Abstract:
A method of aiming a gun ( 14 ) that is mounted on a platform ( 12 ) and that has fired projectile at a target, the firing of the gun ( 14 ) causing the platform ( 12 ) to vibrato. The method includes tracking the projectile and the target, using a tracking device ( 20 ) and inferring an aim error vector from the tracking.

Description:
FIELD AND BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a method and system of fire control and, more particularly, to a fire control method and system for a tank gun, or the like, that is mounted on a platform such as a tank turret.  
           [0002]    Tank fire control includes a series of operations, the propose of which is to aim the tank&#39;s gun so as to hit the target. If the first shot does not hit the target, adjustments must be made in the gun&#39;s aim in the quickest and most accurate way. Wile many efforts have been made in the art to facilitate hitting the target with the first shot, no efficient process and/or apparatus is at present available for permitting a single tank to adjust its fire when the first shot has failed to hit. Fire adjustments are difficult to effect by a single tank for many reasons, including the shocks due to the firing, the difficulty of observing the impact of the projectiles if they do not produce an explosion or, even if they do produce an explosion, because of the presence of dust, the period of time that must pass between the observation of the projectile impact and the firing of the next projectile with an adjusted aim, and so forth. As a result, fire control must be ended to a station separate from the tank or at least requires the collaboration of at least one other tank, and even these means do not provide as prompt an adjustment of the aim as would be desirable. This is very disadvantageous from the viewpoint of the operational autonomy and of the firing efficiency of the tanks.  
           [0003]    There is thus a widely recognized need for, and it would be highly advantageous to have, a method and system of tank fire control that would enable the crew of a tank to correct the aim of the tank&#39;s gun autonomously.  
         SUMMARY OF THE INVENTION  
         [0004]    It is a purpose of this invention to provide a fire control system for a tank&#39;s gun that is entirely autonomous and does not require the collaboration of a separate fire control station or of another tank.  
           [0005]    It is another purpose of the invention to provide such a system that permits rapid and accurate adjustment of the gun&#39;s aim on the part of the tank&#39;s crew.  
           [0006]    It is a further purpose of the invention to provide such a system that permits automatic adjustment of the gun&#39;s aim on the part of the gun&#39;s firing system.  
           [0007]    It is a still further propose of the invention to provide such a system that does not require visual observation of the projectile&#39;s impact. Indeed, the present invention can initiate correction of the aim of the gun even before projectile impact.  
           [0008]    It is a still further purpose of the invention to provide such a system that is not adversely affected by the firing shocks and by the presence of dust.  
           [0009]    Therefore, according to the present invention there is provided a method of aiming a gun that is mounted on a platform and that has fired a projectile at a target, the firing of the gun casing the platform to vibrate, including the steps of: (a) tracking the projectile and the target, using a tracking device, at least a portion whereof is operationally connected to the platform; and (b) inferring an aim error vector from the tracking.  
           [0010]    Furthermore, according to the present invention there is provided A fire control system for a gun that is mounted on a platform and that fires a projectile at a target, including: (a) an antenna that is operationally connected to the platform; (b) a transmitter for transmitting projectile-tracking RF pulses and target-tracking RF pulses via the antenna; (c) a receiver for receiving echoes of the RF pulses via the antenna; and (d) a signal processor for receiving signals, representative of the echoes, from the receiver and transforming the signals into measurement vectors for the projectile and the target.  
           [0011]    The term “trajectory” is used herein to refer to the position and velocity of an object, specifically, of the projectile or of the target, as a function of time. Typically, the trajectory of the projectile is a ballistic trajectory, and the trajectory of the target is whatever motion, if any, the target executes. In the special case of a stationary target, the target trajectory is simply the fixed position of the target.  
           [0012]    The scope of the present invention includes methods and systems of autonomous fire control for any platform-mounted gun. Nevertheless, the focus of the description herein is on autonomous fire control for a tank, in which the platform on which the gun is mounted is the turret of the tank. According to the present invention, at least a portion of a tracking device, for example, the antenna of a radar tacking system, is mounted on the platform. After the gun is fired at the target, and preferably after the vibration (shock) of firing has substantially stopped, the tracking device is used to track both the projectile, in flight, and the target. This tracking allows the determination, in real time, of the trajectory of the projectile and the deviation of that trajectory from the trajectory of the target. An aim error vector, that includes an azimuth error and an elevation error, is inferred from this deviation, and the gun is moved to correct its aim accordingly.  
           [0013]    Preferably, the portion of the tracking device, that is mounted on the platform, is mounted rigidly thereon.  
           [0014]    Preferably, the tracking device is based on radar, the antenna whereof is rigidly mounted on the platform. Most preferably, the antenna is a two-way monopulse antenna. A transmitter transmits, via the antenna, alternately, Doppler RF pulses for tracking the projectile and linear frequency modulated (chirp) pulses for tracking the target. A receiver receives echoes of the pulses via the antenna and provides signals representative of the echoes, typically Σ signals, Δ Az  signals and Δ El  signals, to a signal processor. The signal processor uses a CFAR method to discriminate echoes from the projectile and the target from clutter echoes, and transforms the signals corresponding to projectile echoes and target echoes to measurement vectors of the projectile&#39;s instantaneous position and velocity and of the target&#39;s instantaneous position and velocity. These measurement vectors are input to a post-processor, in which a Kalman filter uses the measurement vectors to update corresponding state vectors. The state vector of the projectile defines the projectile&#39;s trajectory. The state vector of the target defines the target&#39;s trajectory. The post-processor computes the amount by which the projectile&#39;s trajectory misses the target&#39;s trajectory and infers therefrom the aim error vector.  
           [0015]    Preferably, a synchronizer coordinates the transmission of the RF pulses and the reception of the echoes thereof, and also coordinates the alternation between projectile-tracking pulses and target-tracking pulses.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0017]    The sole FIGURE is a schematic depiction of a system of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The present invention is of a fire control method and system which can be used to correct the aim of a platform-mounted gun, that has fired a projectile at a target, while the projectile is in flight. Specifically, the present invention can be used to adjust the aim of a tank gun autonomously.  
         [0019]    The principles and operation of fire control according to the present invention may be better understood with reference to the drawings and the accompanying description.  
         [0020]    Referring now to the drawings, the sole Figure is a schematic illustration of a system of the present invention, as applied to fire control for the firing of a gun  14  mounted an a turret  12  of a tank  10 .  
         [0021]    Also mounted on turret  12  is a two-way monopulse antenna  20 . Preferably, antenna  20  is mounted rigidly on turret  12 . As used herein, the team “mounted rigidly” means that antenna  20  may be rigidly attached to turret  12 , so that antenna  20  points in a direction determined exclusively by the orientation in space of turret  12 ; but also means that antenna  20  may alternatively be mechanically steerable by virtue of being mounted on a mount, such as au altazimuth mount  18 , that in turn is rigidly attached to turret  12 . Preferably, if antenna  20  is rigidly attached to turret  12 , then the field of view of antenna  20  is at least 15 mrad in azimuth and at least 32 mrad in elevation.  
         [0022]    A transmitter  22  generates radio frequency (RF) pulses that are launched by antenna  20  towards the projectile and towards the target. Specifically, transmitter  22  alternates between generating Doppler pulses, that are used to track the projectile, and linear frequency modulated (chirp) pulses, that are used to track the target. Preferably, the RF pulses are in the Ka band. Echoes of the RF pulses are received, via antenna  20 , by a receiver  24 . The received echoes are downconverted in frequency and, in the case of the received chirp echoes, also dechirped. The echoes thus received are passed by receiver  24  to a signal processor  28  as analog signals, specifically, Σ, Δ Az  and Δ El  signals. Signal processor  28  digitizes the analog signals and processes the digitized signals by standard methods. In particular, the signals preferably are processed using Fast Fourier Transforms (FFTs) of appropriate lengths. The FFT length for processing the projectile-echo signals depends on the Doppler pulse repetition frequency and on the required Doppler resolution, which is on the order of one meter per second. Typically, this length is in the hundreds (256 or 512). The FFT length for processing the target echo signals also typically is in the hundreds. A constant false alarm rate (CFAR) method is used to discriminate projectile echoes and target echoes from clutter echoes.  
         [0023]    The output of the processing in signal processor  28  is, for each projectile echo, a measurement vector M P  whose components are projectile range, projectile azimuth, projectile elevation, and three components (range, azimuth, elevation) of the projectile velocity vector; and, for each target echo, a measurement vector M T  whose components are target range, target azimuth, target elevation, and, optionally, three components (range, azimuth and elevation) of the target velocity vector. Projectile range is determined from the round-trip travel time of the projectile echo. Projectile azimuth and elevation are determined from appropriate processing of the corresponding Σ, Δ Az  and Δ El  signals. The range component of the projectile velocity vector is determined from the Doppler shift of the projectile echo. The azimuth and elevation components of the projectile velocity vector are determined from the numerical time derivative of the azimuth and elevation components of successive projectile echoes. Target range is determined from the round-trip travel time of the target echo. Target azimuth and elevation are determined from appropriate processing of the corresponding Σ, Δ Az  and Δ El  signals. Optionally, the three components of the target velocity vector are determined from the numerical time derivative of the range, azimuth and elevation components of successive target echoes.  
         [0024]    A synchronizer  26  coordinates the activities of transmitter  22  and receiver  24 . Specifically, for each projectile-tracking pulse or target-tracking pulse launched by transmitter  22 , synchronizer  26  activates receiver  24  only in a corresponding time gate during which a corresponding echo from the projectile or form the target is expected to arrive at antenna  20 . In addition, synchronizer  26  causes transmitter  22  to alternate between transmitting projectile-tracking pulses (Doppler) and target-tracking pulses (chirp). Preferably, the projectile and the target are tracked almost concurrently, with the time interval between the transmission of a projectile-tracking pause and a target-tracking pulse being on the order of a few milliseconds. Preferably, successive sightings of the projectile and of the target are effected at a rate of about 100 Hz (100 times per second). The total number of sightings depends on the type of projectile and on the type of target, but preferably is at least about 100.  
         [0025]    Tracking of the projectile and of the target is not initiated until the shock of the firing of gun  14  has substantially dissipated. Typically, this time interval between the fixing of gun  14  and the initiation of tacking is several tenths of a second.  
         [0026]    Signal processor  26  passes the measurement vectors M P  and M T  to a post-processor  30 . Post-processor  30  uses these measurement vectors as input to a predictor-corrector algorithm for updating state vectors that represent estimates of the true positions and velocities of the projectile and of the target. The preferred predictor-corrector algorithm is a Kalman filter. The components of the state vectors correspond to the components of the measurement vectors: the components of the projectile state vector are the projectile range, the projectile azimuth, the projectile elevation, and time derivatives thereof (i.e., the projectile velocity vector); and the components of the target state vector are the target range, the target azimuth, the target elevation, and, optionally, time derivatives thereof (i.e., the target velocity vector). The state vectors are initialized when gun  14  is fired. The initial position of the projectile is at gun  14 . The initial velocity of the projectile is the muzzle velocity of the projectile. The illustrated analog components (antenna  20 , transmitter  22 , receiver  24 ) also serve as components of a target acquisition radar system (not shown) that is used to acquire the target and aim gun  14  at the target before gun  14  is fired; and the initial state vector of the target is obtained from this target acquisition system.  
         [0027]    As noted above, the state vectors of the projectile and of the target define the trajectories of the projectile and of the target. Based on these trajectories, post-processor  30  computes an azimuth error and an elevation error for gun  14 . The azimuth error is simply the difference between the azimuth of the projectile trajectory, projected out to the range of the target, and the azimuth of the target. The elevation error is the difference between the actual elevation of the gun and the elevation that would be required for the two trajectories to intersect if there were no azimuth error. This elevation error is computed by post-processor  30  using well-known ballistic equations. The azimuth error and the elevation error are the components of an aim error vector for gun  14 . Note that, even before the projectile impacts, the ballistic equations may be used to predict the remaining trajectory of the projectile. Meanwhile, the fixture behavior (until projectile impact) of the target may be predicted on the basis of the observed behavior of the target. Therefore, the aim error vector may be computed while the projectile is still in flight.  
         [0028]    Post-processor  30  passes the aim error vector along to the crew of tank  10 . The crew of tank  10  corrects the aim of gun  14  in accordance with the aim error vector. Alternatively, if tank  10  is equipped with an automatic system for aiming gun  14 , post-processor  30  sends the aim error vector to the automatic aiming system, which automatically corrects the aim of gun  14 .  
         [0029]    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.