GB2140538A - Projectile guidance system - Google Patents

Abstract

In a guidance system for an 'unpowered' spin-stabilised projectile such as a shell 11 equipped with a lateral thruster 17 and fired from a gun on a leading target-intercepting trajectory, the target, and in its later stages the shell, are tracked by microwave radar 12 and if necessary a new trajectory computed. The characteristics of the lateral thruster 17 are taken into account to determine the optimum trajectory point to fire the thruster. The spin orientation of the projectile is determined by including as part of the shell a radar reflector 19 or microwave transmitter (not shown) which returns or sends a signal modulated with spin orientation and including at the launch station a receiver 20 and decoder 22 of the signal. A deviation from the optimum trajectory point is calculated at which the lateral thruster is in the correct orientation and a command signal generated at 24 to initiate lateral thrust. <IMAGE>

Description

SPECIFICATION Projectile guidance system This invention relates to projectile guidance systems and in particular the guidance in flight of gun launched projectiles without a propulsive source and which spin about an axis along the trajectory.

U.S. Patent No. 4,008,869 describes a projectile control system in which a projectile with or without a propulsive source is fired with a computed lead angle to intercept the trajectory of a target coincidentally with the target. The target is tracked by radar which at least in the terminal stages of projectile flight also tracks the projectile. Any error between the sensed projectile trajectory and its intended trajectory or manoeuvring of the target is used at the tracking and launch station to compute a trajectory correction to an intercept point and a command signal representing this is transmitted to the projectile where such trajectory correction is effected.

The specification does not disclosure how such trajectory correction is to be effected for different types of projectile.

Projectiles which are not spin-stabilised are usually maintained in attitude by guidance surfaces which may readily be moved in response to the command signal to change trajectory (and may be commanded to make several such changes).

Spin stabilised projectiles, on the other hand, are not so readily manoeuvred. One technique proposed is the use of lateral trust producing means acting at an angle, usually 90 , to the longitudinal axis of the projectile either as a conventional thruster or an impulse charge operated upon receipt of a command signal. Such a technique, of course, requires the lateral thrust to be applied at the correct spin orientation of the projectile as the trajectory is defined in three-dimensional space.

The determination of spin rotation has been discussed in several patent specifications and usually employs on-board devies and/or laser tracking of the projectile in flight. Examples of such specifications are U.S. patent No. 3,995,792, U.K. specifications Nos. 1,429,941 (=U.S.P. 3,860,199), 2,041,685 and French specification No. 2,326,676.

In applying such a technique of trajectory changing in the system described in the aforementioned U.S. Patent No. 4,008,869 it appears that the use of a spin stabilised projectile such as an artillery shell does not offer a relatively inexpensive option to a non-spinning projectile with conventional guidance surfaces in view of the extra laser tracking (and transmitting) equipment, the cost of providing laser equipment of sufficient power to operate at up to several miles, and precautions necessary for safe usage of such laser equipment.

It is an object of the present invention to provide a projectile guidance system of the type outlined in the first mentioned specification useful for spin stabilised projectiles which mitigates the expense and disadvantage of the laser based systems.

According to a first aspect of the present invention a projectile guidance system comprises a projectile including lateral thrust producing means and a launch station including a target-tracking microwave radar system, a gun forfiring a spin stabilised projectile, fire control means responsive to target range and direction information from the radar system to compute a projectile trajectory to intercept that anticipated for the target and to cause a projectile to be fired, and guidance means operable to correct the trajectory of the projectile in flight by transmitting thereto by way of command means a command signal initiating a lateral thrust to divert the projectile onto a new trajectory, the system including projectile tracking microwave radar means operable to track the projectile, at least in the vicinity of the target, means in said projectile for causing emission of a microwave signal modulated in accordance with the spin orientation of the projectile, at the launch station a receiving station responsive to said emission to determine the spin orientation of the projectile, trajectory review means responsive to the relationship between tracked target and projectile positions and their respective trajectories and the ballistic characteristics of the lateral thrust producing means to determine an optimum point on the projectile trajectory for the projectile to be directed to a new target intercepting trajectory, and command control means responsive to the spin orientation of the projectile determined by the receiving station to calculate a deviation from said optimum point at which the projectile has the lateral thrust producing means correctly orientated to effect the trajectory change and initiate a command signal by which the thrust producing means is operated to direct the projectile at said deviation from the optimum point onto said new trajectory.

According to a second aspect of the present invention a method of guiding a spin stabilised projectile onto a new trajectory in flight comprising tracking a target by microwave radar and establishing from the trajectory thereof an intercept point and trajectory for a gun spin stabilised projectile of the type including lateral thrust producing means commanded by signals received in flight, firing the projectile, tracking the projectile with microwave radar during at least a terminal portion of flight and transmitting thereto a command signal to initiate diversion of the trajectory by lateral thrust, causing a microwave signal to be emitted from the projectile modulated in accordance with the spin orientation of the projectile and received at a receiving station, forming from the relationship between target and projectile positions and trajectories and the ballistic characteristics of the lateral thrust means an indication of an optimum point on the projectile trajectory for the projectile to be diverted to a new target intercepting trajectory and in accordance with the orientation of the lateral thrust means in the vicinity of said optimum point determine a deviation from said optimum point at which the lateral thrust producing means is correctly aligned with the new trajectory and initiating transmission of a microwave command signal to the projectile to cause a laterial thrust to be produced at said deviated optimum point.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawing, in which the single Figure shows in block schematic form a projectile guidance system included a launch station 10 and a projectile, a spin stabilised artillery shell 11, launched therefrom.

The basic principle of operation of such a system, that is of tracking a potential target, computing a lead angle for fixing a shell on a 'minimum energy' intercepting trajectory, tracking the shell in the latter part of its flight and, if necessary diverting it onto a new target intercepting trajectory are set forth in the aforementioned U.S. Patent No. 4,008,869 and require no more than a brief description for clarity.

The launch station 10 may be a land-or ship-borne establishment including a target-tracking microwave radar 12 which feeds signals to a fire control computer 13. The fire control computer 13 also receives ballistic data to gun and shell performance from store 14, and environmental data, such as weather conditions and station movement, from measuring transducer controller 15 and in known manner produces signals which direct the gun elevation and azimuth and fire a shell along a trajectory which will intercept the target, assuming the target continues at the same speed, follows the same trajectory and no external influences act on the shell.

As the shell approaches the target, which is tracked by the radar 12 with a conically swept or nutating beam, the shell enters the swept region and is also tracked by the radar so that both bodies may have their trajectories reviewed. The signals received from the tracking radar are offered to a trajectory review means 16 which may be formed by the fire control computer 13 if only one shell is in flight at any time, or separately as shown permitting several rounds to be fired.

The shell 11 is similar to a conventional artillery shell in that it is fired from a gun which imparts spin to stabilise it in flight in the absence of control surfaces, It is known to alter the trajectory of such a shell by means of a lateral thrust caused to actthrough its centre of mass, either as a controlled duration thrust proposed in the aforementions U.S. Patent No.

3,995,792 or a relatively short impulsive thrust as proposed in the aforementioned G.B. Patent No.

1,429,941. Again, no detailed description is required other than to state that the shell 11 is provided with such lateral thrust means 17 whose balistic properties as a function of shell orientation and speed are known and held in a store 18 at the launch station.

The store 18 provides the lateral thrust ballistic data to the trajectory review means which in concert with the measured trajectories of target and shell as they converge determines an optimum point on the shell trajectory for the fixed lateral thrust to be imparted to shell and divert it onto a new trajectory at which it should intercept the target.

The above features are considered known, or at least to be inferred from the aforementioned patent specifications.

The feature of the system with which this invention is particularly concerned is producing the lateral thrust so that is acts in the correct direction for the new trajectory, that is, that it is applied at the correct spin orientation of the shell.

The proposals made hitherto in the aforementioned specification are all based upon the use of laser beams which as stated, are expensive to employ and involve a risk of eye injury to friendly personnel both at the base station and in the air.

In accordance with the present invention the shell is illuminated with microwave radiation from a shell tracking radar, which as stated above, is provided by the target tracking radar 12. This radar not only enables the shell position and tajectory to be determined in the usual way but also to be employed for determining shell spin orientation. The rear face or tail 19, of the shell 11 is provided with a radar reflective bevel which reflects a part of the incident radar beam at an angle to its line of incidence. A suitable angle of bevel is considered to be in the region of 10 .

A microwave radio receiver 20 includes an aerial 21 which is steered with the tracking radar aerial to receive such radiation as is reflected from the bevelled tail 19. As the shell spins the direction of the reflected radiation is itself conically scanned in relation to its trajectory such that the amplitude of the received signal varies in accordance with the orientation of the shells, that is, the return signal from the shell is amplitude modulated by the orientation. The relative positions of shell and receiving aerial are such that the strongest signal will tend to be received when the bevelled tail is inclined downwardly towards the aerial and from the signal received for each rotation and the geometry of their relative positions a reference vertical can be determined within the signal.Alternatively, and its accuracy can be replaced over a number of spins or in addition, use may be made of reflection from a suitable terrestrial 'reference' such as the surface of the sea.

The receiver 20 demodulates the signal and feeds the modulation signal to spin decoding means 22 wherein the vertical reference is determined and instantaneous spin orientation is monitored through the tracked portion of the shell flight.

When the optimum point of diversion is calculated by the trajectory review means, in advance of the shell reaching that point, the orientation of the shell at that point is predicted and if it does not accord with the orientation required to divert the shell onto a new directory, a deviation of the optimum trajectory point is ascertained at which the shell is properly orientated in command control means 23.

This deviated optimum point is employed as the timefor initiating lateral thrust and in accordance therewith a command signal is generated in command generator which is transmitted towards the shell by a microwave transmitter 25, the aerial of which is also slaved to the tracking radar 12.

The shell includes a microwave receiving antenna and detector circuit which provides an initiation currentforthethrust means 17. If the command signal is transmitted at the time initiation is required the receiver may be powered directly by the energy of the received signal and initiate detonation of explosive thrust means without the need for an on-board power supply.

Alternatively, the shell may carry a more complex, powered circuit including a timing circuit so that a command signal may be sent in advance of the deviated optimum point comprising data which indicates the delay from reception to initiation of the lateral thrust means at the deviated optimum point.

Presumably an inhibiting means is provided either to prevent the transmission of command signals when the shell is too close to prevent damage by overloading of components.

The above described system is open to alternative ways of performing several of the functions.

For instance, the shell as it passes through the air develops a shock wave which is useful in enabling the shell to be tracked by radar as well as the target, but also may interfere with the reflection of an orientation modulated radiation from the tail 19.

The tail of the shell may alternatively be provided over part of its face with a receive/transmit antenna and a diode rectifying circuit whereby the microwave radar received from an a.c. beam is rectified and re-radiated as a pulsating signal at twice the frequency. Such a system again is essentialy passive in requiring no on-board power supply or additional electrode circuitry. However, if required a more complex method of transmitting a different signal in response to received radiation, i.e. transponding, may be employed.

A further alternative is the provision in the shell of a microwave oscillator and an antenna system by which the signal is emitted for reception by the receiver 20. Such an arrangement could be arranged to conserve power by only operating in response to illumination of the shell by the tracking radar beam.

Furthermore, with the alternative ways described above for causing a beam to be emitted, the beam may be directed at an angle to the longitudinal (spin) axis of the shell for detection similar to simple reflection of the radar beam by bevelled face 19, or may be polarised with respect to a radial line or by antenna construction so that the radiation is more predictably directed to the receiver 20 and contains as a characteristic thereof a reference to a datum orientation.

The shell as described has a single lateral thrust means, preferably an explosive charge applying an impulse to the shell over a fraction, say 0.125, of a revolution. If desired an alternative, slower acting, thruster could be employed but requiring more complex operation, such as pulsing over several revolutions, as the direction in which it acts varies continuously.

Also, the thrust producing means may comprise a plurality of thrusters disposed about the shell which not only permits a potentially smaller deviation from the optimum for application of initial lateral thrust but also gives the opportunity to make subsequent further trajectory changes.

It is also possible to deal with several shells in flight simultaneously, either fired at the same or individual targets. Determination of the optimum point of trajectory charge may be readily achieved by range gating and the determination of orientation and transfer of command signals may be performed by arranging for each shell to have a coded identification means which inputs to the fire control means prior to firing and which is imprinted on any radiation emitted therefrom and has to preface any command signal received thereby.

Other features of the system are that the command signal transmitter, which conveniently operates in the X-band, may with a 1 m diameter parabolic dish produce a circularly polarised beam of adequate power and beamwidth to be received by the shell in any orientation thereby requiring the aerial to be coupled only loosely to movement of the tracker radar 12 and obviating the requirement for complex and expensive servo control.

Similarly the aerial 21 of receiver 20 is only required to be directed approximately with the radar tracker.

It is convenient to employ the same tracking radar 12 for both the target and the shell. However, if it is desired to be able to divert the shell prior to its re-entering the target-tracking beam a separate shell tracking microwave radar may be employed.

Similarly where reflected radiation is employed to establish the shell spin orientation it is convenient to employ the radar beam illuminating the shell. If desired a separate beam may be employed, either slaved to the tracker radar or aimed generally along the shell trajectory with the sole purpose of illuminating the rear of the shell and causing an orientation-modulated emission therefrom.

The above described system, and particularly the preferred system, employs the same tracking radar for both target and shell. It will be appreciated that in some situations, for example, a ship-borne installation, such radar tracking and command facilities already exist in connection with more elaborate guided missiles which exist only in relatively small numbers. The system of the present invention may suitably employ the existing components of such systems with additional features such as the spin orientation signal receiver 20, and the computing facilities necessary to interpret the information. The shells, which are simple and inexpensive in comparison with conventional guided missiles may be carried in larger numbers and used as a back-up after the missiles have been deployed.

Throughout this specification the projectile has been discussed as comprising a shell. It will be understood that the invention is applicable to any equivalent projectile, that is, spin stabilised.

Claims (15)

1. A projectile guidance system comprising a projectile including lateral thrust producing means and a launch station including a target-tracking microwave radarsytem, a gunforfiring a spin stabilised projectile, fire control means responsive to target range and direction information from the radar system to compute a projectile trajectory to intercept that anticipated for the target and to cause a projectile to be fired, and guidance means operable to correct the trajectory of the projectile in flight by transmitting thereto by way of command means a command signal initiating a lateral thrust to divert the projectile onto a new trajectory, the system including projectile tracking microwave radar means operable to track the projectile at least in the vicinity of the target, means in said projectile for causing emission of a microwave signal modulated in accordance with the spin orientation of the projectile, and at the launch station a receiving station responsive to said emission to determine the spin orientation of the projectile, trajectory review means responsive to the relationship between tracked target and projectile positions and their respective trajectories and the ballistic characteristics of the lateral thrust producing means to determine an optimum point on the projectile trajectory for the projectile to be directed to a new target intercepting trajectory, and command control means responsive to the spin orientation of the projectile determined by the receiving station to calculate a deviation from said optimum point at which the projectile has the lateral thrust producing means correctly orientated to effect the trajectory change and initiate a command signal by which the thrust producing means is operated to direct the projectile at said deviation from the optimum point onto said new trajectory.

2. A system as claimed in claim 1 in which the projectile tracking microwave radar means comprises the target tracking microwave radar means.

3. A system as claimed in claim 1 or claim 2 in which the microwave signal emitted from the projectile is formed by the radiation of the projectile tracking microwave radar means.

4. A system as claimed in claim 3 in which the projectile has formed on a rearward facing surface thereof a microwave reflector arranged to return a beam of said incident radiation inclined to the longitudinal (spin) axis of the projectile so that the signal received at the receiving station is modulated in amplitude in accordance with the inclination of said reflected beam to the incident beam.

5. A system as claimed in claim 4 in which said reflector includes frequency varying means operable to cause said returned beam to be at a different (carrier) frequency from the incident beam.

6. A system as claimed in claim 5 in which the incident beam is doubled in frequency by a passive harmonic reflector and returned by way of an aerial.

7. A system as claimed in claim 3 or claim 4 in which the receiving station includes spin decoding means for determining from the returned signal modulation a reference orientation of the projectile.

8. A system as claimed in claim 1 or claim 2 in which the microwave signal emitted from the projectile is produced by a transmitter carried by the projectile.

9. A system as claimed in claim 8 in which the transmitter provides a beam of microwave radiation inclined to the longitudinal (spin) axis of the projectile so that the signal received at the receiving station is modulated in amplitude in accordance with the inclination of the emitted beam to an axis between the projectile and the reception station.

10. A system as claimed in claim 8 in which the projectile includes means operable to determine the datum orientation of the projectile and the transmitter includes modulation means responsive to said spin modulation means to modulate the emitted radiation as a function of the orientation of the projectile.

11. A system as claimed in any one of the preceding claims in which the receiving station includes a directional receiving aerial slaved to be orientated in accordance with the received beam direction of the target tracking radar means.

12. A system as claimed in any one of the preceding claims in which the command signal transmitted to the projectile includes a code charac teristic ofthe shell receiver.

13. A projectile guidance system substantially as herein described with reference to and as shown in, the accompanying drawing.

14. A method of guiding a spin stabilised projectile onto a new trajectory in flight comprising tracking atarget by microwave radar and establishing from the trajectory thereof an intercept point and trajectory for a gun spin stabilised projectile of the type including lateral thrust producing means commanded by signals received in flight, firing the projectile, tracking the projectile with microwave radar during at least a terminal portion of flight and transmitting thereto a command signal to initiate diversion of the trajectory by lateral thrust, causing a microwave signal to be emitted from the projectile modulated in accordance with the spin orientation of the projectile and received at a receiving station, forming from the relationship between target and projectile positions and trajectories and the ballistic characteristics of the lateral thrust means an indication of an optimum point on the projectile trajectory for the projectile to be diverted to a new target intercepting trajectory and in accordance with the orientation of the lateral thrust means in the vicinity of said optimum point determine a deviation from said optimum point at which the lateral thrust producing means is correctly aligned with the new trajectory and initiating transmission of a microwave command signal to the projectile to cause a laterial thrust to be produced at said deviated optimum point.

15. A method of guiding a spin stabilised projectile onto a new trajectory in flight substantially as herein described with reference to, and as shown in the accompanying drawings.