GB1567010A - Tracking arrangements - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO TRACKING ARRANGEMENTS (71) We E M I LIMITED a British company of Blyth Road, Hayes, Middlesex.

do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a correlation system for tracking a target.

A known such system is commonly known as an "autolock follow9' system and is used to track a moving target by means of signals from a sensor, such as a television camera, which may also be mounted on a moving vehicle. Tracking signals which are produced may be used to guide the vehicle in the direction of the target. The sensor used provides representations of a region which is mitially arranged by suitable means to be a region which includes the target. A representation called the "outer patch", is derived from the sensor and stored and updated at regular intervals. A smaller representation including the target, called the "inner patch", is derived from one of the series of outer patches and stored separately for use as a reference representation. The inner patch is compared with each outer patch produced after its derivation.In each comparison the inner patch is displaced relative to the outer patch until a satisfactory correlation is found between the picture points of the inner patch and those of the outer patch. The displacement thus obtained represents the displacement of the target between the production of successive outer patches and can, therefore, be used for the purposes of tracking.

Such systems are known for missiles intended to pursue a target. If the missile is stable in attitude, linear vertical and horizontal displacements of the two patches relative to one another will suffice to achieve correlation between them. However, the missile is often not roll-stabilised, so relative rotation between the two pictures occurs due to the roll.

According to the invention there is provided a correlation system for tracking a target moving relative to the system, including an input for receiving successive representations of a scene including the target, means for storing a reference representation of the target derived from one of the said successive representations, means for producing relative rotational displacements between the reference representation and further representations of an area of the scene including the target derived from the said successive representa tions means for comparing the representations and for producing correlation scores, and control means for causing the comparing means to compare the reference representation with each further representation in each of a plurality of different positions linearly displaced relative to that further representation to determine that one of those linearly displaced positions for which the best correlation score is obtained, and in each of a plurality of different positions having the said rotational displacements relative to that further representation to determine that one of those rotationally displaced positions for which the best correlation score is obtained, and for controlling the derivation of the reference and further representations in dependence upon the said best scores.

The system of the invention provides for separate estimation of the linear motion, and roll. The two estimates are thus provided by individual comparisons of the two patches from which the two errors can be combined to give final tracking data.

In order that the invention may be clearly understood and readily carried into effect one example thereof will now be described with reference to the drawings filed with the provisional specification of which: Figure 1 shows in block diagrammatic form a correlation arrangement operating in known manner and Figure 2 shows in block diagrammatic form a modification to the system of Figure 1 in accordance with the invention.

Figure 1 shows a typical correlation system for autolock follow using known methods for comparison of linear displacements only. The video input information from the sensor is provided at an input 1, in this example in the form of interlaced fields forming a raster of 416x292 picture points.

The data is pre-processed in a pre-processor 2 to give a suitable size for a representation hereinafter called the "outer patch", in this example 44x44 picture points, and is also supplied to a video output which will be referred to hereinafter. Pre-processor 2 also converts the data to digital form for the further processing. The data for the outer patch is then stored in an outer patch store 3 into which it is loaded in response to commands from a control microprocessor 4.

Microprocessor 4 also provides other control functions for the system, such as quantisation signals for automatic gain control (AGC) to pre-processor 2. These functions may be determined by input control signals for example from an operator. Also in response to commands from microprocessor 4 a part of the outer patch held in store 3 is selected to be a reference representation hereinafter called the "inner patch" and is loaded into an inner patch store 5. Typically the inner patch may be variable from say lOx 10 picture points to 24x24 picture points in size with the values along two orthogonal axes x and y, being variable independently.

In operation. data from stores 4 and 5 are supplied to a comparator 6 which carries out under the control of the microprocessor a point by point comparison of the two sets of data for a plurality of displacements of the two patches relative to each other, up to a predetermined maximum, say ten picture points, in any direction, to produce correlation scores for the displacements.

The scores for the correlations in the various displacements are passed to the microprocessor 4 which, via an x-y position counter and character generator 7 and control gate 8, provides suitable information on the video output and also displaces the outer patch to the correct position. The information is also provided via a digital to analogue converter 9 to give system error outputs indicating the determined displacement for the purpose of guidance of the missile.

As a result of expansion of the image in the course of pursuit the degree of correlation possible gradually falls. When this falls below a suitable figure, typically 75% of maximum, the inner patch store is updated from the outer patch store.

As mentioned hereinbefore the arrangement of Figure 1 provides data representing linear displacements, along two axes, of the target from the sensor boresight. At the expense of further complexity comparator 6 may be modified to take account of roll of a missile carrying the sensor.

However in the arrangement of the present invention the roll is accounted for by a modification of the Figure 1 system as shown in Figure 2 for which like reference numerals identify similar features.

The Figure 2 arrangement provides in addition a unit 10 which is arranged to rotate the information of the inner patch through a suitable increment on receipt of instructions from microprocessor 4. An OR gate 11 is also provided to transfer to comparator 6 either the inner patch from store 5 or the related inner patch from unit 10 as commanded by microprocessor 4.

In operation an estimate of the linear displacement is first provided as described hereinbefore and a correlation obtained.

The inner patch is then transferred from store 5 to unit 1() where it is rotated through the required increment. It is then compared in comparator 6 with the outer patch from store 3, at that position which gave the best correlation score in the preceding correlation, to determine if an increased score can be obtained. This is repeated for similar increments over the maximum range of rotation expected to be achieved in one field period, the rotational position giving the highest score being considered the true value. The data representing the best linear position and the best rotational position are then provided to microprocessor 4 which uses them as initial data for the next incoming field.

WHAT WE CLAIM IS: 1. A correlation system for tracking a target moving relative to the system, including an input for receiving successive representations of a scene including the target, means for storing a reference representation of the target derived from one of the said successive representations, means for producing relative rotational displacements between the reference representation and further representations of an area of the scene including the target derived from the said successive representations, means for comparing the representations and for producing correlation scores, and control means for causing the comparing means to compare the reference representation with each further representation in each of a plurality of different positions linearly displaced relative to that further representation to determine that one of those linearly

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. Figure 1 shows in block diagrammatic form a correlation arrangement operating in known manner and Figure 2 shows in block diagrammatic form a modification to the system of Figure 1 in accordance with the invention. Figure 1 shows a typical correlation system for autolock follow using known methods for comparison of linear displacements only. The video input information from the sensor is provided at an input 1, in this example in the form of interlaced fields forming a raster of 416x292 picture points. The data is pre-processed in a pre-processor 2 to give a suitable size for a representation hereinafter called the "outer patch", in this example 44x44 picture points, and is also supplied to a video output which will be referred to hereinafter. Pre-processor 2 also converts the data to digital form for the further processing. The data for the outer patch is then stored in an outer patch store 3 into which it is loaded in response to commands from a control microprocessor 4. Microprocessor 4 also provides other control functions for the system, such as quantisation signals for automatic gain control (AGC) to pre-processor 2. These functions may be determined by input control signals for example from an operator. Also in response to commands from microprocessor 4 a part of the outer patch held in store 3 is selected to be a reference representation hereinafter called the "inner patch" and is loaded into an inner patch store 5. Typically the inner patch may be variable from say lOx 10 picture points to 24x24 picture points in size with the values along two orthogonal axes x and y, being variable independently. In operation. data from stores 4 and 5 are supplied to a comparator 6 which carries out under the control of the microprocessor a point by point comparison of the two sets of data for a plurality of displacements of the two patches relative to each other, up to a predetermined maximum, say ten picture points, in any direction, to produce correlation scores for the displacements. The scores for the correlations in the various displacements are passed to the microprocessor 4 which, via an x-y position counter and character generator 7 and control gate 8, provides suitable information on the video output and also displaces the outer patch to the correct position. The information is also provided via a digital to analogue converter 9 to give system error outputs indicating the determined displacement for the purpose of guidance of the missile. As a result of expansion of the image in the course of pursuit the degree of correlation possible gradually falls. When this falls below a suitable figure, typically 75% of maximum, the inner patch store is updated from the outer patch store. As mentioned hereinbefore the arrangement of Figure 1 provides data representing linear displacements, along two axes, of the target from the sensor boresight. At the expense of further complexity comparator 6 may be modified to take account of roll of a missile carrying the sensor. However in the arrangement of the present invention the roll is accounted for by a modification of the Figure 1 system as shown in Figure 2 for which like reference numerals identify similar features. The Figure 2 arrangement provides in addition a unit 10 which is arranged to rotate the information of the inner patch through a suitable increment on receipt of instructions from microprocessor 4. An OR gate 11 is also provided to transfer to comparator 6 either the inner patch from store 5 or the related inner patch from unit 10 as commanded by microprocessor 4. In operation an estimate of the linear displacement is first provided as described hereinbefore and a correlation obtained. The inner patch is then transferred from store 5 to unit 1() where it is rotated through the required increment. It is then compared in comparator 6 with the outer patch from store 3, at that position which gave the best correlation score in the preceding correlation, to determine if an increased score can be obtained. This is repeated for similar increments over the maximum range of rotation expected to be achieved in one field period, the rotational position giving the highest score being considered the true value. The data representing the best linear position and the best rotational position are then provided to microprocessor 4 which uses them as initial data for the next incoming field. WHAT WE CLAIM IS:

1. A correlation system for tracking a target moving relative to the system, including an input for receiving successive representations of a scene including the target, means for storing a reference representation of the target derived from one of the said successive representations, means for producing relative rotational displacements between the reference representation and further representations of an area of the scene including the target derived from the said successive representations, means for comparing the representations and for producing correlation scores, and control means for causing the comparing means to compare the reference representation with each further representation in each of a plurality of different positions linearly displaced relative to that further representation to determine that one of those linearly

displaced positions for which the best correlation score is obtained, and in each of a plurality of different positions having the said rotational displacements relative to that further representation to determine that one of those rotationally displaced positions for which the best correlation score is obtained, and for controlling the derivation of the reference and further representations in dependence upon the said best scores,

2. A system according to Claim 1, further comprising means for storing each, in succession, of the further representations derived from the said successive representations.

3. A system according to Claim 1 or 2, further comprising a sensor coupled to the said input for providing the said successive representations.

4. A system according to any preceding claim, wherein the means for producing relative rotational displacements is arranged to receive the reference representation from the storing means and to rotate that representation.

5. A system according to Claim 1, 2 or 3, or 4 further including means responsive to said linearly and rotationally displaced positions at which the best scores are obtained to produce an error signal indicative of the difference between the actual and desired positions of the target relative to the system.

6. A guidance system for a vehicle or projectile including a system according to Claim 5 and means for controlling the motion of the vehicle or projectile in dependence upon the error signal.

7. A correlation system substantially as herein described with reference to Figures 1 and 2 of the drawings filed with the provisional specification.