Sighting system

A gun sighting system in which a daylight (visual) sight and a thermal imaging (TI) night sight are mounted on the gun breech. The TI field of view is superimposed on the visual field of view, necessitating accurate alignment between their lines of sight and the gun muzzle boresight. Adjustment of the visual sight causes separation of the visual and TI displayed images which is indeterminate in the absence of distinct target features. In accordance with the invention a visual reference mark is injected on to the field of view, which reference mark is locked to the target scene. Separation of the visual and TI scenes causes corresponding separation of the reference mark and TI sight line marker thus permitting adjustment of the TI field of view to remove this separation and align the visual and TI scenes.

This invention relates to a sighting system particularly, but not 
exclusively, for a weapon aiming system. 
A problem occurs when two or more devices have to maintain line of sight in 
precise alignment each with the other, particularly when these devices are 
required to maintain their alignment throughout the azimuth or elevation 
movements of the system. The problem is also increased by effective shift 
of the line of sight due to instabilities in the scanning and/or relay 
and/or display elements of any one or more parts of the system. 
An example of this problem is the need to maintain precise alignment 
between a gun muzzle, its associated visual sight for a further sight, 
e.g. a night sight, which may employ a scene scanning and display system. 
Ideally, movement of the gun should be precisely followed by both sighting 
systems, but the existence of movement relaying mechanisms introduces 
errors in the accuracy of the resulting alignment, and further errors may 
occur due to the shift of the point of reference of the scanning system or 
the display system. 
In a previously proposed system for checking and maintaining alignment 
between a primary sight (a visual or daylight sight) and a gun muzzle, a 
reference system is employed in which a mirror is mounted at the front of 
the muzzle and a projector at the back. The visual sight is also mounted 
to move with the muzzle and is initially aligned with the muzzle so that 
the boresight of the muzzle and the line of sight of the visual sight 
intersect at some standard target distance. In this condition of initial 
alignment, the mirror and/or projector are adjusted so that the projector 
source image appears in the visual sight field of view in alignment with 
the muzzle-boresight graticule mark which indicates the line of sight of 
the visual sight. 
The visual sight will normally be to one side of the muzzle and the 
reflected reference beam has therefore to be deflected by a prism into the 
line of sight of the visual sight. 
Operational conditions, heating of the muzzle, imperfect relaying of muzzle 
movement to the visual sight (where the sight is not directly mounted on 
the muzzle) etc. may cause the initial alignment of the muzzle boresight 
and the visual line of sight to drift. Such drift can be checked by 
operating the projector and noting the position of the muzzle reference 
(reflected) image in relation to the muzzle boresight mark in the field of 
view. Any discrepancy can be corrected by adjusting the visual sight to 
bring the boresight mark into coincidence with the muzzle reference image. 
The problem previously mentioned arises when a second sight, e.g. an 
infra-red thermal-imaging sight, is employed for night-time use. The 
visual sight can be readily adjusted but it may be impractical or 
otherwise undesirable to use the same, or duplicated, mirror/projector 
reference system for the thermal imaging (TI) sight. 
The TI field of view may be presented on a C.R.T. display and projected on 
to, i.e. superimposed on, the visual sight display so they have a common 
field of view, initially at least. If, therefore, there happens to be a 
distinct target or prominent object in a suitable position, the TI sight 
can be manually adjusted until the visual and TI images of this target are 
superimposed so bringing the lines of sight of the visual and TI sights 
into alignment. 
However, such a convenient target reference cannot be relied upon and the 
difficulty arises of determining what correction has been made to the 
visual sight and transferring this to the TI sight. The two could be 
slaved together, mechanically or electrically, with a suitable coupling 
function, but this may not be practical in view of the coupling tolerances 
and the different corrections that are needed for the two sights as a 
result of their different positions. 
An object of the present invention is therefore to provide a simple method 
of aligning two sights after one has been re-set. 
According to the present invention, a sighting system comprises first and 
second optical sights mounted so as to have substantially the same field 
of view, the field of view of the second optical sight being superimposed 
on that of the first and each being individually controllable within a 
limited angle, the first optical sight having a first line of sight marker 
which is movable with the field of view and the second optical sight 
having a second line of sight marker which is located with reference to 
the scene viewed, the system further including means for injecting a 
reference mark into the field of view of the first optical sight which 
reference mark can be aligned with the projected view of the second line 
of sight marker and is otherwise located with respect to the scene viewed, 
the arrangement being such that alignment of the two fields of view is 
effected by control of the field of view of the second optical sight to 
maintain the relationship between the reference mark and said second line 
of sight marker. 
In use with an artillery gun, the first optical sight may be a visual sight 
adapted to be adjusted for alignment with the gun muzzle and the second 
optical sight may be an infra-red sight. 
There may be included a muzzle reference system having a projector source 
mounted at the rear of the gun, a mirror mounted with reference to the 
mouth of the muzzle to reflect an image of the projector source into the 
superimposed fields of view in coincidence with a boresight mark 
constituting said first line of sight marker, separation of the reflected 
image and the boresight mark indicating a required correction of the line 
of sight of the visual sight and a corresponding correction of the line of 
sight of the infra-red sight by bringing said second line of sight marker 
back into alignment with said reference mark when they are relatively 
displaced on re-alignment of said reflected image and said boresight mark.

Referring to FIGS. 1 and 2, the gun muzzle 2 has an initial boresight 1. 
The visual sight 5 has a wedge prism W (shown in FIG. 2 only) ahead of its 
object lens, the prism W being movable transversely in and out of position 
for setting up purposes. The line of sight 3 of the visual sight is 
initially directed to intersect the boresight 1 at the standard target 
distance, which may typically be 1000 meters. 
A mirror M is mounted on the muzzle 2 at the front end and a projector S is 
mounted on the gun at the breech end 4. The mirror M and projector S are 
arranged so that, when the boresight 1 and line of sight 3 are aligned, a 
spot of light, the reference image, is reflected on to the visual sight 5 
by way of the prism W and so as to coincide with a boresight mark (MBS) 
which indicates the line of sight 3 of the visual sight. This arrangement 
constitutes the muzzle reference system. 
Referring particularly to FIG. 2, if the muzzle moves in operation, such 
that the mirror M moves to a position M', the boresight line will now be 
1' and will not be aligned with the visual line of sight 3. This error is 
corrected by a screw adjustment which tilts the object lens 9 of the 
visual sight in azimuth and/or elevation selectively, until the spot of 
light, the muzzle reference image, is re-aligned with the muzzle boresight 
mark. The visual line of sight 3' is then again correctly aligned with the 
muzzle boresight. 
The muzzle boresight mark indicates both the line of sight of the visual 
sight 5 and also the line of sight of a laser incorporated in the sight 
for rangefinding purposes. The muzzle boresight mark must therefore be 
used for target alignment, rather than, say, the muzzle reference image, 
which does indicate the muzzle boresight. 
A thermal imaging sight TI, sensitive to infra-red radiation, is mounted 
adjacent the visual sight 5 so as to have substantially the same field of 
view. The shaded bars between the various constituents indicate rigid 
connections. The output of the TI sight is displayed on a C.R.T in known 
manner and the displayed infra-red scene is projected into the field of 
view of the visual sight by a prism reflector. The two fields of view are 
thus superimposed and must of course be accurately aligned if the 
gunner/operator is not to be confused. 
The line of sight of the TI sight is indicated by a thermal aiming mark 
(TAM) illustrated as a crosswire. This second line of sight marker is 
produced by a projector 11 which has a `crosswire` slide the image of 
which is projected into the TI object lens 13 by way of a prism reflector 
as for the visual sight. 
Control of the field of view of the TI sight is effected electronically, by 
shifting the raster of the C.R.T. display in each of two directions by a 
controllable D.C. bias imposed on the raster signals. A different portion 
of the raster is thus projected into the visual sight as the bias is 
adjusted. Clearly, the TI line of sight marker will be locked to the 
infra-red scene as the raster, and thus the TI field of view projected 
into the visual sight, is shifted. Because the TI sight field of view is 
controlled so far back in the TI imaging process, the TI aiming mark can 
be introduced into the TI sight even behind the object lens 13, i.e. as 
indicated in FIG. 1. 
The TI sight is initially set up so that its line of sight 7, as indicated 
by its marker, also intersects the muzzle boresight line 1 at the standard 
target distance. 
Referring now to FIG. 3, each of FIG. 3(a), (b) and (c) shows the field of 
view common to the visual and TI sights, i.e. as seen by the gunner. The 
basic marker of the visual sight is the muzzle boresight mark designated 
MBS in the legend. This indicates the visual line of sight (and the laser 
axis) and is required to be kept aligned with the muzzle boresight. The 
latter is indicated by the reflected spot designated "MRS image" in the 
legend. The line of sight of the TI sight is indicated by the injected 
thermal aiming mark, designated TAM in the legend. 
The remaining symbol in FIG. 3 is the visual reference mark, not yet 
mentioned. 
The visual reference mark, shown as a square in FIG. 3, is produced by a 
projector 15 in FIG. 1, the image of the square being projected into the 
(adjustable) object lens of the visual sight by way of reflecting prisms 
17. Since the source of the reference mark is external to the object lens, 
adjustment of the latter will cause the reference mark to move as one with 
the visual scene. The projector 15 is normally inoperative, being switched 
on during the sight alignment procedure. 
In FIG. 3(a) the operator sees two superimposed images of a target tank, a 
visual image 17 derived by the visual sight and an infra-red image 19 
derived by the TI sight (the latter is shown shaded). He also sees the MBS 
mark aligned with the TAM mark but both out of alignment with the spot 11 
of the MRS image. The particular displacement shown would indicate that 
the muzzle had dropped since setting up, as a result of thermal changes 
after firing, perhaps. 
The operator then adjusts the visual sight (by controlling the tilt of the 
object lens 9) until the MBS mark is brought into alignment again with the 
spot 11 of the MRS image. In doing so, the visual scene, including the 
target image 17, moves with the MRS image and the visual and TI scenes 
become separated, as indicated by the separation of the targets 7 and 9. 
If a distinguishable target, such as the tank shown, were present, the TI 
sight could then be re-aligned with the visual sight by manual adjustment 
of the TI sight line until the separated images are again coincident. 
In the absence of such a distinctive target however, the problem of 
aligning the TI sight remains. In the presently described arrangement the 
problem is solved by injecting the visual reference mark, shown as a 
square symbol, into the visual sight, and in such manner that the visual 
reference mark moves with the visual scene as explained above. When the 
visual sight is corrected therefore, as shown in FIG. 3(b) the visual 
reference mark, which was previously in alignment with the muzzle 
boresight mark MBS, is displaced from it by the same amount as was 
necessary to bring the MRS spot and MBS into coincidence. There is 
therefore displayed a measure of the required displacement of the TI line 
of sight irrespective of the presence of any distinguishable target. The 
TI sight is then adjusted as shown in FIG. 3(c) until the thermal aiming 
mark TAM is again in coincidence with the injected visual reference mark. 
The two images of the target will then be found to have coincided.