IL160434A - Spin-stabilized homing projectile - Google Patents

Description

SPIN-STABILIZED HOMING PROJECTILE SPIN-STABILIZED HOMING PROJECTILE FIELD AND BACKGROUND OF THE INVENTION The present invention relates to weapons systems and, more particularly, to a projectile, such as a rocket or an artillery shell, that is spin-stabilized and that homes in on its intended target.

Figure 1 shows the three phases in the flight of a ground-to-ground rocket on a trajectory 10 from a launcher 12 to a target 14: the launch phase 16, the ballistic phase 18 and the terminal phase 20. Launch phase 16 is a phase of powered flight in which the engine of the rocket propels the rocket in the general direction of target 14. During ballistic phase 18, the rocket just coasts. In terminal phase 20, a guidance system o -board the rocket steers the rocket towards target 14.

There are two ways to steer a rocket towards target 14 during terminal phase 20: navigating and homing.

In navigating, the guidance system includes a navigation device, such as an inertial measurement unit (IMU) or a receiver of navigational signals such as a GPS receiver, that tells the rocket what its geographical coordinates are. The geographical coordinates of target 14, according to a digital map of the theater of operation that includes target 14, are loaded into the guidance system before launch. The guidance system steers the rocket towards the geographical coordinates of target 14 according to the readings of the rocket's own geographical coordinates as obtained from the navigation device. One example of a weapons system whose rockets use navigating to reach their targets is the US Army's Guided Multiple Launch Rocket System (GMLRS).

In homing, the guidance system includes one or more sensors that detect target 14. The guidance system steers the rocket towards target 14 according to the signals received from the sensor(s). One commonly used sensor system includes an array of CCD sensors on which is focused visible light from target 14, or an equivalent array of infrared sensors on which is focused infrared light from target 14. The sensor array, used in conjunction with standard imaging hardware, produces a sequence of images of target 14 and its vicinity that are called herein "homing images". The guidance system compares the homing images to a "reference image" of the target that is loaded into the guidance system before launch, and steers the rocket towards target 14 in accordance with the comparison. Homing that relies on comparing homing images with a reference image is referred to herein as "image comparison homing". One typical comparison method includes cross-correlation of the homing images with the reference image, as taught for example by Smith et al. in US Patent No. 5,626,311, which patent is incorporated by reference for all purposes as if fully set forth herein. That patent actually is directed towards a method of guiding an air-to-ground missile; but the principles taught therein apply also to the guidance of ground-to-ground missiles.

The preferred method of guiding a projectile such as a rocket to target 14 according to successively acquired images of target 14 is a method called "proportional navigation", as described, for example, in US Patent No. 6,244,536, which patent is incorporated by reference for all purposes as if fully set forth herein. (The appearance of the term "navigation" in the name "proportional navigation" should not be construed as meaning that this method is a "navigating" method as herein defined.) According to this method, the rate of rotation, with respect to the inertial space, of a line of sight connecting the rocket to target 14 is measured. The guidance system generates a lateral acceleration of the projectile that is proportional to the measured lateral rotation rate. The orientational stability of the imaging system that is required for this method to work precludes the use of spin-stabilization to stabilize the flight of the projectile. One drawback of navigating is that it is less accurate than homing. Even neglecting the inaccuracy in the map coordinates of target 14, which may be as great as about 20 meters, the central error probability (CEP) of a rocket that uses navigating to find its target may be as great as 10-20 meters.

There is thus a widely recognized need for, and it would be highly advantageous to have, a projectile that uses proportional navigation homing to find its target.

SUMMARY OF THE INVENTION According to the present invention there is provided a method of guiding a projectile to a target, including the steps of: (a) launching the projectile along a trajectory; and (b) when the projectile reaches a predetermined point in the trajectory, homing the projectile towards the target.

According to the present invention there is provided a projectile including: (a) a projectile body having a longitudinal axis; (b) a seeker head; and (c) a coupling mechanism for coupling the seeker head to the projectile body so that the seeker head and the projectile body are operative to spin independently about the longitudinal axis.

The method of the present invention is a method of guiding a projectile, such as a rocket or an artillery shell, towards its intended target. The projectile is launched towards the target along a trajectory. When the projectile reaches a predetermined point in the trajectory, homing of the projectile towards the target commences.

Preferably, the trajectory is a ballistic trajectory, i.e. , a trajectory such as trajectory 10 that includes a ballistic phase 18.

Preferably, after the projectile is launched, the projectile is spun about its longitudinal axis to provide directional stability. More preferably, the homing is effected using a seeker head that is decoupled from the spinning, for example by deploying one or more fins from the seeker head, when the projectile reaches the predetermined point along the trajectory at which homing commences.

Preferred homing methods include image comparison homing and proportional navigation.

A projectile of the present invention includes a projectile body (e.g. , the fuselage of a rocket or the casing of an artillery shell) that has a longitudinal axis, a seeker head, and a coupling mechanism for coupling the seeker head to the projectile body so that the seeker head and the projectile body spin independently about the longitudinal axis. "Spinning", as understood herein, includes the special case of zero angular velocity about the longitudinal axis, so that a projectile of the present invention, in its terminal phase 20, typically would have its body spinning at a nonzero angular velocity to provide directional stability while its seeker head "spins" at zero angular velocity to facilitate homing.

Preferably, the coupling mechanism includes a swivel joint that connects the projectile body to the seeker head and that has a rotational axis that substantially coincides with the longitudinal axis of the projectile body.

Preferably, the seeker head includes an optical system for acquiring one or more homing images of a target, a memory for storing a reference image of the target, and a processor for comparing the homing image(s) with the reference image. Most preferably, the projectile also includes a steering mechanism that the processor uses to guide the projectile towards the target in accordance with the comparison(s) of the homing image(s) with the reference image.

Preferably, the projectile also includes a sensing and releasing mechanism that operates the coupling mechanism to decouple the seeker head from the projectile body when the projectile reaches a predetermined point in its trajectory.

Preferably, the projectile also includes a despinning mechanism for causing the seeker head to stop spinning about the longitudinal axis of the projectile body (i.e., causing the seeker head to "spin" about the longitudinal axis of the projectile body at zero angular velocity) while the projectile body continues to spin about its longitudinal axis, and also a sensing and releasing mechanism that operates the despinning mechanism when the projectile reaches a predetermined point in its trajectory. More preferably, the despinning mechanism includes one or more fins operatively associated with the seeker head. Most preferably, the fins(s) is/are stowed within the seeker head while the seeker head is coupled to the projectile body, and the despinning mechanism is operative to deploy the fin(s) from the seeker head.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 shows the three phases of the trajectory of a ground-to-ground rocket; FIG. 2 is a schematic diagram of a rocket of the present invention prior to the homing phase; FIG. 3 is a schematic diagram of a rocket of the present invention during the homing phase.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a projectile and a method of guiding the projectile towards a target. Specifically, the projectile is guided towards the target by homing while spinning about its longitudinal axis for directional stability.

The principles and operation of a projectile according to the present invention may be better understood with reference to the drawings and the accompanying description. The description herein is directed towards the specific case of the projectile being a rocket. It will be clear to those skilled in the art how to apply the principles of the present invention to other projectile-type munitions such as artillery shells.

Referring again to the drawings, Figures 2 and 3 are schematic diagrams of a rocket 30 of the present invention. Rocket 30 includes two principle sections: a fuselage or rocket body 32 and a seeker head 34. Fuselage 32 and seeker head 34 are connected by a swivel joint 36. Swivel joint 36 has a rotational axis 38 that also is a common longitudinal axis of fuselage 32 and seeker head 34. Fuselage 32 and seeker head 34 are free to spin independently about axis 38.

The forward section of seeker head 34 is a transparent dome 42. Behind transparent dome 42 is an optical system 44 for acquiring images of the terrain to the front of rocket 30 and in particular of target 14 and the vicinity of target 14. Optical system 44 includes a sensor array 48, for example a CCD array for imaging in visible light or an analogous array of infrared sensors for imaging in infrared light, and a focusing mechanism, represented schematically by a lens 46, for focusing light from the terrain in front of rocket 30 onto sensor array 48.

Seeker head 34 also includes forward fins 58 that are shown stowed within seeker head 34 in Figure 2 and deployed from seeker head 34 in Figure 3.

Mechanisms for deploying fins 58 from seeker head 34 are well-known in the art and so need not be recited in detail. Fins 58 are deployed during flight, and are subsequently tilted to steer rocket 30, using electric motors 60.

The overall operation of seeker head 34, and indeed of rocket 30, is controlled by a processor 50 in seeker head 34. Other electronic and electromechanical components of seeker head 34 include a memory 52 and a three-component accelerometer 54. The functions of memory 52 and accelerometer 34 are described below.

On the aft portion of fuselage 32 are fixed, curved aft fins 56 that impart a stabilizing spin about axis 38 to fuselage 32. Other components (not shown) of rocket 30 that are housed in fuselage 32 include a rocket engine and a warhead.

Prior to the launch of rocket 30, a reference image of the terrain surrounding target 14 are loaded into memory 52. The reference image is obtained by identifying target 14 in a digital regional image of the theater of operation of the weapons system of which rocket 30 forms a part. In addition, a pixel in the portion of the regional image that represents target 14 is selected by an operator as an aiming point for rocket 30. The coordinates of this pixel (or of the nearest corresponding pixel) in the reference image also are loaded into memory 52. Typically, the regional image is an orthogonal image, and known methods are used to transform the regional image into an oblique image, with an orientation corresponding to the eventual view of target 14 via transparent dome 42, prior to extracting the reference image from the regional image.

Figure 2 shows the configuration of rocket 30 at and immediately subsequent to launch. Curved aft fins 56 impart to both fuselage 32 and seeker head 34 a directionally stabilizing spin about axis 38.

During launch phase 16 and ballistic phase 18, processor 50 receives signals from accelerometer 54 that are indicative of the acceleration of rocket 30. When processor 50 determines, based on the signals from accelerometer 54, that rocket 30 is sufficiently far into ballistic phase 18, processor 50 operates motors 60 to deploy forward fins 58. This places rocket 30 in the configuration illustrated in Figure 3. Alternatively, instead of using accelerometer 54 to determine when rocket 30 is sufficiently far into ballistic phase 18 for forward fins 58 to be deployed, processor 50 uses a timer to measure time from launch, and then deploys forward fins 58 when the timer indicates that enough time has elapsed since launch for rocket 30 to be sufficiently far in to ballistic phase 18 for forward fins 58 to be deployed. While aft fins 56 continue to impart a directionally stabilizing spin about axis 38 to fuselage 32, forward fins 58 slow the spin of seeker head 34 to zero angular speed about axis 38. Seeker head 34 now is in condition for homing phase 20 to commence.

During homing phase 20, processor 50 compares successive homing images, of target 14, that are acquired using optical system 44, to the reference image stored in memory 52. Processor 50 then uses motors 60 to tilt forward fins 58 so as to steer rocket 30 towards target 14, in accordance with a standard image comparison homing method, for example in accordance with proportional navigation as described in US Patent No. 6,244,536.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims (5)

9 160434/4 ^WHAT IS CLAIMED IS:

1. A method of guiding a projectile comprising a multi-finned seeker head to a target, comprising the steps of: (a) launching the projectile along a trajectory; (b) spinning the projectile about a longitudinal axis thereof; and (c) when the projectile reaches a predetermined point in said trajectory subsequent to said launching: (i) decoupling the seeker head of the projectile from said spinning, and (ii) homing the projectile towards the target, using said seeker head.

2. The method of claim 1, wherein said trajectory is a ballistic trajectory.

3. The method of claim 1, wherein said decoupling is effected by steps including deploying at least one fin from said seeker head.

4. The method of claim 1, wherein said homing includes image comparison homing.

5. The method of claim 1, wherein said homing is effected using proportional navigation. Mark M Friedman Advoe!ate, Patent Attorney MoMe Aviv Tower 54th floor Jabotinsky 52520 Ramat Gan