Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
THREE AXIS AERODYNAMIC CONTROL OF GUIDED MUNITIONS
Document Type and Number:
WIPO Patent Application WO/2006/086532
Kind Code:
A2
Abstract:
An apparatus for 3-axis aerodynamic control of multi-caliber munition bodies comprising three shafts each having at least one canard attached thereto. In one embodiment, a set of three miniature stepper motors, each driving a zero backlash spur assembly is attached to the canard drive shafts by bevel gears. In a second embodiment, the stepper motors are connected to the canard drive shafts by a gear mechanism comprising a planetary gear, worm gear and pitch gear. The stepper motors are high torque two phase, 18 degree motors. Aero-mechanical guidance equipment is integrated into an optical seeker assembly located in the nose of the munition body and is used to provide steering signals to the stepper motors to steer the body toward the target thereby improving the Circular Error Probability (CEP) of the munition. A first shaft has a canard mounted on opposed ends of the shaft to provide roll control of the launched munition body and two additional shafts, each has an attached canard, one of which controls the yaw (left/right steering) and the other controls the pitch (up/down steering) of the munition body.

Inventors:
CARLSON, Mark, A. (20 Criket Hill Road, Amherst, NH, 03031, US)
ZEMANY, Paul, D. (27 Pulpit Run, Amherst, NH, 03031, US)
MAYNARD, John, A. (6 Sargent Quarry, Amherst, NH, 03031, US)
Application Number:
US2006/004538
Publication Date:
August 17, 2006
Filing Date:
February 07, 2006
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
BAE SYSTEMS INFORMATION AND ELECTRONIC SYSTEMS (Integration Inc, 65 Spit Brook RoadNashua, NH, 03060, US)
CARLSON, Mark, A. (20 Criket Hill Road, Amherst, NH, 03031, US)
ZEMANY, Paul, D. (27 Pulpit Run, Amherst, NH, 03031, US)
MAYNARD, John, A. (6 Sargent Quarry, Amherst, NH, 03031, US)
International Classes:
F42B10/00
Attorney, Agent or Firm:
LONG, Daniel, J. (BAE SYSTEMS INFORMATION AND ELECTRONIC SYSTEMS, Integration Inc.65 Spit Brook Roa, Nashua NH, 03060, US)
Download PDF:
Claims:

CLAIMS

1. An apparatus for three axis aerodynamic control of a body while in flight, said apparatus comprising: first, second and third shafts; a canard attached to spaced ends of the first shaft; a canard attached to one end of each of the second and third shafts, a gear assembly operatively connected to each of the shafts; a drive motor for driving each , of the shafts through the gear assemblies; and a controller operatively connected to each of the drive motors for integration of guidance controls to the canards to steer the body.

2. The apparatus defined in claim 1 wherein the gear assembly includes a planetary gear, a worm gear and a pitch gear.

3. The apparatus defined in claim 2 wherein the planetary gear is connected between the drive motor and the worm gear; and in which the pitch gear is connected between the worm gear and the canard shaft.

4. The apparatus defined in claim 2 wherein the planetary gear has a gear ratio of approximately 256/1.

5. The apparatus defined in claim 4 wherein the planetary gear has an output torque of approximately 1.77 in/lb.

6. The apparatus defined in claim 1 wherein each of the drive motors fs a high torque two phase stepper motor.

7. The apparatus defined in claim 6 wherein the stepper motors have a generally 18 degree step size; and in which the gear assembly is a generally 30/1 zero backlash spur reduction gear assembly.

8. The apparatus defined in claim 2 wherein a fold pivot and a pivot lock is located between the gear assembly and canard when the canard is a retractable canard.

9. The apparatus defined in claim 1 wherein each of the canards have a double wedge configuration consisting of first and second wedges with symmetrical surfaces and with a pivot located at a junction of said wedges, said junction being the longitudinal center of pressure of the canard providing torque balanced aerodynamic surfaces for said canards.

10. The apparatus defined in claim 9 wherein the junction of the canard is the thickest portion of the canard; and in which the first and second wedges taper inwardly outwardly from said junction.

11. The apparatus defined in claim 1 wherein a Ram Air Turbine (RAT) and a safe/arm rotor assembly are mounted in the body to prevent errant rounds from exploding.

12. The apparatus defined in claim 1 wherein the gear assembly includes a spur gear operatively connected to the drive motor and a bevel gear operatively connected between the spur gear and each of the shafts.

13. The apparatus defined in claim 12 wherein the spur gear is a zero backlash reduction assembly.

14. A guided munition comprising: a projectile body; a stabilizing fin assembly mounted at the rear of the projectile body; a guidance assembly mounted in a forward end of the projectile body, said guidance assembly including: a) at least four canards movably mounted with respect to the projectile body,

b) a first shaft having first and second of said canards mounted on respective ends of said first shaft, c) a second shaft having a third of said canards mounted thereon, d) a third shaft having a fourth of said canards mounted thereon; e) an optical sensor mounted in the projectile body; and f) a controller operatively connected to each of the three shafts for independently rotating said shafts to move said canards to change the ballistic path of said projectile body in response to guidance signals received from the optical sensor.

15. The guided munition defined in claim 14 wherein the controller is a Proportional-lntegral-Derivative (PID) controller.

16. The guided munition defined in claim 14 wherein the optical sensor is an array of optical detectors located in a forward part of the projectile body.

17. The guided munition defined in claim 14 wherein the controller is connected to each of the shafts by a stepper motor and a gear assembly.

18. The guided munition defined in claim 19 wherein each of the gear assemblies includes a planetary gear connected to the stepper motor, and a worm gear located between and operatively connected to the planetary gear and a pitch gear, said pitch gear being attached to the shaft.

19. The guided munition defined in claim 17 wherein each of the gear assemblies includes a spur gear operatively connected to the drive motor and a bevel gear operatively connected between the spur gear and each of the shafts.

20. The guided munition defined in claim 14 wherein the front end of the projectile body is a conical nose; in which the four canards are rotationally non-foldably mounted on the nose and are located within an imaginary cylinder defined by the projectile body.

Description:

THREE AXIS AERODYNAMIC CONTROL OF GUIDED MUNITIONS

CROSS REFERENCE TO RELATED APPLICATION This application claims rights under 35 USC 119(e) from U.S. application Serial No. 60/650,705, filed February 7, 2005; the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to armaments and more particularly to guided munitions. Even more particularly, the invention relates to a three axis control system and mechanism for guided mortar shells or for other small aerodynamic bodies.

2. BACKGROUND INFORMATION

Mortars are one of the most commonly employed weapons in a ground combat unit. The traditional role of mortars has been to provide close and continuous fire support for maneuvering forces. Military history has repeatedly demonstrated the effectiveness of mortars. Their rapid, high-angle, plunging fires are invaluable against dug-in enemy troops and targets in defilade, which are not vulnerable to attack by direct fires. One of the major disadvantages of mortars is their comparatively low accuracy, and as a result mortars are becoming less effective in today's precision combat environment. Equipping a mortar round with a precision guidance package will increase its accuracy, enabling the mortar to be a precision munition that will be significantly more effective in wartime situations. For maximum utility, the guidance package preferably should be an inexpensive retrofit to current munitions, with a cost in production that allows its use in all situations, either as a guided or unguided weapon.

Unguided munitions are subject to aim error and wind disturbances. These factors, along with other more subtle error sources, may cause the munition to miss the target completely or require many rounds to complete the fire mission due to the resulting large CEP (Circular Error Probability). Current approaches to

Case 20040126 PCT 1

guided weapons are expensive and are used on larger, long range weapons. The approach of the present invention results in significantly lower cost and smaller size. This allows use with small to medium caliber weapons and significantly improves CEP which also results in a significant reduction in the quantity of rounds required to complete the fire mission which in turn results in lower overall cost and improved crew survivability. In addition, another benefit to this approach is the virtual elimination of collateral damage due to errant rounds impacting non- targeted areas. Furthermore, complete integration of a seeker/guidance error can be used in a modification to the existing fuse in order to "safe" errant rounds which are failing to meet the established CEP ground rule which further controls unwanted collateral damage by preventing detonation of off target rounds.

Mortars are typically unguided or guided by an expensive G&C (Guidance and Control) system. The cost is high for current guided mortars and unguided mortars may have poor accuracy. Also, unguided mortars may result in unacceptable collateral damage, excess cost due to the large number of rounds required to blanket the target area, and may expose the mortar crew to counterbattery fire due to the large time required to drop the necessary shells to saturate the target.

The control of a small caliber body, (or any multi-caliber munition or Micro- Air Vehicle, MAV), necessitates a space efficient, light weight, reliable, low cost hardware embodiment. Furthermore, in order to optimize performance, a full 3- axis control system is required to maximize maneuver load capability and overall control, hence minimizing CEP (or miss distance) and for flight trajectory optimization. The combination of a full 3-axis control system using, in this case canards, (although traditional aileron, rudder, and elevator control can be incorporated), in a low cost, volumetrically efficient package which does not adversely effect center of gravity has been developed. The approach of the present invention does not require shaft encoders or resolvers and integrates feedback from other onboard sensors to effect control over the body utilizing the 3-axis control scheme.

Therefore, there is a need for an accurate and cost effective means for guiding mortar munitions. There is also a need for an ultra low cost G&C approach for mortars which is compatible with a large class of rounds.

Furthermore, subsequent need exists for preventing detonation of errant rounds to minimize non-target specific damage and non-combatant loss of life. Likewise, there is a need for a highly integrated control mechanism for small caliber and MAV aerodynamic bodies which provides full 3-axis control with minimal weight, cost, volume, and power impacts.

BRIEF SUMMARY OF THE INVENTION

In one embodiment of the present invention, a set of three miniature stepper motors, each driving a zero backlash spur gear assembly, are used to drive bevel gears attached to a set of three shafts. In another embodiment the stepper motors each drive a planetary gear assembly which drives a worm gear and a pitch gear connected to each of the three shafts. Attached to each shaft are the canards. A common shaft has a canard attached at opposite ends of the shaft. Two individual shafts each have a single canard affixed at an end thereof. In this manner, roll, pitch, and yaw steering commands can be executed through the canard frame to maneuver the munition in order to increase system accuracy. This technique is robust and also can be adapted to conventional MAV airframes to improve aerodynamic maneuvering performance.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is further described with reference to the accompanying drawings wherein:

Figure 1 is a diagrammatic perspective view of the guided munition of the present invention. Figure 2 is an exploded perspective view of the guidance control mechanism located in the nose of the munition shown in Fig. 1.

Figure 3 is a front end view of the nose of the munition of Fig. 1. Figure 4 is a fragmentary diagrammatic sectional view taken on line 4-4, Fig. 3 showing the improved guidance control mechanism mounted in the front of the munition.

Figure 5 is a top plan view of one of the guidance canards. Figure 5A is a side elevation of the canard of Fig. 5.

Figure 6 is a diagrammatic perspective view of a first embodiment of the guidance canards and drive mechanism.

Figure 6A is a diagrammatic top view of the guidance canard drive mechanism of Fig. 6. Figure 7 is a diagrammatic perspective view of a second embodiment of the guidance canard and drive mechanism.

Figure 7A is a diagrammatic top view of the guidance canard drive mechanism of Fig. 1.

Figure 7B is a perspective view of a stepping motor and two gear clusters used in the guidance mechanism of Figs. 7 and 7A.

Figure 8 is a system block diagram of a preferred embodiment of an optical guidance system for the three axis control mechanism of the present invention.

Figure 8A is a schematic diagram of a preferred embodiment of a guidance control model for use in the present invention. Figure 9 is a schematic block diagram of the Proportional-lntegral-

Derivative (PID) roll axis controller:

Figure 10 is a depiction of the control implementation methodology of the present invention depicting roll, pitch and yaw control capabilities.

Figure 11 is a pair of graphs showing that the canard equipped 60mm mortar round has sufficient aerodynamic performance to reach targets within a 100 m maneuver basket with better than a 4 m CEP. ,

Similar numbers refer to similar parts throughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION A preferred embodiment of the three axis aerodynamic controlled munition of the present invention is indicated generally at 1 , and is shown in assembled position in Fig. 1. Munition 1 preferably is a mortar, such as a 60mm mortar, and includes a main body 3 formed with a hollow interior 5 (Fig. 4) in which an explosive charge is contained. At the rear of body 3 will be usual aerodynamic stabilizing fins 7 with a propellant charge being located within an adjacent housing 9. Alternate stabilizing fin assemblies can be used in conjunction with this design. As shown in Fig. 4 and in accordance with one of the features of the invention, a nose indicated generally at 11 , is mounted on the front of body 3, preferably by a

threaded connection at 13, and includes a tapered portion 12. Nose 11 replaces a standard nose/fuse construction used with body 3 when the munition follows a ballistic path to a target after being launched from a mortar launcher or other type of propelling device without any guidance control. This enables modified nose 11 to be used with existing munition bodies 3 thereby avoiding the redesign and reconstruction of the entire mortar shell. Thus, only the currently used fuse is replaced with modified nose 11 which contains the improved guidance and control system described further below.

Munition 1 can be an optically guided mortar shell such as shown and described in detail in a pending patent application entitled, Optically Guided Munition, being filed concurrently herewith, the contents of which are incorporated herein by reference. At the forwardmost end of nose 11 is located an optical seeker subsystem 15 for detecting an optical illuminator or other type of radiator located at a target. The details and manner of operation of one type of optical seeker which can be used in munition 1 is shown and described in detail in a pending patent application entitled Optically Guided Munition Control System And Method being filed concurrently herewith, the contents of which are incorporated herein by reference. Operationally connected with optical seeker assembly 15 is the three axis aerodynamic control system of the present invention indicated generally at 17.

Nose 11 is formed with a Ram Air Turbine (RAT) 14 which includes a plurality of air ducts 19 which supply air through openings 21 formed in taper nose portion 12 for controlling an alternator and switch plate assembly 23 and a safe/arm rotor assembly 25 (Fig. 2) to prevent errant rounds from exploding. Alternate air duct embodiments are permissible and do not effect system performance. An end cap 27 secures the components as shown in Fig. 2 in nose 11 on main body 3. A booster pellet 29 preferably is located adjacent end cap 27 and is positioned within the end of main body 3. An array of batteries 31 is mounted forwardly of end cap 27 for supplying the required power for the control and guidance system of the present invention, including the three axis control system of the present invention.

A first embodiment of the three axis control system 17 is shown in detail in

Figs. 6 and 6A. System 17 includes four canards 33, an example of which is

shown in Figs. 5 and 5A. Canards 33 preferably are identical or very similar to each other, each having a double wedge configuration consisting of a first wedge 35 and a second wedge 37. Each wedge is formed with symmetrical opposed surfaces which taper inwardly outwardly from a junction 41 in which is located a pivot 43. The junction 41 and pivot 43 are located at the thickest portion of the canard which also is the longitudinal center of pressure of the canard, which provides for torque balanced aerodynamic surfaces for the canard.

Referring to Figs. 6 and 6A, two of the canards indicated at 33A, are connected to a common shaft 45, each through a wing pivot lock 47 and a wing fold pivot 49. Lock 47 and pivot 49 enable the canard to be retracted against nose 11 enabling certain munitions to be launched from a mortar launcher, afterwhich the canards move outwardly and lock into position for subsequent rotation for guiding the munition. Individual canards 33B and 33C are each connected to a separate shaft 53 and 55 respectively. Although canards 33 are shown mounted on tapered nose portion 12, they can be mid-body wings or similar aerodynamic control surfaces without departing from the concept of the invention.

In accordance with one of the features of the invention, common shaft 45 and individual canard shafts 53 and 55 each are driven through a gear assembly 51 by three drive motors, each of which is indicated generally at 57. Each drive motor 57 preferably is a high torque two-phase stepper motor, and in the preferred embodiment has an 18° step size. Each^ motor 57 is connected to a planetary gear 59, which in turn is connected to a worm gear 61 which drives a pitch gear 63 that is connected either directly to a respective canard or to the pivot lock and fold pivot therefore. In this preferred embodiment, planetary gear 59 has a gear ratio of approximately 256/1 and an output torque of approximately 1.77 in/lb.

In a second embodiment as shown in Figs. 7, 7A and 7B, a modified gear assembly indicated generally at 52, includes stepper motors 57 which are connected to the respective canard shafts through a spur reduction gear 65 or 65A, each preferably having a 30/1 zero backlash reduction drive. The spur reduction gears in turn are connected to a first bevel gear 67 which engages and drives a second bevel gear 68 attached to a respective one of the canard shafts.

Thus, drive motors 57, which preferably are stepper motors, can be connected by various gear assemblies, such as gear assemblies 51 and 52 to the canards, which if desired will be through a pivot lock 47 and fold pivot 49, and in another embodiment directly to the canards. In either construction the gear assemblies and drive motors will rotate the canards about their pivots 43 to control the pitch, roll and yaw movement of the munition body after passing through apogee and in its descent toward a target along a ballistic path. The particular canard shown in Figs. 1-4 and in Figs. 7 and 7A do not have pivot lock 47 and fold pivot 49 thereon, but are connected directly to the ends of shafts 45 through gear assemblies 52, due to their mounting on the tapered portion 12 of nose 11. These canards, when in operational position as shown in Figs. 1 and 4, lie within an imaginary cylinder of the rear cylindrical portion of nose 11. This arrangement avoids the need of pivot lock 47 and fold pivot 49 making their construction less expensive and complicated. However, either type can be utilized without affecting the concept of the invention.

Although the above-described three-shaft and associated gear assembly and drive motor is the preferred construction, it is understood that a four-drive motor configuration could be utilized wherein each supports roll and pitch, as well as a single or two-motor drive motor configuration which requires higher complexity transmission with independent shaft shifting functions and various gear arrangements, which would provide the desired results but in a more complicated and complex manner.

Common shaft 45 is driven through gear assemblies 51 or 52 by a stepper motor which is attached to one of the three zero backlash reduction spur gear heads or to the planetary gear, worm gear, pitch gear and spur gear arrangement. This common shaft is capable of generating a force normal to the plane of the canards in both positive and negative senses and is utilized in a left / right or up / down mode only. The two independently driven canards 33B and 33C, each of which is attached to a separate shaft by a gear assembly are capable of bi- directional control, and thus when used in a common mode are capable of generating the same normal force as described for the common shaft assembly in either a positive or negative direction, and is capable of supporting both the left / right or up / down maneuver directions. However, when driven in a differential

mode, the pair of canards are capable of generating a roll torque, thus positioning the total canard frame, (in this case the mortar roll orientation), in the angular orientation required to support maximum maneuver capability and for removing spin generated due to any tail asymmetries or manufacturing defects. The stepper motors also provide the angular state of the canards based on the magnet pole energized. Minor corrections can be applied in a single step or multiple step modes and active feedback from a guidance algorithm used to null out oscillations about the intended course line, thus implicitly correcting any potential angular misorientation of the canards caused by launch conditions. In this way complicated shaft encoders or resolvers are eliminated further reducing cost.

One manner in which the three axis control system and mechanism of the present invention is used to control an aerodynamic body, such as a munition, is shown in diagrammatic block form in Figs. 8-10 as set forth below. Figs. 8 and 8A is an example of a preferred optical guidance and control system for controlling the three axis control mechanism of munition 1 as shown in Figs. 1-7B. Fig. 8 diagrammatically shows optical seeker subsystem 15 which comprises broadly a seeker optics subassembly and a detector array connected to a control processor subsystem 16, which in turn is connected to a canard or flight control subsystem 18.

One particular manner in which the drive and control mechanism is used to control the flight of munition 1 is set forth below.

Flight Control Subassembly - The overall projectile center of gravity is controlled in order to maintain static and dynamic stability and to optimize maneuverability. The aerodynamic control surfaces are canards 33 with a trapezoidal planform, double wedge (symmetrical) surfaces, which are deflected to generate normal force, which in turn generates a pitching moment on the mortar shell causing the new trim attitude to be achieved and accelerating toward the target. The canard panels are located relative to the drive shafts at the longitudinal center of pressure location providing a torque balanced aerodynamic surface. This minimizes the torque required of stepper motors 57 to generate the requisite normal force from the canard surface enabling low cost miniature motors to be incorporated into the design.

The choice of a double wedge planform, while not normally preferred for subsonic rounds, is a cost effective planform from the productibility perspective with the thick portion 41 of the airfoil ideally suited to incorporation of the canard hinge or pivot (Figure 5). The longitudinal center of pressure location has been evaluated over the proposed launch envelope and the canard hinge 43 is located as close as practical to the locus of center of pressure points ensuring a near zero hinge moment (aerodynamically balanced) and minimizing the motor torque requirements. This control method is also compatible with mid-body wing embodiments for aerodynamic control. Illustrated in Figure 1 1 is the performance for the baseline 60 mm mortar and the canard effect at 2 degrees of deflection and 4 degrees of deflection. The canard equipped 60 mm mortar is capable of ≥0.5 g's maneuver force in the direction of the target when commanded in the manner prescribed.

The actual drive motors and gear assembly options for the canards are miniature, high torque, 2 phase stepper motors with an 18 degree step size. The motors are driven at a rate of between 0 and 15,000 rpm which translates, through the gear assemblies to a rate of up to 3000 degrees / second well in excess of the required Bandwidth of the control system. The motors are stepped at a high rate then paused to allow the effect of each command to settle (aerodynamically) through the system. Each motor is capable of 0.5 A total current draw at stall. The motors are attached via the gear mechanisms to the canard shafts in one embodiment using right angle bevel gears at a 1/1 ratio (Figure 7) and in another embodiment using a planetary gear, worm gear and pitch gear arrangement (Fig. 6). Roll Control - Zero roll is maintained by using a Proportional-Integral-

Derivative (PID) control loop 70 shown diagrammatically in Figs. 8A and 9. The "proportional" and "integral" inputs come from pitch and yaw rate gyros 71 and 72, respectively. The "derivative" term comes from a roll rate gyro 73. These three terms are combined to estimate the instantaneous roll rate component and steer the canards appropriately to offset this effect.

Steering Control - Steering control has two separate components: YAW (left/right) control, in which the canards, acting in pairs, provide horizontal displacement, and Elevation (up/down) in which the canards, again operating in

pairs, provide an increment or decrement to the projectile range. Two of the diagonally opposed canards also provide the roll control discussed above. Figure 10 demonstrates this effect.

Left/Right Steering Correction - The horizontal steering correction term is determined from the left/right centering error determined from the sensor array. This error is used to determine the necessary correction to drive the canards to correct any lateral aiming error through a L/R steering loop 75 (Fig. 8A). The control system monitors the bore sight angle of the round and accounts for any angle of attack (AOA) developed because of the steering command and repositions the canards accordingly. An outline of this process is shown in Figure 10. In the actual control processor, the canard positions will be continually updating, therefore the angle of attack will be constantly adjusted. Thus, the instantaneous illumination of a homing illuminator will slowly oscillate back and forth across the centerline of the detector array. Up/Down Steering Correction - Vertical steering correction is done in a similar manner to the horizontal steering correction. However, unlike the left/right correction where the desired horizontal angle of attack is known and equals zero (at the detector array centerline as shown in Fig. 8), the up/down correction requires a vertical angle of attack which is dependent on the mortar trajectory and time to impact. By using the RAT (Ram Air Turbine) 14 time-to-apogee, an estimate of the mortar trajectory and remaining time of flight can be determined. A table in processor subsystem 16 will store the allowable mortar trajectories and will fit the best match to the true trajectory. Using this desired trajectory the desired vertical angle of attack can be determined at each 10 Hz update point. The true vertical angle of attack can then be compared to this desired angle of attack and the necessary correction can be made. The up/down steering correcting is performed by an U/D steering loop 77 and is combined appropriately with the roll correction to deflect the canards as appropriate.

The trajectory correction approach involves estimation of the trajectory by a trajectory estimator 78 and a determination of the impact point relative to the target. If the mortar is on course, the target will be centered with respect to the left/right center line. It will also be aimed at the proper elevation angle vs. time.

Thus, the downward look angle will follow a specific time history. See body axis graph 80 in Fig. 8.

For a cross track error the location of the target with respect to the left/right of the bore sight center line is the "horizontal" error signal. This error is used to deflect the canards to correct the cross track error. In this case a trajectory estimate is not needed. Only an estimate of down is required to roll the mortar to zero degree roll angle.

To correct along-track errors, the "vertical" error signal is computed from the difference between the nominal bore sight look down angle and the bore sight look down angle measured by the seeker. To implement this approach, the trajectory is estimated (using time to apogee and launch speed). This trajectory estimate is then used to provide the nominal look down angle to the impact point.

This nominal angle is time dependent and decreases a few degrees per second.

This nominal value is compared to the seeker value. If the nominal value exceeds the seeker value then the current trajectory will pass over the target. In this case a downward correction is applied. If the nominal value is less than the measured value, an upward correction is applied.

In the absence of gravity, the nominal bore sight look down angle would be zero. In this case we have the "direct fly in" approach. The effect of gravity diminishes as the mortar closes on the target for short range shots using high quadrant elevation (>45 degrees) because the approach angle is closer to vertical. Thus in the end the ballistic correction approach morphs into a direct fly in approach.

When a maneuver command is applied, the mortar develops an angle of attack (AOA). This AOA shifts the look angle to the target. For a 0.2 g maneuver a 6-DOF (degrees of freedom) model shows that the AOA will be about 1.9 degrees. Thus, if the projectile is initially aimed 1 degree to the right of the target in the horizontal direction and a 0.2 g left maneuver is commanded, the target look angle will be 0.9 degrees to the right. As the mortar velocity vector turns left towards the target, the look angle will move further to the right. This does not indicate an over shoot. In fact the turn must be continued until the look angle is

1.9 degrees to the right. At this point the canards are zeroed and the AOA trims back to zero. With zero AOA and the velocity vector pointing to the target, the

look angle will be zero. It is critical to account for this AOA effect when steering because the effect the expected AOA will be of comparable magnitude to the aim angle error.

Further details of one type of guidance and control system for use with the three axis control apparatus of the present invention is shown and described in pending patent application entitled, Ballistic Guidance Control For Munitions, being filed concurrently herewith, the contents of which are incorporated herein by reference.

While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.