Lead computing sight

Abstract: The gun sight comprises a cathode ray tube (21) to generate an aiming image and optical means (22,23) to project said generated image into a line of sight through the gun sight. Velocity of movement of the gun sight is sensed by gyros and in response thereto the cathode ray tube (21) generates an aiming image at a location displaced from a sight-stationary position by an amount dependent on the velocity and direction of movement of the gun sight and a predetermined target range.

Description: The present invention relates to a lead computing sight. More particularly it relates to a sight which enables a gunner to track targets with the required lead angle offset automatically predicted. Such sights are particularly advantageous in cases where the target is fast moving and as such find particular utility as sights for surface to air artillery. In such sights, there is generally shown a crosswire display that is aimed on the target. As the gun traverses, either horizontally and/or vertically, the crosswire may be moved to have a deflection from a central point dependent on the lead angle required for any particular range, which can be calibrated beforehand. Such devices are shown in US-A-3727514 and EP-A-0054489. Lead angle computing sights are also known which include one or more free gyros having a mirror attached to the motor axis of the or each gyro. The crosswire is projected by means of the mirror or mirrors which must be aligned accurately to give the lead angle for a particular range. The optical systems involved in such a sight are complex, and the image produced is affected by the damping of the gyro or gyros. A lead angle computing sight is disclosed in GB-A-2121934 (or in corresponding FR-A-2 524 978) in which the problems of the mechanical crosswire moving systems of the prior art are overcome by providing a crosswire system in the form of a liquid crystal display which is associated with appropriate drive circuitry to produce a crosswire image. This system suffers from the disadvantage, however, that the display is located in the line of sight of the gunner and accordingly inhibits a clear view of the target. Also, the available position of the horizontal and vertical crosswires in a liquid crystal display system is quantized and is limited to the number of available display segments. Thus, the actual position of the crosswires produced will in general not correspond precisely with the required offset and this leads to inaccuracy in the sighting system. The problem of positioning the image generating components in the line of sight of the gunner is overcome by the device of US-A-4794430 since, in this case, the crosswire image is generated out of the line of sight of the gunner and is projected into the line of sight of the gunner by an optical system. In this case, however, separate light emitting diode displays are used for generating the horizontal and vertical crosswires of the aiming image respectively. Again, such devices are only capable of producing lines at quantized and discretely spaced locations. In the described embodiment of the invention of US-A-4794430 each display includes 80 lines. As in the case of GB-A-2121934, therefore, the position of the aiming image will, in most cases, be an approximation to the required position necessary to produce the correct offset of the gun. On occasions when the required offset coincides with the intersection of horizontal and vertical lines of the light emitting diode displays the gun will be laid accurately. At all other times, there will be an inherent inaccuracy as a result of the quantized nature of the light emitting diode lines. Also, in the case of US-A-4794430 a split prism system is required to combine the images of the horizontal and vertical light emitting displays. The inclusion of this component adds to the cost of the optical system and, at least to some extent, degrades the quality of the image produced. It is an object of the present invention to provide a gun sight which overcomes the above disadvantage. According to the present invention a gun sight in accordance with the pre-characterising portion of claim 1 is characterised in that the aiming image is formed on the screen of a cathode ray tube and the position of the aiming image relative to the centre of the field of view of the viewing means is varied in response to signals indicative of the velocity and range of the target by varying the input signals to the cathode ray tube to vary the position of the aiming image formed on the screen. By forming the aiming image on the screen of a cathode ray tube the problem of quantization of the position of the aiming image associated with GB-A-2121934 and US-A-4794430 is avoided. Also, the circuitry necessary to control the position of a crosswire image formed on a cathode ray tube is relatively simple compared with that necessary to activate an individual bar of a liquid crystal or light emitting diode display. It is also to be noted that the brightness of the image produced on the screen of a cathode ray tube can readily be varied over a wide range and accordingly the system of the present invention in which the image projected into the line of sight of the gunner is derived from the screen of a cathode ray tube is particularly desirable for use under varying light conditions. The aiming image preferably includes a crosswire formed by intersecting horizontal line and vertical lines. The means to sense velocity of movement may be a pair of gyros, operable about orthogonal axes, one to sense a vertical component of movement and one to sense a horizontal component of movement. Preferably, each gyro is a rate gyro which, on sensing movement, outputs a voltage, the magnitude of which is dependent on the velocity of movement and which is fed to control means for the cathode ray tube to vary the position of the corresponding line of the crosswire. An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 is a longitudinal cross-section through a sight embodying the invention; FIGURE 2 is a schematic circuit diagram of a control circuit for the sight; FIGURES 3 to 5 show schematically the generation of a vertical line of the crosswire; FIGURE 6 shows schematically the generation of a horizontal line of the crosswire; FIGURE 7 shows views through the sight for motion in a horizontal plane; and FIGURE 8 shows views through the sight for motion in a vertical plane. The sight may be fitted to any weapon by means of a dovetail bore sighted to the cannon axis. It is especially suitable for weapons in the 20-35 mm calibre range, although it may be used with other calibre weapons. Referring now to Figure 1 of the drawings, the gun sight comprises a cathode ray tube 21. This may be a 1 inch (2.5 cm) monitor set at a focal distance of 100 mm from a lens 23 and an optical prism 22 which projects the image displayed by the CRT at infinity into the sighting path. The image generated on the monitor 21 is a crosswire formed of an intercepting vertical line and horizontal line. When the gun and therefore the sight is stationary, the crosswires appear at a predetermined point in the sight path (see Figs. 7 and 8), which point is generally central but need not necessarily be so. However, for convenience, it will be referred to as a central point. When a target is picked up in the sight, it is held at the junction of the crosswires and the gun and sight moved to track the target. The movement is sensed and the position of the crosswires is moved from the central position in accordance with the speed and direction of movement. For example, in Figure 7 there are shown positions for the crosswire when the sight is being moved to port, is stationary, and being moved to starboard respectively. Similarly, Figure 8 shows a view through the sight fo various directions of elevational movement. If the target is moving in a diagonal line, obviously both horizontal and vertical lines of the crosswire move accordingly. In general, assuming smooth tracking of the target, the central point (and therefore the gun) will be spaced from the crosswire intersection by such a distance and in such a direction that a target seen at the intersection of the crosswires would be seen at the central point after a time interval allowing a shell from the gun to reach the point where is the target. This time delay period will obviously depend on range and the muzzle velocity of the gun among other factors, and these factors need to be programmed into the sight in order to make it most effective. The rate of movement of the sight is sensed by two independent gyros, mounted at 90 o one to another for azimuth and elevation sensing. The preferred gyros are each a Smiths Industries 930 RGS1 Rate Gyro which gives an output of approximately ± 200 mV/deg. per sec. Each gyro feeds directly into an Op. Amp circuit which provides offset bias, gain control and low pass filter. Referring now to Figures 2 and 6, the output of the elevation gyro Op. Amp provides a controlling voltage for adjusting the frequency of a horizontal line oscillator Osc. 2, which has a constant current source circuit to enable the frequency to change linearly with the changing control voltage. In order to generate the horizontal line of the crosswires, the frame sync pulses trigger the oscillator timer Osc. 2 to generate a horizontal position and to initiate start and synchronising pulses to lock the vertical line with reference to the horizontal line. Osc. 2 triggers Osc. 3 which provides a start trigger for the horizontal line width timer Osc. 4, the output of which feeds into a video mixer. The output of the azimuth gyro Op. Amp provides a controlling voltage for adjusting the frequency of a vertical line oscillator Osc. 1, which also has a constant current source circuit. In order to generate the vertical line of the crosswires, the line sync pulses trigger Osc. 1 to a varying time period dependent on the control voltage, which represents the start of the vertical line bar, the width of which is controlled by a differentiator. The resulting output pulse inputs a gate and is switched through for a time period generated by oscillators Osc. 5 and Osc. 6. Osc. 5 is triggered by Osc. 2 to govern the position of the vertical line with reference to the horizontal line, and Osc. 6, triggered by a pulse from Osc. 5, generates a pulse representative of a vertical line length. This is shown in Figures 3 to 5. The gate output is then combined at the video mixer to provide a mixed composite output to interface with a standard 625 line monitor 21. The sight is preferably self-contained and power is provided by ten 1.4 V nicad cells, or via a DC/DC converter utilising an external source of electrical power. The power source should be monitored so that a fixed crosswire display appears when voltage falls below a minimum level. As can be seen, the sight is extremely compact with movement of the crosswires being caused by electronic input to a cathode ray tube. The image generated by the cathode ray tube moves from a central point by an amount determined by the two gyros, whereby the lead angle is automatically computed.

Claims: A gun sight comprising: means defining a line of sight for viewing a zone containing a target to be engaged by a gun associated with the gun sight; means for generating an aiming image out of the line of sight; a fixed optical system for projecting the aiming image into the line of sight superimposed on the zone of the target so that by movement of the gun sight a gunner may align the aiming image with the target to lay an associated gun on the target; and means for varying the position of the superimposed image relative to the centre of the field of view of the viewing means in response to variations in the range of the target and the velocity of movement of the gun sight as the gunner tracks the target such that aligning the aiming image with the target produces an offset in the direction of fire of the associated gun to compensate for movement of the target during travel of the projectile fired from the gun, characterised in that the aiming image is formed on the screen of a cathode ray tube (21) and the position of the aiming image relative to the centre of the field of view of the viewing means is varied in response to signals indicative of the velocity and range of the target by varying the input signals to the cathode ray tube (21) to vary the position of the aiming image formed on the screen. A gun sight as claimed in claim 1, characterised in that the aiming image comprises a crosswire formed by an intersecting horizontal line and vertical line. A gun sight as claimed in either claim 1 or claim 2, characterised in that the velocity of movement of the gun sight is detected by a pair of gyros, operable about orthogonal axes, one to sense a vertical component of movement and one to sense a horizontal component of movement. A gun sight as claimed in claim 3, characterised in that each gyro is a rate gyro which, on sensing movement, outputs a voltage, the magnitude of which is dependent on the velocity of movement and which is fed to control means for the cathode ray tube to vary the position of the corresponding line of the crosswire. A gun sight as claimed in claim 4, characterised in that the output of a vertical movement sensing one of said pair of gyros is fed to a first oscillator to generate a start position of a horizontal line. A gun sight as claimed in claim 4, characterised in that the output of a horizontal movement sensing one of said pair of gyros is fed to a second oscillator to generate start positions of a vertical line, the width of which is controlled by differentiator means.

Inventors: JACKSON LYNN, PIKE JOHN ERNEST, POLZIN DAVID HERMANN, JACKSON, LYNN, PIKE, JOHN ERNEST, POLZIN, DAVID HERMANN
Applicants: ASTRA HOLDINGS PLC
Application year: 1990
Main classification: F41G3
IPC codes: F41G3