Patent GB-1336451-A (GB)
Title: Automatic video contrast tracker
IPC: G01S3/786
Filed: 1971-04-19
Abstract: 1336451 Optical target tracking SAAB-SCANIA AB 19 April 1971 [11 Feb 1970] 21601/71 Heading H4D [Also in Division G4] A pattern recognition system for a contrast tracking system, such as may be used by a guided missile or an aircraft landing system, comprises a correlator for determining numbers indicating the degree of match between a reference pattern or target and any pattern present in an electronic window that is moved across signals from a scanner, i.e. in effect moved across the field of view of the scanner. The position (relative to the window) of the pattern that gives the highest correlation number is stored and used to adjust the window-generating circuitry so that the window auto-follows the target, maintaining the pattern at the centre of the window. Signals from a camera 1, Fig. 2, having a television-type raster are fed to a display 2, a unit 46 superimposing the position of the window on the display. The signals are also fed to window - generating circuitry comprising counters 15, 16, 20, 21, 28, 30. Each line sync. signal causes clock pulses to be gated to counters 15, 16, 20, 21. The clock pulse frequency is equal to the rate at which elemental areas of a line of the field of view is scanned, the elemental areas being of a width equal to the distance between successive scan lines so that the field is effectively divided into a rectangular matrix of areas. Counters 16, 20 define the x-ordinate position of the top left hand corner of the window, and counters 15, 21 the y-ordinate. The x-ordinate of the window is changed, so that the window scans over the field of view, by altering the phase of counter 20 relative to counter 16, the y-ordinate being changed similarly. At each successive position of the window its width and height are defined by counters 28, 30, their output controlling the superimposition of the window on the display 2 by unit 46 and also enabling gate 31. The video signals passing via gate 31 represent the data in the window in the field of view and are fed to a circuit 3 comprising summing units 33, 34 and comparators 35, 36 to provide a digital signal which has a 1-state only when the amplitude lies above or below a selected and adjustable range of values centred on the average value of the video signals. The digital signals are fed to a correlator 6, e.g. Fig. 3a or 3b (not shown), which outputs to comparator 39 a number indicative of the degree of match between the pattern contained in successive portions of the window and a reference target. The number may be weighted according to the distance of the pattern from the centre of the window. The number is compared at 39 with the correlation number of the best match so far obtained during investigation of successive portions of the window, this number being stored at 40. If the present number is greater than the best so far a signal indicating the present position of the portion of the window replaces that indicating the position of the best so far in store 44 and the present number replaces the previous best so far in store 40. Dependent upon the number in store 40 unit 41 widens or narrows the range of the triggering values of units 33, 34 so as to tend to maintain the highest correlation number so far obtained at about a certain fixed value. The signal in store 44 indicating the position of the portion giving the best so far number passes to unit 45 which controls the relative phases of the counters 15, 16 and 21, 20 to maintain the window on the target. Instead of a single digitizing unit 3, there may be three such units having ranges of differing widths. The three digital outputs are fed to respective correlators and the ranges are uniformly adjusted in width if the highest correlation number obtained by way of the unit having an intermediate range is not higher than that obtained by way of the units having wider and narrower ranges.