracker calculates the normalized distances between each subimage that it has acquired and the selected aimpoint in block 54. These distances are stored in a matrix whose particular form is described above. In blocks 56 and 58, the tracker reacquires the subimages and estimates the location of the aimpoint from the previously calculated normalized distances using the mathematical model described above. If the missile is closing on the target, some of the original subimages may leave the field of view of the sensor. Meanwhile, new trackable subimages may be found as target details appear. These new subimage locations may be added to the matrix of subimage locations, typically in block 56, as needed. The tracker may then adjust sensor pointing, block 60, to maintain the aimpoint at the center of its field of view. The final three steps depicted in FIGURE 7 are repeated until the missile impacts its target or the tracker otherwise ends its program. It should be understood that block 60 may comprise any number of related steps such as issuing commands to an operator to follow the aimpoint or displaying a crosshairs to pinpoint the location of the aimpoint in a display. The tracker may also be mounted in a stationary environment where it simply follows an aimpoint in its field of view without actively pursuing the target. FIGURE 8 is an elevational view of a targeted tank in which the tank 62 is moving from left to right with a velocity v(t). A trackable clutter object 64 in the upper left correlator (indicated by the burning oil well pump) will introduce a bias into the original aimpoint 66. If the clutter happens to be more trackable than the upper left edges of the tank, the aimpoint will slide toward the back of the tank (indicated by the cross 68). As the aimpoint slides, a higher percentage of clutter will be incorporated into future correlation reference snapshots to the exclusion of subimages of the target. Eventually, the aimpoint may be off the target altogether. The rate of sliding is a function of target velocity, the amount of clutter in any one tracker subimage, and the infrared ("IR") strength of the clutter relative to the target if the tracker is IR sensitive. The same scenario will occur if, instead of using four correlator groups as depicted, any number of arbitrary subimages were employed. The reason why the aimpoint can slide in scenarios like FIGURE 8 is that the aimpoint is computed by assuming that all the individual subimages have the same magnification around the aimpoint. If the magnification of one or more subimages are bad, then the magnification errors will bias the aimpoint. If the bad subimage tracker can be detected and removed, the aimpoint may be recomputed based on the positions of the remaining good subimages. Bad subimages may be detected by looking for large differences between the measured subimage location x and the estimated measurement x and : δ x = x - x Each estimated subimage location is determined from the estimated aimpoint. Typically this is accomplished by adding the product of an estimated magnification factor and the normalized offset vector to the estimated location of the aimpoint. The estimated aimpoint is itself determined by the location of all subimages, both good and bad, as described above. FIGURE 9 depicts the location of the measurements and the estimated measurements in a scenario such as FIGURE 8. The measured locations of the subimages, x 1 , x 2 , x 3 , and x 4 are indicated by squared points while the estimated measurements, x and 1 , x and 2 , x and 3 , and x and 4 are indicated by circled points. The estimated and true aimpoints are indicated by x and A and x A respectively. Here, the residual, or error, of the upper left subimage x 4 - x and 4 is larger and of a different direction compared to the other three subimages. A robust statistical procedure may be used to detect the drift where: M = median (δx i ), i = 1, 2, ... N MAD = median ( δ x i - M ) , i = 1, 2, ... N MAD stands for the median absolute deviation. Other mathematical differences may also be used to test the behavior of each subimage. The MAD is a robust estimator of the scale or spread of a distribution of values. The ratio MAD/0.6745 approaches the standard deviation for a Gaussian distribution as N becomes large. A robust T-test may be used of the form: T i = (δ x i - M ) 2 ( MAD /0.6745) 2 , i = 1, 2, ... N If T i is greater than a predetermined displacement threshold, then the tracker measurement is considered bad and is deleted. The T i statistic will not be corrupted by bad subimages as long as the majority of the trackers are good. The test is performed for both the x and y image dimensions. The robust T-test would eventually detect and reject the bad subimage of FIGUREs 8 and 9. By recomputing the aimpoint and magnification estimates using a subset of the subimages of only the good measurements, the reestimated aimpoint will be much closer to the true aimpoint. FIGURE 10 depicts the steps described above and is a detailed diagram of step 58 of FIGURE 7. An aimpoint is estimated in block 70 initially using all subimages available to the tracker at a certain time. In block 72, each subimage location is estimated using the aimpoint estimate of block 70. This calculation may typically be accomplished by adding the product of a range normalized distance and magnification factor or factors (TIMF) to the aimpoint location. Each subimage has a range normalized distance that relates its location to the location of the aimpoint. A difference is calculated in block 74 between the estimated subimage location from block 72 and the measured location of each subimage in block 56 (FIGURE 7). This difference is then statistically compared to a threshold value in block 76. If the difference exceeds the threshold value, then the subimage is rejected and deleted from the memory or database containing all of the subimages. In block 78, a new aimpoint is calculated using the revised aimpoint database.

Claims: A method for rejecting subimages associated with an aimpoint (66) comprising the steps of: determining (56) the location of a set of subimages associated with an aimpoint (66) at a first time; estimating (58, 70) the location of the aimpoint based on the location of the set of subimages at the first time and on a geometric relationship between the positions at a second time prior to the first time of the subimages and the aimpoint (66), characterized in that it comprises the steps of estimating (72) a set of locations of the subimages, each estimated location based on the estimated aimpoint at the first time; setting a displacement threshold; calculating (74) a difference between each estimated subimage location and the corresponding measured location; and rejecting (76) each subimage whose difference exceeds the displacement threshold. The method of claim 1, comprising the step (78) of recalculating the location of the aimpoint based on all of the remaining subimages of the certain set. The method of claim 2 wherein the recalculated location of the aimpoint (66) is calculated (78) using a least squares technique. The method according to any of claims 1 to 3, wherein the difference is a T-test. A method for tracking an aimpoint comprising a method according to any one of claims 1 to 4, and comprising the steps of: acquiring (50, 52) the aimpoint and a set of subimages arbitrarily associated with the aimpoint at the said second time; calculating (54) the normalized distance from each subimage to the aimpoint; and at a later time reacquiring (56) at least one of the subimages. The method according to claim 5, comprising the step (58) of re-estimating the aimpoint (66) based on the remaining later subimages. The method according to claim 6, wherein the estimated (58) and re-estimated (78) locations of the aimpoint (66) are determined using a least squares technique. A method according to claim 6 or claim 7, wherein the step of acquiring (50, 52) the aimpoint on the target and the set of subimages associated with the aimpoint and the step of reacquiring (56) at least one of the subimages are carried out using a sensor (16) operable (18, 28-32) to change its displacement relative to the aimpoint; and comprising the step of changing the displacement of the sensor (16) relative to the target (12) in accordance with the re-estimated location of the aimpoint (66). A guidance system for tracking an aimpoint (66) by rejecting subimages associated with said aimpoint, the guidance system comprising: an aimpoint designator (14) for establishing an aimpoint on a target (12); a sensor (16) for periodically acquiring (52) a set of subimages arbibrarily associated with the aimpoint; a normalized distance calculator (24) responsive to the aimpoint designator (14) and the sensor (16) for calculating (54) the distance between each subimage from a first set of subimages and from the established aimpoint; characterized in that it comprises a processor (24) coupled to the aimpoint designator, the sensor and the normalized distance calculator for periodically estimating and re-estimating (58, 78) the subsequent location of the aimpoint and for estimating the subsequent location of a set of subimages (72), the estimated location of the aimpoint (58) based upon a set of periodically acquired subimages (56) and on the calculated normalized distances (54), and the re-estimated aimpoint (78) based upon a subset (76) of the set of subimages and upon the normalized distances, and the location of the set of subimages (72) based upon the calculated normalized distances (54) upon the estimated aimpoint (58); a comparator coupled to the processor for calculating a difference between the location of the periodically acquired subimages (56) and of the estimated subimages (72) and for creating the subset of the subimages, the subset of subimages comprising those subimages whose difference does not exceed a threshold; and memory (26) coupled to the processor for storing the normalized distances. The guidance system of claim 9, comprising a control system (18, 28-32) for moving the sensor (16) towards each of the subsequent locations of the aimpoint (66). A missile comprising a guidance system (16-26) according to claim 9 or claim 10; fins (18) for guiding the missile (10) responsive to the re-estimated aimpoints; and a motor (32) for propelling the missile.

Inventors: MCWILLIAMS JOEL K, RHEEDEN DON R VAN, MCWILLIAMS, JOEL K., RHEEDEN, DON R. VAN
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1992
Main classification: F41G7
IPC codes: F41G7, G01S3