Patent Publication Number: US-11385024-B1

Title: Orthogonal interferometry artillery guidance and navigation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 62/738,059, filed Sep. 28, 2018, the content of which is incorporated by reference herein its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure gee rally relates to accurately guiding projectiles and more particularly to guiding projectiles in GPS-denied or GPS-limited environments using, at least partially, radio frequency (RF)/orthogonal interferometry (OI) techniques. 
     BACKGROUND 
     The dominant approach currently used for guiding a weapon, projectile, UAV, or other similar asset is the global positioning system (GPS). The weapon, projectile, UAV, or the like measures its earth position in latitude, longitude, and altitude, to calculate and execute a trajectory towards a GPS located target. This approach has been in use for many years but is now becoming vulnerable to GPS jamming, both denied and spoofing. Other techniques to extend the GPS approach involve pseudolites, or pseudo-satellites, which are devices that are placed along the path to the target and which utilizes GPS-like transmissions to aid the navigation of the asset. One issue with this approach is the delivery/placement of the pseudolites along the path to the target. The risk to the installer is high given these pseudolites are typically in hostile regions and they are also susceptible to jamming since they are analogous to systems using GPS waveforms. Other pseudolite deployments utilize air platforms, which complicate the engagement logistically. 
     Wherefore it is an object of the present disclosure to overcome the above-mentioned shortcomings and drawbacks associated with conventional projectile guidance systems especially in GPS-denied and GPS-limited environments. 
     SUMMARY 
     It has been recognized that there is a need to replace GPS navigation with a new, jam resistant navigation system for success in today&#39;s tactical environment. One aspect of the present disclosure is a navigation method within a GPS-denied or a GPS-limited environment that utilizes a local domain RF illuminator for weapon, projectile, UAV, or other asset guidance that can be deployed away from the engagement area. In certain embodiments, the RF system provides GPS navigation-like performance, but is resistant to jamming. 
     One innovation on this present disclosure is the employment of the Orthogonal Interferometer (OI) principle to achieve a GPS denied guidance and navigation capability by capturing the object measurements using the RF based OI configuration ( FIG. 2B ) The OI based design outperforms the conventional interferometer (CI) design by leveraging the multiple in and multiple out (MIMO) radar technologies. It uses multiple orthogonal waveforms to achieved enhanced performance over the CI principle while reducing the multi-path effect. The OI sensor technology has been developed, tested via simulation, and carefully analyzed via actual data collection. As a result, its applications to munition guidance and control is considered a great contribution in the context of robustness and simplicity (i.e., eliminating both conventional IMU and GPS receiver). 
     One aspect of the present disclosure is a flight navigation system directing at least one projectile to a target area, comprising: a radio frequency orthogonal interferometry array providing a reference frame, via a projected grid, in the direction of the target area; the reference frame providing azimuth and elevation information for use in guidance of the at least one projectile from a distance from the target to a hand-off point; the flight navigation system deriving location or orientation information using a projectile state estimator and not an inertial measuring unit; and a short range guidance system configured to accept hand-off from the radio frequency orthogonal interferometry array and begin guidance of the at least one projectile from the hand-off point to the target area. 
     One embodiment of the flight navigation system directing at least o projectile to a target area is wherein the radio frequency orthogonal interferometry array is aligned via a north finding device. In some cases, the distance from the target is about 100 km. In certain embodiments, the radio frequency orthogonal interferometry array accuracy is about ±5 m in range and about ±100 m in azimuth and elevation. In some embodiments, the hand-off point is less than about 10 km. 
     Another embodiment of the flight navigation system directing at least one projectile to a target area is wherein a CEP-50 is about 30 m. In some cases, the short range guidance system is a semi-active laser seeker having a hand off error of less than 0.1 degree. In certain embodiments, the short range guidance system is an image automatic target recognition system having a hand-off error of less than 2 m. 
     Yet another embodiment of the flight navigation system directing at least one projectile to a target area is wherein the at least one projectile is two or more projectile and each projectile receives unique target and guidance information. 
     Still yet another embodiment is wherein each of the at least one projectile comprise an RF receiver, an on-board processor, a communication module, and at least one other detector for use in short range guidance. 
     Another aspect of the present disclosure is a munition flight navigation method comprising: initiating a fire command via a fire control system; loading and firing at least one munition; powering up the at least one munition after launch; powering up a radio frequency orthogonal interferometry (RF/OI) array, wherein the radio frequency orthogonal interferometry array provides a reference frame, via a projected grid, in the direction of a target area; receiving target location and waveform mission code information, at the at least one munition; collecting, via a RF detector on the at least one munition, unique waveform data from the radio frequency orthogonal interferometry array; determining via a processor on the at least one munition, azimuth, elevation and range data for the at least one munition via the RF detector, the munition flight navigation system deriving munition location information using a projectile state estimator and not an inertial measuring unit; powering up a short range guidance system located on each of the at least one munition; designating a target via each short range guidance system; calculating target navigation waypoints for respective short range hand-off for each of the at least one munition; handing off guidance from the RF/OI array to a respective short range guidance system for each of the at least one munition; guiding the at least one munition to the target via the respective short range guidance system; and signaling the detonation of the at least one munition. 
     The present disclosure offers a GPS-denied navigation solution without explicitly requiring an onboard Inertial Measurement Unit (IMU) while providing the Guidance and Autopilot the ability to guide and intercept a designated target (of interest.) 
     These aspects of the disclosure are not meant to be exclusive and other features, aspects, and advantages of the present disclosure will be readily apparent to those of ordinary skill in the art when read in conjunction with the following description, appended claims, and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the disclosure will be apparent from the following description of particular embodiments of the disclosure, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the disclosure. 
         FIG. 1A  is a diagram of one embodiment of the system of the present disclosure. 
         FIG. 1B  is a diagram of one embodiment of the system of the present disclosure. 
         FIG. 2A  is a conventional interferometer (CI) according to the principles of the present disclosure. 
         FIG. 2B  is an orthogonal interferometer (OI) according to the principles of the present disclosure. 
         FIG. 2C  shows the reduction in angle error with an OI compared to a CI with equivalent signal-to-noise ratio (SNR) according to the principles of the present disclosure. 
         FIG. 2D  shows a typical product of a real beam pattern and an electrically large interferometric ambiguity according to the principles of the present disclosure. 
         FIG. 2E  shows a zoomed in view of the product of the real beam pattern and electrically large interferometric ambiguity of  FIG. 2D . 
         FIG. 2F  shows the interaction of two lobe spacings whose product yields a substantial reduction in lobe amplitude according to the principles of the present disclosure. 
         FIG. 3  is a flow chart one embodiment of a method of the present disclosure. 
         FIG. 4  is a flow chart of another embodiment of the method of the present disclosure for guiding multiple assets. 
         FIG. 5  is a depiction of the projectile according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment of the system of the present disclosure, an Orthogonal Interferometry (OI) RF illuminator is located behind the weapon system (e.g., at 0 to 100 km) and an RF/OI receiver is mounted on an asset and receives the OI waveforms (distinguishable waveforms referenced to respective phase centers) to determine azimuth and elevation and range information to guide the asset to a target. In some embodiments, the azimuth and elevation information has an accuracy of about 100 to 300 μrads depending on the transmitter configuration. In some cases, the system range information has an accuracy of about +/−20 to 40 meters depending on various system operating parameters. In certain embodiments, the asset is given the target&#39;s location prior launch or via RF communications link after launch within the RF/OI frame of reference. The asset then calculates the trajectory for the target intercept using on-board guidance laws on an on-board processor. 
     The approach to local domain guidance control of the present disclosure allows the user to deploy an RF/OI illumination system anywhere in the world given the portability of the system (e.g., it fits on a small utility trailer), the system&#39;s range &gt;100 km, and the system&#39;s accuracy. This system&#39;s performance is similar to GPS systems, but has the added benefit of inherent Jam resistance due to the use of custom coding of the RF/OI waveform, the illuminator&#39;s signal strength, the deployment geometry, and the antenna configurations. Unlike the GPS navigation waveforms which are published, the RF/OI illumination system would be classified or otherwise held secret. The system operator could select frequency, Pulse Repetition Interval (PRI) and pulse duration. Assuming a 100 nanosecond pulse, frequency hopping with varying PRI could be utilized in a code format loaded just prior to launch or during flight. In addition the rearward looking antenna on the projectile provides receiver isolation from the Jammers forward or below the projectile. The combination waveform control and antenna spatial selectivity provides inherent counter measure immunity. Ground based jammers have the additional burden of being direct line of sight of the RF/OI illuminator, thereby making detecting its presents difficult due to the curvature of the earth. 
     Referring to  FIG. 1A , a diagram of one embodiment of the system of the present disclosure is shown. More specifically, the figure illustrates how the three OI transmitters or illuminators such as from AESA arrays that are employed to communicate with the projectile and provide two angles and range measurements needed to derive an IMUless navigation solution via the Projectile State Estimator (PSE) also sometimes referred to as an orientation state estimator that is able to determine properties of the projectile as detailed herein. The equivalent GPS denied navigation solution has been demonstrated via a high fidelity simulation environment to allow the Guidance and Control subsystems to achieve a projectile target engagement at a miss distance of less than one meter. Here, at least one asset is launched from a launch area  104  and the at least one asset is directed at a target  100  some distance away  118  from the launch area  104 . In some cases, the distance  118  is about 200 km. After launch, the asset (e.g., munition, projectile, air-borne device, etc.) travels along a trajectory  106  toward the target  100 . A circular error probable (CEP-50)  102  is defined as a circular area having a radius that encompasses where 50% of the munitions land. CEP-50  102  is a common measure of accuracy for ballistics. In certain embodiments of the system of the present disclosure, the CEP-50  102  is about 30 m. In some cases, the CEP-50  102  is limited by the performance of the airframe, its limited control authority, the asset&#39;s ability to perform a high G maneuvers, and the like. 
     Still referring to  FIG. 1A , a radio frequency (RF) array  108  is used to track (if equipped with a fire control) and guide the one or more assets to the target(s). In one embodiment, the RF array comprises three active electronically scanned array (AESA) panels, where an AESA is one type of phased array antenna that is a computer-controlled. There, the RF waves may be electronically steered to point in different directions without physically moving the antenna. In some embodiments of the system of the present disclosure, the array panels can also move. In one embodiment, the RF array is compact, with dimensions  110  of about 1.5 m×1.5 m×0.75 m. 
     In some embodiments, the RF array  108  guides (and tracks if equipped with fire control system) the one or more assets along the trajectory  106  with accuracy of about ±5 m range and ±10 m azimuth and elevation  112 . In certain embodiments, the RF array uses orthogonal interferometry (OI) methods to project a reference frame, or a projected grid, in the air, which is analogous to a polar coordinate azimuth and elevation. The polar coordinates can be mapped to a standard grid coordinates utilizing latitude and longitude. In some cases, the RF/OI system  108  produces a reference frame that is aligned using a north finding gyro, or the like, such that the one or more projectiles or assets do not require separate north finding capabilities. This also tempers the need for precise alignment of the assets—center mass aiming—and thus, minimizes operator workload. In certain embodiments, the RF/OI system can provide 10°, 20°, or 30° fields of engagement. In some embodiments, the system provides for adjustable accuracy/guidance precision based, in part, on the RF/OI transmit power, antenna spacing, and deployment angle, where the cross range accuracy is equal to angular resolution times range. 
     Additionally, the system of the present disclosure eliminates the varying energetics and the cross wind impact of each of the one or more projectiles by maintaining the desired trajectory using the RF/OI system array for long range tracking and guiding. This technique reduces the complexity and the cost of the control actuator system (CAS) by simplifying the components needed on the projectiles. In some cases, an RF receiver and RF apertures are present on each round. In some cases, by using the RF/OI system, no azimuth aiming is required and minimal elevation adjustment is needed for each projectile, thus allowing the flight navigation system to make the course corrections accounting for the range differential due to energetics and aiming errors. 
     In one embodiment of the system of the present disclosure, the RF/OI system  108  “hands off” the tracking and guidance of the one or more projectiles at a certain hand-off point  114 . Hand off refers to a transition from the use of the RF/OI guidance to a secondary form for guidance, to increase the accuracy of the projectile. In some cases, the hand-off point  114  is about six km to about ten km from the target along the flight path. In some cases, the hand-off point  114  is located a distance above a plane  116  on which the target is located. In some cases the distance  116  is about two km to about three km above the plane. In some cases, the target is on land. In some other cases the target is on the surface of water. The hand-off can be accomplished as a timed event starting from launch or the hand-off can be event driven. In certain embodiments, an event driven hand-off may be when a short range guidance system (e.g., a semi-active laser or image seeker) detects the target and initiates terminal guidance. In other cases, the hand-off point can be the point where LOS limitation for the RF/OI system effect the accuracy of terminal guidance. 
     The navigation approach of the present disclosure can be adapted for airborne targets, such as UAV but a tracking subsystem on the ground providing target location updates to the airframe would be needed. The fire control system would track the UAV, providing azimuth, elevation and range information in the RF/OI reference frame and then this information would be uplinked to the guided projectile/weapon to complete the guidance loop. The uplink can be accomplished by an EO/IR or RF modality, or the like. 
     In certain embodiments, the hand-off is from the RF/OI array to a semi-active laser (SAL) seeker. In some cases, each of the one or more projectiles has a unique SAL hand-off associated with it. In certain cases, the hand-off error is less than 0.1 degree. In some embodiments of the present disclosure, a SAL seeker is located on the front of the asset and the asset is guided by a designator (laser) coming from a forward observer on the ground, a UAV, or an aircraft. The use of coding technology in the SAL seeker mitigates false locks onto a second target where multiple designators are in the same engagement space or counter countermeasures (CCM) are being employed to defeat the weapons accuracy. In some cases, the SAL seeker is capable of detection at ten km with a one mrad target angle error. In some embodiments, the SAL seeker has a field of view (FOV) ranging from about 40 to about 70 degrees. 
     In other embodiments, the hand-off include projectile state estimation used in the last four to five seconds prior to detonation. In certain embodiments, the RF/OI illuminator provides sufficient guidance control even when guidance is ended at two to three meters above a target. The use of projectile state estimation, as known in the art, can adequately provide the orientation and/or location approximation needed to accurately deploy the projectile. 
     In still other embodiments, the hand-off is to an image-based homing and navigation system. In certain embodiments, a library of images exists for a given target area. The library is available to the one or more assets for guidance purposes and when one or more images in the library are matched to images from the field as view of the asset, once the asset is within a certain distance, the munition may be detonated. In some cases, automatic target recognition (ATR) is used. Generally, image-based methods work best for fixed targets, such as buildings, and the like. Wind and the energetics of launch can affect the trajectory of a munition but the RF/OI system is used, in part, to keep the correct steering for the munition until a short range guidance system takes over. 
       FIG. 1B  depicts a diagram of one embodiment of the system of the present disclosure is shown. More specifically, in this figure the RF/OI system is co-located with the launch point  104  for the one or more projectiles (one flight path is shown  106 ). In some cases, the RF/OI system is located well behind the launch point  104  to provide protection for the RF array. In some cases, the RF/OI system can be located a distance  118  from the target having a known CEP-50  102 . In certain embodiments, the distance  118  is about 100 km and the CEP-50  102  is about 30 m. In contrast, a conventional radar system has range limitations for two-way radar, and may need to be forward deployed, thus placing radar in front of the launch area endangering the equipment by subjecting it to crossfire and or direct targeting by enemy forces. 
     As seen in  FIG. 1B , the RF/OI system produces a RF reference frame  120 . The munition trajectory  106  is located within that reference frame  120 . The reference frame does not require active scanning and thus provides for simplified flight control management. The reference frame also provides for tracking of multiple rounds at the same time by projecting a grid in the air as a reference frame. The hand-off point  114 , e.g., where a SAL seeker takes over the short range tracking and guidance for the one or more projectiles, is also shown. In certain embodiments, RF communication links on each round allows for programming the trajectory during flight for each round, including SAL codes. In some cases, the guidance for the asset begins at the moment of firing or early in the flight trajectory. With the present system, no pre-firing program or precise aiming of the weapon system is needed. Instead, guidance can be handled directly from a mission computer. 
     Still referring to  FIG. 1B , the line of sight (LOS)  122  is limited over the distance  118  due to the curvature of the earth. In one embodiment, the distance above the plane of the target  116  for the LOS  122  is about 800 m. In certain embodiments, the distance above the plane of the target  116  for the base of the RF reference frame is about 1400 m, thus making the hand-off point  114  at which terminal guidance is handled by a SAL seeker, or the like, very important for high accuracy in targeting. In some cases, a magnetometer IMU is also used to supplement the guidance of the one or more projectiles. 
     The LOS prevents the weapon from seeing the RF/OI illuminator below the horizon. In addition, the RF/OI receiver&#39;s wave form is controlled to mitigate multipath due to the earth and influencing the accuracy of the position measurement. Waveforms allow multipath mitigation and allow the receiver to post process the impact of multipath out of the position results. These techniques yield a safe zone of navigation that corresponds to a slant angle of about one degree  126  from a transmitter (e.g., a tank) to the projectile at point  124  from the RF/OI illuminator or a height restriction  116  which is range dependent. 
       FIG. 2A  and  FIG. 2B  compare the path lengths and system components of a conventional interferometer (CI)  200  and an Orthogonal Interferometer (OI)  202  for a notional two dimensional case. For a CI measurement, a transmitter  204  illuminates the target  206  and the phase of the returns at two separate receivers  208   a ,  208   b  provides a differential path length difference (Δφ), shown as  216   a , and  216   b  that leads to a target angle estimate of θ  210 . In the case of OI  202 , two phase centers  212   a ,  212   b  each transmit orthogonal transmissions which are individually decorrelated on respective receptions. The fundamental concept behind the orthogonal interferometer is the use of at least two coherent transmit/receive antennas  212   a ,  212   b  that transmit nearly orthogonal coded waveforms. For example the orthogonal transmission from  212   a  travels to target  206  and returns to both transmit/receive antennas  212   a , and  212   b , this is shown by path  218   b . Additionally an orthogonal transmission from  212   b  travels to target  206  and returns to both  212   a  and  212   b , shown by path  218   b . On reception, the separation of the signals is achieved by decoding against a particular code and exploiting the cross-correlation suppression of the orthogonal coded waveforms. Orthogonal coding in this sense can entail some combination of time, frequency and/or code modulation—as long as the receiver can perform a decorrelation and form an estimate of the received signal keyed to a particular transmit phase center. 
     As depicted the CI case  200  has a common transmit path  214  and distinct receive paths  216   a ,  216   b  while the OI case  202  has distinct transmit and receive paths  218   a ,  218   b  at each receiver  212   a ,  212   b . Decoding OI has achieved a double path length dependency, which provides twice the target angle  210  sensitivity as compared to CI with an equivalent SNR. The phase difference relationship of an interferometry is defined as 
     
       
         
           
             
               
                 Δϕ 
                 = 
                 
                   
                     K 
                     ϕ 
                   
                   ⁢ 
                   
                     D 
                     
                       2 
                       ⁢ 
                       πλ 
                     
                   
                   ⁢ 
                   sin 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     θ 
                     ) 
                   
                 
               
               ; 
               
                 
                   K 
                   ϕ 
                 
                 = 
                 
                   1 
                   ⁢ 
                   
                     ( 
                     CI 
                     ) 
                   
                 
               
             
             , 
             
               2 
               ⁢ 
               
                 ( 
                 OI 
                 ) 
               
             
           
         
       
     
     where D is the interferometer baseline (array phase center separation)  224 , λ is the nominal operating wavelength), and K ϕ  represents the phase gain factor that depends on path length. This expression highlights the physical advantage of a system with an electrically large baseline 
             (     D   λ     )         
in that yields a greater Δϕ for the same target offset θ; the geometric “gain” of the larger interferometric baseline yields a larger Δϕ relative to SNR dependent phase estimation noise σ Δϕ   2  and provides a more precise measurement of θ. In many signal processing applications the localized performance of an estimator can be bounded by the Cramer-Rao Lower Bound (CRLB). This bound on the θ estimation error for a CI radar or an OI radar is:
 
                     σ   θ     CI   ,   OI       :     =     λ       K   ϕ     ⁢   2   ⁢   π   ⁢           ⁢   D   ⁢     √   SNR           ;       K   ϕ     =     1   ⁢     (   CI   )           ,     2   ⁢     (   OI   )             
Note that for the same interferometer baseline (D) and same SNR the OI angle accuracy is a factor of two better than the CI angle accuracy.
 
       FIG. 2C  depicts the reduction in angle error with an OI compared to the CI with equivalent SNR; the OI radar achieves twice the precision (or the effective baseline) as compared to the CI radar.  FIG. 2C  compares CI case, D=50λ  224 , and two OI cases, D=50λ  226 , D=100λ  228  against with the ambitious angular precision goal σ θ =25 μrad,  230 . It should also be noted that with respect to precision, a factor of two improvement in λ/D is worth a factor of four improvement in SNR. 
     This increase in the local precision of the angular estimate of θ due to an increased 
             D   λ         
comes at the cost of an increased chance of an ambiguous θ estimate. Angle ambiguity is a fundamental tradeoff that must be resolved for the potential of this increased estimator precision to have a real world benefit. There are a range of techniques used to suppress interferometer ambiguity. Depending on the particular application a combination of these techniques (discussed briefly herein) can provide effective angle disambiguation.
 
     For interferometer baselines with D&gt;&gt;λ, Δϕ can greatly exceed 2π so the determination of angle-of-arrival using phase difference 
     
       
         
           
             
               sin 
               ⁡ 
               
                 ( 
                 θ 
                 ) 
               
             
             = 
             
               
                 λΔϕ 
                 
                   4 
                   ⁢ 
                   π 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   D 
                 
               
               + 
               
                 2 
                 ⁢ 
                 π 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 N 
               
             
           
         
       
     
     will be ambiguous by N 2π wraps where N is the ambiguity number. 
       FIG. 2D  and  FIG. 2E  depict a typical product of a real beam pattern  232  and an electrically large interferometric ambiguity  234 . Note that there are many closely spaced 
             λ   D         
lobes within the main lobe—all reflecting the same Δϕ (modulo 2π measurement). Two important points should be taken from the “zoom” portion shown in  FIG. 2E : First, σ θ   CI,OI , the angular precision of a local radius of a
 
             λ   D         
lobe  234  trace is much finer than the physical beam pattern. Trying to disambiguate these closely spaced lobes based on a model of the amplitude difference from the main lobe&#39;s much broader response will require very high SNR and a highly consistent signal model that is unlikely to be available in a tactical system.
 
     Still referring to  FIG. 2D , the  236  trace represents a prior probability that would be part of a recursive tracking filter. CRLB is the radius of the local lobe. Trace  232  represents the array beam pattern and trace  234  represents the interferometer lobes. The figure shows large interferometer baselines D=100λ gain precision with increased ambiguity. 
     Another approach to ambiguity mitigation for the OI-tracer application would exploit the high prior information on the projectile trajectory, which provides the opportunity to incorporate accurate kinematic models. In this case, the  236  trace can be interpreted as a prior estimate in a non-linear estimation/tracking formulation where a specific 
             λ   D         
lobe&#39;s probability is updated via a Bayesian recursion and the local covariance is update via a Kalman Filter. A physical example of exploiting prior information would involve an OI radar with
 
               λ   D     =     1   100           
or 1 meter at 100 m range which is still extremely coarse as compared to the “close-in” CEP of the projectile.
 
     For a projectile guidance application, where all the projectiles are cooperative, and there are well timed targets, this approach would be naturally integrated into a tracking filter that can be incorporated the aero-ballistic modeling. A final approach to ambiguity suppression involves multiple measurements at distinct λ/D values forming multiple interferometric baselines. For each available λ/D baseline, the relationship among feasible ambiguity numbers scales (by λ/D) but since the true target angle θ is independent of 
             λ   D         
the unwrapped 0 th  lobe experiences no shift.
 
       FIG. 2E  depicts (for θ=0) the interaction of two lobe spacings whose product yields a substantial reduction in lobe amplitude. Ambiguity can be suppressed by combining different 
             λ   D         
measurements.
 
     
       
         
           
             
               sin 
               ⁡ 
               
                 ( 
                 θ 
                 ) 
               
             
             = 
             
               
                 
                   
                     
                       λ 
                       1 
                     
                     ⁢ 
                     
                       Δϕ 
                       1 
                     
                   
                   
                     4 
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       D 
                       1 
                     
                   
                 
                 + 
                 
                   2 
                   ⁢ 
                   π 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     N 
                     1 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   and 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     sin 
                     ⁡ 
                     
                       ( 
                       θ 
                       ) 
                     
                   
                 
               
               = 
               
                 
                   
                     
                       λ 
                       1 
                     
                     ⁢ 
                     
                       Δϕ 
                       2 
                     
                   
                   
                     4 
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       D 
                       2 
                     
                   
                 
                 + 
                 
                   2 
                   ⁢ 
                   π 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     N 
                     2 
                   
                 
               
             
           
         
       
     
     This lobe-wise product will only admit an θ ambiguity where the two lobe spaces overlap closely; in the combination of 
               λ   D     =     1   125             238  and of
 
               λ   D     =     1   100             240  or the 125/100 case, the first significant overlap  242  occurs at the 5 th  lobe
 
               λ   D     =     1   125           
and the 4 th 
 
               λ   D     =     1   100           
lobe. Hence, there is another ambiguity suppression approach that involves the projectile priors and the interferometer design. In sum, achieving very high precision angle and trajectory estimates via large baseline interferometry incurs the additional complexity of angle ambiguity. Successful mitigation of the ambiguity challenge in an operational system requires substantial integration of the interferometer, array, and aero-ballistic modeling—the details each depending on the particular system configuration under consideration.
 
     In certain embodiments of the system of the present disclosure, the RF system via an orthogonal interferometry (OI) reference frame operates at a frequency of about 5-10 GHz and has a signal-to-noise ratio (SNR) of about 20 dB. In some embodiments, the antenna gain is about 15-20 dB. In some cases, the baseline is about 1.5 m with an angular precision of less than 1 mrad. In some cases, the angular accuracy is about 0.45 mrad. This accuracy is in contrast to conventional radar systems that have angular accuracy of about one to two degrees. Conventional radar systems are also limited by bandwidth. Additionally, radar has cross range accuracy at 100 km of about 2.5 km (1.5° beam width) as compared to a 45 m cross range accuracy for the RF/OI system disclosed herein. At 50 km, the present system has 22 m accuracy, or hand-off error. This accuracy provides for accurate hand-off positioning. The present system provides actual location within GPS norms. In contrast, conventional radar systems produce a beam that is too broad to implement an angle transfer as described herein. 
     In some cases, the power requirement for the system ranges from about 100-200 W. The power needed is much lower than for a conventional radar system (e.g. 100 kW). Additionally, the RF/OI system is preferred due to inherent jamming resistance as compared to radar systems. In some embodiments, the projectiles have rear looking antennas for use with the RF/OI system. In some cases, the RF/OI illuminator can control multiple weapon batteries or UAV against multiple targets. The RF/OI reference frame is analogous to the localized GPS where several weapons platforms, air vehicles, and weapons can use the same RF/OI reference frame for navigation. In some cases, the pulse width for the system of the present disclosure is about 1.7 μsec which encapsulates the RF/OI waveform. 
     The RF/OI reference frame is analogous to GPS, the spatial extent of the illuminator provides elevation and azimuth bearings from the OI illuminator Like GPS, knowing where the satellites are in GPS constellation you can determine your position with the RF/OI illuminator; knowing it&#39;s latitude and longitude position and the zero phase line direction relative to true north. In the multiple weapons scenario, the weapons can be preprogrammed the information prior to launch and then guided using the RF/OI as a reference. This information can also be programmed after launch via a communications link. 
     Referring to  FIG. 3 , a flow chart  300  of some of the functional elements for one embodiment of the method of the present disclosure is shown. More specifically, in this embodiment, a fire control system initiates a fire command for a single asset  302 . In this case, the asset is an artillery round (AR) or other munition. The AR is then loaded and fired  304 . The AR is powered up after launch  306 . In some embodiments, the AR has a rear-facing RF detector. In some cases, the AR has a communications module for receiving and/or transmitting information to a fire control system. In certain embodiments, the AR has an on-board processor, memory, and/or additional detectors for use in guidance of the AR to a target. In some cases, the RF/OI illuminator powers up after the launch of the AR  308 . The AR receives target information and waveform mission code from an RF communication link, or the like  310 . The RF detector on the AR collects the RF/OI waveform data from the RF/OI illuminator and determines the azimuth, elevation, and range from the asset to the target  312 . The AR calculates target navigation waypoints  314  as it navigates to the target  316 . 
     Referring to  FIG. 4 , a flow chart  400  of some of the functional elements for one embodiment of the method of the present disclosure is shown. More specifically, in this embodiment, a fire control system initiates a fire command for multiple assets  402 . In this case, the assets are artillery rounds (ARs) or other munitions. The ARs are then loaded and fired  404 . The ARs are powered up after launch  406 . In some embodiments, each AR has a rear-facing RF detector. In some cases, each AR has a communications module for receiving and/or transmitting information to a fire control system. In certain embodiments, each AR has an on-board processor, memory, and/or additional detectors for use in guidance of the AR to a target. In some cases, the RF/OI illuminator powers up after the launch of the ARs  408 . 
     In some embodiments, multiple rounds are coordinated in one RF/OI reference frame. In some cases a full battery of Howitzers, or the like, are used and each round has customized trajectories for the particular target type or for masking the round&#39;s location. In some cases this is limited to the weapons control authority and its ability to fly an azimuth arc from the point of launch thereby disguising it&#39;s launch point from enemy counter fire radar. 
     In certain embodiments, the RF/OI reference frame is extended to about 100 km and provides location to within about 100 m. In some cases the reference frame is extended to about 50 km and provides location to within about 50 m. The system utilizes one way illumination with rear-looking antennas on the projectiles. The system of the present disclosure provides for RF jam hardening. 
     In some cases the round may be programmable during the initial flight path, which can reduce the time to fire. By equipping the RF/OI reference frame with a high quality north seeker, the system allows for “on the go” alignment for all of the rounds. No azimuth aiming is required with the RF/OI reference frame, and only minimal elevation adjustment is needed to account for a range differential. The RF/OI can be designed to cover various fields of engagement. In some cases, the field of engagement may be 10, 20 or 30 degrees. 
     Still referring to  FIG. 4 , each of the multiple rounds receives target information and unique waveform mission codes from an RF communication link, or the like  410 . An RF detector on each AR collects the RF/OI waveform data from the RF/OI illuminator and determines the azimuth, elevation, and range from each asset to the target  412 . The RF/OI method requires only minimal electronics costs embedded into each round, such as an RF receiver and RF apertures. In certain cases, the system hands off the guidance for the multiple rounds at about 6-10 km from the target to a short range guidance system. Each short range guidance system powers up 414 and each short range guidance system designates a target  416 . Each AR calculates target navigation waypoints along the flight path for use in a respective short range handoff  418 . Each AR hands off AR guidance from the RF/OI system to a respective short range guidance system  420 . Each short range guidance system guides a respective AR to a target  422 . Detonation of the AR can be signaled  424 . In some cases, the detonation is signaled by a fire control system. In some cases, detonation is signaled by the short range guidance system at a certain distance. In some cases, detonation is signaled by the short range guidance system at a certain time point to maximize war head effectiveness. 
     In some cases, the RF/OI illuminator&#39;s guidance of a munition is handed off to a SAL seeker, or the like. There, the round is equipped with laser detection ROIC or the like. In some cases, the short range guidance system is small, e.g., about 1 in 3 , including the optics. In one embodiment, the SAL seeker is capable of detection at 10 km with 1 mrad target angle error with a FOV ranging from 40 to 70 degrees. In certain embodiments, a SAL seeker located on the front of the asset is guided by a designator (laser) coming from a forward observer on the ground, a UAV, or an aircraft. In some cases, the short range guidance system utilizes full counter countermeasure (CCM) filtering with spatial and temporal filtering and/or full pulse repetition frequency (PRF) and pulse interval modulation (PIM) decoding with multiple designators within the FOV. 
     In some cases, the RF/OI illuminator&#39;s guidance of a munition is handed off to an image based homing and navigation system. In certain embodiments, the short range guidance system utilizes image ATR (automatic target recognition). ATR is generally better suited for fixed targets, including, but not limited to buildings or structures. A series of images are stored in a database and either loaded onto a round or accessible by a round. The images are for areas of interest and/or for particular types of assets. When a round is within a certain range, the round can “recognize” the target form the images stored in the library. In some cases the library of images comprises items viewed at a distance of 1000 to 3000 meters. The processed imagery for stationary targets would need to factor in the attack angle for both elevation and azimuth direction of flight relative to the target; i.e. building or fortification. Moving targets (missile launcher) would need to also consider various poses; missiles in firing stowage or launch position would represent two exemplary different images or poses. 
     In some cases, the RF/OI illuminator&#39;s guidance of a munition is handed off to an IMU during the final four to five seconds, or the terminal phase. The IMU would be the primary means of navigating to the target in the last few seconds when the RF/OI illuminator in hindered by the LOS of the system. In some cases, this method is utilized in a grid pattern for area bombing. 
     Referring to  FIG. 5 , a perspective view of the projectile  500  is shown that employs the RF/OI processing for navigation and guidance to the target. The projectile  500  can be a missile, rocket, artillery round or similar guided munition. The projectile has a front portion  505  that that typically houses the warhead and fuze elements such that the fuze detonates the warhead at the appropriate point for the desired result. On the rear or tail portion of the projectile  510  is an optional rocket engine that can be deployed to provide thrust to extend the range of the projectile. In one example, the projectile is launched without a rocket engine such as from a launch platform that achieves a certain altitude and is guided to the target. Examples of launch platforms include anti-tank guns, mortars, howitzer, field guns and railguns. The projectiles from the launch platforms may or may not have a rocket engine. 
     Referring again to  FIG. 5 , the midsection tends to house the electronics, communications, and guidance/navigation systems. A rear facing antenna  525  is typically use to obtain the RF/OI waveforms for the reference frame that enable determination of the azimuth and elevation with respect to the illuminator. In one example, the processing involving firmware/software is performed on one or more processors that execute software residing on memory that is coupled to the processors. While labels are placed on certain items for descriptive purposes, the processing may be all done on a single circuit card. In this example an RF receiver  530  is coupled to the antenna  525 . The RF receiver  530  has a downconversion stage to process the analog inputs from the antenna and may include mixer(s), filter(s) and low noise amplifier(s) to process the analog signals. The downconverted signals are input to an analog-to-digital converter (ADC) to provide digital information that is then processed by one or more processing units such as in a digital signal processor. 
     A short range guidance section  540  is used when the projectile reaches a hand-off point near the terminal end of the trajectory near the target area. The short range guidance section  540  in one example is a SAL seeker that receives a signal such as reflected laser signal from the target. Another example is an imaging section that uses a camera to view the target area and compares the captured image to stored images to identify the target and guide the projectile to the target. The processing includes having a state estimator to provide orientation information without relying upon GPS or an IMU. In one example, after the RF/OI communications is no longer available, the projected waypoints provide estimated target locations. By using the short range guidance systems such as imaging electronics, the orientation can be determined and used to continue along the trajectory with the proper orientation. The short range guidance systems may also enable highly accurate target location information to help guide the projectile and maintain orientation. 
     It will be appreciated from the above that portions of the invention may be implemented as computer software, which may be supplied on a storage medium or via a transmission medium. It is to be further understood that, because some of the constituent system components and method steps depicted in the accompanying Figures can be implemented in software, the actual connections between the systems components (or the process steps) may differ depending upon the manner in which the present invention is programmed. Given the teachings of the present system provided herein, one of ordinary skill in the related art will be able to contemplate these and similar implementations or configurations of the present invention. 
     It is to be understood that the present invention can be implemented in various forms of hardware, software, firmware, special purpose processes, or a combination thereof. In one embodiment, the present invention can be implemented in software as an application program tangible embodied on a computer readable program storage device. The application program can be uploaded to, and executed by, a machine comprising any suitable architecture. The computer readable medium as described herein can be a data storage device, or unit such as a magnetic disk, magneto-optical disk, an optical disk, or a flash drive. Further, it will be appreciated that the term “memory” herein is intended to include various types of suitable data storage media, whether permanent or temporary, such as transitory electronic memories, non-transitory computer-readable medium and/or computer-writable medium. 
     While various embodiments of the present invention have been described in detail, it is apparent that various modifications and alterations of those embodiments will occur to and be readily apparent to those skilled in the art. However, it is to be expressly understood that such modifications and alterations are within the scope and spirit of the present invention, as set forth in the appended claims. Further, the invention(s) described herein is capable of other embodiments and of being practiced or of being carried out in various other related ways. In addition, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items while only the terms “consisting of” and “consisting only of” are to be construed in a limitative sense. 
     The foregoing description of the embodiments of the present disclosure has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the present disclosure to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the present disclosure be limited not by this detailed description, but rather by the claims appended hereto. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the scope of the disclosure. Although operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. 
     While the principles of the disclosure have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the disclosure. Other embodiments are contemplated within the scope of the present disclosure in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present disclosure.