GB1574022A - Target tracking systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO TARGET TRACKING SYSTEMS (71) We, SIEMENS AKTIENGESELL SCHAFT, a German Company of Berlin and Munich, German Federal Republic, 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 invention relates to target tracking systems of the type in which scanning seqentially covers a specific zone containing the target, and which a sensor is provided, together with an analysis circuit which produces follow-up signals from the detected target, where any one image sequence is correlated with the preceding image sequence.

The German Patent Specification No.

2,258,046 describes a target tracking process for use where a target possesses a plurality of zones, which are separate from one another and have different grey values.

Four spaces tracking gates are provided, which mark the edges of the target, i.e.

zones having a specific grey value. The grey value is determined by amplitude investigation. Through the intersection point of their two connection lines the four space tracking gates mark a measuring window, which is assumed to be the target centre, and is used in the tracking process to produce control parameters. However, determination of the target centre solely by grey value differentiation produces only very rough and inaccurate target information, and impairs the accuracy of the control parameters which serve for tracking.

The German Patent Specification No.

2,106,035 discloses a target tracking system in which a given target diagram is stored and by means of a window the entire detection zone is scanned consecutively in stages.

The actual target information contained in the window in each scan is logic-linked to the given, rigid target diagram in a correlation circuit, whereby a correlation number is formed which indicates the degree of of identity between the target diagram and the actual target. The percise zone having the highest correlation number is considered to be that containing the target from which the control parameter for tracking is then formed. The disadvantage of this system is that a predetermined target diagram has to be used, which cannot readily take into consideration all possible and different target shapes. This is particularly true in view of the fact that the target diagram varies with the range of the target.

Furthermore, the simple yes into pattern which results from a correlation in this known prior art is very prone to suffer from any interference because in the case of a moving target, the background also changes rapidly, and is therefore included in the analysis. It is also possible that the variations occurring as the target line of sight travels over the target, so-called glint, may lead to inaccuracies. In extreme cases, the target can be lost entirely.

One object of the present invention is to provide a target tracking system which substantially overcomes these difficulties, and can ensure a better and more accurate alignment and tracking of the line of sight to a specific point within the target.

The invention consists in a target tracking system in which there is provided a scanning arrangement comprising a sensor that, when operating, sequentially sweeps a scanning pattern over a specific zone containing the target to produce a sequence of output signals representing quantised image elements of the scanned zone, and so produce an image formed by the distribution in the sequence of the output signals whose individual amplitudes represent the heat value (thermal signature) and/or the bright ness value (optical signature) of said target during scanning, each said scanning sequence being fed into an image store and a comparator store being provided for storing the signals of the preceding image sequence and enable the contents of the image store to be releated to the stored elements of the preceding image, taking into account the distribution and amplitude values of individual image elements, using an analysis circuit which effects a multiplication of the related store contents, image element by image element, as each new image element output signal is entered into said image store, thus providing a correlation of the incoming image sequence with the preceding image sequence to detect any change in the position of the correlation maximum in successive scanning sequences, the coordinate values of the new position being used as an indication of the new target position, and to control any requisite re-adjustment of the line of sight of the scanning arrangement.

Due to the fact that two separate items of infoirmation concerning the target are logic-linked to -one another, namely the contour together with the optical and/or thermal signature of the target, for inclusion in the analysis, the consequences of undesired movements of the line of sight can be largely suppressed. Even in the case of relatively fast moving targets, the direction of the line of sight remains substantially constant at a specific region of the target during the tracking process.

The invention will now be described with reference to the drawings, in which : -- Figure 1 schematically illustrate a display of an image area with two different states of a target: Figure ~ 2 schematically illustrates the distribution of signals showing the target and the optical andlor the thermal signature for a given target state; Figure 3 is a block schematic circuit diagram of one exemplary embodiment of a system constructed in accordance with the invention: Figure 4 is an explanatory diagram of the composition of an image by image points: Figure 5 is a block schematic circuit diagram of part of a system in accordance with the embodiment shown in Figure 3, this being that part required to effect correlation:: Figure 6 illustrates an exemplary display providing a signal with individual amplitude values indicated; and Figure 7 schematically illustrates various steps of the correlation process with the image shown in Figure 6.

Figure 1 illustrates the screen of a viewing device SG, on which can be seen the resultant signals produced by row-wise scanning of a target zone in the direction x, where parallel rows a, b, c, d . . . z are provided. Instead of this television raster type scanning, different forms of scanning, e.g. radial scanning, can be used in known manner. The scanning can be effected by passive sensors (detectors) e.g. elements sensitive in the infra-red range for example, preferably using a plurality of photo-diodes arranged in a row. The scanning can also be effected sequentially in a series of rows, or using an intermeshed scanning of alternative rows in respective image frame periods.

The viewing device SG is provided with an image window BF, which is set at a detected target during the vector process.

Then, using corresponding control devices in known manner, with any movement of the target, the line of sight of the sensor is displaced in both the x direction and y direction by means of adjusting quantities x0 and y0 so that the target is held at a specific point of the image window. The details of this procedure can correspond to that used in known target tracking radar devices, as described for example in Skolnik's book entitled "Radar Hand book", pages 21-1 to 21-53.

In the present example the contour of an aircraft viewed from the front is represented in the image window BF. For the N-th scanning period (image No. N) of the target it will be assumed that signals are produced in the positions of a scanning sequence shown by circles, with an expansion in the contour due to the body of the aircraft, clearly visible at the centre of the contour. In the following scanning period N+1 (image No. N+1) the Jtarget has moved and its contour now possesses a centrally expanded contour at a position in the scanning sequence indicated by image elements x. The known tracking devices normally operate only on the basis of such simple Yes-No information regarding the contour, which information is obtained by checking whether a threshold value is overshot or undershot by each echo signal.

In addition to the actual target information, a series of interference signals will be present, such as those referenced SS1 to SS4, which originate from background disturbances.

The viewing device screen shown in Figure 2 has the same construction as that in Figure 1 and possesses an image window BF, and shows the state of the (N+ I)th scanning period indicated by signal elements x to show the target contour. A further item of information, which contains the signal strength (amplitude) of the respective received signals is indicated by a respective number of horizontal lines. For example, this may be an item of greater or lesser strength brightness information (optical signature) and/or strength of temperature radiation (thermal signature). The receiver, and its detectors must be designed accordingly, especially remembering that background signals SSl to SS4 have a corresponding optical and/or thermal signature.

For the target tracking analysis i.e. in order to obtain the adjusting quantities x and yO, the two items of information are logic-linked, i.e. the control quantities are dependent upon the contour together with the optical and/or the thermal signature of the target. This produces a considerably more accurate alignment of the line of sight to a specific zone of the target, e.g. to an engine of an aircraft, than has hitherto been readily available.

In the block schematic circuit diagram shown in Figure 3 the signals pass from a video-detector (not shown) which is provided in a receiving device ER incorporating a sensor SO, to be fed via a threshold stage SW, and an AjD converter stage AD, to produce signals of digitalised form with quantised amplitude values which are supplied to an image store BS. The latter stores the received signals for one scanning period (image period) under the control of a pulse generator TG. The image store BS is bridged by a line UL which leads to one input of a correlator KR. The second input of this correlator is supplied from the output of the store BS. In the correlator KR the logic linking of the two items of information, namely the contour of the target with the optical and/or thermal signature of the target is carried out.In the device NG, adjusting quantities x0 and y0 required for accurate realignment of the line of sight VL to a specific zone of the target are then produced. For this purpose these adjusting quantities are transmitted to the sensor SO where there are provided control devices (not shown) for the line of sight adjustment in known manner. Devices for this purpose are described for example in "Applied Optics", April, 1966, Vol. 5, No. 4 pages 497 to 505. In this embodiment the analysis circuit therefore comprises the converter AD, store BS, correlator KR and device NG.

By way of further explanation Figure 4 shows an image area (e.g. image window) of a zone which is to be scanned. The image consists of rows a, b, c . . . z and columns 1, 2, 3 . . . n. Thus a total of n.z image points are provided. The scanning is effected in such manner that firstly the row a is scanned and then the row b, row c, etc. up to row z.

A detailed illustration of the analysis circuit showing the various store arrangements which are required in this embodiment for the implementation of the correlation is shown in Figure 5. The amplitude values of the relevant image points for image N are entered in full in the image store BS1 consecutively in accordance with the row sequence shown in Figure 4. Thus, leading from the right there is first the row a, then the row b etc. to the row z, and in each of these rows the image points 1 to n are recorded to form an entire target image N, giving both the individual signal amplitudes as stored in the store BS1 and the contour, with reference to the sequence in which the scanning has been carried out.

At the beginning of the next scanning period, i.e. for producing the image N+1, the full data relating to the image N is stored in the image store BS1, whilst the following image N +1 is entered element by element into a comparator image store BS3 in accordance with the scanning programme. Arranged in parallel to the image store BS3 there is a comparator store BS2, whose function will be explained later. The comparator image store BS3 must possess at least the same storage capacity as the image store BS1 and comparator store BS2.

The corresponding store positions in the image store BS1 and comparator store BS3 are fed into each stage of a common multiplication element array. To simplify the drawing, only a part of the multiplication array has been shown, with multiplication element Vnz for the last image point of the row z to the element Vlz for the first image point of the row z, and the element Vla for the row a. In these multiplication elements the amplitude values in the relevant register positions of the image store BS1 are multiplied with those of the comparator image store BS3. The products obtained in this way are fed to a common line GL and added. The line GL leads to a maximum level stage MLS, which in practice is expediently provided with interconnected adder circuits, which have now been illustrated here, however, to simplify the drawing.The stage MLS operates as follows: Commencing from an initial threshold value (threshold value zero or a specific minimum threshold value), every higher sum value that appears on the line GL is stored in the stage MLS and used as a new threshold value which is retained until there is another higher sum value, whereupon the latter forms the new threshold value etc. This ensures that the relevant threshold value is always equal to the highest amplitude value. At the end of the scanning of an image the threshold value is reset by the pulse generator TG to an initial threshold value, and the cycle recommences. Thus the behaviour of the stage MLS is similar to that of known maximum value stores.

The pulse generator TG is provided for the control of the various scanning processes, and triggers the beginning of scanning, i.e. to commence with row a at point 1. At each subsequent signal instant, any incoming echo pulse or target signal must be quantised and its amplitude value entered into a first register position of the store BS3. Thereafter the pulse generator TG supplies shift pulses for the comparator image store BS3 to move the data along, so that any contour is compared with all positions in BS1. The pulse generator TG also emits counting pulses, which are fed to an image point counter BPZ.Thus, this image point counter indicates which image point has been fed into the comparator image store BS3 at any specific instant of time, and how many image points have been scanned in a specific period of time from the beginning of each scanning period.

Following the scanning of one complete image point counter has counted up to z.n. If the image contents of the comparator image store BS3 are displaced further to the right, the image point counter must also count on beyond n.z, for such time as the displacement process lasts.

On the occurrence of a maximum output value from the stage MLS the associated counting value in the image point counter BPZ is selectively passed through to an assignment store ZOS. Thus the relevant count value Zp is entered in this assignment store. This assignment store is operated in such manner that on the occurrence of a new, greater maximum value at the stage MLS, the previous value of Zp during that sequence is erased and replaced by the corresponding new value.

Since a displacement of the image contour of a detected target from one image N to the next image N +1 may have occurred and normally will in fact have occurred, the displacement of the image N +1 in the comparator image store BS3 should be continued after scanning has reached image point n in row z. This additional displacement is dependent upon how rapidly any moving target can have moved on within any one image sequence. Thus the said additional displacement is a function of the target speed and of the scanning frequency.

As already mentioned, the shift pulse trains for the counter BPZ should also be continued for a specific time at the end of each actual scanning period. Sufficient time is normally available for this purpose, since there is usually a certain dead time between the end of one scanning of an image and the beginning of a new scanning.

On the other hand, the shift pulse trains for the comparator image store BS2 are terminated at that moment at which the last image point in the image window BF has been entered. Thus this comparator image store BS2 contains, at that time, a complete set of image element signals for the window BF. At the end of the comparison process, i.e. at the end of the passage of the image N+1 through the comparator image store BS3, the entire contents of the comparator image store BS2 are transferred in parallel mode into the image store BS1 by one single switching command that is expediently provided by the pulse generator TO by means (not shown) so that the contents of the image store BS1 relating to image N are erased.In this way the signals for the next image N+2 can then be put into the comparator image stores BS2 and BS3 and at the same time the new correlation between the images N+1 and N+2 can be effected.

Simultaneously to the storage transfer from the comparator image store BS2 into the image store BS1, the N+1 image contents of the comparator image stores BS3 and BS2 are erased so that all the stores are prepared for the processing of the next image sequence N+2.

At the end of the comparison process the assignment store ZOS contains the counting value Zp, associated with the maximum value Mp, for the image N+1. The counting value Zp is then fed to a differenceforming circuit DS which detects what is the amount of the difference between the new counting value Zp of the current maximum and the counting value Zp' of the previous maximum value, relating to image N, which is contained in the intermediate store ZWS. This difference value Zp'-Zp indicates by what amount the target has become displaced in the raster sequence between the scanning of image N and the next scanning of image N+1, taking into consideration the amplitude distribution. Then, in known manner, the locating device is adjusted in such a manner that the target continues to lie at the same, or at a previously determined point of the image window shown in Figure 4.

After the scanning of image N+1, the counting value Zp is entered in the intermediate store ZWS and is kept available for comparison with the value obtained when scanning the image N+2.

Figure 6 shows a simple example of the contours defined by an amplitude distribution, assuming that there is a scanning raster of five rows a to e with six image points in each row. Thus a total of thirty image points must be scanned consecutively in any one single scanning process. It will be assumed that in the N-th scanning, the amplitudes of the output signals are distributed in the manner indicated by rectangles drawn as solid lines. Thus in row b the second image point has the amplitude 1, the third image point has the amplitude 3 and the fourth image point has the ampli tude 1. In addition the third image point in row c has the amplitude 1. Two further image points indicate typical background or noise signals, namely the first image point in row d and the last image point in row e.In the scanning of the next image N +1, it is assumed that the target has moved upwards and to the right, so that amplitude values 1, 3 and 1 now lie in the first row at image point 3, 4 and 5 as indicated in broken-line rectangles. The underlying amplitude value 1 lies at the fourth image point in row b. Thus overall the target has travelled by one image point to the right and by one row upwards. If the image points are continuously counted, the new maximum, namely the amplitude value 3, now occurs five image points before that of the signature occurring in the earlier image N.

For explanation of the various multiplication processes, the upper half of Figure 7 shows how the amplitude distribution contours are stored in the image store BS1, as obtained for the image N in Figure 6.

The lower half of Figure 7 shows the movement of data in the store BS3 as successive element amplitude values are entered during entry of the image information when scanning the image N+1. The uppermost row shows the situation when a 23rd timing pulse has been fed to the store BS3. This means that 23 consecutive items of image point information have been fed into the store BS3. The first associated product value produced by the multiplication devices corresponding to Figure 5 is referenced SK 23 and has a value 1.1=1. It is formed by the coincidence of the fourth image point in row b of the store BS1 (amplitude value 1) with the third image point counted from the right in the upper row shown for the store BS3 (amplitude value 1).

In the case of the next image point when the data has the position indicated by arrow SK 24, the information in the store BS3 has been displaced by one register position further towards the right. A coincidence then occurs between two elements in row b of store BS1, namely image point No. 4 (amplitude value 1) and image point No. 3 (amplitude value 3), with the corresponding elements having the amplitude values 3 and 1 respectively in the second row shown in store BS3. When added, the associated total product 1.3+3.1 has a value of 6.

After the next shift pulse, the distribution of the amplitude values indicated by the arrow SK25 is reached. Now the amplitude values 1, 3 and 1 from row b of store BUS 1 coincide with the corresponding amplitude values 1,3 and 1 of store BS3.

The correlation value obtained as a result of product formation has an overall value of 1.1+3.3+1.1=11.

As there is also a coincidence product 1.1 in element 3 of row c of the store BS1 with a 1 in store BS3, the total sum is 12.

With the next shift timing pulse the information is advanced again by one register position, as indicated by arrow SK26, and produces multiplication values 1.3+3.1=6 as a total added output value. The following register position, indicated by arrow SK 27, only results in one coincidence correlation, between two values, 1.1=1, and thus gives the correlation value 1. The threshold value in MLS thus moves from an initial value of 1 (SK 23) to a value of 6 (SK 24) and then to a value of 12 (SK 25), where it remains. As SK 25 is the actual maximum for that sequence, the counting value 25 is retained as the count value for Zp in the assignment store ZOS.At the end of the comparison process the associated counting value Zp=25 is fed into the store ZWS and compared in the difference circuit DS with the previous value Zp' which was formed by correlation of the image sequence N with the preceding image sequence N-1 (not shown). We can assume there was then no change, so that maximum Zp'=22 (row b image point No. 4).

The difference formation of the counting value of the correlation maximum for the image N (ZZp'=22) and of the following correlation maximum (Zp=25) produces a difference value of 4 image points. The maximum value of the image sequence N + 1 thus occurs 5 image points earlier than the maximum value of the image sequence N. Thus the displacement of the target has been accurately detected in respect of value by the correlation process.

The sign of the difference indicates the direction of the displacement required (forwards or backwards in the scanning direction) to achieve re-alignment with the target. Thus the displacement indicated in Fig. 6 has been converted into a counting value from which the x0 and y0 values can be directly obtained for the necessary adjustment. If it is desired to commence counting from the front, i.e. from image point 1 in row , the given SK values of z.n=30 must be deducted.

If a target, e.g. an aircraft, is moving rapidly, then a change in the outer contour can occur between one image and the next.

The relative position of the maximum of thermal or optical contour is normally retained however, e.g. because the engines remain, as before, in the same region of the target. Considering the example in Fig. 6, in the broken-line boxes (image N +1), an amplitude value 1 could be added (e.g. row a, image point 6, i.e. a change could occur in the outer contour) without this producing a change in the counting value Zp. The line of sight VL in Fig. 3 thus continues to be aligned towards the same region of the target (e.g. the engine), and wandering or jumping (glint) is avoided.

Instead of multiplicative correlation, it is possible to employ an additive correlation between the amplitude values of the image N and N+1. The correlation maxima which then occur are not so marked, however, as in the case of the multiplication in the manner described with reference to Fig. 5. As a consequence the outlay circuitry is also is somewhat lower.

WHAT WE CLAIM IS:- 1. A target tracking system in which there is provided a scanning arrangement comprising a sensor that, when operating, sequentially sweeps a scanning pattern over a specific zone containing the target to produce a sequence of output signals representing quantised image elements of the scanned zone, and so produce an image formed by the distribution in the sequence of the output signals whose individual amplitudes represent the heat value (thermal signature) and/or the brightness value (optical signature) of said target during scanning, each said scanning sequence being fed into an image store and a comparator store being provided for storing the signals of the preceding image sequence and enable the contents of the image store to be related to the stored elements of the pre ceding image, taking into account the distribution and amplitude values of individual image elements, using an analysis circuit which effects a multiplication of the related store contents, image element by image element, as each new image element output signal is entered into said image store, thus providing a correlation of the incoming image sequence with the preceding image sequence to detect any change in the position of the correlation maximum in successive scanning sequences, the coordinate values of the new position being used as an indication of the new target position, and to control any requisite readjustment of the line of sight of the scanning arrangement.

2. A system as claimed in Claim 1, in which two parallel fed stores are provided in said comparator store, one of which is connected via said analysis circuit to said image store and the other serving as an intermediate store whose contents are transferred in parallel into said image store after the completion of each correlation.

3. A system as claimed in Claim 2, in which said analysis circuit contains a pulse generator which controls input into the comparator stores and controls an image point counter in which, during each new image scanning process the count value of the image point having maximum amplitude is entered.

4. A system as claimed in Claim 3, in which said pulse generator controls any subsequent deflection of the sensor.

5. A system as claimed in Claim 3 or Claim 4, in which the count obtained when the correlation maximum is reached for the relevant image is read out, and the information for adjusting the line of sight of said scanning arrangement is derived by subtracting the resultant count from the count obtained for the previous image.

6. A target tracking system substantially as described with reference to Figures 3 and 5.

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

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. a change in the counting value Zp. The line of sight VL in Fig. 3 thus continues to be aligned towards the same region of the target (e.g. the engine), and wandering or jumping (glint) is avoided. Instead of multiplicative correlation, it is possible to employ an additive correlation between the amplitude values of the image N and N+1. The correlation maxima which then occur are not so marked, however, as in the case of the multiplication in the manner described with reference to Fig. 5. As a consequence the outlay circuitry is also is somewhat lower. WHAT WE CLAIM IS:-

1. A target tracking system in which there is provided a scanning arrangement comprising a sensor that, when operating, sequentially sweeps a scanning pattern over a specific zone containing the target to produce a sequence of output signals representing quantised image elements of the scanned zone, and so produce an image formed by the distribution in the sequence of the output signals whose individual amplitudes represent the heat value (thermal signature) and/or the brightness value (optical signature) of said target during scanning, each said scanning sequence being fed into an image store and a comparator store being provided for storing the signals of the preceding image sequence and enable the contents of the image store to be related to the stored elements of the pre ceding image, taking into account the distribution and amplitude values of individual image elements, using an analysis circuit which effects a multiplication of the related store contents, image element by image element, as each new image element output signal is entered into said image store, thus providing a correlation of the incoming image sequence with the preceding image sequence to detect any change in the position of the correlation maximum in successive scanning sequences, the coordinate values of the new position being used as an indication of the new target position, and to control any requisite readjustment of the line of sight of the scanning arrangement.

2. A system as claimed in Claim 1, in which two parallel fed stores are provided in said comparator store, one of which is connected via said analysis circuit to said image store and the other serving as an intermediate store whose contents are transferred in parallel into said image store after the completion of each correlation.

3. A system as claimed in Claim 2, in which said analysis circuit contains a pulse generator which controls input into the comparator stores and controls an image point counter in which, during each new image scanning process the count value of the image point having maximum amplitude is entered.

4. A system as claimed in Claim 3, in which said pulse generator controls any subsequent deflection of the sensor.

5. A system as claimed in Claim 3 or Claim 4, in which the count obtained when the correlation maximum is reached for the relevant image is read out, and the information for adjusting the line of sight of said scanning arrangement is derived by subtracting the resultant count from the count obtained for the previous image.

6. A target tracking system substantially as described with reference to Figures 3 and 5.