GB2280563A - Apparatus for detecting and indicating the position of a source of transient optical radiation - Google Patents

Abstract

An apparatus for detecting and indicating the position of a source of transient optical radiation (e.g. the flash from a gun) comprising a sensor (1, 2 or 3) incorporating an array (5) of photosensitive elements. A lens (4) focusses an image of a real world scene onto the array so that radiation from a source in the field of view of the sensor is directed onto a corresponding region of the array. The outputs of the photosensitive elements are scanned and stored, and the stored outputs utilised in a signal processor (7) to produce electrical signals representing the angle and elevation of the source relative to the sensor. A display means uses the signals to provide a visual display (13) indicating the source position in the real world scene. When the photosensitive elements are responsive to infra-red radiation, the apparatus finds particular application in providing information on the position of the origin of gunfire. <IMAGE>

Description

APPARATUS FOR DETECTING AND INDICATING THE POSITION OF A SOURCE OF TRANSIENT OPTICAL RADIATION This invention relates to apparatus for detecting and indicating the position of a source of transient optical radiation, more especially, though not exclusively, to such apparatus for use in pinpointing the position of the origin of hostile gunfire.

A common enemy faced by both civilian police forces, and government armed forces is the threat of terrorist gunfire from sniping positions. In most instances it is the time taken to define the origin of such gunfire that allows the terrorist to make an effective escape.

Depending upon the quantity and characteristics of the chemical propellant, along with the gun geometry, various effects are produced as the propellant gases emerge from the muzzle of a gun. Three of the effects most commonly recognised by an observer are flash, sound and smoke. The short duration of the flash lakes eyeball detection difficult, especially with high ambient light levels, the sound of a shot being fired can be difficult to locate accurately due to sound reflections from buildings and other obstacles, and even during daylight when smoke is visible it may drift or be dispersed by wind conditions.

The main object of the invention, therefore, is to provide a means of giving rapid angular and/or elevation information on the position of such gunfire. However, the invention could have other applications as will be apparent from the following description.

According therefore to the invention in its broadest aspect, apparatus for detecting and indicating the position of a source of transient optical radiation comprises a sensor incorporating an array of photosensitive elements responsive to said radiation, a lens disposed so as to direct any such radiation from a source in a field of view on to a corresponding region ofsaid array, means for scanning and storing the outputs of said elements, signal processing means responsive to said stored outputs for producing electrical signals indicative of the angle and/or elevation of said source relative to the sensor, and display means responsive to said signals for indicating the position of said source.

The choice of sensors will depend upon the characteristics of the radiation that is to be detected. Of the three effects produced by a gun being fired the most likely to provide reliable information about the position of the weapon being fired is the flash. This is produced by the re-ignition of super-heated particles making contact with the atmosphere. Various ways, including chemical and mechanical, have been devised for reducing visible flash, but whatever method is used for minimising the visible flash from a firearm to a human observer, there will always remain a considerable proportion of invisible flash in the infra-red spectrum that will be almost impossible to hide.

Accordingly, for providing information on the position of gunfire, the sensors are preferably responsive to infra-red radiation, and in such a case an optical filter is preferably provided for excluding visible light from the array in order to reduce background effects, and thus improve contrast between signals derived from gunfire radiation and those resulting from ambient light levels. Preferably the filter is a narrow pass-band dichroic or interference filter tuned to the frequency of the infra-red radiation. Additional stop band filters tuned to selected visible light frequency band may also be required in certain instances e.g.

where sunlight and street lamps need to be blocked.

Preferably the apparatus incorporates means for reducing the effect of slowly changing background illumination, on the contrast between the signals representative of detected transient radiation, and signals representative of the background illumination.

The sensor is conveniently constituted by one or more CCD (charge coupled device) cameras, although a PIN diode array or arrays might alternatively be used. In such an arrangement each row of elements of the sensor array(s) will be scanned in much the same way as in a video camera, line by line until the complete array has been covered, the rate of frame scan being chosen to suit the particular application. Each complete frame scan is conveniently transferred to a temporary frame store which is continually overwritten until instructed otherwise. When instructed the scan in the temporary frame store is moved to a more permanent memory area so that processing may be carried out. The criteria for taking the video data into the memory is as follows.A signal processor performs an analysis on each video line of information pixel by pixel, and looks for high level peaks of radiation in the background information. When a peak is detected, the next video line is inspected to see if pixels in the same column are active, and if pixels do coincide, then a mapping routine begins; the processor then calculates the angle and elevation of the position of origin of the radiation relative to the sensor.

The information so derived may be displayed as a light image in the form of a spot, cross or other distinctive shape, superimposed in the appropriate position on a television screen showing a picture of the real world scene in the field of view, thereby presenting an operator a visual indication of the precise firing position in the real world scene. Alternatively it may provide, for example on a liquid crystal display, a display of the information as co-ordinates of the firing position relative to a datum e.g. the position of the sensor. Moreover in some cases the data may be fed to a gunnery fire position to initiate immediate return fire to the indicated firing position.

For use on mobile detectors a plurality of cameras are preferably provided so as to provide a 360- field of view around the detector. This may be achieved by the use of three cameras disposed at 120' to each other, each designed to cover 120 in azimuth. In elevation a maximum field of view of 90 above the horizontal would be ideal, but in practice 60 may be all that is needed. With a camera having a square array of photosensitive elements, this means that any image formed on the array would only use half the number of element rows.Where a 40 field of view in elevation is adequate, only a third of the element rows would be used, so that the signal processor designed to process all of the data from the full element array from one camera could be modified to accept three smaller data arrays from three separate cameras.

The precise form of optical arrangements employed will depend upon the particular requirements. Thus the requirements in open country will be different from those in town where tall buildings and narrow streets will dictate a different angular specification.

Apparatus in accordance with the invention may also be used to determine and indicate the range of a muzzle flash, by disposing two or more detectors in a triangular formation and correlating the azimuth information.

The invention will be further explained by way of example with reference to Figures 1 to 4 of the accompanying schematic drawings, in which Figure 1 illustrates one suitable form of camera configuration for apparatus in accordance with the invention for detecting and indicating the position of origin of hostile gunfire, Figure 2 illustrates the field of view obtainable with such a camera configuration, Figure 3 illustrates diagrammatically the various units of the apparatus, and Figure 4 illustrates a single line of video information derived from one of the cameras of Figure 1.

Referring first to Figure 1 of the drawings the apparatus incorporates three CCD cameras 1, 2, 3 disposed at angles of 120 to each other about a central vertical axis V. Each camera incorporates a lens 4, a square array 5 of photosensitive elements, disposed in a vertical plane, and has a field of view of 120' both horizontally and vertically. The combination of cameras thus has a total field of view of 360* in azimuth as indicated in Figure 2. It will be seen that, in an ideal situation, coverage by each camera need only be 90' in elevation, so that some of the element array 5 of each camera will be redundant in such a case. In practice it may be found that only 60' or in open country only 40 elevation coverage will be required.With 60' coverage any image formed on the square array would only use a half of the available element rows or a third if a 40' coverage is adequate. In the latter case an electronic unit designed to process all the data from the full array from one camera could be modified to accept three smaller data arrays from three separate camera sources.

Each camera is also fitted with a filter 6 to exclude all visible light, but capable of passing infra-red emissions produced from the muzzle of a gun.

Figure 3 illustrates the three camera multiplex system having 40' coverage in elevation; as mentioned above, only a third of the active area of the array 5 in each camera is used, i.e. the scene viewed by, say, camera 1 is formed in the top third of its array 5, camera 2 uses the centre section, and camera 3 the lower section. Once a muzzle flash of a gun is detected a suitable digital signal processing unit 7 is used to carry out geometric calculations. In order to do this the processing unit 7 must also receive compass information from an electronic compass 8; this can be provided by a flux gate direction finding device or a global positioning satellite (GPS) system, if available. The processing unit also receives signals from a multiplexing system 9 which combines the inputs from the three sensor arrays 5.A single line of information from each of the three cameras 1, 2, 3 is sampled in turn, incrementing down by a line at a time until the complete field is covered. The system may alternatively use three separate electronic units and process the information in parallel. This approach will allow faster scanning speeds and may improve overall resolution.

The video data containing the point of origin of a small arms muzzle flash would be obtained as a fra snap shot. In addition to the array 5 of light sensitive elements known as the active area, each of CCD cameras 1, 2 and 3 includes an equal number of memory transfer cells (not shown) known as the transfer area.

During a period dictated by the electronic shutter speed, the active area is exposed to light via the lens system 4. The period during which the shutter is open, known as the integration time, can range from 20ms or longer down to 100 us. When the shutter inclosed the information in the active area is transferred to the transfer area, where it is serially clocked out by buffers, row by row on a first-in first-out basis to the processing unit 7.

The precise shutter speed for any particular application of the invention may readily be found by calculation or trial. The minimum requirement from a processing point of view is that a suspect image must fall across three adjacent pixels horizontally and three vertically.

Figure 4 shows a single line of video information between two sync pulses 64 us apart. In the absence of the optical filter 6 background noise 10 will be present as shown in 4(a). With the optical filter 6 present the video information between the line sync pulses 11 will be flattened as in 4(b). The effect of a muzzle flash at the pass band frequency of the filter 6 is illustrated in 4(c), the position of the vertical line 12 in the X axis contains azimuth information, and the height of the line contains elevation information.

For portable detector arrangements it will be necessary to mount the sensor and television cameras on gimbals to ensure that elevation angles are measured from the horizontal.

When the minimum requirement for processing, as described above is detected in the signal fed to the processing unit 7 from the cameras 1, 2 and 3, the whole frame of pixel signals wherein the requirement is detected is moved from a temporary frame store in the unit 7 to a memory area in the unit 7 wherein it is held during carrying out of the calculations required to determine the angle and elevation of the position of the muzzle flash in the field of view of the cameras 1, 2 and 3. The position of the muzzle flash may be displayed as a bright spot, cross or other distinctive shape superimposed in the detected position on a television picture 13 of real world scene in the field of view of the sensor cameras 1, 2, 3.

An arrow 14 may be included in the television picture 13 to make the bright spot etc. more readily identifiable. The television picture 13 is produced by a conventional television camera (not shown) which is automatically slewed to view the part of the sceme containing the detected position.

For some applications, however, it may be sufficient for the information on the origin of a muzzle flash to be displayed as co-ordinates e.g. on a simple liquid crystal display.

Various other modifications are clearly possible. Thus more than three suitably arranged sensor cameras may be used to provide a 360 field of view in azimuth. Also by feeding the derived information on the position of a muzzle flash to a gunnery fire control system it may be possible to initiate immediate return fire directly at that position.

The range of the flash may be. obtained, if desired, by the use of two or more similar detector units suitably disposed such that triangulation can provide the required information from signals derived from each of the units, the range conveniently being displayed on a liquid crystal display or, in the case where a gunnery fire control system is activated automatically by data provided by the equipment, it could be used to provide a more accurately directed return fire.

Claims (12)

1. An apparatus for detecting and indicating the position of a source of transient optical radiations comprising a sensor incorporating an array of photosensitive elements responsive to said radiation, a lens disposed so as to direct any such radiation from a source in a field of view on to a corresponding region of said array, means for scanning and storing the outputs of said elements, signal processing means responsive to said stored outputs for producing electrical signals indicative of the angle and/or elevation of said source relative to the sensor, and display means responsive to said signals for indicating the position of said source.

2. An apparatus according to Claim 1 wherein said photosensitive elements are responsive to infra-red radiation.

3. An apparatus according to Claim 2 wherein said sensor incorporates optical filter means for excluding visible light from said array.

4. An apparatus according to Claim 3 wherein said filter means includes a narrow pass-band dichroic or interference filter tuned to the frequency of said infra-red radiation.

5. An apparatus according to Claim 3 or Claim 4 wherein said filter means further includes at least one stop band filter tuned to a selected visible light frequency band.

6. An apparatus according to any one of the preceding claims including means for reducing the effect of slowly changing background illumination on the contrast between the signals representative of detected transient radiation, and signals representative of the background illumination.

7. An apparatus according to any preceding claim wherein said sensor produces successive frames of data corresponding to said field of view,said processing means is responsive to said outputs to determine when the data in a frame of data indicates the presence of a said source of radiation in said field of view, and said frame of data is then held in memory means whilst said processing means produces said electrical signals indicative of angle and/or elevation from said frame of data held in said memory means.

8. An apparatus according to any one of the preceding claims wherein said sensor is constituted by one or more CCD cameras.

9. An apparatus according to any one of Claims 1 to 7 wherein said sensor is constituted by one or more PIN diode arrays.

10. An apparatus according to any preceding claim wherein said display means produces a television picture of a real world scene in said field of view and indicates said position by a light image superimposed on said picture.

11. An apparatus according to any one of Claims 1 to 9 wherein said display means provides a display of the co-ordinates of said position relative to a datum.

12. An apparatus according to Claim 1 substantially as hereinbefore described with reference to the accompanying drawings.