Method and apparatus for tracking an aimpoint on an elongate structure

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

Description: TECHNICAL FIELD OF THE INVENTION This invention relates to imaging and guidance systems and more particularly to tracking an aimpoint on an elongate structure with arbitrary subimages. 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, 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. 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 discloses a method of recognising the target by identifying the discrete object nearest to the centre of the field of view. FR-A 2 656 106 discloses a method of recognising faint objects by detecting a pixel of maximum brightness and identifying an object when such a maximum occurs in closely adjacent pixels in successive images. 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. Certain subimages will appear to move at rates that are dependent on the location of the subimage 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 an aimpoint or an elongate target by using any number of subimages arbitrarily related to the aimpoint without range data. The invention provides a method for estimating the location of an aimpoint on an elongate target comprising the steps of: acquiring an aimpoint and a set of related subimages on an elongate target at a first time with a sensor; calculating the normalized distance in a first dimension d x and in a second dimension d y from each subimage to the aimpoint; at a second time reacquiring at least one of the subimages at an image position; and estimating the position of the aimpoint at an aimpoint image position (x A , y A ) wherein the position ( x, y) of each subimage at the second time is related to the aimpoint position by the formulas: x = x A + M o d x + bd x d y y = y A + M o d y + bd 2 y where M o and b are constants determined at each periodic time. The invention also provides a tracker for tracking the location of an aimpoint on an elonate target comprising: an aimpoint designator for establishing an aimpoint at an image position (x A , y A ) on an elongate target; a sensor for periodically acquiring a set of subimages at an image position (x, y) arbitrarily associated with the aimpoint; a normalized distance calculator responsive to the aimpoint designator and the sensor for calculating the distance in a first dimension and in a second dimension from each subimage of a first set of subimages from the established aimpoint; a processor coupled to the aimpoint designator, the sensor and the normalized distance calculator for periodically estimating the subsequent location of the aimpoint based upon the formulas: x = x A + M o d x + bd x + bd x d y y = y A + M o d y + bd 2 y where M o and b are constants determined at each periodic time; and memory for storing the normalized distances. The subimages may be arbitrarily associated with the aimpoint. It is a technical advantage of the invention that an aimpoint located on an elongate target 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 calculated at an initial time and saved for subsequent steps. At each subsequent time, a vector 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 due to uniform and non-uniform magnification due to closure. 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 view of an elongate target which may be tracked by the disclosed invention; FIGURE 2 is a part schematic, part cross-sectional diagram of a "fire and forget" missile which may incorporate the disclosed invention; FIGURE 3 is a flow chart of a subimage tracker which incorporates the disclosed invention; FIGURES 4 and 5 are one and two-dimensional models, respectively, of a tracker targeting an aimpoint with one subimage on an elongate object; and FIGURE 6 depicts the magnification of a subimage as a function of its position in the tracker's field of view. DETAILED DESCRIPTION OF THE INVENTION The preferred embodiment of the present invention and its advantages are best understood by referring to FIGUREs 1 through 6 of the drawings like numerals being used for like and corresponding parts of the various drawings. FIGURE 1 depicts an elongate target 10. Here target 10 is a runway but might also be a bridge, a train or a building relatively large in the horizontal dimension as compared to the vertical. An operator of the tracker, or an automatic algorithm, initially selects an aimpoint on runway 10 such as its geometric center. A tracker targeting runway 10 then acquires trackable subimages such as control tower 12 or airplane 14 from which it can track the center of the runway 10. The tracker then determines range normalized distances from each subimage to the designated aimpoint and saves these values for future calculations. This distance may be the actual number of pixels between the aimpoint and a subimage as sensed by the tracker's sensor or may be further normalized so that at least one subimage is a particular distance, such as "1", away from the aimpoint. The tracker reacquires the subimages at subsequent times and uses the range normalized distances and a mathematical model as will be explained in connection with FIGURES 3 through 6 throughout to estimate the location of the aimpoint at those subsequent times. If the tracker is part of a device such as a "fire and forget" missile, it will continually adjust its course to intercept the initial aimpoint until it reaches the aimpoint. As the tracker approaches runway 10, the subimages of runway 10 will exhibit complex motion relative to one another. This motion may be characterized as comprising a uniform and a non-uniform component. The uniform component of motion will cause the subimages to radially move away from the aimpoint as the tracker nears the aimpoint and the target image fills more and more of the tracker's field of view. The non-uniform component causes each subimage to move depending on its location in the field of view of the sensor. For instance, if a tracker targets an aimpoint on runway 10 between control tower 12 and airplane 14 and approaches runway 10 along its longitudinal axis from the left side of the figure, control tower 12 will appear to move toward the bottom of the field of view at one rate while airplane 14 will appear to move toward the top at a much smaller rate. Without accounting for the non-uniformities, the aimpoint would likely slide from the true aimpoint toward the bottom of the field of view as the tracker attempted to find a compromise position between control tower 12 and airplane 14. The compromise position would be one that would make the magnification of the subimages appear equal along the length of runway 10. FIGURE 2 depicts a "fire and forget" missile 16 which may incorporate the disclosed invention. Missile 16 delivers a warhead 18 which detonates upon impact with a target. The missile 16 contains a passive imaging sensor 19, such as a forward looking infrared camera ("FLIR"), that is sensitive to radiation emitted by the missile's target. The sensor 19 periodically acquires images within its field of view during operation. A latch 20 temporarily saves the information received by sensor 18 so that it is more accessible by central processing unit ("CPU") 22. CPU 22 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 and a normalized distance calculator for calculating the distance between each subimage and the aimpoint. CPU 22 has associated with it a memory 24. Memory 24 may contain the routines which CPU 22 runs and stores data necessary to the disclosed invention. CPU 22 controls the direction of missile 16 through fin control unit 26. Fin control unit 26 manipulates each fin 28 through a servo 30. Missile 16 is propelled by rocket motor 32. FIGURE 3 depicts a high level flow chart of a subimage tracker which incorporates the disclosed invention. An aimpoint is initially selected 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 (block 52). The tracker calculates the normalized distances between each subimage that it has acquired and the selected aimpoint in block 54. These values are saved for later use at each subsequent time, the tracker reacquires the subimages and estimates the location of the aimpoint from the previously calculated normalized distances in blocks 56 and 58 respectively. The math and the particular normalized distances are more fully described below. The tracker may then adjust sensor pointry (block 60) to maintain the aimpoint at the center of its field of view. These final three steps 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 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. The imager could, in fact, recede from the target and the tracker would still maintain the aimpoint properly. The approach to multiple subimage tracking without range estimates is based on a generalized geometric model. This model is based on the assumption that though the target and thus the subimages will be growing in the image during closure, 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 + d x i + n x i y i = y A + d y i + n y i where ( d x i ,d 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 + d x + n The key to accurately modeling each subimage position is the accurate determination of how the offset vector varies as a tracker approaches an elongate target. FIGURE 4 depicts the mathematical framework for determining the offset vector related to an elongate target in a two-dimensional universe, the vertical plane through the missile and the aimpoint. Here an aimpoint A is downrange of and below a missile "M" by a distance "R". A trackable subimage is located at B. B is further downrange of A by a distance " x ". For a large distance R relative to x , the angle between the subimage, missile and aimpoint may be expressed as: d y = x sin Θ R + x cos Θ This angle is distance from the aimpoint to the subimage which a tracker actually "sees" when it acquires an image. (In FIGURE 4 R is shown as on the same order of magnitude as X for purposes of clarity.) The vertical "magnification" of a subimage associated with an elongate target at a particular time may then be defined as: M = d y ( R 1 ) d y ( R 0 ) where d y ( R 0 ) is the distance between the subimage and aimpoint at an initial range of R 0 and d y ( R 1 ) is the distance between the subimage and aimpoint at a subsequent range R 1 . If the tracker is a missile designed to intercept the target, then R 1 will be less than R 0 . The magnification M may be used to model the behavior of the subimages between successive times and thus, may be used to predict the subsequent position of the aimpoint. This model will be more fully described in connection with FIGUREs 5 and 6. FIGURE 5 depicts a mathematical framework for determining the offset vector of a subimage associated with an elongate target in a three-dimensional universe. The four vectors R M , R T , R T/M and R C/T may be expressed as: where X, Y and Z are positive in the directions indicated by the depicted coordinate system. The distances between the subimage and aimpoint are indicated as x C and y C for the X and Y dimensions respectively. The coordinate system in FIGURE 5 may be transformed to a platform ("(p)") coordinate system at the missile position through the transformation: where The vectors R T/M and R C/M may be transformed into the new coordinate system as: The vector R C/M( P ) may be used to calculate the magnification function M as was done in connection with FIGURE 4. The Y and Z platform components correspond to the distances between the subimage and the aimpoint in the platform coordinate system. These may be divided by the total platform downrange distance R s + x C cos Θ to yield angular distances between aimpoint and subimage: or rearranging, d x R s + d x x C cosΘ = y C and d y R s = -( d y cos Θ + sin Θ) x C The distances d x and d y correspond to the distances between the aimpoint and the subimage in the horizontal and vertical axes of the image plane of the passive sensor. The distances d x and d y may be used to solve for ( x C , y C ) for a given geometry with the series of equations: These equations, are used to create test cases for simulation of the tracker and to illustrate the results in the following paragraph. FIGURE 6 depicts the magnification versus vertical image position for one particular geometry. Specifically, the solution is depicted in terms of M H and M v as a function of d y (6000) where M H = d x ( R 1 )/ d x ( R o ) , M v = d y ( R 1 ) / d y ( R o ) , R 1 = 3,000 and R o = 6,000 . The solution indicates that M H and M v are identical and are closely related to d y ( R 0 ) by an equation having the form of a line, M 0 + bd y ( R 0 ) . M 0 are b are constants. Combining these empirical results with the general equations: x i = x A + d x i + n x i y i = y A + d y i + n y i leads to the set of equations for the location of the image of the i th subimage at each successive time: x i = x A + M H d x i ( R 0 ) + n x i y i = y A + M v d y i ( R 0 ) + n y i or x i = x A + M o d x i + d y i d x i b + n x i y i = y A + M o d y i + d y i d y i b + n y i where d x i and d y i are understood to be determined when the aimpoint is first acquired. For N subimages, the above equations for one subimage may be expanded as: This equation itself can be more conveniently expressed as: x = H  + n where and H is the 2N x 4 matrix depicted above. At each successive time the tracker reacquires the subimages, all variables are known except those in  and the noise vector n . The vector  and hence the aimpoint may be estimated by several statistical methods, including a least squares technique:  = ( H T H ) -1 H T x where  and is the estimate of . This method will minimize the effect of the noise vector n . Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims: A method for estimating the location of an aimpoint on an elongate target (10) comprising the steps of: acquiring (50,52) an aimpoint and a set of related subimages (12,14) on an elongate target (10) at a first time with a sensor (19); calculating (54) the normalized distance in a first dimension d x and in a second dimension d y from each subimage to the aimpoint; at a second time reacquiring (56) at least one of the subimages (12,14) at an image position ( x,y ) ; and estimating the position of the aimpoint at an aimpoint image position ( x A , y A ) wherein the position ( x, y ) of each subimage (12,14) at the second time is related to the aimpoint position ( x A , y A ) by the formulas: x = x A + M o d x + bd x d y y = y A + M o d y + bd 2 y where M 0 and b are constants determined at each periodic time. The method of Claim 1 wherein said estimating step (58) comprises the step of calculating the subsequent location of the aimpoint using a least squares technique on a matrix of normalized subimage distances. The method of Claim 1 or claim 2 wherein said selecting step (58) further comprises the step of selecting a subimage using the criteria of subimage contrast. The method of any one of Claims 1 to 3, wherein said selecting step (58) further comprises the step of selecting a subimage using the criteria of subimage brightness. A tracker for tracking the location of an aimpoint on an elongate target (10) comprising: an aimpoint designator for establishing (50) an aimpoint at an image position ( x A , y A ) on an elongate target (10); a sensor (19) for periodically acquiring a set of subimages (12,14) at an image position ( x,y ) arbitrarily associated with the aimpoint; a normalized distance calculator responsive to the aimpoint designator and the sensor for calculating the distance in a first dimension d x and in a second dimension d y from each subimage of a first set of subimages from the established aimpoint; a processor (22) coupled to the aimpoint designator, the sensor (19) and the normalized distance calculator for periodically estimating the subsequent location of the aimpoint based upon the formulas: x = x A + M o d x + bd x d y y = y A + M o d y + bd 2 y where M 0 and b are constants determined at each periodic time; and memory (24) for storing the normalized distances. The tracker of claim 5, comprising a control system (26-32) for moving the sensor (19) towards each of the subsequent locations of the aimpoint. A missile (16) comprising: a track (19-24) according to claim 5 or claim 6; movable fins (28) for guiding the missile (16) 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