Method and apparatus for tracking an aimpoint with arbitrary subimages

Abstract: A method is disclosed for tracking an aimpoint (50) with a set of arbitrarily related subimages. A dimensional relationship between the subimages (52) and the aimpoint is initially determined (54) and saved for later calculations. Subsequently, at least one of the subimages is reacquired (56). The aimpoint at the subsequent time is then determined using the later acquired subimages and the saved dimensional relationship (58).

Description: TECHNICAL FIELD OF THE INVENTION This invention relates to imaging and guidance systems and more particularly to tracking an aimpoint with variable subimages. In certain computer control applications, it is necessary to track and measure an 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 or tracking system. 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. The target might also fire its own weapon 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. EP-A 0 423 984 discloses an object tracking system in which a "correlation tracker" (not described in detail) tracks a reference region centred on a target within the field of view of a sensor, while an "object tracker" (not described in detail) tracks the object itself. In each successive image from the sensor, the correlation tracker locates the reference region, and passes the information on to the object tracker. The object tracker, starting from the centre of the reference region and the extrapolated movement of the object relative to the scenery, recognises and locates the object, and passes the information to the correlation tracker, which then selects for future use a new reference region centred on the new position of the object. US-A 4 796 187 (on which the preamble of claim 1 is based) discloses a method of recognising the target by identifying the discrete object nearest to the centre of the field of view. 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 connected 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. As the tracker nears the target, however, the subimages will appear to move with the respect to each other. The position of the subimages with respect to one another may also change 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 elongate 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. 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. This system also lacked the ability to use trackable subimages that were not in the requisite pattern. Therefore, a need has arisen for a passive subimage tracker which is able to track any number of subimages arbitrarily related to an aimpoint without range data. The invention provides a method for tracking an aimpoint, comprising the steps of acquiring an aimpoint on a target and a set of subimages arbitrarily associated with the aimpoint by a sensor in a field of view; for a first time, calculating a geometric relationship between subimage and aimpoint; for a second time, reacquiring at least one of the subimages; and estimating a subsequent location of the aimpoint based on the subsequent location of the subimages and on the geometric relationship. The invention also provides a guidance system for tracking an aimpoint, the guidance system comprising: a sensor for initially acquiring an aimpoint and for periodically acquiring a set of subimages arbitrarily associated with the aimpoint; and a processor for calculating a geometric relationship between the first set of subimages and the aimpoint, and for estimating subsequent locations of the aimpoint based on the periodically acquired subimages and the geometric relationships. The invention further provides a missile comprising a guidance system according to the invention; fins for guiding the missile responsive to the estimated aimpoints; and a motor for propelling the missile. It is a first technical advantage of the invention that an aimpoint may be tracked without range data using subimages that are arbitrarily related to the aimpoint. A normalized distance from the aimpoint to each subimage is preferably calculated at an initial time and saved for subsequent steps. At each subsequent time, a target image magnification factor is calculated which maintains the same normalized distance from each subimage to the aimpoint. The subsequent location of the aimpoint may be maintained despite apparent movement of the subimages. A second technical advantage of the invention is its flexibility. The target image magnification factor may be slightly adjusted to model different scenarios as the user's needs merit. For instance, the target image magnification factor may be a single parameter, a pair of parameters that directly represent the magnification of the image, or a pair of parameters related to the magnification of the image. The first case will provide results using a model which assumes uniform target magnification. The second and third cases may be used to accurately target an image rotating about one of several axes. 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 depict the image and trackable subimages of the target from FIGURE 1 at a first time and at a subsequent time, respectively; FIGUREs 7a-c illustrate perspectively the apparent decrease in width of a turning target; FIGURE 8 illustrates a coordinate transformation of subimages used in a third preferred embodiment of the disclosed invention; and FIGURE 9 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 1-9, 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 these methods, a sensor identifies subimages according to the hottest (brightest or warmest) spot within a specified region and by finding a previously identified geometric pattern, respectively. 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 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 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 + δ y 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 + δ x + n The key to accurately modeling each subimage position is the accurate representation of the offset vector. This vector may be modeled several ways depending upon the accuracy desired or the expected behavior of the target. Single Magnification Model This model assumes that the subimages will move radially outward from the aimpoint (in an intercept mode) at a rate which is identical for the vertical and for the horizontal dimensions of the image. The offset vector may then be expressed as where S is the target image magnification factor ("TIMF") and ( d x i , d y i ) are the size normalized distances between a subimage i and the aimpoint A. 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 model leads to the following matrix representation for a subimage i : For N subimages, the previous equation may be expanded: This equation may be rewritten as x = H + n where =[ x A y A s ] T and H is the 2N x 3 matrix of "1"s, "O"s, and normalized distances 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  is an estimation of . This method will minimize the effect of the noise vector n . This model is suitable for applications where the target is generally in a plane perpendicular to a plane containing the weapon's trajectory, and when the viewing aspect of the target by the missile does not change significantly during closure. In such cases, the apparent motion of the subimages is caused primarily by the closure of the tracker and not by the rotation of the target with respect to the tracker. Dual Magnification Model FIGUREs 7a-c depict a physical complication to the Single Magnification Model. Here, a target is rotating about an axis generally parallel to the focal plane of the sensor onboard the tracker. Although the apparent height of target is constant through the rotation of the target, the apparent length of the target, l 1 , l 2 , and l 3 , decreases as the target turns. Two exemplary subimages (indicated by dashed boxes) located generally on each end of the target appear to move together. Conversely, if the target were to turn towards the tracker, its apparent length would increase and the two subimages would move apart. There would be no apparent movement, however, between two subimages located immediately above and below each other. Mathematically, therefore, the magnification differs for the two dimensions of the target. The offset vector δx may be expressed as: where ( S x , S y ) are the TIMF's and ( d x i d y i ) are the size normalized distances between subimage i and the aimpoint A. By using two magnification factors in the TIMF, the tracker can mathematically compensate for rotation of its target as depicted in FIGUREs 7a-c. Again ( d x i , d y i ) are range independent normalized distances as described in connection with the Single Magnification Model. These are typically normalized to the size of the target. This offset vector leads to the expression: for a single subimage i , and for N subimages. The previous expression may be more conveniently written as x = G  / + n where '=[ x A s x y A s y ] T and G 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 subimage, 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  / = ( G T G ) -1 G T x where  and / is an estimate of  and / . This method will minimize the effect of the noise vector n . Generalized Magnification Model FIGURE 8 depicts a rotation of the tracker's target image through an angle Θ. Such a rotation occurs in the real world when, for instance, a target pitches as it passes over the crest of a hill. If the change in target axes is not accounted for in such a situation, the aimpoint may be mistakenly discarded as inaccurate or may in fact be inaccurate and used by the tracker. The aimpoint may be considered inaccurate if the tracker implements an error checking routine that considers the variance of the actual location of subimages and an estimated location of each subimage. Such a variance calculation could yield a result that was too great. The tracker would then typically indicate to its related guidance system to "coast" along a course towards the last known aimpoint. The aimpoint used may be inaccurate if it rotates off of the target and onto background clutter. For instance, a long target with subimages selected on one end and with the aimpoint on another would cause such a failure if the target rotated. The tracker would select an aimpoint above or below the true aimpoint. In FIGURE 8, an aimpoint A, at ( x A , y A ) is is tracked using three subimages at ( x 1 , y 1 ), ( x 2 , y 2 ), and ( x 3 , y 3 ). After the target.rotates through an angle Θ and the weapon approaches the target, each subimage i at ( x / / i , y / / i ) is related to the aimpoint A by the equation: where M is a magnification factor for both dimensions of the image and the coordinates ( x A , y A ) are understood to be in the rotated coordinate frame. Weapon closure is allowed but is not necessary. That is, M could be 1 indicating the same range since initialization. Receding targets are also allowed in this model as in the other two models. In such a case, s, s x , s y , or M would be less than 1. The previous equation may be rewritten as: where: α = M cosΘ β = M sinΘ In this case, the 2 x 2 matrix, containing α, β, and -β depicted above is the TIMF. For N subimages the following set of equations relates the rotated subimages to the aimpoint: The previous expression may be more conveniently written as x = F // + n where  // =[ x A y A αβ] T and F 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. The vector  // and hence the aimpoint may be estimated by several techniques including a least squares technique where  // = ( F T F ) -1 F T x where  and // is an estimate of  // . This method will minimize the effect of the noise vector n . Once  // is estimated, M and Θ may be determined by the relations: α 2 + β 2 = M 2 FIGURE 9 depicts a flow chart of a subimage tracker which may incorporate all three embodiments of the disclosed invention. 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 depends on which mathematical model is implemented. In blocks 56 and 58, the tracker reacquires the subimages and estimates the location of the aimpoint from the previously calculated normalized distances using one of the particular mathematical models described above. If the tracker 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 its trajectory, block 60, to maintain the aimpoint at the center of its field of view. The final three steps depicted in FIGURE 9 are repeated until the tracker impacts its target or 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.

Claims: A method for tracking an aimpoint, comprising the steps of acquiring (50) an aimpoint on a target (12) and (52) a set of subimages arbitrarily associated with the aimpoint by a sensor (16) in a field of view; for a first time, calculating (54) a geometric relationship between subimage and aimpoint; for a second time, reacquiring (56) at least one of the subimages; and characterized in that it comprises the steps of estimating (58) a subsequent location of the aimpoint based on the subsequent location of the subimages and on the geometric relationship. A method according to claim 1, wherein the geometric relationship is a normalized distance from each subimage to the aimpoint. The method of claim 1 or claim 2, wherein said estimating step (58) comprises the steps of calculating the subsequent location of the aimpoint using a least square technique on a vector of subimage locations. The method of any one of claims 1 to 3, wherein said step of reacquiring (56) comprises the step of computing a target image magnification factor to determine the amount of movement of said at least one subimage within the field of view from the first time to the second time. The method of claim 4, wherein said estimating step (58) comprises the step of computing a single target image magnification. The method of claim 4, wherein said estimating step (58) comprises the step of computing a dual target image magnification factor. The method of claim 6, wherein the dual target image magnification factor contains a first element corresponding to a horizontal magnification factor and a second element corresponding to a vertical magnification factor of the target in the field of view. The method of claim 6, wherein the dual image magnification factor contains at least one element indicating the angular rotation of the target (12) about an axis generally perpendicular to the focal plane of the sensor. A method according to any one of claims 1 to 8, wherein the step of acquiring (52) a set of subimages of the target arbitrarily associated with the aimpoint is provided by the step of using predetermined subimage trackability criteria using an image sensor (16) operable to change its displacement relative to the target (12); wherein between a first time and a second time the displacement of the image sensor (16) relative to the target (12) is changed; and wherein the step of estimating (58) comprises the step of calculating a location of the aimpoint at said second time using the geometric relationship between the at least one subimage and said aimpoint. The method of claim 9, wherein said calculating step (58) comprises the step of calculating two magnification factors. The method of claim 10, wherein the two magnification factors comprise at least one factor indicating the angular rotation of the target about an axis generally perpendicular to the focal plane of the sensor. The method of claim 9, wherein said calculating step (58) comprises the step of calculating a single magnification factor. The method of claim 9, wherein said reacquiring step (56) comprises the step of reacquiring a set of subimages with a sensor (16) mounted on a missile (10). The method of any one of claims 1 to 13, wherein said acquiring step (52) comprises the step of selecting a subimage using the criteria of subimage hotspots. The method of any one of claims 1 to 14, wherein said acquiring step (52) comprises the step of selecting a subimage using the criteria of subimage contrast. The method of any one of claims 9 to 15, wherein said second calculating step (58) comprises the step of estimating the second aimpoint bases on a least squares technique of each location indicated by each reacquired subimage. A guidance system for tracking an aimpoint, the guidance system comprising: a sensor (16) for initially acquiring an aimpoint and for periodically acquiring a set of subimages arbitrarily associated with the aimpoint; and a processor (24) for calculating (54) a geometric relationship between the first set of subimages and the aimpoint, said processor being characterized in estimating (58) subsequent locations of the aimpoint based on the periodically acquired (56) subimages and the geometric relationships. A guidance system according to claim 17, wherein the geometric relationships are normalized distances from the first set of subimages to the aimpoint. A guidance system according to claim 18, comprising memory (26) for storing the normalized distances. The guidance system of any one of claims 17 to 19 comprising means (18, 28-32) for moving the sensor (16) towards each of the subsequent locations of the aimpoint. A missile comprising a guidance system according to any one of claims 17 to 20; fins (18) for guiding the missile (10) responsive to the 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: G01S3, F41G7