Abstract:
A missile guidance system and method for guiding a missile, mainly horizontally flying, to pass a target at a desired passage height.

Description:
[0001]    The present invention refers to a method and a system for guiding a missile, and also to a missile provided with such a system. In particular, it refers to such guidance systems for missiles using passive target seeker, where the missile is devised not to hit the target dead on, but to pass by at a predetermined distance. 
       BACKGROUND 
       [0002]    When using missile systems it is sometimes desirable to let the missile miss the target with a certain distance. One example comprises an antitank missile travelling approximately horizontally and provided with a shaped charge devised to hit at an angle downwards/forwards. Said missile should pass approximately one meter over the tank to enable the shaped charge to achieve good effect at the tank. It should be mentioned that most conventional tanks usually are well protected against direct hits from the front, side and behind. The missiles “NLAW” and “Bill” are examples of missiles using such a method, although they are not utilising target seeking mechanisms. 
         [0003]    Another example concerns attacks using a ground target missile, where the target seeker is not able to see the target, but where it has been possible to determine the target position in relation to one or more other objects that can be seen by the target seeker. 
         [0004]    Most known missile systems for antitank warfare use missiles that approach the target from above. There seems to be few if any known systems of today that combine the benefits of a target seeker, a horizontally flying missile having a shaped charge devised to hit downwards, with means for steering the missile in such a way that it passes a predetermined distance above the target. 
         [0005]    U.S. Pat. No. 5,932,833 discloses a fly over homing guidance system for terminal homing missile guidance which comprises a fire and forget missile guidance method wherein on board target sensing tracks the target and guides the missile to the target, but instead of being guided to a direct impact as is conventionally done, the missile is guided towards a precise distance over the top of the target, intentionally avoiding impact. 
         [0006]    The use of target seeking and inertial navigation system data in order to accomplish a direct hit is well known in the art. The use of the same information to accomplish that the missile “misses” the target with an appropriate distance is less known. 
       SUMMARY OF THE INVENTION 
       [0007]    It is a purpose of the present invention to provide a missile guidance system for use in a missile, where said system is capable of guiding said missile to pass a predetermined distance above a target. 
         [0008]    Said missile guidance system comprises a gamma-ref calculation unit capable of calculating a reference value of a vertical flight direction angle which, if used to adjust a current vertical flight direction angle γ ref  of said missile, would cause the missile to pass the target at a desired passage height (h des ). 
         [0009]    Said gamma-ref calculation unit calculates the reference value of the vertical flight direction angle (γ ref ) based on the following parameters:
       an elevation angle (σ)   a desired passage height (h des )   a line-of-sight rotation ({dot over (σ)})   a missile velocity (V).       
 
         [0014]    In a preferred embodiment said reference value is calculated as 
         [0000]    
       
         
           
             
               γ 
               ref 
             
             = 
             
               
                 
                   
                     
                       h 
                       des 
                     
                      
                     
                       σ 
                       . 
                     
                   
                   V 
                 
               
               - 
               σ 
             
           
         
       
     
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Preferred embodiments of the present invention is described with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  is a schematic illustration defining directions, distances and angles of a missile guidance system according to a preferred embodiment of the invention, 
           [0017]      FIGS. 2A and 2B  is a flowchart of a method of a missile guidance systems according to a preferred embodiment of the invention, 
           [0018]      FIG. 3  shows a system overview of a missile guidance system according to a preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]      FIG. 1  shows a schematic illustration defining directions, distances and angles according to a preferred embodiment of the present invention. An antitank missile  101  is travelling with a velocity V. The velocity vector forms a vertical flight direction angle γ with the horizontal plane  110 . The antitank missile  101  has a centre of gravity  103 . A target total distance r between the centre of gravity  103  of the missile  101  represents the line of sight between said centre of gravity and a top surface  122  of a target  120 . The target distance r forms an elevation angle σ with a horizontal x-axis. A target vertical distance from the centre of gravity  103  of the missile  101  to the top surface  122  of the target  120  is designated h. 
         [0020]    As can be derived from  FIG. 1 , the travelling path of the missile aims such that if all parameters are left unchanged, the missile would pass over the target at a target vertical distance h, where h can be estimated from the following formula 
         [0000]        h=r (sin(σ)+cos(σ)(sin(γ)  (1) 
         [0021]    Assuming the target total distance r is greater than the target vertical distance h, the target vertical distance h can be approximated with the formula 
         [0000]        h′=r (σ+γ)  (2) 
         [0000]    where h′ designates an estimated target vertical passing distance, here also called estimated passage height. The time derivative of the elevation angle σ, also called the line-of-sight-rotation σ, fulfils the equation 
         [0000]        r{dot over (σ)}=V  sin(γ+σ)  (3) 
         [0022]    Using the equation (3) to solve the total (missile to) target distance r from the height expression (2) and utilising again that the elevation angle σ and the vertical flight direction angle γ are small, you arrive at: 
         [0000]    
       
         
           
             
               
                 
                   
                     h 
                     ′ 
                   
                   = 
                   
                     
                       
                         V 
                          
                         
                           ( 
                           
                             γ 
                             + 
                             σ 
                           
                           ) 
                         
                       
                       2 
                     
                     
                       σ 
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0023]    Assuming a straight flight path of the missile, the estimated passage height h′ and the vertical flight direction angle γ will be constant, even though the velocity V, the elevation angle σ and the line-of-sight-rotation {dot over (σ)} may vary. From the expression (4) above, the inventors have chosen to form a reference value for the vertical flight direction angle γ according to the following expression 
         [0000]    
       
         
           
             
               
                 
                   
                     γ 
                     ref 
                   
                   = 
                   
                     
                       
                         
                           
                             h 
                             des 
                           
                            
                           
                             σ 
                             . 
                           
                         
                         V 
                       
                     
                     - 
                     σ 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0024]    where “h des ” designates the desired passage height. This expression (5) will be referred to as “the law of guidance” in the following. 
         [0025]    In order for the law of guidance to function properly, it is necessary to first check if the desired passage height h des  and the line-of-sight-rotation {dot over (σ)} have the same sign. This since it is necessary to have a positive expression under the square root sign. If the signs are different, it is advisable to steer the missile such that the vertical flight direction angle γ becomes positive for desired passage heights h des  greater than zero, and such that the vertical flight direction angle γ becomes negative for desired passage heights h des  less than zero. 
         [0026]    The missile  101  is provided with an inertial navigation system. The missile  101  is also provided with a target seeking system. The target seeking system could be any type of present or future passive or active target seeking systems based on, but not restricted to, one or more of the following principles: laser, infra-red, radio, radar, heat and/or optical. With the aid of the target seeking system information about the direction, or the direction and the distance to the target  120 , a method and a system according to an embodiment of the present invention easily calculates the necessary values of the elevation σ and the line-of-sight rotation {dot over (σ)}. 
         [0027]    Simulations have shown that the law of guidance (5) works best when the total target distance r is greater than approximately ten times the desired passage height h des . One of the advantages with the law of guidance (5) is that when the missile is set for the correct desired passage height h des , the reference value of the vertical flight direction angle γ ref  will be constant, despite variations in the velocity V, the elevation angle σ and the line-of-sight rotation {dot over (σ)}, i.e. corrections of the vertical flight direction angle γ will be minimised. 
         [0028]    The law of guidance (5) works less good according to performed simulations when the distance r is less than ten times the desired passage height. In practice this is not a problem since during the time left there is no time to perform any manoeuvre. 
         [0029]      FIG. 2  A is a flowchart of a method of a missile guidance system according to a preferred embodiment of the present invention. Said method comprises the following steps:
       Setting a desired passage height h des ,  205 .   Obtaining value of current elevation angle σ,  210 .   Obtaining value of current line-of-sight rotation {dot over (σ)},  212 .   Obtaining value of current velocity V,  215 .   Forming a reference value of the vertical flight direction angle γ ref  as a function of desired passage height h des , line-of-sight rotation {dot over (σ)}, velocity V and elevation angle σ,  225 .   Steering the missile such that the vertical flight direction angle γ becomes closer to said reference angle γ ref ,  230 .         
         [0036]    For the method to be efficient, the inventors have realised that the case when the desired passage height and the line-of-sight rotation have different signs, has to be handled separately. In one embodiment this comprises the following step:
       Checking if the desired passage height h des  and the line-of-sight rotation {dot over (σ)} have the same sign,  220 , and if so, performing the following steps:   Checking if the desired height h des  is positive or negative,  240 .   If positive, steering the missile such that the vertical flight direction angle γ becomes slightly greater than zero,  245 .   If negative, steering the missile such that the vertical flight direction angle γ becomes slightly less than zero,  250 .       
 
         [0041]      FIG. 2  B is a flowchart of part of an alternative preferred embodiment of the present invention. As described above, the case where the desired height h des  and the line-of-sight rotation {dot over (σ)} have different signs is handled separately. This case is handled in a method of a further embodiment of the present invention comprising the following steps:
       Checking if the desired passage height and the line-of-sight rotation have the same sign,  260 .   If they have, setting the reference value of the vertical flight direction angle γ ref  to a value being a function of the desired passage height h des , the line-of-sight rotation {dot over (σ)}, the velocity V and the elevation angle σ,  210 .   If said variable does not have the same sign, and the desired passage height h des  is greater or equal to zero, setting the reference value of the vertical flight direction angle to a value greater than zero,  270 .   If said variable does not have the same sign, and the desired passage height h des  is less than zero, setting the reference value of the vertical flight direction angle to a value less than zero,  275 .         
         [0046]    As can be seen from the above, the function for determining the reference value of the vertical flight direction angle γ ref  comprises the following variables: the desired passage height h des , the line-of-sight rotation {dot over (σ)}, the velocity V and the elevation angle σ. In one embodiment, the reference value of the vertical flight direction angle γ ref  is formed as, or derived from, the difference between the square root of the desired height h des  multiplied with the line-of-sight rotation {dot over (σ)} divided by the velocity V and the elevation angle σ. 
         [0047]      FIG. 3  shows a system overview of a missile guidance system according to a preferred embodiment of the invention. 
         [0048]    A target seeking system  305  is connected to an elevation angle σ estimator unit  315 . Said target seeking system  305  is also connected to a line-of-sight rotation {dot over (σ)} estimator unit  320 . 
         [0049]    An inertial navigation system  310  is connected to said elevation angle σ estimator unit  315 , to said line-of-sight rotation estimator unit  320 , and also to a velocity V estimator unit  325 , and a vertical flight direction angle γ estimator unit  330 . The inertial navigation system  310 , the target seeking system  305 , and the missile steering system  360  should be viewed at as conventional ditos. The navigation system  310  is preferably of a strapped-down type as explained in e.g. D. H. Titterton and J. L. Weston “Strapdown inertial navigation technology” ISBN 0 86341 260 2. The estimator units  315 ,  320 ,  325 ,  330  may also be part of the target seeking system  305  or the inertial navigation system depending on selected level of integration. 
         [0050]    Said elevation angle estimator unit  315  is further connected to a gamma-ref calculation unit  350 . 
         [0051]    Said line-of-sight rotation estimator unit  320  is connected to a sign comparing unit  340 , and also to said gamma-ref calculation unit  350 . 
         [0052]    Said velocity estimator unit  325  is further connected to said gamma-ref calculation unit  350 . 
         [0053]    Said vertical flight direction angle estimator unit  330  is further connected to a missile steering system  360 . 
         [0054]    Said sign comparing unit  340  is connected to a desired passage height obtaining unit  345 , and to the gamma-ref calculation unit  350 . 
         [0055]    Said gamma-ref calculation unit  350  is further connected to the missile steering system  360 . 
         [0056]    The target seeking system  305  measures the direction to the target and provides values representative of this direction to the elevation angle estimator unit  315 , and to the line-of-sight estimator unit  320 . The elevation angle estimator unit  315  receives values from the target seeking system representative of the direction to the target. Said elevation angle estimator unit makes an estimate of the current elevation angle {dot over (σ)} based on the values from the target seeking system and values from the inertial navigation system  310 , representative of the missiles own flight parameters, such as altitude angles and translational and rotational velocities. 
         [0057]    The line-of-sight rotation estimator unit  320  estimates in a similar way the line-of-sight rotation {dot over (σ)} based on values from the target seeking system  305  and the inertial navigation system  310 . The velocity estimator unit  325  estimates the velocity based on values from the inertial navigation system  310 , representative of the velocity V. 
         [0058]    In an alternative embodiment, the velocity estimator unit  325  is also connected to the target seeking system  305 , and the velocity is estimated based on both values from the inertial navigation system  310  and from the target seeking system  305 . 
         [0059]    The gamma estimator unit  330  receives values from the inertial navigation system and estimates a vertical flight direction angle γ. Said gamma estimator unit  330  communicates said estimated vertical flight direction angle γ to the missile steering system  360 . 
         [0060]    The desired height obtaining unit  345  obtains the desired height. Said obtaining can be effected by manual setting or automatic setting by a computer program, or another suitable method. The value representing the desired passing height h des  is communicated to the sign comparing unit. The sign comparing unit  340  compares the signs of the designated passage height and the line-of-sight rotation {dot over (σ)}. The result is communicated to the gamma-ref calculation unit  350 , which calculates a reference value for the vertical flight direction angle γ ref  according to the method explained above. The reference value γ ref  is then communicated to the missile steering system  360 , which makes the necessary adjustments of the missile ailerons, control surfaces, or other means for adjusting the course of the missile to get the vertical flight direction angle γ closer to the reference value γ ref . Such course changes are obtained in one embodiment by steering in vertical direction according to the following expression: 
         [0000]        a   c   =K (γ ref −γ)  (6) 
         [0061]    where a c  is the commanded acceleration and K is a constant. γ ref  and γ as explained above. 
         [0062]    In a further embodiment such course changes are obtained by steering in vertical direction according to the following expression: 
         [0000]    
       
         
           
             
               
                 
                   
                     a 
                     c 
                   
                   = 
                   
                     
                       CV 
                       c 
                     
                      
                     
                        
                       
                          
                         t 
                       
                     
                      
                     
                       ( 
                       
                         γ 
                         ref 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0063]    where C is another constant and V c  is the commanded velocity. 
         [0064]    It is understood the missile guidance system also comprises a horizontal guidance function. This is however not part of the invention and is not described here. 
         [0065]    The scope of the invention is only limited by the claims below.