Fire control system with aiming error compensation

A fire control system for a weapon having an operator-aimed barrel compri a sighting piece carried by the barrel and having a reticle adjustable in position with respect to the bore axis of the barrel. A camera fast with the barrel has a field coaxial with the bore axis. The apparent movement of the target in the field of the camera is measured as well as the angular movements of the barrel by the operator. The position to be given to the reticle is computed from variations of the aiming direction of the barrel and from the apparent movement and the position of the reticle is consequently adjusted. The reticle may consist of a LCD.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention relates to fire control systems for a weapon having a barrel 
which can be aimed by an operator. 
It is particular suitable in systems comprising a weapon or several 
mutually coupled weapons, of small or medium caliber. 
2. Prior Art 
Fire control systems are known (U.S. Pat. No. 4,020,324 to Buscher) of the 
type having a sighting piece carried by the barrel of the weapon, whose 
reticle is movable with respect to the bore axis of the barrel, with means 
for assessing the offset angle and controlling the position of the 
reticle. The offset angle, i.e. the angle between the direction of the 
reticle and the axis of the weapon (direction of fire), is determined so 
that, if the reticle is aligned with the present or actual target, the 
weapon is aimed towards a "future target". The offset angle is computed by 
taking into account the angular velocity of the target, the speed of the 
ammunition round or bullet and the assessed distance between the target 
and the weapon. The distance between the target and the weapon is 
generally estimated or measured, while the speed of the ammunition round 
is stored in a memory. The angular speed of the target with respect to a 
fixed reference point is assumed to be equal to the angular speed of the 
weapon controlled by the gunner who holds it aimed at the target. Due to 
that assumption, measurement of the angular speed of the target is 
affected by noise due to the aiming errors by the gunner and possibly to 
instabilities of the support. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a fire control system with an 
improved determination of the offset angle to be given to the reticle, the 
latter term being used for designating not only an ocular sighting 
reference but also a reference taking into account by an automatic fire 
control system automatically aiming a weapon from plotting of the 
trajectory of the target. 
To this end, there is provided a system of the above-defined type further 
comprising a wide field camera fast with the weapon barrel, means for 
measuring the apparent movement of the image of the target in the field of 
the camera and for computing the offset angle from angular variations of 
the aiming direction of the weapon barrel measured by sensors, for example 
tachometric or gyrometric sensors, and from the apparent movement of the 
image in the field of the camera. 
Thus a function is provided equivalent to that of a stabilized remote 
aiming control station operating as a master station for a weapon system, 
but in a much simpler way. Measurement of the apparent movement in the 
field of the camera simply gives a corrective term, i.e. a minor 
correction of the aiming direction removing the noise caused by the 
incapacity of the gunner to permanently and accurately follow the movement 
of the target. In other words, the trajectory plot of the target becomes 
independent of the quality of tracking the target. Filtering for 
minimizing the influence of aiming errors may be simplified. 
Furthermore, coupling between aiming and introduction of the fire 
correction term is without effect on the calculation of the latter. 
The invention also provides a fire installation having a range finding 
capacity while only using simple means. For that, the invention uses a 
wide field camera (which may be the same camera as used for measuring the 
aiming error) and means for measuring the apparent diameter of the target 
image in the field of the camera. A simple process makes it possible to 
determine the distance of the target from the apparent diameter of the 
target, the value of the focal length of the camera and the actual preset 
dimensions of the target. 
A camera using a matrix of charge coupled sensors (less expensive than a 
"Vidicon" camera) may be used for implementing the invention. This camera 
may, for night use, be coupled to a light intensifying tube. The system 
can then be used not only by day but also by night. 
It is important to note that range measurement may be permanently 
maintained, provided that the target remains in the field of the camera. 
The camera need not be aimed exactly at the target, while it is necessary 
in the case of a narrow angle telemeter for example. 
It is still another object of the invention to provide a movable reticle 
for fire control systems, making it possible to define the aiming 
direction for the gunner accurately using only simple means. 
For that, the invention provides a fire control system with a sighting 
piece whose reticle is formed by a liquid crystal matrix display, making 
it possible to give a particular brightness to at least one pixel. 
The matrix may also be used for displaying the image as seen by a wide 
angle camera, such as that required for the above-mentioned functions, to 
the gunner. This use is of particular interest when the camera is suitable 
for operating with a low light level, for example when it uses a light 
intensifier or a "NOCTICON" matrix of sensitive elements: night firing 
becomes then possible.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The aiming system shown schematically in FIGS. 1 and 2 is intended for a 
small caliber weapon 10, such as 20 mm twin guns, with manual aiming by 
rotation about a bearing axis 12 and an elevation axis 14. Axis Xa of the 
weapon (i.e. the firing direction) must not be directed towards the 
instantaneous position of the target at the time of firing, which will be 
called "actual target", but towards the position which the target will 
have when the ammunition is closest, i.e. towards a "future target" 
indicated with broken lines in FIG. 1. For easier aiming by the gunner, 
the sighting high piece fixed to the weapon is provided with means for 
offsetting the reticle in a direction and by an amount such that, when the 
sighting axis Xr is aligned with the actual target, the axis Xa of the 
weapon barrel is aligned with the future target. The use of offset means 
requires estimating or computing the offset angle .alpha. between Xr and 
Xa, and the direction of the offset. 
The means shown in FIGS. 1 and 2 for defining the amount and direction of 
.alpha. comprise a unit for measuring the aiming angle (or the speed of 
variation thereof) and a camera equipped with an error measuring device. 
The measuring unit 16 may be of conventional construction. As shown in 
FIG. 2, it is carried by the weapon and may comprise for example two 
gyrometers which permanently supply angles or speeds to a computing unit 
18. When the mount of the weapon is perfectly stationary, tachometric or 
angular sensors may be used instead of gyrometers. 
Due to the imperfections and time constants of the system for motorizing 
the mount and due to the limitations inherent in manual a-ming, tracking 
of a target whose angular position varies in time as shown by a straight 
line in FIG. 3, may be represented by an irregular line, such as line 20. 
In a conventional system, the influence of the aiming error is reduced by 
low-pass filtering of the signal, which is not without drawbacks. The 
system of the invention as shown in FIGS. 1 and 2 overcomes this 
requirement. For that, it comprises a camera 22 carried by the weapon 
barrel and whose field .theta. is coaxial with the direction of the weapon 
barrel (fire direction). The field of the camera must be sufficient for 
the actual target not to leave it, even when the distance between the 
actual target and future target is maximum. 
The video signal delivered by camera 22 is applied to an image processing 
circuit 24. An object of circuit 24 is to provide, at an output 26, the 
characteristics (modulus and angle) of the position and of the speed of 
the target image in the field, i.e. data of optical measurement. 
By combining the speed or position measurements delivered by unit 16 and by 
circuit 24, as well as an indication of target range, the computing unit 
18 determines angle .alpha. (FIG. 1). The range may be input manually to 
the computing unit from an output 28 of a console 30. It may also be 
provided by the image processing circuit 24, as will be seen further on: 
the console 30 may then comprise an output 32 for selecting the operating 
mode, permitting the computing unit 18 to take into account either a 
preset distance from the console or a range delivered by the image 
processing circuit. 
It will often be useless to assist the image processing circuit by 
indicating the target image in the field manually. In fact, the computing 
unit 18 may deliver to the image processing circuit 24, over a channel 34, 
an approximate indication of the position where the target will be found 
in the field, which allows discrimination. 
The image processing circuit 24 may be designed for analysing the image of 
the target, extracting therefrom its geometric dimensions (apparent 
diameter) and delivering them to the computing unit 18 through an output 
36. It is then sufficient to store, in the computing unit, typical target 
dimensions, for different angular positions in which they may appear, so 
that the computing unit has available the elements required for range 
finding. A few typical dimensions are sufficient. It is consequently 
enough to provide a target manual selector on console 30. 
Reticles of very various kinds may be used; however, in an advantageous 
embodiment of the invention, the reticle comprises a matrix of display 
elements, typically a liquid crystal matrix. To make the reticle appear, 
the arrangement shown schematically in FIG. 1 may be used. A 
semi-reflecting mirror 42 makes it possible to superimpose the image of 
the actual target and the pattern delivered by matrix 44. The computing 
unit 18 energizes at least one line and one column of the matrix, so as to 
cause two bright crossed lines to appear, or energizes a pixel. The 
arrangement may be reversed, so that the reticle appears in black on a 
white background. Contrast reversal may be provided by modifying the 
control of the matrix or inverting the direction of a polarizer used with 
the liquid crystal matrix display. The computing unit computes the 
position of the column and of the line to be energized. It is of 
particular advantage to energize two lines 48 and two columns 50 so as to 
form a rectangular reticle which can be used for a range assessment by the 
gunner. 
It is also possible, particularly for night firing, to use a light 
intensifying camera and then to use the matrix to display the reticle and 
the image delivered by the camera (FIG. 4). Night firing is thus made 
possible. 
Numerous modifications of the invention are possible. Two orthogonal strips 
may for example be used, instead of a matrix addressable by lines and 
columns; for time multiplexing of the control circuit without loss of 
contrast, the matrix may comprise transistors for storing the energization 
between two write-in operations.