GB2183315A - Determining gun muzzle displacement - Google Patents

Abstract

An arrangement for quantitatively determining muzzle displacements such as bending of a long elevatable gun barrel 11 comprises an electronic CCD camera 25 pivotally mounted at 26, coupled to a sight 15, pivoted at 20, by a linkage rod (27) so that the camera field of view receives an image of a muzzle portion of the barrel as it moves in elevation with the sight. To avoid gun follow elevation error, that is, a change in muzzle image position with respect to the camera field of view as the sight is rotated through its elevation range the disposition of pivot points 20 and 26 length of rod (27) and distances from pivot points 20 and 26, at which the rod is connected, form a quadrilateral linkage whereby rotation of the camera in elevation differs from rotation of the sight by the proportion of the gun follow elevation error for that elevation angle. This enables the muzzle portion which carries a straight edged marker 33 of known dimensions to fill a larger part of the image and for signal processing means (45) to determine, in a calibration mode, a datum image position and the number of resolvable image pixels occupied by the marker to give a resolution factor. Subsequently during gun firing displacement of the muzzle image is multiplied by the resolution factor to determine quantitatively the value of muzzle displacement. A smaller gun follow azimuth error, causing azimuth movement of the image in the camera, is corrected for by the processing means or may also be corrected by appropriate coupling between camera and sight. <IMAGE>

Description

SPECIFICATION Correction of Gun Aiming Errors This invention relates to guns and in particular to guns of the type having a long cantilevered barrel supported in a gun cradle adjustably mounted for rotation in elevation on trunnions.

Such long-barrelled guns are found, for example, in battle tanks and inaccuracies inherent in the physical structure of the gun and its mounting support have given rise to numerous arrangements for accurate directing, or aiming. This invention is particularly concerned with aiming inaccuracies due to a condition conveniently called muzzle displacement whereby the position of the muzzle with respect to the carrier vehicle differs from that ostensibly assigned to it by the gun training mechanism and which muzzle displacement may vary significantly during operation, depending on its cause.

One form of muzzle displacement is due to displacement of the whole gun barrel resulting from wear in the trunnion bearings or gun cradle whereby the gun moves within its mounting when fired or when the mount moves.

A second form of muzzle displacement is barrel bend in which muzzle displacement occurs relative to the trunnion. It is conventional to consider such bending movements, and therefore of muzzle displacement in general in mils, where 6400 milts=360".

Thermal bending of a gun barrel due to the effects of either solar heating or cooling winds of one side thereof may cause muzzle displacement in excess of 1 mil in time periods as short as 15 minutes. Rapid firing of the gun also results in heating effects which may create a 1 mil displacement. A firing backlash effect between muzzle and trunnions from high velocity ammunition may result in a bending displacement of several mil and a moving vehicle may also introduce vibrational bending of the massive barrel of several milts with a period measured in seconds.

All these effects may occur simultaneously and act in the same or different directions and may combine to effect a muzzle displacement having a deleterious effect on the accuracy of the gun.

Muzzle displacement effects as considered in this specification are also discussed in U.S. Patent Specification No.4,020,739, U.K. Patent Specification No. 1,587,714 and UK Patent Applications Nos. 2,069,105 and 2,119,069, which specifications describe arrangements for compensating for the effects in aiming the gun.

That the arrangements described therein do not offer a completely satisfactory solution to this problem of muzzle displacement may be better understood by considering further the structure of a typical tank, in which the long-barrelled gun is mounted for rotation in elevation on a turret rotatable in azimuth to cause the gun to traverse, a target being observed through a periscope sight also mounted on the turret but spaced from the gun.

The gun has to move through a considerable angular range in elevation for different target distances and target elevations with respect to the tank.

An observer's line of sight to a target by way of the periscope must be movable in elevation to accommodate target elevation and distance variations and is usually linked to elevation of the gun with provision for controlling relative elevation, also called superelevation, between the gun boreline and periscope sightline to account for such range differences.

The link may be mechanical, with elevation differential being incorporated by a suitable mechanism, or electromechanical/electrohydraulic under the control of an electronic ballistics computer which takes into account other factors affecting the trajectory of projectiles; small differences in elevation and azimuth may be effected by altering the position of an aiming mark in the sight so that the observers sightline to the target is offset from the 'natural' or central sightline of the sight.

There are also two main forms of target sight in general use the so-called telescopic sight and periscopic sight, the latter form being more commonly employed with tanks and to which form the present invention is directed. Periscopic sights are divided into two types both of which are used.

One type is the so-called swinging periscope sight in which the periscopic reflectors are fixed within a periscope tube which itself is pivoted about an axis parallel to the gun trunnions to permit the sightline to be moved in elevation. The other type is the socalled moving mirror type in which the upper reflector element of an essentially fixed periscope is rotatable about an axis parallel to the gun trunnion and permits the sightline to be moved in elevation without physically moving other elements of the periscope.

To determine muzzle displacement principally due to barrel bending the first three aforementioned specifications rely upon reflection of a beam of optical radiation, that is, in the visible or infra-red part of the spectrum, from a reflector fixed to the muzzle the reflected beam being received as a spot on a two dimensional detector whereby bending of the barrel, which alters the angle of incidence between radiation and mirror, is manifested as a change in position of the spot at the detector.

The emission of optical radiation which may be detected by an enemy is considered undesirable and the radiation source itself presents a potential failure point of the arrangement.

The disadvantages of emitting optical radiation forward of the gun are also addressed in the aforementioned U.K. Patent Application No.

2,119,069 and an arrangement proposed which views an image of the muzzle portion of the barrel itself formed on a television detector whereby displacement of the image at the detector from an expected position is indicative of muzzle displacement due to one of the above outlined causes.

To form an image of the muzzle the detector has to be located laterally of the barrel and to receive an image, despite large movements of the muzzle in elevation, the field of view of the camera extends by way of a rotatable reflector of a swinging mirror periscope sight so that the muzzle is kept within the field of view of the detector for all elevation angles of the sight and barrel.

Whilst this arrangement overcomes some of the disadvantages of earlier systems there are many requirements or desiderata which do not appear satisfied.

Firstly the arrangement does not provide a quantitative evaluation of muzzle displacement suitable for use within a ballistics computer but rather suggests for any elevation angle a "window" of possible muzzle displacements and qualitatively determines a displacement relative to the window boundaries for controlling the position correction of an aiming mark in the sight eyepiece.

The arrangement thereby requires a modified form of sight and despite the apparent simplicity of the principle involves a considerable cost penalty either in modifying existing sight designs for new manufacture or in retro-fitting the arrangement to existing sights.

It is believed that the system as therein described, by sharing an optical path within the sight and by maintaining the muzzle portion within the field of view for all elevation angles, requires a field of view such that the image available is of such a size that it cannot be resolved positionally by the detector to provide the required accuracy of-a quantitative measurement.

The specification does not dwell on this point and deals only implicitly with the need to consider "gun follow error" of the field of view throughout the range of elevation angles, the effect of gun follow error on resolution being considered fully hereinafter.

In this specification the term "gun follow elevation error" is used to mean the variation in elevation angle subtended between a line joining the camera/sight position to the muzzle and a line (the sightline) joining the camera/sight position to a parallax, ortarget, point over the angular range of elevation of the gun barrel.

The term "gun follow azimuth error" is used analogously to mean the variation, over the angular range of elevation of the gun barrel, in azimuth angle subtended between a line joining the camera/ sight position to the muzzle and a line (the sightline) joining the camera/sight position to a parallax, or target point, over the angular range of elevation of the gun barrel.

Furthermore, the arrangement by which the detector field of view extends by way of the rotatable, but positionally fixed, reflector of the periscope arrangement limits its operation to such sight types and precludes its operation with a swinging periscope sight wherein the whole reflector is physically displaced about a remote pivot of the periscope.

It is an object of the present invention to provide an arrangement for quantitatively determining muzzle displacement of a cantilevered barrel of a gun having a displaced periscopic sight and which mitigates at least some of the disadvantages identified within known muzzle displacement measuring arrangements.

According to the present invention an arrangement for quantitatively determining muzzle displacement (as herein defined) of a cantilevered gun barrel rotatable in elevation about support trunnions and coupled to elevation motion of a periscopic target sight comprises an imaging system including (a) an electronic camera, supported on a carrier of a gun with which used separately from a gun barrel and sight, said camera having field of view defining means pivotal in elevation and coupled for rotation in elevation with a pivotal element of the sight about an axis in a plane substantially parallel to the plane of the gun trunnions, said camera being directed at a muzzle portion of the barrel to produce an electrical signal corresponding to a received image thereof and constrained by said coupling to maintain said muzzle portion position in the field of view for all elevation angles of the gun barrel, and (b) signal processing means operable in a calibration mode to determine from the camera signals, dimensions of the muzzle portion image and, in combination with known dimensions of the muzzle portion, an imaging system resolution factor, and operable in a gun firing mode to determine displacement of the muzzle image from a predetermined expected position within the field of view related to gun barrel elevation and produce from said displacement and said resolution factor signals representing the magnitude of muzzle displacement.

Preferably the camera is located close to the sight and contained within an armoured protective housing with the sight.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 (a) shows a sectional elevation through a gun barrel and periscope target sight of a battle tank illustrating the approximate relative disposition and the elevational range of rotation of the gun barrel, Figure 1 (b) shows a plan view of the gun barrel and sight disposition of Figure 1(a), Figure 2(a) is a sectional elevation through a portion of a swinging periscope sight and housing showing an imaging system according to the present invention including an electronic camera coupled to the sight and a gun barrel with a visual marker thereon, Figure 2(b) is a plan view of the sight and housing of Figure 2(a) showing the coupling between sight periscope and camera, Figure 3 is a schematic representation ofthe mechanical linkage between the sight periscope and camera, Figure 4(a) is a schematic representation of the sectional elevation of Figure 1 (a) showing the gun barrel at maximum elevation and illustrating angular relationships between barrel bore line and sight line to a target parallax point, Figure 4(b) is a schematic representation similar to Figure 4(a) but showing the gun barrel at minimum elevation (maximum depression), Figure 4(c) is a schematic representation of the plan view of Figure 1 (b) and illustrating the angular relationships between barrel bore line and sight line to the target parallax point, Figure 5(a) illustrates a possible image of a muzzle portion of the gun barrel formed in the camera and at a datum portion superimposed on a grid representing the pixel resolution of the camera image, Figure 5(b) illustrates a similar image to that shown in Figure 5(a) but displaced as a result of muzzle displacement during firing, Figure 6 is a schematic circuit representation of signal processing means of the imaging arrangement, Figure 7 shows in block form the implementation of the processing means in digital computer form, Figure 8 shows a sectional elevation view, corresponding to that of Figure 2(a), of an imaging system according to the present invention in which the camera is coupled to a rotatable mirror element of an alternative form of periscopic target sight, and Figure 9 shows a sectional elevation view, similar to Figure 2(a) illustrating an alternative form of camera.

Referring to Figure 1,a gun 10 having a barrel 11 is carried by a vehicle 12 which supports the barrel in a gun cradle 11' with trunnions 13, extending in a nominally horizontal or azimuth plane, for rotation thereabout in elevation. Assuming the carrier vehicle is on level ground the range of barrel rotation is through an elevation angle 8 as delimited by broken lines 14 which may typically extend from +200 with respect to a horizontal plane 14' at full or maximum elevation to -100 at full depression, or minimum elevation as it is called in this specification.

The gun 10 also includes a target sight 15 displaced from the gun barrel to enable the supported end of the barrel, sight and operating personnel to be contained within an armoured turret. To enable a target to be viewed by way of the sight those parts of it, such as an optical element 16 extending above the general turret body are enclosed in an armoured housing 17, a target sightline 18 being defined byway of a window 19.

The optical element 16 comprises a reflector of a periscope arrangement by which the sightline 18 is deflected into the turret and is rotatable in elevation about a sight azimuth axis 20 extending parallel to the gun trunnions 13.

The sight arrangement shown is of the so-called swinging periscope type in which the whole sight, including periscope reflectors, focussing arrangement and aiming graticules etc., is pivotable about axis 20 in elevation in correspondence with elevational rotation of the gun by mechanical or electrical linkage with the gun cradle.

The elevation position of the gun cradle may be measured by rotation sensor 21 which provides a representation of the angle to a ballistics computer 22, of known form.

The sight 15, and therefore the sightline 18 moves in elevation with the gun and is boresighted with the gun barrel boreline 23 such that the two lines 18 and 23 converge on a parallax point P, which represents a target point at a real or nominal distance from the gun. Atypically average range of 1500 metres may be used.

An aiming mark is positioned within the field of view of the sight and marks the natural sightline to the parallax point and the ballistics computer may be caused to inject a further optical mark into the sight offset in elevation and azimuth from the aiming mark in accordance with a computed superelevation figure determined from known and measured parameters of the gun, ammunition, atmospheric conditions and trajectory.

The gun sight and ballistics computer thus far described are conventional and described only to the extent necessary for an understanding of the invention.

As stated earlier a major source of aiming errors is due to muzzle displacement, that is, movement of, or bending of, the boreiine 23 with respect to the sightline 18 caused by the aforementioned barrel bending, or play at the trunnions or sight exacerbated by gun firing, and it has been proposed to accommodate such additional source of actual boreline error by detecting muzzle displacement by continuously viewing an image of a muzzle portion of the barrel with television camera directed through a pivotal reflector of the sight. Elevation of the reflector with the barrel permits the camera a reasonably narrow field of view in elevation and for the form of image processing employed adequate resolution despite the large elevation range of the muzzle portion and the field of view being fixed in relation to the sightline 18.

Referring now to Figure 2(a) this shows in greater detail the housing 17 containing the swinging periscope sight 15 and pivotal axis 20. In accordance with the present invention an imaging system 24 is provided which includes an electronic camera 25 supported in the housing 17 on an axle 26 which extends substantially in the same azimuth plane as the sight rotation axis 20 and gun trunnions 13 but inclined thereto and permits rotation of the camera in elevation.

The camera is separate from the sight in respect of their optical paths and components but is coupled thereto by a rigid rod 27 pivotally connected to both the sight periscope and camera such that the camera rotates in elevation with movement of the sight.

Referring to Figure 2(b) which shows a plan view of the coupled periscope and camera it will be seen that the optical axis of the camera, that is, the centre of its field of view, shown at 28 is inclined to the sightline in azimuth so as to be directed not at a distant target point but to a portion of the gun barrel adjacent the muzzle. The axle 26 extends perpendicularly to this direction 28 and therefore inclined to the sight pivot axis 20.

Referring now to Figure 3 as well as Figure 2(a) the coupling between camera and sight effects a quadrilateral linkage, the rod 27 being pivotally connected at one of its ends to the camera 25 at a first distance D25 from its pivotal axis 26 and pivotally connected at its other end to the sight 15 at a second distance D15 from its pivotal axis 20, the distance between the pivotal axes 20 and 26, length of rod 27 and first and second distances D25 and D15 being chosen such that as the sight 15 rotates through an elevation angle 6 (substantially equal to t;ie barrel angular range of Figure 1) the camera pivots in elevation through a second angle 6'.

The angular difference between 6 and 6', is equal to the gun follow elevation error angle as defined hereinbefore, may be explained with reference to Figure 4.

Referring firstly to Figure 4(a) this shows schematically the gun barrel 11 pivoted atT (trunnion axis 13) and inclined at a full elevation angle tp, with respect to the nominal horizontal plane 14'. A sight 15 is conveniently considered as located at a point S behind and above the point T by distances TO and OS respectively. The sight and gun are boresighted to the aforementioned target, or parallax, point P. The gun muzzle is shown as point M and the line MP represents the boreline as an extension of barrel bore TM, SP represents the sightline and SM the line joining the sight to the muzzle which makes an angle a, to the sightline at maximum elevation.

Referring to Figure 4(b) which shows a similarly schematic diagram but with the gun barrel (TM) set for minimum elevation 4)2, that is, full depression. In this configuration the line SM joining the sight and muzzle departs from the sightline SP by an angle a2.

It can be readily shown that for conventionally dimensioned gun barrel and sight placing and barrel elevation angle range the angle a varies approximately 30 (52 mil) from a, to a2 over the elevation angle range (4)i +4)2) corresponding to the range of elevation movement of the sight and gun, angle 8. That is, the sight to muzzle line has a gun follow elevation error angle of (a,-a2).

Furthermore, the variation in a is a linear function of elevation angle from one extreme so that in the linkage of Figure 3 (and illustrated in Figure 2) the angular elevation range 6' of the camera 25 is smaller than the angular elevation range 6 of the gun/sight by an amount (a1 -a2). The gun follow elevation error is thereby effectively eliminated such that in the absence of any muzzle displacement the muzzle portion remains fixed in position within the field of view of the camera for all elevation positions ofthe barrel and camera.

The above analysis relates to gun follow error in elevation and similar consideration may be given to the analogous gun follow azimuth error angle as depicted in Figure 4(c). This error results from the lateral displacement of the sight S from the bore line OTP and the angle between the line SM joining sight and muzzle and the bore line is y. Again employing simple trigonometrical relationships it will be seen that the angle y is given by y=cos~ (OM/SM) where OM=OT+TM and TM is itself proportional to the cosine of the gun barrel elevation angle. The value of 8 decreases with gun elevation motion, both positive and negative, from the horizontal.Again, substituting typical dimensions and barrel elevation range it can be shown that between maximum and minimum barrel elevation the value of y decreases by less than 5 mil, that is, less than 0.25 .

As stated, the camera 25 is supported with axle 26' inclined to sight axis 20 so as to be directed at a portion of the barrel adjacent the muzzle and the form of image produced at the camera is of the form shown in Figure 5(a). The camera is preferably of the solid-state type having a photosensitive surface, on which the image 30 is formed, comprising a coordinate array of discrete semiconductor elements 31 each of which provides a signal defining one picture element, or pixel, of the image.

The grid 32 superimposed on the image of Figure 5 corresponding to the location of the semiconductor elements and represents the defined pixels of the image which are produced as, and processed in the form of, electrical signals representing the average image intensity of each pixel.

Conveniently, but not necessarily, the coordinate directions of the array are arranged to be aligned with respect to the pivot axis 26 of the camera, that is, extend in the elevation and azimuth directions defined with respect to the gun trunnions.

To assist in image processing the gun barrel has fitted to the end thereof adjacent the muzzle a visual marker element 33 of known dimensions arranged to be upstanding with respect to the barrel in Figure 2. The marker 33 appears in the image of the barrel portion seen by the camera in Figure 5(a) and is so disposed with respect to the barrel that the image thereof appears in silhouette against the sky giving good contrast.

The marker 33 also has straight well-defined edges aligned such that the edges of the image are aligned with the coordinate directions of the camera photosensitive array, that is, of grid 32.

It will be appreciated from the above analysis of gun follow errors inherent in a displaced sight (and camera) location that gun follow elevation errors are considerably greater than azimuth errors but that the coupling arrangement between the rotatable sight element and separate camera reduces the gun follow elevation error to zero.

The camera is arranged with the working part of the muzzle portion image, that of marker 33, at a datum position substantially central in the field of view and as may be expected any muzzle displacement which occurs during firing of the gun will result in the position of the marker image moving with respect to the camera array, such as is illustrated in Figure 5(b). The camera field of view in elevation is chosen to include, but not be significantly greater than, the range of the possible muzzle displacements in elevation whereas the field of view in azimuth is chosen to be at least as great as such range of possible muzzle displacements in azimuth plus the smaller gun follow azimuth mirror.

It is desirable for processing that the image be as close to binary levels of contrast, that is, true black and white as possible and although the marker 33 is designed to maximise the image contrast between the muzzle portion and background, which will often be sky it is desirable to clean the image signals to ensure the binary level nature.

The camera 25 is shown in block form in Figure 6 comprises focussing/image forming optical lens 35, the aforementioned photodetector array 31 which may typically be a CCD, an array drive circuit 36 which scans the individual detectors of the array and produces on line 37 a serial signal representing as a series of pulses the two dimensional image.

The line 37 applies the signal to contrast enhancement means 38 comprising a signal threshold level detector 39 and a signal pulse amplitude detector 40. The pulse amplitude detector 40 determines the maximum amplitude of signal for a complete scan of the array 31 and in accordance therewith sets the threshold level in detector 39 for the next scan, thereby compensating for changes in illumination level and the like. The threshold detector 39 thus produces on line 41 a binary level signal suitable for storage and further processing in digital form.

The imaging system 24 also comprises signal processing means 45 shown in more detail in Figure 7. The signal processing means comprises an input interface 46 at which are received the binary level input signals from the camera 25 on line 41', elevation signals from the gun sensor 21 and a manual mode select signal on line 47. In addition to the input interface the processing means also comprises a central processing unit 47, main memory 48 in which the operating instructions are stored, a reference image store 49, a temporary, or updated, image store 50, an output interface 51 and interconnecting bus 52. The output interface 51 provides an output on line 53 to the aforementioned known ballistics computer 22 and optionally an output on line 54 to a display 55 of the extent of muzzle displacement in azimuth and elevation, conveniently in alphanumeric form.

Operation of the arrangement is divided into two modes selected manually by signal on input line 47, these being conveniently labelled a calibration mode and a gun firing mode.

The calibration mode is selected prior to operation of the gun and after the sight has been boresighted to the gun as outlined above. The camera is aligned with the part of the muzzle portion of the barrel including visual marker 33 such that the marker 33 appears in the camera image as shown in Figure 5(a) and defines a datum position image and the binary level signals corresponding to the pixels of the camera image are bit-mapped into the reference image store 49.

The precise position of the image datum position in relation to the field of view of the camera is not important provided it is sufficiently central to accommodate all image displacements due to muzzle displacement in operation and in said calibration mode centralising the image of marker 32 with respect to the camera field of view is assisted by a television type monitor 56 temporarily connected to receive and display the output of the camera 25, the monitor thereafter being removed.

The data representing the datum position image is then analysed conventionally by the processorto determine the number of pixels in each coordinate direction occupied by the marker 33 and, from its known dimensions, a resolution factor for the imaging system, that is, that linear dimension at the muzzle portion in each coordinate direction which corresponds to (the smallest measurable) one pixel of the image. Conveniently, but not necessarily, this is the same in both coordinate directions and the value of the resolution factor, or each component thereof, is stored in the main memory 48.

The extent of the gun follow azimuth error may be calculated from the dimensions of the gun but conveniently is measured from the camera image in the calibration mode.

The gun is moved in elevation between a number of points between full elevation and full depression and at each point the azimuth displacement of the image and elevation angle stored to provide a lookup table relating them.

The gun firing mode is then selected and at regular intervals thereafter the binary camera image signals are bit-mapped to the updated image store 50. Upon receipt of each image, typically of the form shown in Figure 5(b), the processor compares the image data with that of the reference image and determines the number of pixels in each coordinate direction by which the image has been displaced.

The processor then extracts the value of the resolution factor from the memory 48 and multiplies this by said number of pixels of image displacement for each coordinate direction, deriving a value of muzzle displacement in said coordinate directions, that is, in elevation and azimuth.

In assessing the magnitude of displacement in the azimuth direction the processing means determines from the sensor 21 the elevation angle of the gun and by reference to the look-up table determines the gun follow azimuth error angle for that elevation angle and varies the apparent muzzle displacement in azimuth to give a corrected value indicative of a true muzzle displacement.

These values are converted by the output interface 51 into a form suitable for feeding to the ballistics computer 22, wherein additional aiming corrections are made in known manner, and/or to a visual display 55 by which an operator can vary use of the gun or make aiming adjustments manually.

The operation of the arrangement described above has been simplified to assist in its comprehension. For instance it has been assumed that muzzle displacement occurs in the image plane, that is, orthogonally to the centre of the camera field of view, the line SM in Figure 4. It will be appreciated, however, that such displacement will be more closely represented as occurring orthogonally to the bore line TMP particularly in respect of bending of the barrel about its trunnions support point. Consequently a movement of the camera image will be seen to be smaller than such actual displacement as a function of the cosine of the angle (such as yin Figure 4(c)) between the centre of the camera field of view and the bore line.

Afuller trigonometric analysis may be employed to show that for a barrel length TM from trunnions to muzzle and the camera inclination y that the actual displacement due to barrel bend is given as a function of the apparent image displacement d by tan-l (d/TMcosy).

The constant TM of this relationship may be readily stored within the processing means 45 and employed with the elevation related value of 8 to correct the muzzle displacement components provided at the output interface.

The arrangement described this far represents one embodiment which is open to modification or variation of parts thereof without departing from the scope of the invention. For instance, the signal processing means 45 for the camera signals could form part of the ballistics computer, possibly with the addition of extra memory for the image stores.

The marker 33 is employed principally to enable calibration of the arrangement and this calibration may be conducted either prior to each operational engagement of the gun or at intervals during such operation, this being facilitated by the marker being in the received image for all barrel/sight elevation angles.

It will be appreciated that during daylight hours the marker will appear darkly contrasting against a lighter sky. To facilitate operation during the hours of darkness the viewed marker surface may be provided with a source of chemical illumination such as luminescent paint. The sense of the image contrast will of course be reversed and the image processing means may be arranged to accommodate this change of binary signal levels by manuai switching or automatically by means of daylight sensing photocell. Alternatively the camera 25 may be arranged to operate in low light conditions, for example with image intensification or may be replaceable by, or ganged for parallel operation with, thermal imaging means.

The use of a readily distinguished straight-edged marker element eases the calibration operation by which the resolution factor is determined. It will be appreciated however that a separate marker is not essential and any suitable imaged part of the muzzle portion of known dimensions may be employed to determine the number of pixels occupied in each coordinate direction, and therefore, the resolution factor.

Similarly it will be appreciated that the camera image may comprise essentially just the straight edged marker, rather than including any part of the muzzle portion, movement of the image being readly quantified by detecting movement of the regular marker boundaries.

Also, the arrangement describes coupling of the camera to a rotatable element of the sight which is the periscope tube of a swinging periscope sight. It will be appreciated that the arrangement may be employed with a fixed periscope sight as illustrated in sectional elevation in Figure 8 as 60 in which at least one reflector 61 of the periscope is pivotable in elevation about an axis 20' and coupled to elevation motion of the barrel such that rotation of the mirror through an angle 6M causes elevation of the gun barrel and the sightline through an angle 65=26M In accordance with the present invention an imaging system 24' comprises an electronic camera 25' and signal processing means 45' functioning as described above, the camera being coupled to the rotatable reflector element 61 or its drive mechanism by a link arm 27'.

In accordance with the analysis given above in relation to Figure 4, the length of rod 27', separation of pivotal axes of reflector and camera and distances between said pivotal axes and mounting points of the rod 27' are chosen such that as the reflector element rotates in elevation through an angle OM the camera field of view moves through an angle 0'5 which differs from Es (=2elm) by the above described gun follow elevation angle. Clearly the coupling rod 27' may be connected to a part of the sight other than the reflector element shown, such as to a linkage which controls motion of the reflector element.

The arrangement according to the present invention is therefore suitable for both known forms of periscopic sight employing an element rotatable in elevation and the form of coupling between the camera and sight element may be other than a rod which, in combination with pivoting positions and radii, defines a quadrilateral linkage. For example, the camera may be coupled for rotation with the sight element by other mechanical means such as conventional rotational gearing having the appropriate ratio related to the angles 8 and 6' or electrically or hydraulically by a position control servo system.

As stated above the sight, or rotatable element of the sight, is coupled to the gun elevation mechanism so that the two elevate in synchronism.

It will be appreciated that the camera may be coupled not only to the pivotal sight/sight element or its actuating linkage but also to the gun which represents the origin of the sight movement, the camera still of course being coupled to the pivotal sight/sight element by virtue of the coupling between gun and sight/sight element.

In the above described embodiment the field of view of the camera is defined by a conventional lens system fixed in position with respect to the photosensitive region of the camera and the whole camera is pivotable in elevation to elevate this field of view. It will be appreciated that coupled elevation of the pivotal element of the sight and the camera field of view may be effected by elevation of a field of view defining means separately movable in relation to a fixed photosensitive region of the camera.

Such an arrangement is shown in elevation in Figure 9 which resembles Figure 3 except that the camera 25" comprises a camera unit fixed in relation to the support vehicle 12, the field of view being defined as to direction by a reflector 71 pivotable in elevation about an axis 72 parallel to the sight axis 20. The reflector 71 is coupled to the sight periscope 15 by a rod 73 (corresponding to rod 27 of Figure 3) pivotally connected to both the periscope and the reflector 71. The distances between the periscope axis 20 and rod connection and between reflector axis 72 and rod connection are chosen, along with the length of rod 73 and position of reflector axis 72 such that a quadrilateral is formed which provides the appropriate angular coupling between the periscope sightline and camera field of view. It will be appreciated that the reflector 71 will be required to undergo only half of the angular motion required by the whole camera in the arrangement of Figure 3 in order to move the field of view through the requisite angular range. The form of camera shown in Figure 9 may of course be employed with the rotating mirror type of periscope sight shown in Figure 8, the coupling rod 73 extending between the camera reflector 71 (Figure 9) and periscope reflector 61 (Figure 8).

In the above described embodiment the form of coupling has been limited to eliminating the larger gun follow elevation angle whilst the much smaller gun follow azimuth angle is accommodated within the azimuth field of view of the camera and correction made within the processing means. If desired, the camera may be mounted such that the coupling with the sight causes the camera field of view to swing in azimuth as it is pivoted in elevation in accordance with sightline elevation, the azimuth swing being an elevation-related function of the gun follow azimuth error described above with reference to Figure 4(c).

The elimination of such gun follow azimuth error may enable the camera to be operated with a narrower field of view and therefore greater magnification of, and resolution, of the image.

However, muzzle displacement in elevation may be greater than in azimuth due to the effects of gravity on the massive barrel and the limitations on the ability to increase image magnification in the elevation direction may not justify the elimination of gun follow azimuth error.

It is emphasised however that the elimination of the larger gun follow elevation error is fundamental to the invention enabling a currently available solid state camera to resolve an image of the muzzle portion sufficiently to enable quantitative measurement of muzzle displacements with the accuracy required to effect compensatory gun aim correction.

Considering in further detail the camera required, it is well established that for a ballistics computer to effect correction for muzzle displacement due to barrel bend the displacement requires quantitative measurementtoan accuracy of 0.1 mil (or lateral displacement of some 0.53 cms for a 5.3 metre barrel).

A typical state of the art camera has a CCD imaging array of 604x575 10 cm cells which at a camera/muzzle viewing distance of some 6.3 metres requires a viewing area of at most some 32x30 cms.

Considering now the low magnification imaging of a muzzle portion including a part of the barrel and the marker with the image positioned as shown in Figure 5; a typical muzzle portion has a diameter of some 16 cms and occupies at least half of the image in each coordinate direction to provide sufficient image data to enable operational comparison between image positions by correlating pixels.

Accordingly, it will be seen that the imaging system is capable of coping with a muzzle displacement of some +6 cms (*10 mil) without losing image information but incapable of accommodating large movements of the image at the camera due to gun follow elevation error for correction by the signal processing means.

It is therefore believed that any known imaging system sited remotely from the gun barrel which ignores gun follow error is incapable of measuring muzzle displacement with sufficient accuracy to be used in ballistic corrections whereas any imaging system which accommodates image displacement due at least to gun follow elevation error and corrects for it by image signal processing is incapable of providing adequate image resolution to give meaningful quantitative evaluation of the muzzle displacement.

Also the fact that the camera is separate from the existing sight, except for a potentially simple form of coupling, and shares no optical components, means that it is simple to implement in incorporating into new sights of existing design and retro-fitting to existing sights with minimum modification thereof.

Claims (12)

1. An arrangement for quantitatively determining muzzle displacement (as herein defined) of a cantilevered gun barrel rotatable in elevation about support trunnions and coupled to elevation motion of a periscopic target sight, said arrangement comprising an imaging system including (a) an electronic camera, supported on a carrier of a gun with which used separately from a gun barrel and sight, said camera having field of view defining means pivotal in elevation and coupled for rotation in elevation with a pivotal element of the associated sight about an axis in a plane substantially parallel to the plane of the gun trunnions, said camera being directed at a muzzle portion of the barrel to produce an electrical signal corresponding to a received image thereof and constrained by said coupling to maintain said muzzle portion position in the field of view for all elevation angles of the gun barrel, and (b) signal processing means operable in a calibration mode to determine from the camera signals, dimensions of the muzzle portion image and, in combination with known dimensions of the muzzle portion, an imaging system resolution factor, and operable in a gun firing mode to determine displacement of the muzzle image from a predetermined expected position within the field of view related to gun barrel elevation and produce from said displacement and said resolution factor signals representing the magnitude of muzzle displacement.

2. An arrangement as claimed in claim 1 in which the coupling comprises a mechanical linkage between the camera and pivotal element of the sight.

3. An arrangement as claimed in claim 2 in which the mechanical linkage comprises a quadrilateral linkage comprising a rigid rod member pivotally connected at one end thereof to the camera at a first distance from its pivotal axis and pivotally connected at the other end thereof to the rotatable sight element at a second distance from its pivotal axis, the separation of said pivotal axes, rod length and first and second distances being related such that rotation of the sight elementthrough first angle causes rotation of the camera through a second angle differing from the first as a function of the gun follow elevation error angle (as herein defined).

4. An arrangement as claimed in any one of claims 1 to 3 in which the field of view ofthe camera in azimuth subtends an angle which is greater than the possible total range of muzzle displacement in azimuth by a gun follow azimuth error angle (as herein defined).

5. An arrangement as claimed in any one of the preceding claims in which the camera comprises a solid state device in which the muzzle portion image is formed on an array of discrete semiconductor elemental areas each of which provides a signal defining one pixel of the image and wherein the signal processing means comprises means operable in said calibration mode to determine how many pixels of the image in at least one coordinate direction of the array are occupied by a part of the muzzle portion of known dimensions in that coordinate direction to determine said resolution factor and from the positions of said pixels in the array a datum position of the image, and operable in said gun firing mode to count the number of pixels in each of said coordinate directions by which the muzzle part image is displaced from said datum position in each of said coordinate directions and to multiply the numbers by said resolution factor to determine the magnitude of muzzle displacement during said gun operation.

6. An arrangement as claimed in claim 5 in which said coordinate directions of the array are arranged to be aligned with respect to the pivot axis of the camera and extend in elevation and azimuth respectively with respect to the gun trunnions.

7. An arrangement as claimed in claim 5 or claim 6 in which the field of view of the camera in the elevation direction at any position is arranged to be not significantly greater than the possible total range of muzzle displacement in elevation.

8. An arrangement as claimed in any one of claims 5 to 7 including a visual marker element arranged to be attached to the gun barrel adjacent the muzzle and form said part of the muzzle portion of the image.

9. An arrangement as claimed in claim 8 in which the marker element is arranged to be upstanding with respect to the barrel and to have optical properties which enhance contrast between the marker element and a background.

10. An arrangement as claimed in claim 8 or claim 9 in which the marker element is arranged to have substantially straight edges of known dimensions extending in said coordinate directions.

11. An arrangement for quantitatively determining muzzle displacement (as herein defined) of a cantilevered gun barrel rotatable in elevation about support trunnions and coupled to elevation motion of a periscopic target sight, said arrangement being substantially as herein described with reference to, and as shown in, Figures 1 to 7 or in Figure 8 or Figure 9 of the accompanying drawings.

12. A gun having a cantilevered barrel rotatable in elevation about trunnions supported on a carrier and coupled to elevation motion of a periscopic target sight also supported on the carrier and an arrangement as claimed in any one of the preceding claims for quantitatively determining muzzle displacement (as herein defined) of the barrel.