Precision guided antiaircraft munition

A small diameter, 20 mm to 50 mm, guided projectile is used in antiaircraft defense. A pulsing laser designator illuminates the target aircraft. Energy reflected from the aircraft is received by the guided projectile. The guided projectile is fired from a standard weapon but the spining caused by the riflings are removed before active tracking and guidance occurs. The received energy is focused by immersion optics onto a bridge cell. AC coupling and gating removes background and allows steering signals to move extended vanes by means of piezoelectric actuators in the rear of the guided projectile.

BACKGROUND OF THE INVENTION 
This invention relates generally to antiaircraft munitions and, in 
particular, relates to guided antiaircraft munitions. 
In the past, several types of munitions have been used in point defense 
against aircraft attack. One type is the shoulder fired guided missile 
which is either guided by a laser designator or a trailing wire. Although 
this weapon may be effective for its intended purpose, it lacks a high 
fire rate because each round must be loaded individually in the shoulder 
launcher. Another type of point defense is the radar directed gun such as 
the DIVAD. The disadvantages are its high cost and a very high rate of 
fire needed to put up a wall of "lead" to insure a kill with the smaller 
caliber munition used. 
SUMMARY OF THE INVENTION 
The present inventon sets forth a guided projectile fired from a gun having 
a bore of about 20 to 50 mm. 
The present invention uses a conventional small caliber antiaircraft gun. 
The rounds therein have a typical metal cartridge with a conventional 
propellant onto which the guided projectile is fitted. In addition to the 
above, the present invention uses a pulsing target laser designator for 
illuminating the intended aircraft target. 
Within a fraction of a second or so after leaving the gun muzzle, 
derotating and steering vanes are deployed from the projectile, for 
example, which cause the projectile to stop rotating within a few seconds. 
After the projectile has stopped spinning, a laser guidance system steers 
the projectile to the target. 
An immersion optical lens in the front of the projectile focuses the 
received laser radiation onto a bridge cell positioned directly behind the 
lens. Depending upon the position of a focused spot on the cell, 
appropriate drive signals will actuate the steering vanes to maneuver the 
projectile to the target or near the target for detonation. 
It is therefore one object of the present invention to provide a guided 
projectile for a small diameter round usable in a conventional weapon. 
Another object of the present invention is to provide better point defense 
against aircraft with a round that is more likely to hit the target. 
Another object of the present invention is a guided projectile that has a 
lower minimum range, has a greater chance of survival against counter 
measures, has a higher fire rate, and has a lower launch signature. 
These and many other objects and advantages of the present invention will 
be readily apparent to one skilled in the pertinent art from the following 
detailed description of a preferred embodiment of the invention and the 
related drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a guided projectile 10 is shown in cross section. The 
diameter of projectile 10 ranges from 20 mm to about 50 mm and is used in 
a conventional weapon, not shown. In addition, a conventional laser 
designator, also not shown, is used by the weapon crew to select a target 
aircraft. The aiming of the laser designator can be by radar or optical 
tracker. 
Projectile 10 has a casing 12 with rotating lands 14 about casing 12. An 
immersion lens 16 is mounted in the nose of projectile 10 and causes 
received laser radiation to be focused onto a quadrant detector 18 being a 
bridge cell tracker. Detector 18 is connected into an electronic steering 
section 20. Power is provided by a battery 22 activated by firing. A 
detonator 24 causes high energy explosive 26 to detonate upon impact. A 
vane control section 28 is placed at the rear of projectile 10. Vanes 30, 
two shown of four, are used to derotate projectile 10 and to steer 
projectile 10 to the target aircraft. 
Because projectile 10 is fired from a weapon having riflings, the rotation 
must be stopped before active guidance can be started. This may be 
achieved several ways. A rotating sabot may be placed about projectile 10 
to minimize the spin imparted by the riflings. The sabot would then drop 
away after projectile 10 clears the muzzle of the weapon or rotating lands 
14 may be used. In either case, projectile 10 would still have some spin 
upon leaving the muzzle. Very shortly after leaving the muzzle, vanes 30 
would be automatically deployed by springs 32 into a position to maximize 
derotation at which time a derotation switch 34, FIG. 2, allows a detector 
36 to output signals 46, 48 and 50. 
Referring to FIG. 3, vane 30 is shown in the extended position. Initially 
vane 30 is depressed onto an arm 38 being box shaped to prevent rotation 
of vane 30 about arm 38. After leaving the weapon muzzle, vane 38 is 
extended by spring 32 acting against a retainer 40 attached to vane 30. 
Arm 38 is attached to an actuator 42 that can turn either one vane or two 
vanes 30. Vanes 30 can be held in place by the sabot, not shown or by 
other such means, that breaks away upon leaving the muzzle whereupon vanes 
30 are extended. 
Actuator 42 is an electromechanical device that provides motion 
piezoelectrically. Actuator 42 is a multiple ring actuator in which the 
actuator shaft is driven through a mounting by repeated peristatic 
strokes. The stroke length is unlimited and the steps are as rapid as 
pulses can be applied. 
Referring to FIG. 1, immersion lens 16 is connected directly to a bridge 
cell 44. Table I provides typical parameters. 
TABLE I 
______________________________________ 
Laser type 
Operating wavelength 10650 A 
Operating mode Repetitive Q-switch 
Laser pulse energy 1 V 
Pulse length 15 nanosec 
Laser beam divergence 0.2 mrad 
Beam spreader transmission 
90 
Photons per pulse 5.36 .times. 10.sup.18 
Pulse repetition rate 30 sec.sup.-1 
Range of laser to spot 
5000 m 
Target hemispherical reflectance 
15 
Range of detector to spot 
1500 m 
Atmospheric condition Standard Clear 
Height of path worst case 
-0 
Atmospheric attenuation coefficient 
0.115 Km.sup.-1 
Total atmospheric transmission 
47.4 
Collector diameter 1.6 cm 
Collector field of view 
30.degree. 
Detector diameter 4.0 mm 
Immersion material Sapphire 
Immersion material index 
1.76 
Beam convergence at detector 
72.degree. 
Required angular resolution 
10 mrad 
Required lens resolution 
13.1 p/mm 
Filter band width 300 A 
Filter transmission 75 
Transmission of optics 
80 
Total transmission 60 
Band width of AC system 
25 MHz 
Time constant of detector 
10.sup.-7 sec 
Detector noise, 1 Hz bandwidth 
4.0 .times. 10.sup.-13 W 
Detector noise, operating bandwidth 
5 .times. 10.sup.-9 W 
Detector quantum efficiency 
75 
______________________________________ 
Bridge cell 44 is a single photodiode having four independent anodes or 
quadrants. The ratio of the quadrant output defines the location of the 
focused spot on cell 44. 
Referring to FIG. 2, when X output 46, Y output 48, and I (energy pulse) 
output 50 of bridge cell 36 are AC coupled to amplifiers 52, 54 and 56, 
the outputs correspond to that part of the image which changed within one 
time constant. In 10.sup.-7 sec the background is effectively stationary, 
and produces no signal. Thus any variation in the outputs is produced 
solely by the laser spot, the only part of the input that varies fast 
enough. This circumvents the tendency of the bridge cell of detecting the 
centroid of all sources present instead of the one source one is 
interested in. 
Whenever an I output 50 is sensed, the X and Y amplified outputs 58 and 60 
are gated by gate control 66. As the guidance becomes more and more 
accurate, X and Y become smaller as they are measures of the deviation 
from the "true" direction. When projectile 10 is on a perfect track, there 
will be no X and Y outputs 46 and 48 when I output 50 is detected, and no 
tracking inputs 68 and 70. As the electronic steering is a nulling one, 
the values of X and Y, the amplifier gain, and the guidance responsivity 
require only gross estimation, not calibration. As the system senses 
angular deviation, it becomes better the closer its gets to the spot, and 
so can be aligned quite permissively. 
The size of the laser spot is not critical as the centroid will be 
detected. However, at very close ranges the spatial reflectivity variation 
within the spot will shift its centroid. Fortunately, this happens at only 
very close ranges where small angular errors are insignificant. 
The maximum range equation for projectile 10 assuming no atmospheric loss 
and a target approaching along the line of sight is 
##EQU1## 
where R.sub.M =maximum range 
R.sub.A =average range 
a=air drag velocity loss factor 
v.sub.o =initial velocity 
t.sub.D =lag time for target maneuver acceleration 
L.sub.D =lift drag ratio 
.alpha..sub.T =maximum target maneuver acceleration 
v.sub.t =target velocity 
Here we have for the range at intercept 
##EQU2## 
the time at intercept 
##EQU3## 
the velocity at this point and the maneuvering capacity here is 
EQU .alpha..sub.M =a L.sub.D v.sup.2 (5) 
The time duration of the guidance phase is 
##EQU4## 
Clearly, many modifications and variations of the present invention are 
possible in light of the above teachings and it is therefore understood, 
that within the inventive scope of the inventive concept, the invention 
may be practiced otherwise than specifically claimed.