Abstract:
Systems and methods for contiguously and accurately updating target object information during an entire target engagement period are provided. The target tracking system includes a database for storing starfield information, an optical beam source configured to illuminate one or more optical beam pulses, first and second camera systems, and a processor. The processor instructs the first camera system to track the object based on recordation of the tracked object, instructs the second camera system to stabilize the tracking image based on the instructions sent to the first camera system, and determines inertial reference information of the tracked object based on the stabilized image and starfield information associated with the stabilized image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a Continuation Application of U.S. patent application Ser. No. 10/721,580, filed on Nov. 25, 2003 now U.S. Pat. No. 7,007,888 and is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   This invention relates generally to target tracking and, more specifically, to systems and methods for accurately identifying the inertial position of a target. 
   BACKGROUND OF THE INVENTION 
   When line of sight tracking is used to determine the inertial position of objects, accuracy and engagement, timelines may be difficult to achieve. In one approach, an inertial reference unit (IRU) star calibration update is performed before and after the engagement sequence thereby requiring a longer engagement timeline. When star calibration is performed before and after the engagement is performed, the IRU information diverges between the calibrations, thereby, resulting in degraded inaccurate target object position accuracy. 
   One process to generate more accurate information of the target object would be to temporarily suspend tracking of the target object during the time of engagement. While tracking is suspended, an IRU star calibration update is performed. However, suspension of tracking during an engagement period may be problematic. For example, if the target object changes course during the suspension, then reacquiring the target object may become difficult. 
   However, there exists an unmet need to provide contiguous, accurate target object information during an entire target engagement period. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides systems and methods for contiguously and accurately updating target object information during an entire target engagement period. The accurate target object information is used to instruct a weapon or defense system about the target. 
   In an embodiment of the present invention, an exemplary target tracking system includes a database for storing starfield information, an optical beam source configured to illuminate one or more optical beam pulses, first and second camera systems, and a processor. The processor instructs the first camera system to track the object based on recordation of the tracked object, instructs the second camera system to stabilize the tracking image based on the instructions sent to the first camera system, and determines inertial reference information of the tracked object based on the stabilized image and starfield information associated with the stabilized image. 
   In one aspect of the invention, one or more platform information sources may be coupled to the target tracking system to send platform information to the target tracking system for use in determining inertial reference information of the tracked object. In another aspect of the invention, the system may be hosted on a platform that may include satellite, an aircraft, or a ground based system. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
       FIG. 1  is a block diagram of an exemplary target tracking system formed in accordance with an embodiment of the present invention; 
       FIG. 2  illustrates an exemplary process performed by the system of  FIG. 1 ; 
       FIG. 3  illustrates a geometric representation of an analysis performed by the system of  FIG. 1 ; 
       FIG. 4  illustrates an exemplary stabilized image generated by a component of the system of  FIG. 1  for analyzing inertial reference information; and 
       FIG. 5  is a time graph of a modulated optical beam used in the system of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a target tracking system  22  included within a platform  20 , such as without limitation a ground based facility, an aircraft, or a satellite, provides near continuous updating of inertial reference unit (IRU) information while performing uninterrupted optical target tracking. In addition, the platform  20  includes platform information sources  26 , such as a flight data computer, and an Inertial Reference System (IRS) that are coupled to the target tracking system  22 . 
   In one embodiment, the target tracking system  22  includes a tracking processor  30 , a high signal light source component  32 , a first camera  34 , a second camera  36 , a first fast steering mirror (FSM)  38 , a second FSM  40 , a beam splitter  42 , and various reflecting mirrors  44 . The tracking processor  30  is operatively coupled to the high signal light source component  32 , the first and second cameras  34  and  36 , the first and second FSMs  38  and  40 , and a database  46 . The database  46  stores starfield reference information for use by the processor  30 . 
   The tracking processor  30  includes an inertial reference unit (IRU)  50 . The IRU  50  determines and adjusts inertial reference information received from the IRS based on an optical output of an optical beam source  58  of the high signal light source component  32  and an image received by the second camera  36 . In addition, the tracking processor  30  includes a target tracking component  54  that tracks a target displayed within an image generated by the first camera  34  and determines inertial reference information of the tracked target based on the inertial reference information determined by the IRU  50  and any information from the sources  26 . The target tracking component  54  generates an instruction to the second FSM  40  for stabilizing the tracked image, thereby allowing the second camera  36  to record a stabilized image of the starfield. The IRU  50  may be located remote from the processor  30  or the target tracking system  22 . 
   Referring now to  FIG. 2 , a process  100  performed by the target tracking system  22  ( FIG. 1 ) provides nearly continuous inertial reference information updating using starfield information stored in the database  46  without loss of contact of an optically tracked target. The process  100  begins after the target tracking system  22  has acquired a target in its view. In other words, the first camera  34  has acquired a target within its field of view and the target tracking component  54  has analyzed images generated by the first camera  34  and instructed the first FSM  38  to track the identified target. The process  100  begins at a block  102  where the target tracking component  54  determines the inertial angular rate of the target scan. The target scan inertial angular rate is the speed at which the first FSM  38  moves in order to track the target. At a block  106 , the starfield image received by the second camera  36  is stabilized based on the determined target scan rate. The determined target scan rate is sent to the second FSM  40  for de-scanning the starfield image received from the first FSM  38 . 
   Referring back to  FIG. 2 , at a block  108  the second camera  36  records the stabilized image over a predetermined period of time. At a block  112 , during the period of time that the second camera  36  records the starfield image, the IRU optical beam generator  58  generates an optical beam that is pulsed over a finite period of time. At a block  114 , the tracking processor  30  or components thereof identifies the location within the stabilized image of when the IRU optical beam was turned on and off during each optical beam pulse. Referring to  FIG. 3 , time t 1  identifies the location within a stabilized image  300  where an optical beam pulse was initiated and time t 2  is the location within the image  300  that identifies when the IRU optical beam pulse was turned off. Referring back to  FIG. 2 , at a block  118 , centroids of each optical beam pulse are determined based on respective times t 1  and t 2 . 
   At a block  120 , the processor  30  compares the centroids to one or more stars located within the stabilized image based on starfield information stored in the database  46  and adjusts inertial reference information received from the IRU  50  based on the comparison. At a block  122 , the processor  30  determines inertial reference information for the target based on present target tracking information produced by the target tracking component  54 , the adjusted inertial reference information, and any information relating to the platform  20 , such as without limitation GPS location information, pitch, roll, yaw, or other orientation information received from the other sources  26 . Platform information may include position, velocity, and attitude from separate inertial navigation system for transforming target position into a platform body-fixed coordinate system. 
   In one embodiment, the optical beam direction is referenced to the target based on target tracking information generated by the target tracking component  54  and sent to the light source component  32 . Because the optical beam is referenced to the tracked target, the pulses track across a stabilized image. 
   Referring now to  FIG. 3 , a stabilized image  300  includes a plurality of stars  302  that are identified by a starfield analysis component included within the tracking processor  30 . The stabilized image  300  includes a plurality of optical beam pulses recorded by the second camera  36 . Centroids  306  of each optical beam pulse  304  are identified based on identified t 1  and t 2  of the respective pulse  304 . The centroids  306  are simply the center location of each pulse  304 . The processor  30  determines the location of the centroids  306  relative to the starfield pattern  302  within the stabilized image  300  in order to generate highly accurate inertial reference update information of the target.  FIG. 4  illustrates an exemplary de-scanned (stabilized) image that is recorded by the second camera  36 . Satellite  1  is a tracked target. 
   The resulting series of pulses from the beam combined with the knowledge of the precise time of each pulse allows accurate measurement of the optical beam pointing direction relative to the starfield. This measurement is input to a Kalman Filter which estimates the IRU errors, thereby allowing accurate reporting of the track object position in inertial frame coordinates. The Kalman Filter algorithm is a standard, recognized estimation algorithm for estimating IRU errors. The Kalman Filter would be applied in the same manner as if the track were interrupted and the star measurements taken independent of the track process. The errors in the inertial system which would be estimated are the three components of inertial attitude, the three components of gyro bias, and the 3 components of gyro scalefactor. The Kalman Filter would therefore be at least a 9 state estimation algorithm. The equations for the Kalman Filter are given in the literature but are shown here for completeness: 
   
     
       
             
             
           
             
           
         
             
                 
             
           
           
             
               K n  = P n H T   n (H n P n H T   n  + R) −1   
               Kalman Gain 
             
             
               x n+1  = x n  + K n (z n  − H n x n ) 
               State Update 
             
             
               P n+1  = P n  − K n H n P n   
               Co variance Update 
             
           
        
         
             
               K n  = 9 × 9 Initial Co variance Matrix (Identity), 
             
             
               H n  = 2 × 9 Measurement Matrix, 
             
             
               P 0  = 9 × 9 Initial Co variance Matrix (Identity), 
             
             
               R = 2 × 2 Measurement Noise Matrix 
             
             
               x 0  = 9 × 1 State Vector (Zero Vector) 
             
             
               z n  = 2 × 1 Measurement Vector (Inertial Angles to Each Star Observed) 
             
             
                 
             
           
        
       
     
   
   The Kalman Filter equations are iterated over each exposure time of the streak camera  36  with a star measurement comprising the z n  vector at each exposure time. 
   The optical beam has a signal-to-noise ratio of approximately 100. The beam can be very intense relative to the signal returned from the target, thereby allowing centroid measurement of the optical beam streaks to approximately 1/100 of the camera  36  pixel angular extent. 
   Referring to  FIG. 5 , a time graph  350  illustrates example on/off times of an optical beam. The following are exemplary system values associated with the graph  350 :
         Aperture size: 50 cm (mirror # 44 )   Optical Magnification=10 x  1 (mirror # 44 )   Slew Rate (ω): 1 degree/second 917.4 mrad/sec) (system # 22 )   Second camera  36 
           1 degree field-of-view   4096×4096 array   4.25 μrads IFOV   
           Optical beam pulse repetition frequency: 300 hz   Angular length of each streak (pulse): 10 pixels (42.5 μrads)   Time length of each pulse: 42.5×10 −6 /17.4×10 −3 =2.4 msec   De-scan time=7.8 msec (assuming two complete pulses in scan time)   Required De-scan FSM  40  angle range=7.8×10 −3 ×17.4×10 −3 ˜136 μrads (output space)       

   While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Also, the steps in the process  100  may be performed in various order. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.