Abstract:
A methodology determines the offset distance between a threat missile plume and its hardbody during boost phase to aid in guiding a kinetic weapon (KW) or interceptor missile to the threat missile hardbody using the KW infrared sensor of the interceptor missile in conjunction with a radar sensor.

Description:
FIELD OF THE INVENTION 
   This invention relates to directing antimissile weapons against threat missiles in their boost stage, and more particularly to methods for compensation for the difference between the location of the plume relative to the hardbody of the threat missile. 
   BACKGROUND OF THE INVENTION 
   Antimissile efforts may use directed-beam weapons, in which the future target missile or target location is not of particular interest, as the speed of the beam is so great that the missile motion is irrelevant. In those cases in which antimissile interceptors with explosive warheads are used, the interceptor speed is of the same order as that of the target missile, and the estimated future location of the target missile is of great importance. The estimated future location of the target missile can be determined by the use of radar. A great deal of effort has been put into antimissile interceptor guidance schemes which predict the future location of the target missile, the interceptor missile, or both, so as to attempt to cause the interceptor to get within a given range of the target missile such that the explosive warhead destroys the target missile. These guidance systems require measurements of the target missile so as to determine its current location, and also require estimates of its projected trajectory. 
   It is known that rocket engine or rocket motor plumes are hot, and radiate energy across the entire electromagnetic spectrum, including the infrared (IR) portion of the spectrum. The radiated energy constitutes a signature which may allow the rocket engine to be identified or characterized. Different missile systems using either liquid or solid propellant display different infrared (IR) signatures at various altitudes, mach numbers, and aspect angles. These IR signatures have been used for many years to warn of Intercontinental Ballistic Missile (ICBM) launches or to characterize tactical threat systems. 
   Kinetic weapons (KW) are known for use against threat missiles. Such weapons do not use an explosive charge for destroying the threat missile, but rather rely upon the kinetic energy of a moving object impacting on the hard body of the threat missile. Such schemes have been tested and can be effective. Some antimissile weapons use infrared (IR) schemes for terminal guidance, so as to result in the desired impact between the antimissile weapon and the missile to be destroyed. One of the problems associated with the use of infrared guidance of a kinetic weapon against a boosting missile lies in the inability of the kinetic weapon&#39;s infrared seeker to accurately determine the location of the hard body of the target missile in the presence of a hot IR plume from the boost engine. 
   A proposed solution to the problem of inability of the infrared seeker to distinguish between the hard body of the target missile and the hot plume lies in the use of multiple IR sensors, which respond to different portions of the IR signature, and can distinguish between the hard body and the plume. This solution may be effective, but requires that two or more IR seekers with different characteristics be used. Terminal guidance of a kinetic weapon is facilitated when the sensing and the signal processing are performed on-board the kinetic weapon so as to avoid delays associated with ground-based detection and processing, and data transmission delays. On-board IR sensing with different signatures requires that the kinetic weapon carry two or more different IR sensors, which undesirably adds weight, complexity, and cost to the weapon. 
   SUMMARY OF THE INVENTION 
   A method according to an aspect of the invention is for directing a weapon toward a boosting missile. The method comprises the steps of sensing the boosting missile with an infrared sensor to thereby generate an infrared signature representing the boosting missile, and determining from the infrared signature the infrared centroid. The boosting missile is illuminated with a radar to thereby generate a radar cross-section representing the boosting missile. From the radar cross-section representing the boosting missile, the radar cross-section of the hardbody and of the plume of the boosting missile are determined. From the radar cross-sections of the hardbody and plume, the radar cross-section centroids of the hardbody and plume are determined. The centroids are processed to determine the location of the hardbody relative to the infrared centroid in the plume. A particularly advantageous mode of the method further comprises the step of directing the weapon toward a location offset from the infrared centroid by the difference between the location of the hardbody relative to the infrared centroid. 
   In another version of this method, the step of processing the centroids to determine the location of the hardbody relative to the infrared centroid comprises the steps of determining a line representing the boosting missile velocity vector. The difference is taken between the location of the infrared centroid and the location of the plume radar cross-section centroid to establish a first difference vector. The difference is taken between the location of the plume radar cross-section centroid and the location of the hardbody radar cross-section centroid to form a second difference vector. The first and second difference vectors are vector summed to establish the offset vector between the location of the hardbody radar cross-section centroid and the location of the infrared centroid. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a simplified representation of a scenario in which a boosting threat missile is observed by the IR seeker of a kinetic weapon and by a radar system; 
       FIG. 2  is a representation of the locations along the instantaneous trajectory vector of the infrared centroid of the plume, the radar cross-section centroid of the plume, the radar cross-section centroid of the hardbody and the defined difference vectors. 
       FIG. 3  is a representation of the radar cross-section (RCS) return obtained by a radar for the hardbody and plume of a boosting target missile. 
   

   DESCRIPTION OF THE INVENTION 
   In the scenario  10  of  FIG. 1 , a threat missile  12  includes a hard body  12   h . The missile  12  is boosting, so it produces a plume  12   p  of hot gas coaxial with its instantaneous trajectory  12   t . A weapon  14 , which may be a kinetic weapon, includes an infrared seeker  14   s  which senses the boosting missile  12 . The sensor  14   s  will respond most strongly to the plume  12   p  of the boosting missile  12 . Due to the infrared contrast between the relatively cool hard body  12   h  and the hot plume  12   p  of the threat missile  12 , infrared seeker  14   s  may not even perceive the existence of the hard body  12   h . A radar system designated generally as  16  illuminates the threat missile  12 , including its hardbody  12   h  and its plume  12   p . The radar cross-section of the radar return to the radar system  16  from the target missile  12  may include a component attributable to the hard body  12   h  and another component attributable to the plume  12   p.    
   The sensor of the IR seeker  14   s  of  FIG. 1  will generate a signal representing the IR signature of the plume at the frequencies to which the IR sensor responds. The IR signature of the plume is processed in known fashion to obtain the plume infrared centroid. The location of the plume infrared centroid is illustrated as  12   p   cir  in  FIG. 1 . Those skilled in the art know that the plume infrared centroid can be obtained by the general method of calculating any distributed property centroid, as indicated, for example, in the text  Vector Mechanics  by Beer and Johnston. 
   Illumination of the boosting threat missile  12  of  FIG. 1  by radar  16  results in a reflected signal which represents the radar cross-section (RCS) of the missile hardbody  12   h  and plume  12   p . Plot  310  of  FIG. 3  represents one possible reflected signal plotted as a function of range. Plot  310  includes a peaked or spiked portion  312  which represents the hardbody  12   h  of the target missile, and also includes a broad peak region  314  which represents the plume  12   p . The RCS thus includes components arising from the hardbody  12   h  and the plume  12   p . These components are separable, as known in the art. The separation of RCS components is performed by examining the RCS return, determining the sharp spiked portion of the return is the hardbody component and the extended peak region is the plume component. The RCS components from both the hardbody and the plume are processed to obtain their RCS centroids as described above. Thus, processing of the RCS information returned from the boosting threat missile hardbody  12   h  and the plume  12   p , which may be performed in a processor (Proc) illustrated as a block  18 , separately determines the instantaneous locations of the RCS centroid  12   h   crcs  of the hardbody  12   h  and the RCS centroid  12   p   crcs  of the plume, which in general is at a different location than the infrared centroid  12   p   cir . While processor  18  is illustrated as being separated from kinetic weapon  14  and radar system  16 , the processing may be performed at any location, including at the site of the radar system  16 , on-board the kinetic weapon  14 , elsewhere, or it may be distributed among any or all of these locations. If the radar system  16  is ground-based and the processing is on-board the kinetic weapon  14 , some means, such as a transmission path, must be provided for making the radar information available to the processor in the kinetic weapon  14 . The need for such a data transmission path can be obviated by placing the radar system on-board the kinetic weapon. 
   Regardless of the location of the processor(s) which perform the calculations, simple calculations are used to determine the offset to be applied to the location of the hardbody  12   h  of the threat missile  12  relative to the infrared centroid  12   p   cir .  FIG. 2  illustrates the relevant geometry. In  FIG. 2 , distance along the instantaneous trajectory or path  12   t  of the threat missile is denominated S. Thus, Sh crcs  is a location on the hardbody as indicated by the centroid of the radar cross-section of the hardbody. At some distance behind (in the direction of motion of the threat missile) the RCS centroid hardbody location Sh crcs  is the location Sp crcs  of the RCS centroid of the plume  12   p , both as determined by the radar cross-section. The vector representing this distance is denominated  S   pcrcs−hcrcs . Similarly, the vector representing the distance between the location of plume RCS centroid Sp crcs  and the location of infrared plume centroid Sp cir  is given as  S   pcir−pcrcs . 
   The various centroid locations are in practice changing during many sequential measurements, so that at any time they are calculated as
 
 Sh   crcs =Σ( rcs   i   S   i ) h   /RCS   h  
 
 Sp   crcs =Σ( rcs   i   S   i ) p   /RCS   p  
 
 Sp   cir =Σ( ir   i   S   i ) p   /IR   p  
 
where:
 
   (rcs i S i ) h  is the local value of RCS for the hardbody times its distance along S; 
   (rcs i S i ) p  is the local value of RCS for the plume times its distance along S; 
   (ir i S i ) p  is the local value of IR for the plume times its distance along S; 
   RCS h  is the total hardbody RCS return; 
   RCS p  is the total plume RCS return; and 
   IR p  is the total plume IR return. 
   The kinetic weapon  14  of  FIG. 1  sees the threat missile  12  by means of its infrared sensor as being located at the instant of the measurement at the IR centroid of the plume, Sp cir . The offset distance between the threat missile as seen by the IR sensor at Sp cir  and the actual location of the hardbody  12   h  is defined by the vector equation
 
   S     hcrcs−pcir =−(   S     pcrcs−hcrcs   +  S     pcir−pcrcs )
 
This calculation is performed repeatedly during tracking to update the information, and the kinetic weapon is directed toward a location offset by  S   hcrcs−pcir  in the direction of motion along track  12   t  from the apparent location of the missile Sp cir  as indicated by the IR seeker  14   s.  
 
   An advantage of the described system is that the kinetic weapon can use a conventional single IR sensor, and the information is supplemented by information from a radar system, which is often available in situations in which a kinetic weapon is used. The supplemental information identifies the offset which must be applied to the apparent location of the target missile as indicated by the IR sensor in order to hit the hard body. This avoids the need for multiple IR sensors aboard the kinetic weapon. 
   A method according to an aspect of the invention is for directing a weapon ( 14 ) toward a boosting missile ( 12 ). The method comprises the steps of sensing the boosting missile with an infrared sensor ( 14   s ) to thereby generate an infrared signature representing the boosting missile, and determining the infrared centroid ( 12   p   cir ), as known in the art. The boosting missile is illuminated with a radar ( 16   i ) to thereby generate a radar cross-section representing the boosting missile ( 12 ). From the radar cross-section representing the boosting missile, and determining centroid of the radar cross-section ( 12   h   crcs ;  12   p   crcs ) of the hardbody ( 12   h ) and of the plume ( 12   p ) of the boosting missile ( 12 ) are determined as in the prior art. The centroids ( 12   h   crcs ;  12   p   crcs ;  12   p   cir ) are processed to determine the location  S   hcrcs−pcir  of the hardbody relative to the plume infrared centroid Sp cir . A particularly advantageous mode of the method further comprises the step of directing the weapon toward a location offset from the infrared centroid by the difference−(  S   pcrcs−hcrcs +  S   pcir−pcrcs ) between the location  S   hcrcs−pcir  of the hardbody ( 12   h ) relative to the infrared centroid  12   p   cir . 
   In another version of this method, the step of processing the centroids ( 12   h   crcs ;  12   p   crcs ;  12   p   cir ) to determine the location of the hardbody  S   hcrcs−pcir  relative to the infrared centroid  12   p   cir  comprises the steps of determining a line ( 210 ) representing the boosting missile ( 12 ) velocity vector. The difference is taken between the location S p   cir  of the infrared centroid and the location of the plume radar cross-section centroid S p   crcs  to establish a first difference vector  S   pcir−pcrcs . The difference is taken between the location of the plume radar cross-section centroid and the location of the hardbody radar cross-section centroid to form a second difference vector  S   pcrcs−hcrcs . The first and second difference vectors are vector summed−(  S   pcrcs−hcrcs +  S   pcir−pcrcs ) to establish the offset vector  S   hcrcs−pcir  between the location S h   crcs  of the hardbody radar cross-section centroid and the location S p   cir  of the infrared centroid.