Abstract:
A sensor system uses ground emitters to illuminate a projectile in flight with a polarized RF beam. By monitoring the polarization modulation of RF signals received from antenna elements mounted on the projectile, both angular orientation and angular rate signals can be derived and used in the inertial solution in place of the gyroscope. Depending on the spacing and positional accuracies of the RF ground emitters, position information of the projectile may also be derived, which eliminates the need for accelerometers. When RF signals of ground emitter/s are blocked from the guided projectile, the sensor deploys another plurality of RF antennas mounted on the projectile nose to determine position and velocity vectors and orientation of incoming targets.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to a guidance system for a projectile launched by firing the projectile from a gun. This invention relates particularly to a sensor system for measuring position and velocity vectors and orientation of a guided gun-launched projectile. 
     During launch, gun-fired munitions are subjected to extremely high setback accelerations. Here, the term “gun-fired munitions” is also intended to include mortar shells. A gun-fired munition receives all of its kinetic energy during its launch phase. Peak launch accelerations occur during the first 3 to 4 milliseconds from initial movement of the projectile, at which time a projectile has typically moved only a few feet. Shortly after gun barrel exit, the projectile has stopped accelerating and experiences a set-forward acceleration typically about ten percent of the peak set-back acceleration level. Once the round has exited the barrel, no more energy is available to the round during the remainder of the flight, except that for guidance and control actions and when boosters are used to extend the range of the round. 
     The prior art uses accelerometers and gyroscopes to determine the position and orientation of the round during the flight for guidance and control purposes to ensure precision target acquisition. In the gun-fired munitions applications, these inertia-based accelerometers and gyroscopes are required to withstand extreme harsh launch environments (up to 120,000 g acceleration), yet be sensitive enough to yield the required position and orientation precision up to the target area. 
     There are two primary challenges with inertial devices currently used as guidance sensors in gun fired munitions for closed loop feedback control. The first challenge of current inertial technologies is gun survivability of devices that have the needed sensitivity for flight measurements. Prior inertial devices are not able to survive when the full-scale dynamic range exceeds 5% of the maximum force experienced during the launch. For guidance applications in gun fired munitions, it is required that the full scale dynamic range during flight be in the order of 0.2% of the maximum force experienced during launch. This challenge is very specific to the environment of a gun-fired munition. The second challenge devices constructed using prior inertial technologies have long settling times that are on the order of a few milliseconds. These limitations of the prior art inertial technologies significantly affect their use as guidance sensors for gun fired munitions. 
     Precise end game targeting also requires extremely fast activation of the inertia sensor after the initial setback. At an approximate exit velocity of 3000 m/s it is necessary to ensure that the inertia sensors react extremely quickly to avoid badly missing a target. Less than one quarter of a millisecond settling time would significantly advance current inertia sensors for gun fired munition systems—an improvement of more than an order of magnitude that would improve target acquisition by a similar amount. 
     SUMMARY OF THE INVENTION 
     The present invention provides means for overcoming the above two challenges and for increasing the sensor sensitivity, even if these devices need to survive extreme harsh environments such as those found during the launch of gun-fired munitions. 
     The present invention eliminates the prior art requirement for gyroscopes and GPS devices by illuminating the projectile in flight with a polarized RF beam. By monitoring the polarization modulation of RF signals received from antenna elements mounted on the projectile, both angular orientation and angular rate signals can be derived and used in the inertial solution in place of the gyroscope. Depending on the spacing and positional accuracies of the RF ground emitters, position information of the projectile may also be derived, which eliminates the need for accelerometers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a flight profile for a projectile after being fired from a launcher toward a target; 
         FIG. 2A  is a perspective view of an incoming munition which represents a typical target; 
         FIG. 2B  is a perspective view of a guided projectile/munition; 
         FIG. 3  illustrates four antenna elements embedded into the nose of a guided munition; 
         FIG. 4  illustrates a monopulse configuration for measuring direction of arrival of an incoming munition; 
         FIG. 5  is a block diagram showing a monopulse method for determining angular deviation of an incoming munition; 
         FIG. 6  graphically illustrates position vectors of a plurality of antennas embedded into the fins of a guided munition; 
         FIG. 7  coordinates of antennas mounted into the fines of a guided munition; 
         FIG. 8  graphically illustrates position vectors of a guided munition with respect to each of three RF emitters; 
         FIG. 9  illustrates a configuration of s single RF polarized sensor; and 
         FIG. 10  schematically illustrates a neural network that may be used for determining angular rotations from measured RF polarized data. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an example of a flight profile for a projectile  10  after being fired from a launcher  12  toward a target  14 . The word “projectile” is also used in this disclosure to represent a guided munition. The word “target” is also used to represent an incoming munition. A typical range of about 50 Km is assumed. Immediately after being launched the projectile  10  is in an ascent phase (phase  1 ) wherein guidance is accomplished using RF triangulation and polarization sensing. After reaching a maximum height, the projectile  10  enters a descent phase (phase  2 ) wherein projectile guidance is accomplished using sensed inertial acceleration and RF polarization sensing. The descent velocity is typically about 350 m/s for descent duration of about 100 sec. It should be noted that the projectile  10  contains no gyroscopic rotation sensors. 
     An incoming munition  15  has an axis of symmetry ( FIG. 2A ). Its backscattered signals are independent on the symmetry angle φ and depend only on the incident angle α. This facilitates inferring the relationship relating the incident angle α and the backscattered polarized signals (direct scattering) and hence extracting values of the incident angle from the backscattered signals (inverse scattering). Fortunately, values of the incident angle α determine the orientation of the incoming munition  15  (target) relative to the guided munition (source). 
       FIG. 2   b  shows a guided projectile  16  that typically has an elongated shape. The projectile  16  may have a plurality of canards  32 - 35  attached thereto near a nose  26  and may have a plurality of fins  36 - 39  attached near a tail section  42 . 
       FIG. 3  shows a sensor  19  that includes four antenna elements  20 - 23  that are embedded in a nose section  26  of a guided munition  16 . These antenna elements  20 - 23  are used to illuminate incoming targets when the RF emitter&#39;s signals are blocked from the guided projectile (seeker mode). 
     Referring again to  FIG. 3 , the antenna elements  20 - 23  are preferably aligned along two perpendicular axes (x, y) located in a plane normal to the lengthwise axis of the munition  16 . The fields of each of the antenna elements  20 - 23  are set parallel to the axis where the antenna element is located. The antenna elements  20 - 23  are preferably equally spaced from the munition axis. The main beam of the sensor is aligned along the munition axis. The antenna elements  20  and  22  aligned on the x-axis are chosen to be active such that they both transmit and receive RF signals. The other two antenna elements  21  and  23  aligned on the y-axis are chosen to be passive such that they only receive. 
     Each two elements located on the same axis have the same orientation and the same phase center to ensure the equality of both magnitude, and direction of electric fields transmitted or received by each one of the two antenna elements. Taking only two antenna elements  20  and  22  to be active and the other two antenna elements  21  and  23  to be passive minimizes the coupling (interference) between the antenna elements and reduces power consumption. Having magnitudes and phases of both like and cross-polarized backscattered signals enables the extraction of the target (incoming munition) shape from both signals. 
     The like polarized field E y  is the sum of the fields measured by the antenna elements  20  and  22 . The cross-polarized field E x  is the sum of the fields measured by the antenna elements  21  and  23 . The sensor  19  also measures the phase difference Φ of like and cross-polarized fields. 
     Knowing the relationship between magnitude and phase difference of like and cross-polarized backscattered fields and the incident angle α and the distance L provides the capability of obtaining the orientation of the incoming munition. This relationship, which is known as inverse scattering, can be inferred through either an analytic approach or a neural network approach. 
     The analytic approach is based on:
         Analysis of the database to infer the dependence of backscattered signals on the incident angle α and the distance L between the scattering center and the center of gravity for the incoming munition;   Capturing the dependence into mathematical formulae; and   Implementing the formulae into the payload of the sensor for future use.       

     The neural network approach is based on:
         Using the database of in training a neural network for extracting the incident angle α and the distance L from the data; and   Implementing the trained network into the payload of the sensor for future use.       

     The operating frequency of a radio frequency polarized (RFP) sensor is determined by considering several factors such as size of the incoming munition, weather condition, weight requirements of the guided munition, etc. At lower frequencies (longer wavelength) different parts of an incoming munition contribute to the backscattered polarized signals. However, at lower frequencies the RFP sensor requires antennas with larger sizes and apertures, which may not fulfill the requirements of the guided munition. 
     At higher frequencies (shorter wavelengths) antennas with smaller sizes can be used. In this case backscattered signals stem from both munition parts perpendicular to the direction of the transmitted signals, and irregularities such as fins, canards, etc. ( FIG. 2 ) located on the munition surface. Furthermore, the sensor signals are more attenuated by rain and ice particles, especially at millimeter wave frequencies. 
     The RFP sensor frequency typically is chosen to be around the X and Ku bands (8-18 GHz). These frequencies provide the following advantages:
         Mitigating impacts of weather conditions such as rain, hail, snow etc.;   Minimizing contributions of small irregularities on incoming munitions surfaces to the sensor response; and   Avoiding the atmospheric absorption bands due to both oxygen (centered at 60 and 118 GHz) and water vapor absorption (centered at 22.235, 183 and 323).       

     In either case a database of backscattered polarized signals will be created and analyzed. The database is created at different configurations for both sensor and incoming munition. Creating the database is known as a direct scattering or forward scattering. In analyzing the data polarization transformation from the local frame of the incoming munition (target) to the local frame of either the guided munition or a ground station is required. A polarization transformation can be employed. 
     Referring to  FIG. 4 , the incoming munition  15  has two directions of arrival with respect to the boresight (main beam) of an RFP sensor  44  located at the origin of a coordinate system (X,Y,Z). These directions are the horizontal angle of arrival β 1 , and vertical angle of arrival β 2  as shown in  FIG. 4 . A monopulse radar technique ( FIG. 5 ) can be applied to extract the horizontal angle of arrival from the difference and sum of the voltages measured by the two antenna elements  20  and  22 ; and to extract the vertical angle of arrival from the voltage measured by the other two antenna elements  21  and  23 . Signals from the munition  15  are input to a summing circuit  46  and to a differencing circuit  48 . The output Δ of the differencing circuit  48  and the output Σ of the summing circuit  46  are input to a divider  50  that calculates the quotient 
             Δ   ∑         
to produce a monopulse ratio. A multiplier  52  multiplies the monopulse ratio by a sensor slope factor to determine the angular deviation of the munition  16  from the RFP sensor&#39;s line of sight (LOS), and hence the axis of the guided munition  15 .
 
     To illustrate how the monopulse technique can be applied to obtain the horizontal direction of arrival let the voltage measured by the antenna element  21  to be v a  and the voltage measured by the antenna element  22  to be v c . The monopulse ratio R ac  associated with the two antenna elements  20  and  22  is constructed as 
                     R   ac     =           v   a     -     v     c   ⁢                     v   a     +     v   c         .             (   1   )               
Getting values of the monopulse ratio using Equation (1) gives the capability to extract values of the azimuth direction of arrival β 1  such that
 
β 1   =κR   ac   (2)
 
In the above κ is a constant slope factor that depends on the antenna element geometry and the operating frequency.
 
     The vertical direction of arrival is similarly obtained from the monopulse ratio between the difference and sum of the responses of the antenna elements  21  and  23  such that 
     
       
         
           
             
               
                 
                   
                     
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     The linear velocity vector of an incoming target may be extracted as follows:
         Measuring the Doppler frequency shift in the response of each antenna element  20 - 23 ;   Getting the component of the linear velocity vector along the direction between each of the antenna elements and the incoming munition, which leads to three algebraic equations in the perpendicular components for the velocity vector;   Solving the equations to obtain the magnitudes of the linear velocity components; and   Calculating the magnitude and direction of the velocity vector from the velocity components.       

     The target velocities associated with each Doppler frequency shift are calculated and designated as u a , u b , u c  and u d . Also let the linear target velocity vector be Ū(U x , U y , U z ). In addition, the unit vectors along the directions connecting the antenna elements  20 - 23  and the scattering center of the incoming munition expressed as:
 
 â=a   x   {circumflex over (x)}+a   y   ŷ+a   z   {circumflex over (z)}   (6)=
 
 {circumflex over (b)}=b   x   {circumflex over (x)}+b   y   ŷ+b   z   {circumflex over (z)}   (7)
 
 ĉ=c   x   {circumflex over (x)}+c   y   ŷ+c   z   {circumflex over (z)}   (8)
 
 {circumflex over (d)}=d   x   {circumflex over (x)}+d   y   ŷ+d   z   {circumflex over (z)}   (9)
 
     Taking the scalar vector products of Equations (6)-(9) with the velocity vector Ū yields the following set of four algebraic equations.
 
 u   a   =a   x   U   x   +a   y   U   y   +a   z   U   z   (10)
 
 u   b   =b   x   U   x   +b   y   U   y   +b   z   U   z   (11)
 
 u   c   =c   x   U   x   +c   y   U   y   +c   z   U   z   (12)
 
 u   d   =d   x   U   x   +d   y   U   y   +d   z   U   z   (13)
 
     Three Equations (10)-(13) can be solved to obtain the components of target velocity vector Ū. The fourth equation should be ignored because it is dependent on the other three equations. 
     In the presence of plural incoming munitions, a monopulse technique can be used. The technique has the capability of detecting the presence of two targets within a scan of the RFP sensor  44 , and extracting direction of arrivals of each munition. The technique can be generalized under certain conditions to detect the presence of more than two incoming munitions and extracting direction of arrival of each munition. 
       FIGS. 6 and 7  illustrate the basic concept of the invention using a single RF emitter  54  located at coordinates (X 0 ,Y 0 ,Z 3 ) in an emitter reference frame (X,Y,Z). The axis of symmetry of the guided munition  15  is located at (x t ,y t ,z t ) in the emitter reference frame (X,Y,Z). The vector from (X 0 ,Y 0 ,Z 0 ) to (x t ,y t ,z t ) is the target munition position vector. The guided munition local reference frame is (X′,Y′,Z′). The Z′ axis is aligned through the axis of symmetry of the guided munition  15 . The X′ and Y′ axes are in a plane perpendicular to the axis of symmetry. 
     The orientation angles (roll, yaw, pitch) relate the local frame of the target munition to the emitter reference frame. Three RF antennas a, b and c are located a distance r from the center of symmetry of the target munition. As shown in  FIG. 6 , the antennas are positioned at distances R a , R b  and R c , respectively, from the RF emitter  54 . Referring to  FIG. 7 , in the local frame of the guided munition the components of the position vector of the antenna a are 
     (x′ a ,y′ a ,z′ a )
 
 x′   a   =x′   t   +r  cos α a   (14)
 
 y′   a   =y′   t   +r  sin α a   (15)
 
 z′   a   =z′   t   (16)
 
The positions of the other two antenna b and c can be similarly expressed in the local frame of the guided munition.
 
     Applying triangulation using the three antennas yields the position vector (x′ t ,y′ t ,z′ t ) of the target munition: 
                       X   0   ′     -     x   t   ′       =       1   Δ     ⁡     [         (       sin   ⁢           ⁢     α   c       -     sin   ⁢           ⁢     α   b         )     ⁢     (       R   a   2     -     R   b   2       )       +       (       sin   ⁢           ⁢     α   c       -     sin   ⁢           ⁢     α   b         )     ⁢     (       R   b   2     -     R   c   2       )         ]               (   17   )                   Y   0   ′     -     y   t   ′       =       1   Δ     ⁡     [         (       cos   ⁢           ⁢     α   c       -     cos   ⁢           ⁢     α   b         )     ⁢     (       R   a   2     -     R   b   2       )       +       (       cos   ⁢           ⁢     α   c       -     cos   ⁢           ⁢     α   b         )     ⁢     (       R   b   2     -     R   c   2       )         ]               (   18   )                     (       Z   0   ′     -     z   t   ′       )     2     =       -     [         (       X   0   ′     -     x   t   ′       )     2     +       (       Y   0   ′     -     y   t   ′       )     2     +     r   2       ]       +         2   ⁢   r       1   -   λ       ⁢     {         (       X   0   ′     -     x   t   ′       )     ⁢     (       cos   ⁢           ⁢     α   a       -     λ   ⁢           ⁢   cos   ⁢           ⁢     α   b         )       +       (       Y   0   ′     -     y   t   ′       )     ⁢     (       sin   ⁢           ⁢     α   a       -     λ   ⁢           ⁢   sin   ⁢           ⁢     α   b         )         }           ⁢     
     ⁢           ⁢   where           (   19   )                 Δ   =     2   ⁢     r   ⁡     [       (       cos   ⁢           ⁢     α   b       -     cos   ⁢           ⁢     α   a         )     ⁢       (       sin   ⁢           ⁢     α   c       -     sin   ⁢           ⁢     α   b         )     ++     ⁢     (       cos   ⁢           ⁢     α   c       -     cos   ⁢           ⁢     α   b         )     ⁢     (       sin   ⁢           ⁢     α   b       -     sin   ⁢           ⁢     α   a         )       ]           ⁢     
     ⁢           ⁢   and           (   20   )                       ⁢     λ   =         (       R   a       R   b       )     2     .               (   20   )               
Transferring the position vector (x′ t ,y′ t ,z′ t ) to the reference frame of the emitter  54  gives the required target munition position vector (x t ,y t ,z t ).
 
     The guided munition  16  has a velocity vector with the unknown velocity components V x , V y  and V z  along the X, Y and Z axes of the emitter reference frame; and the antennas a, b and c measure the Doppler velocities V a , V b  and V c  respectively. The Doppler velocities V a , V b  and V c  are related to the velocity vector components V x , V y  and V z  through the following identities: 
     
       
         
           
             
               
                 
                   
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       FIG. 8  illustrates a system  56  that uses three emitters  60 - 62  with three different frequencies for measuring position and velocity vectors of a guided projectile. The emitters  60 - 62  are located at coordinates (X 1 ,Y 1 ,Z 1 ), (X 2 ,Y 2 ,Z 2 ) and (X 3 ,Y 3 ,Z 3 ), respectively. The system  56  records signal travel time between each of the emitters  60 - 62  and an RF antenna embedded in the projectile. A triangulation process is used to measure distances between each of the emitters  60 - 62  and the RF antenna to obtain the position vector of the projectile. The system  56  uses the measured Doppler frequency associated with each emitter and the derived position vector to obtain the velocity of the projectile. The signals from the three emitters  60 - 62  may also be used to obtain angular rotation rates. Each emitter signal has its own direction and polarization, which provide additional information for extracting angular rates. 
     The position vector components (x t ,y t ,z t ) of the projectile in terms of components of the position vectors R i &#39;s (i=1, 2, 3) and the distance between the projectile and each of the emitters R it &#39;s (i=1, 2, 3) may be written as the following matrix equation: 
     
       
         
           
             
               
                 
                   
                     
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                   24 
                   ) 
                 
               
             
           
         
       
     
     The matrix equation governing the components of the velocity vector (v x ,v y ,v z ) is written in terms of the position vector components (x t ,y t ,z t ), the distances (R 1t ,R 2t ,R 3t ) between the emitters and the three velocity components (v 1d ,v 2d ,v 3d ): 
     
       
         
           
             
               
                 
                   
                     
                       [ 
                       
                         
                           
                             
                               ( 
                               
                                 
                                   x 
                                   t 
                                 
                                 - 
                                 
                                   X 
                                   1 
                                 
                               
                               ) 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   y 
                                   t 
                                 
                                 - 
                                 
                                   Y 
                                   1 
                                 
                               
                               ) 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   z 
                                   t 
                                 
                                 - 
                                 
                                   Z 
                                   1 
                                 
                               
                               ) 
                             
                           
                         
                         
                           
                             
                               ( 
                               
                                 
                                   x 
                                   t 
                                 
                                 - 
                                 
                                   X 
                                   2 
                                 
                               
                               ) 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   y 
                                   t 
                                 
                                 - 
                                 
                                   Y 
                                   2 
                                 
                               
                               ) 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   z 
                                   t 
                                 
                                 - 
                                 
                                   Z 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                         
                           
                             
                               ( 
                               
                                 
                                   x 
                                   t 
                                 
                                 - 
                                 
                                   X 
                                   3 
                                 
                               
                               ) 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   y 
                                   t 
                                 
                                 - 
                                 
                                   Y 
                                   3 
                                 
                               
                               ) 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   z 
                                   t 
                                 
                                 - 
                                 
                                   Z 
                                   3 
                                 
                               
                               ) 
                             
                           
                         
                       
                       ] 
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           
                             
                               v 
                               x 
                             
                           
                         
                         
                           
                             
                               v 
                               y 
                             
                           
                         
                         
                           
                             
                               v 
                               z 
                             
                           
                         
                       
                       ] 
                     
                   
                   = 
                   
                     [ 
                     
                       
                         
                           
                             
                               R 
                               1 
                             
                             ⁢ 
                             
                               v 
                               
                                 1 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 d 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                               R 
                               2 
                             
                             ⁢ 
                             
                               v 
                               
                                 2 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 d 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                               R 
                               3 
                             
                             ⁢ 
                             
                               v 
                               
                                 3 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 d 
                               
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   25 
                   ) 
                 
               
             
           
         
       
     
       FIG. 9  illustrates a basic configuration for a single RF polarized sensor system  70  for measuring orientation angles of the guided projectile and their change rates (angular velocities). The sensor system  70  includes an active sensor  72  used as an illuminator/emitter and a passive sensor  74  that is understood to be embedded on the fins of a guided munition. For simplicity, two similar flared feed horn antennas are used as RF polarized sensors for both the active and passive sensors  72  and  74 , respectively. The direction of the active illuminator antenna  72  is kept fixed. The direction of the passive munition antenna  74  is varied by varying its roll, yaw and pitch angles. The RF power received by the munition passive sensor antenna  74  is determined as a function of its angular rotations. 
     Referring to  FIG. 10 , a neural network  80  may be used to extract values of angular rotation from measured RF polarized power. The neural network  80  includes an input layer  82  that receives measured RF polarized data output from a plurality of RF sensors. A plurality of hidden layers  84  contain neurons that calculate signals that are output at an output layer  86 . Extracting angular rotations from measured RF polarization data is based on two steps. The first step is to train the neural network  80  on extracting angular rotation by using simulated or real data. The second step is to employ the trained network in extracting angular rotation from other measured RF polarized data. In training the neural network RF polarized data is obtained at different scenarios for sensors and munition configurations and for variations in the surrounding environment. 
     The structures and methods disclosed herein illustrate the principles of the present invention. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as exemplary and illustrative rather than restrictive. Therefore, the appended claims rather than the foregoing description define the scope of the invention. All modifications to the embodiments described herein that come within the meaning and range of equivalence of the claims are embraced within the scope of the invention.