Projectile Trajectory Correction System

Abstract

A system for correcting the terminal portion of the trajectory of a projectile in free fall so as to minimize the error between the actual and the intended impact point. The projectile may be an artillery rocket, cannon shell, or a similar ballistic body, which is caused to roll during its free fall trajectory. The intended target or impact point is illuminated by a light source and this light is received at the projectile by a sensor consisting of optics and a plurality of detectors arranged in a plane, and along an annular area. The sensor is made to have a hollow conical field of view, such that the ground area in the vicinity of the target covered by the field of view is reduced as the projectile approaches the target. Thus, when the target appears in the field of view its image will fall on one of the detectors to determine the polar coordinates of the target with respect to the uncorrected impact point. Electronic means are provided for firing a lateral thruster at a predetermined time commensurate with the polar position of the detector that has detected the target image. This will apply a lateral impulse to the projectile to change the trajectory so as to minimize the terminal error.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to projectile trajectory correction systems and particularly relates to a system for minimizing the error between the actual impact point and the intended impact point of a free fall projectile.

Usually it is desired to direct a projectile toward a particular target. An example of this is a cannon shell which is directed toward a specific target. The same applies to other ballistic bodies which are in free fall or an air dropped bomb. In general, once a projectile is launched and in free fall, it is difficult to correct its trajectory. The alternative would be some form of guided missile but it is well known that missile guidance systems are expensive, rather sophisticated and not always reliable.

It is accordingly desirable to provide a system capable of correcting the trajectory of a projectile in free fall. More correctly such a system will minimize the impact error. This, in turn, will insure that the projectile comes much closer to its intended target. As a result, the expenditure of projectiles required to achieve a desired effect is minimized.

It is accordingly an object of the present invention to provide a relatively inexpensive system for correcting the terminal portion of the trajectory of a projectile in free fall.

Another object of the present invention is to provide a system of the type discussed which will much reduce the impact error of a projectile so that projectiles equipped with the system will, on the average, impact much closer to the intended target than otherwise equivalent projectiles.

A further object of the present invention is to provide a sensor which will provide the information required to minimize the impact error of a projectile in free fall.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a system for correcting the terminal portion of the trajectory of a projectile in free fall directed toward a target. The target is illuminated by a light source. Also the projectile is caused to roll at a predetermined rate, for example, by providing suitable fins. Disposed on the projectile is a sensor apparatus including a sensor for sensing the light of the light source reflected by the target. Means are associated with the sensor apparatus for limiting the field of view to a cone of predetermined angle and preferably a hollow cone with fixed, interior and exterior angles. Furthermore, a thruster is disposed on the projectile for supplying upon firing to the projectile a predetermined lateral impulse. Finally, electronic means are coupled to the sensor and to the thruster for energizing the thruster when the sensor receives light from the illuminated target within its field of view.

As a result the thruster is energized in accordance with the polar coordinates of the image of the target at the projectile. It is energized at a certain distance from the ground as determined by the angle of the field of view cone so that the thruster's impulse acting on the projectile will deviate the trajectory so as to minimize the target error.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevational view of the terminal portion of the trajectory of a projectile in free fall, its uncorrected and corrected trajectory as well as the light source illuminating the intended target;

FIG. 2 is a sectional view on enlarged scale of the sensor with detector and associated lens and electronic components;

FIG. 3 is a schematic top plan view along the roll axis of the projectile at the sensor illustrating the detectors disposed along a portion of an annulus and the thruster shown in its initial and firing positions;

FIG. 4 is a top plan view somewhat similar to that of FIG. 3 but illustrating the offset angle of the thruster required to compensate for the buildup and decay time of the thrust; and

FIG. 5 is a schematic diagram in block form of the electronics required to fire the thruster in response to the image of the target received by one of the detectors.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and particularly to FIG. 1, there is illustrated a projectile 10 in free fall. The projectile 10 is provided with fins 11 which are shaped in such a manner that they impart a roll to the projectile so that the projectile rotates at a relatively constant predetermined rate of revolution. The projectile 10 is also provided with a lateral thruster 12 which is so positioned to impart a single lateral impulse to the projectile upon firing.

A straight line approximation to the projectile's free fall trajectory is shown at 14 and if uncorrected will cause the projectile to hit an impact point 15 on the ground. The actual target is shown at 16. The trajectory 14 and an extension of the projectile roll axis makes an angle with a horizontal line indicated at .theta..sub.t.

It is therefore desired to provide a terminal correction to the trajectory 14 of the projectile. The corrected trajectory is shown at 17 corresponding to the corrected impact point 18. It will be noted that the corrected impact point 18 is much closer to the desired target 16 than is the uncorrected impact point 15. The angle between the original trajectory 14 and the corrected trajectory 17 is .theta..sub.c. The angle between the projectile roll axis and the line of sight from the projectile to the target is .theta..sub.i. Finally, as will be explained hereinafter, the projectile 10 is provided with a sensor which may have a hollow conical field of view, the interior angle and exterior angle of which are given by .theta..sub.i and .theta..sub.o respectively from the sensor axis which is prealigned with the projectile roll axis.

It should also be noted that a light source 21 is provided on a convenient hill or other location which directs a light beam 22 toward the target 16 for illuminating it. The light source 21 could be any suitable light source capable of illuminating the target 16, as well as the various impact points 15 and 18, with sufficient light intensity to enable operation of the sensing system of the projectile 10 on light reflected from the target. Conveniently, the light source 21 may consist of a pulsed solid state laser utilizing, for example, a YAG (yttrium aluminum garnet) host doped with neodymium or a glass host doped with erbium or a ruby.

The YAG neodymium laser generates infrared light having a wavelength of 10,600 A (angstrom). For certain applications it may be desirable to use infrared light which is invisible. It may also improve the reception of the target image at the projectile. Accordingly, it will be understood that the term "light source" includes a source that emits light in either infrared, visible or ultraviolet wavelength regions. The light source 21 may develop a continuous light beam or it may be pulsed say at the rate of 10 pulses per second and each pulse may have a duration of 10 to 30 nanoseconds.

Instead of placing the light source 21 at a neighboring hill it is also possible to illuminate the target from a helicopter or the like. The projectile 10 may be a projectile from a gun, a rocket or a bomb in free fall.

The sensor disposed in the nose of the projectile 10 will now be described and is illustrated in FIG. 2 to which reference is now made. The geometric position of the detectors and the thruster 12 is shown in FIGS. 3 and 4 which will be subsequently explained.

As shown in FIG. 2 there may be provided a cylindrical container 25 including a cylindrical element 26 containing a plurality of detectors 27 arranged in a plane and along the major portion of an annular space as shown in FIGS. 2 and 3. The detectors 27 may, for example, consist of PIN silicon or PN germanium. Alternatively, for a different spectral region a detector of gallium arsenide may be used. The detectors may, for example, be manufactured on a single semiconductor chip in a conventional manner. The container 25 also includes a lens 28 which may be mounted in a ring 30 and retained by a lens retainer 31 which is threadably connected to the ring 30 which in turn is threaded on the container 25. Ahead of the detectors 27 there may be provided a light filter 32 for passing substantially only the light of the laser source 21. Such an optical filter may be a dielectric interference type filter or an absorption type filter for operation with a YAG laser source emitting at 10,600 A. Such filters can be readily made to cut off all light below 9,000 A. On the other hand a silicon detector is not responsive to infrared light above 11,000 A. Accordingly, between the optical filter and the detector only light between 9,000 A and 11,000 A is sensed including that generated by a YAG laser. This will reject most of the daylight background light.

A plurality of connector pins such as shown at 33 interconnect each detector to the electronics shown at 34 having an output cable 35 which in turn is connected to the thruster 12 for firing it.

The lens 28 and the detectors 27 are so arranged that the field of view of the detector system preferably is a hollow cone with predetermined interior and exterior angles; namely, the angles .theta..sub.i and .theta..sub.o. These angles are determined by the size of the detectors and the focal length of the lens 28. The angles .theta..sub.o and .theta..sub.i are also shown in FIG. 1 between the trajectory or projectile roll axis 14 and lines 20 and 29.

The operation of the detectors 27 and how they correct the projectile trajectory 14 will now be explained by referring to FIGS. 3 and 4. As shown in FIG. 3 there are disposed, for example, 15 detectors in a plane about an annular space extending through an angle of 270.degree.. It will, of course, be understood that it is possible to have an annular space completely taken up by detectors 27. However, by providing a gap of detectors through a predetermined angle such as 90.degree., errors related to the uncertainty of roll rate are minimized.

Assuming that the arrow 37 points toward the target 16, as seen from the projectile, the image 38 of the target will eventually be received by one of the detectors, say detector 27'. Since the projectile provides a conical field of view it will be understood that the area viewed by the detector decreases as the projectile approaches the ground. Due to the direction of roll of the projectile shown by the arrow 40, the image of the target will follow a spiral diverging or outward-going with time in the plane of the detectors 27 until eventually the image 38 of the target falls on one of the detectors, say detector 27'. The projectile roll axis is shown at 43.

12' indicates the circumferential position of the thruster 12 when the target is first detected. 12" shows the position of the thruster 12 when it receives a firing impulse generated by the electronics 34 in a manner which will later be explained. In this manner the thruster is properly positioned so that its impulse causes a trajectory deviation which reduces the impact error. Accordingly, the angle .alpha. which indicates the offset in polar coordinates between thruster positions 12' and 12" considering the roll rate of the projectile corresponds to the time delay caused by the electronics 34. It will be understood that the angle .alpha. depends on the angular roll rate of the projectile and the time delay generated by the electronics.

FIG. 4 illustrates schematically the arrow 37 pointing toward the target, and the circle 41 representing a cross section of the projectile body with its roll axis 43. As shown at 42 rather schematically, the thruster 12 does not fire instantly upon receipt of the ignition signal but requires a finite time for both buildup and decay of the thrust produced. Therefore, the thruster's impulse vector which represents the thrust integrated over time, is offset by an angle .theta..sub.f from the position of the thruster at the time of ignition.

To compensate for this offset, the thruster 12 should be correspondingly offset by the angle .theta..sub.f as shown at 12'", that is, its position should be retarded with respect to the direction of rotation shown by the arrow 40. This will ensure that the impulse of the thruster is directed in the proper direction. The effect of the impulse on the projectile trajectory is explained for a two-dimensional case by the vector diagram of FIG. 1. v.sub.m indicates the initial velocity vector. In order to obtain the desired new velocity vector v.sub.r it is necessary to provide a lateral velocity v.sub.e all as shown in the vector diagram so as to reduce the impact error such that the angle between the two velocity vectors v.sub.m and v.sub.r is .theta..sub.c. The lateral velocity vector v.sub.e is that imparted to the projectile by the thruster 12.

The electronics 34 is shown in more detail in FIG. 5 to which reference is now made. The detectors are shown schematically at 27. Each detector is connected to an amplifier 45 which generates an amplified signal 39 and in turn is connected to a threshold circuit or amplifier 46 which generates an output pulse as shown in 47 when the input exceeds a predetermined level. Each threshold circuit is coupled to a shift register 48 having 15 storage positions corresponding to the 15 detectors 27. The shift register 48 is actuated by a clock pulse generator 50 which generates clock pulses 49 fed by a lead 51 into the shift register 48.

Outputs from all of the threshold circuits 46 are connected to an OR gate 52. Accordingly, the OR gate 52 will generate an output signal to start the clock pulse generator 50, upon the occurrence of an output signal from any one of the 15 threshold circuits 46. In other words, as soon as the image of the illuminated target is received by one of the detectors 27, an output signal is generated by that detector which is loaded into the shift register 48. At the same time the OR gate 52 will cause the clock pulse generator 50 to start and to send pulses into the shift register 48. This in turn will cause the previously loaded input signal to propagate through the shift register 48 until it is passed to a switch circuit 54 which then generates an output pulse 55 for firing the thruster 12. It will now be appreciated that the time delay caused by the electronics corresponds to the angle .alpha. through which the thruster must rotate to be properly positioned for firing. It will also be understood that the first pulse received by any one of the detectors initiates the cycle of operation and that subsequent pulses do not influence the time of firing of the thruster.

By way of example, it may be assumed that the projectile rotates at the rate of 7.5 revolutions per second. It may also be assumed that there are 15 detectors covering 270.degree. of the field of view. Accordingly, the clock pulse rate is 7.5 .times. (360.degree./18.degree.) = 150 pulses per second, where the 18.degree. corresponds to the angle through which one detector extends. With this assumption the thruster can be made to fire within the same revolution when the image of the target is first received by one of the detectors. This will minimize the effects of variations in the rate of roll of the projectile. Therefore, for all practical purposes, the thruster will fire within three-fourths of a revolution or less.

It will be appreciated that the field of view of the detector, that is the interior angle .theta..sub.i of the hollow cone determines the point in the terminal portion of the path of the projectile when the correction system operates. Thus, by way of example, the correction system may operate during the last 15 percent of the projectile's trajectory. It may also be assumed that the velocity of the projectile is 700 ft. per second. As indicated before, the duration of the illuminating light pulse may be 10 to 30 nanoseconds while the build up time of the thrust may amount to 10 milliseconds with a total build up and decay time of about 25 milliseconds. It will now be appreciated that the impulse delivered by the thruster must be such as to provide a desired angle of deviation which should be equal to the interior angle of the field of view. Because the angle of view of the detector system is known it can be determined how far from the ground the projectile will be when its trajectory is corrected. It should also be understood that the correction reduces the miss distance of the error but may not completely correct the trajectory. However, the miss distance is corrected to such an extent as to minimize the impact error commensurate with the complexity and cost of the correction system.

There has thus been disclosed a correction system for the terminal portion of the trajectory of a projectile. The system includes a sensor consisting of a series of detectors for receiving an image of the illuminated target and appropriate electronics. The electronics will provide a firing impulse to a thruster which provides a lateral impulse to the projectile to correct the trajectory toward the target. The correction is effected during one revolution of the rotating projectile. It occurs when the conical field of view has become sufficiently small so that the target image is received at the inner edge of one of the detectors. The polar coordinates of the target image determines the time of firing of the thruster so that the trajectory is corrected in the proper direction.

Claims

What is claimed is:

1. The method of correcting the terminal portion of the trajectory of a projectile in free fall and rotating about its axis, the projectile being directed toward a target, said method comprising the steps of:

a. illuminating the target;

b. detecting the light reflected from the target within a predetermined hollow cone extending between the rotating projectile and the ground as the projectile approaches the target;

c. applying a single lateral impulse to the projectile in accordance with the image of the light reflected from the target; and

d. determining the polar coordinates of the image of the target received at the projectile, thereby to determine the instant of time when the single lateral impulse is applied to the projectile, whereby the direction of the impulse applied to the projectile and the instant the impulse is applied is determined by the polar coordinates of the target image.

2. A system for correcting the terminal portion of the trajectory of a projectile directed toward a target, said system comprising:

a. a light source for illuminating the target;

b. means for causing said projectile to roll at a predetermined rate;

c. a plurality of detectors disposed on said rolling projectile and responsive to the light of said light source reflected by the target, said detectors being disposed in a plane and along an annular area;

d. means associated with said detectors for limiting the field of view of said detectors to a cone of predetermined angle;

e. a thruster disposed on said projectile for supplying to said projectile a single predetermined lateral impulse; and

f. electronic means coupled to said detectors and said thruster for energizing said thruster upon a particular one of the rotating detectors receiving light from the illuminated target within the field of view thereof.

3. A system as defined in claim 2 wherein said detectors extend over a portion only of said annular area to provide a detector-free annular segment.

4. A system as defined in claim 3 wherein electronic means is connected to each of said detectors for generating an electronic output signal in response to light falling on a particular one of said detectors, said electronic means being coupled to said thruster for energizing it at an instant of time depending on the position of said particular detector with respect to said segment for minimizing the error of the projectile's impact point.

5. A system for correcting the terminal portion of the trajectory of a projectile in free fall directed toward a target, said system comprising:

a. a light source for illuminating the target;

b. means for causing said projectile to roll at a predetermined rate;

c. a plurality of detectors, each for detecting the light reflected by the illuminated target, said detectors being arranged in a plane along an annular area extending over a portion only of said annular area to provide a detector-free annular segment;

d. means for limiting the field of view of said detectors to a hollow cone of predetermined interior and exterior angles, whereby said cone covers successively smaller areas as said rolling projectile approaches ground;

e. a thruster connected to said projectile for imparting thereto a predetermined lateral impulse;

f. electronic means connected to each of said detectors for creating an output signal in response to the image of the illuminated target falling on a particular one of said detectors, said electronic means being coupled to said thruster for energizing said thruster at an instant of time depending on the angular position of said particular detector with respect to said segment, thereby to impart a lateral impulse to said projectile in a direction to minimize the errors of the projectile's impact point.

6. A system as defined in claim 5 wherein said thruster is disposed circumferentially at such an angle with respect to said segment as to correct for the buildup and decay of the thruster's thrust.