Patent Publication Number: US-2022234756-A1

Title: System and method for net-capture of unmanned aerial vehicle

Description:
FIELD OF DISCLOSURE 
     The disclosure relates to a system and method for capturing an unmanned aerial vehicle. 
     DESCRIPTION OF THE RELATED ART 
     Various applications may require capturing of fixed-wing unmanned aerial vehicles (UAVs). For example, testing fixed-wing UAVs may include capturing the UAVs without damaging the UAV due to the cost of expending UAVs and to meet predetermined time tables for testing the UAVs. Some UAVs may also carry payloads that are not expendable or expensive to replace. Prior attempts to capture fixed-wing UAVs include using global positioning systems (GPS) and operator remote control. However, operator remote control may be ineffective for autonomous missions of a UAV or when the UAV is out of a communication range of the operator. For example, the UAV may travel into a GPS-denied environment in which GPS of the UAV may not be used to control the UAV. 
     SUMMARY OF DISCLOSURE 
     The present application provides a system and method for capturing a fixed-wing unmanned aerial vehicle (UAV) that uses an infrared emitter arrangement. The infrared emitter arrangement includes infrared emitters that are arranged around a net. The UAV includes an infrared sensor that is configured to detect the infrared emitter arrangement to initiate the terminal flight of the UAV in which the UAV travels toward the net for capture. When the infrared emitter arrangement is detected, the UAV enters a guidance mode in which corrections are made to the UAV to adjust at least one of an elevation or an azimuth to ensure the UAV flies into the net. The corrections are made using algorithms that are stored in the UAV processor and configured to be executed based on the detected infrared emitter arrangement in a field-of-view of the infrared sensor. 
     The corrections to the UAV may include changing the heading of the UAV depending on the infrared emitters of the infrared emitter arrangement that are seen in the field-of-view of the sensor. The algorithms used may be in a roll-corrected frame of reference such that corrections in elevation and azimuth may command the UAV to make corrections in yaw, pitch, and roll. If only some of the infrared emitters are seen in the field-of-view at a point during the terminal flight, the UAV may receive a correction until all of the infrared emitters are seen in the field-of-view. The algorithms for adjusting the elevation or azimuth of the UAV are stored in guidance and signal processing logic of the UAV processor. The system is advantageous in that the UAV is adjusted to capture the entire infrared emitter arrangement within the field-of-view of the infrared sensor to ensure that the UAV is captured in the net. 
     The infrared emitters may be light sources or thermal sources and one of the infrared emitters is a reference infrared emitter arranged behind the net. The net may be tilted. A plurality of infrared emitters may be arranged at corners of the net to define the shape of the net for detection by the UAV. The reference infrared emitter may be arranged in a housing having a shutter for obfuscating the reference infrared emitter during the initial detection to determine where the infrared emitter arrangement is in the field-of-view, e.g. where the upper and lower infrared emitters are or where the left and right side infrared emitters are. 
     The system may also be configured to make elevation or azimuth corrections to a gimbal if the sensor is mounted to a nose of the UAV by a gimbal. The sensor may include a fix-post camera or a gimballed arrangement. The corrections to the gimbal may be made to put the reference infrared emitter of the infrared emitter arrangement in a boresight of the sensor after the initial detection of the infrared emitter arrangement. 
     According to an aspect of the disclosure, a system for capturing a UAV may include a net having an infrared emitter arrangement and an infrared sensor mounted to the UAV. 
     According to an aspect of the disclosure, a system for capturing a UAV may be configured to adjust at least one of an azimuth or an elevation of the UAV based on a detected infrared emitter arrangement in a field-of-view of a sensor. 
     According to an aspect of the disclosure, a system for capturing a UAV may include a net having an infrared emitter arrangement formed by a reference infrared emitter and infrared emitters arranged at corners of the net. 
     According to an aspect of the disclosure, a system for capturing a UAV may be configured to adjust at least one of an azimuth or an elevation of a gimbal for an infrared sensor mounted to the UAV based on a detected infrared emitter arrangement in a field-of-view of the sensor. 
     According to an aspect of the disclosure, a method for capturing a UAV may include capturing images of an infrared emitter arrangement arranged proximate a net. 
     According to an aspect of the disclosure, a method for capturing a UAV may include adjusting at least one of an azimuth or elevation of UAV based on a detected infrared emitter arrangement in a field-of-view of an infrared sensor. 
     According to an aspect of the disclosure, a system for capturing an unmanned aerial vehicle includes a net configured to receive the unmanned aerial vehicle, an infrared emitter arrangement including a plurality of infrared emitters arranged around the net, an infrared sensor mounted to the unmanned aerial vehicle and configured to detect the infrared emitter arrangement, and a processor that is in communication with the infrared sensor and configured to adjust at least one of an azimuth or an elevation of the unmanned aerial vehicle based on the detected infrared emitter arrangement in a field-of-view of the infrared sensor. 
     According to an embodiment of any paragraph(s) of this summary, each of the plurality of infrared emitters may be a light source or a thermal source. 
     According to an embodiment of any paragraph(s) of this summary, the system may include a power source configured to heat each of the plurality of infrared emitters to produce a predetermined amount of thermal emissivity for detection by the infrared sensor. 
     According to an embodiment of any paragraph(s) of this summary, the plurality of infrared emitters may include a reference infrared emitter arranged behind the net relative to a capturing face of the net and configured to be captured in the field-of-view of the infrared sensor. 
     According to an embodiment of any paragraph(s) of this summary, the reference infrared emitter may be arranged in a housing having a shutter for obfuscating the reference infrared emitter from the infrared sensor for predetermined intervals. 
     According to an embodiment of any paragraph(s) of this summary, the plurality of infrared emitters may include two or more infrared emitters that are spaced relative to the reference infrared emitter. 
     According to an embodiment of any paragraph(s) of this summary, the two or more infrared emitters may include four infrared emitters that are each arranged at a different corner of the net. 
     According to an embodiment of any paragraph(s) of this summary, the net may be tilted relative to a vertical orientation. 
     According to an embodiment of any paragraph(s) of this summary, the processor may be configured to determine a slant range between the unmanned aerial vehicle and the net based on the field-of-view and distances between the plurality of infrared emitters. 
     According to an embodiment of any paragraph(s) of this summary, the processor may be configured to adjust at least one of a yaw, pitch, or roll of the unmanned aerial vehicle based on the determined slant range. 
     According to an embodiment of any paragraph(s) of this summary, the processor may include guidance and signal processing logic for determining the slant range and adjusting the azimuth or the elevation of the unmanned aerial vehicle. 
     According to an embodiment of any paragraph(s) of this summary, the net may include logic configured to control the infrared emitter arrangement. 
     According to an embodiment of any paragraph(s) of this summary, the system may include a gimbal arranged to mount the infrared sensor to a nose of the unmanned aerial vehicle. 
     According to an embodiment of any paragraph(s) of this summary, the processor may be configured to adjust at least one of an elevation or an azimuth of the gimbal to put a reference infrared emitter of the plurality of infrared emitters in boresight. 
     According to an embodiment of any paragraph(s) of this summary, the infrared sensor may be configured to view the infrared emitter arrangement for a predetermined number of frames before the processor adjusts the azimuth or the elevation. 
     According to another aspect of the disclosure, a method for capturing an unmanned aerial vehicle in a net includes detecting an infrared emitter arrangement arranged proximate the net in a field-of-view of an infrared sensor arranged on the unmanned aerial vehicle, and adjusting at least one of an azimuth or elevation of the unmanned aerial vehicle based on the detected infrared emitter arrangement in the field-of-view. 
     According to an embodiment of any paragraph(s) of this summary, the method may include determining a slant range between the unmanned aerial vehicle and the net based on the field-of-view of the infrared sensor and distances between the plurality of infrared emitters. 
     According to an embodiment of any paragraph(s) of this summary the method may include adjusting at least one of a yaw, pitch, or roll of the unmanned aerial vehicle based on the determined slant range. 
     According to an embodiment of any paragraph(s) of this summary, the method may include adjusting at least one of an elevation or azimuth of a gimbal and sensor arrangement on the unmanned aerial vehicle to put a reference infrared emitter of the infrared emitter arrangement in boresight after an initial detection of the infrared emitter arrangement. 
     According to an embodiment of any paragraph(s) of this summary, the method may include obfuscating the reference infrared emitter for predetermined intervals during detection to determine a position of the infrared emitter arrangement in the field-of-view. 
     To the accomplishment of the foregoing and related ends, the disclosure comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the disclosure. These embodiments are indicative, however, of but a few of the various ways in which the principles of the disclosure may be employed. Other objects, advantages and novel features of the disclosure will become apparent from the following detailed description of the disclosure when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The annexed drawings, which are not necessarily to scale, show various aspects of the disclosure. 
         FIG. 1  shows a side view of a net for capturing an unmanned aerial vehicle (UAV) and an infrared emitter arrangement arranged around the net. 
         FIG. 2  shows a front view of the net and infrared emitter arrangement of  FIG. 1 . 
         FIG. 3  shows a system for capturing the UAV that includes the net and infrared emitter arrangement of  FIG. 1  and an infrared sensor mounted on the UAV. 
         FIG. 4  shows a schematic drawing of the system for capturing the UAV of  FIG. 3 . 
         FIG. 5  shows a flowchart of a method for capturing an unmanned aerial vehicle in a net. 
     
    
    
     DETAILED DESCRIPTION 
     The principles described herein may be used in applications that implement unmanned aerial vehicles (UAVs). Small UAVs may be suitable, such as fixed-wing UAVs having a wingspan less than 3.6 meters (12 feet). Larger UAVs may also be suitable in some applications. The UAV may be tube-launched and capable of performing surveillance imagery, targeting, near real-time damage assessment, and identification and elimination of threat UAVs. Any suitable platform may be used to launch the UAV, such as a land vehicle, sea vessel, aircraft, or spacecraft. Exemplary applications for the UAV may include targeting assistance, perimeter security, and research missions. Military and non-military application may be suitable. In an exemplary application, the system described herein may be used for testing a fixed-wing UAV and capturing the UAV without damaging the UAV prior to executing a mission. 
     Referring first to  FIGS. 1 and 2 , a net  10  for capturing a UAV is shown. The net  10  may be formed of braided nylon or any other suitable material for capturing the UAV without damaging the UAV. The width W of the net  10  may be longer than the height H of the net  10  and the net  10  may be rectangular in shape. Other shapes of the net  10  may be suitable for other applications. In an exemplary embodiment, the net  10  may have a width W that is between 63.5 and 88.9 centimeters (between 25 and 35 inches) and a height H that is between 50.8 and 76.2 centimeters (between 20 and 30 inches). The net  10  may be arranged on a surface  12  and mounted above the surface  12 . The surface  12  may be a ground surface or another stationary surface. In other exemplary embodiments, the surface  12  may be movable, such as on a movable platform. 
     An infrared emitter arrangement  14  is arranged proximate the net  10  and may be mounted to the net  10 . The infrared emitter arrangement  14  includes a plurality of infrared emitters  16 ,  18  arranged around the net  10 . The plurality of infrared emitters  16 ,  18  includes a reference infrared emitter  16  arranged behind the net  10 . Two or more infrared emitters  18 ,  18   a ,  18   b ,  18   c ,  18   d  may be spaced relative to the reference infrared emitter  16 . The two or more infrared emitters  18  may include four infrared emitters  18   a ,  18   b ,  18   c ,  18   d  that are each arranged at a different corner of the net  10  such that the infrared emitters  18  define the shape of the net  10  for detection by the UAV. The upper infrared emitters  18   a ,  18   c  are spaced by the width W of the net  10  and the upper infrared emitters  18   a ,  18   c  are spaced from the lower infrared emitters  18   b ,  18   d  by the height H of the net  10 . The reference infrared emitter  16  may be arranged proximate a center of the net  10 . In other exemplary embodiments, the reference infrared emitter  16  may be arranged at any location along the net  10 . 
     Any suitable infrared light source or thermal source may be used for each of the infrared emitters  16 ,  18 . In an exemplary embodiment, the infrared emitters  16 ,  18  may include 500-watt halogen bulbs and the UAV may include a long-wave infrared sensor. Any number of infrared emitters  16 ,  18  may be used. Four infrared emitters  18  and one reference infrared emitter  16  may be suitable. Fewer than four or more than four infrared emitters  18  may be suitable in some applications. The arrangement of the infrared emitters  16 ,  18  may be symmetrical. 
     The reference infrared emitter  16  may be arranged behind the net  10  relative to a capturing face  20  of the net  10  that faces the UAV. The net  10  and the reference infrared emitter  16  may be spaced by a distance D 1 . In exemplary embodiments, the distance D 1  may be between 4.5 and 7.6 meters (between 15 and 25 feet). The reference infrared emitter  16  may be mounted above the surface  12  by a distance D 2 . For example, the distance D 2  may be between 2.1 and 3.4 meters (between 7 and 11 feet). Many other dimensions may be suitable. 
     The net  10  may be tilted relative to a vertical orientation relative to the surface  12  by an angle A that is between zero and 15 degrees. In exemplary embodiments, the angle A may be approximately 10 degrees. Accordingly, the infrared emitters  18  arranged on the net  10  may be tilted with the net  10  whereas the reference infrared emitter  16  may be mounted in a vertical orientation behind the net  10 . Each of the infrared emitters  16 ,  18  may be held in position above the surface  12  by a pole  22 . The arrangement of the net  10  and the infrared emitter arrangement  14  described herein are exemplary and other configurations of the infrared emitters  16 ,  18  may be suitable for certain applications. 
     Referring in addition to  FIGS. 3 and 4 , a system  24  for capturing a UAV  26  includes the net  10 , the infrared emitter arrangement  14 , and an infrared sensor  28  mounted to the UAV  26  and configured to detect the infrared emitter arrangement  14 . The UAV  26  travels in a direction T toward the net  10 . The infrared sensor  28  may detect the infrared emitter arrangement  14  when the UAV  26  is within a predetermined range of the net  10  and the infrared emitter arrangement  14 , such that the UAV  26  begins its terminal flight for net capture of the UAV  26 . During the terminal flight, the engine of the UAV  26  may be turned off and the UAV  26  may be switched into a guidance mode in which the system  24  is used to adjust the elevation or azimuth of the UAV  26  to ensure the UAV  26  flies directly into the net  10 . 
     The predetermined range at which the UAV  26  begins its terminal flight may be defined by the UAV  26  having a slant range SR and above ground level (AGL) relative to the net  10 . The AGL may be between 25 and 50 meters. The UAV  26  may have a maximum speed of 45 meters per second and a stall speed of 20 meters per second. At the predetermined slant range SR and the predetermined above ground level AGL, such as at a slant range SR of approximately 200 meters and an above ground level AGL of approximately 50 meters, the UAV  26  may begin its terminal flight. The UAV  26  may thus have an ingress angle B relative to the net  10  of approximately 30 degrees and an angle of attack of approximately three degrees. Other slant ranges and AGLs may also define the range in which the UAV  26  begins its terminal fight. 
     For the initial detection of the infrared emitter arrangement  14 , the system  24  includes a power source  30  arranged proximate the net  10  for heating the infrared emitter arrangement  14 . Power may be supplied to the thermal sources of each of the infrared emitters  16 ,  18  until the measured temperature exceeds 250 degrees Celsius, or any other predetermined temperature. The infrared emitters  16 ,  18  may then produce enough thermal emissivity to be treated as a black body from one to 13.5 microns. The infrared emitters  16 ,  18  may be configured to produce any suitable thermal emissivity for detection by the infrared sensor  28  of the UAV  26 . 
     After the initial detection of the infrared emitter arrangement  14 , the UAV enters the guidance mode in which corrections are made based on the infrared emitters  16 ,  18  that are seen in the field-of-view of the infrared sensor  28 . The system  24  includes a processor  32  arranged in the UAV  26  that is in communication with the infrared sensor  28  of the UAV  26  and configured to adjust at least one of an azimuth or elevation of the UAV  26 . The infrared sensor  28  continuously captures images of the infrared emitter arrangement  14  such that the corrections to the UAV  26  are made based on changes in the position of the infrared emitter arrangement  14  that are seen in the captured images and indicate that the UAV  26  should be adjusted to maintain all of the infrared emitters  16 ,  18  in the field-of-view. Advantageously, the corrections made to the UAV  26  ensure that the position and orientation of the UAV  26  is suitable for capture as the UAV  26  travels toward the net  10 . 
     The infrared sensor  28  includes a camera for capturing images of the infrared emitter arrangement  14 . The camera may have a microbolometer that is configured to detect longwave infrared having a 7.5 to 13.5 micron wavelength, or any other thermal emissivity produced by the infrared emitters  16 ,  18 . The infrared sensor  28  may include any suitable lens. In an exemplary embodiment, the lens may have a focal length of approximately 18 millimeters, a nominal horizontal field-of-view of approximately 24.4. degrees, and a nominal vertical field-of-view of approximately 19.5 degrees. The infrared sensor  28  may have any suitable frame rate for capturing images of the infrared emitter arrangement  14 , such as a frame rate configured at approximately 30 hertz. The frame rate may be variable. Other configurations of the infrared sensor  28  may be suitable. One or more sensors  28  may be used depending on the application. 
     The processor  32  includes signal processing algorithms and is configured to receive the data, e.g. the captured images, from the infrared sensor  28  and execute the algorithms to make the corrections to the UAV  26 . When the infrared emitters  16 ,  18  are treated as black bodies, there is enough contrast with the surrounding area around the net  10  that enables the algorithms of the processor  32  to perform. The processor  32  may be connected to a mission computer  34 . For example, the processor  32  may be connected to the mission computer  34  for the UAV  26  via an Ethernet switch on a circuit board that houses the mission computer  34  and guidance, navigation, and control (GNC) algorithms for the UAV  26 . 
     The processor  32  includes an Inertial Navigation System (INS) including the guidance and signal processing logic  36  that stores the signal processing algorithms. The guidance and signal processing logic  36  is configured to execute algorithms such as determining the slant range SR and sending commands to adjust the UAV  26 . The mission computer  34  may be configured to receive the commands from the INS to make the corrections to the UAV  26 , such as an elevation and azimuth correction. The corrections to the UAV  26  may include changing the heading of the UAV  26  depending on the infrared emitters of the infrared emitter arrangement  14  that are seen in the field-of-view FOV of the sensor  28 . The algorithms used may be in a roll-corrected frame of reference such that corrections in elevation and azimuth may command the UAV  26  to make corrections in yaw, pitch, and roll. The processor  32  and mission computer  34  may include any suitable circuitry, microprocessors, controllers, etc. 
     During the terminal flight of the UAV  26 , if all of the infrared emitters  16 ,  18  are seen in the field-of-view FOV of the infrared sensor  28 , the guidance and signal processing logic  36  is configured to determine the slant range SR throughout the terminal flight based on the field-of-view FOV of the infrared sensor  28  and the fixed distances between the infrared emitters  16 ,  18 , such as the widths W and heights H shown in  FIG. 2 . The guidance and signal processing logic  36  is then able to send commands to the UAV  26  pertaining to a pitch adjustment for the UAV  26  based on the determined slant range SR. 
     Alternatively, during the terminal flight, the infrared sensor  28  may only see the infrared emitters  18   a ,  18   b  or the infrared emitters  18   c ,  18   d  on one side of the net  10  such that the UAV  26  is heading toward the net  10  too far to the left or too far to the right of the net  10 . If the reference infrared emitter  16  is present in the field-of-view FOV, the slant range SR and the known distances between the infrared emitters  18 , e.g. the widths W and the heights H, are used to steer the UAV  26  to the left or right, e.g. make a yaw correction to the UAV  26 . Similarly, if the UAV  26  is heading toward the net  10  too far above or too far below the net  10 , a pitch correction may be made to the UAV  26  to steer the UAV  26  up or down. 
     The net  10  also includes a processor  40  and logic  42  to control the infrared emitter arrangement  14 . Once the infrared emitter arrangement  14  is initially detected, the reference infrared emitter  16  may be arranged in a box  44  having a shutter to obfuscate the reference infrared emitter  16  from the infrared sensor  28  for a predetermined number of frames. The reference infrared emitter  16  is arranged proximate the center of the net  10  or anywhere along the net  10  that is in the field-of-view FOV of the infrared sensor  28 . After the initial detection, the processor  40  and logic  42  may be configured to operate the shutter to obfuscate the reference infrared emitter  16  from the infrared sensor  28  for two consecutive frames during every four consecutive frames. The flickering reference infrared emitter  16  provided by the shutter is used to determined which infrared emitters  18   a ,  18   b ,  18   c ,  18   d  are being viewed in the field-of-view FOV of the infrared sensor  28 . For example, as shown in  FIG. 2 , the infrared sensor  28  may only view right side infrared emitters  18   a ,  18   b , top infrared emitters  18   a ,  18   c , left infrared emitters  18   c ,  18   d , or bottom infrared emitters  18   b ,  18   d.    
     The UAV  26  may travel to a specific distance relative to the infrared emitter arrangement  14  at which the detected infrared emitters  18   a ,  18   b ,  18   c ,  18   d  may be resolved and determined by the processor  32 . The infrared sensor  28  may be configured to view the infrared emitter arrangement  14  for a predetermined number of frames, such as for approximately 30 frames, before the processor  32  of the UAV  32  executes the correction algorithms. Viewing the infrared emitter arrangement  14  for the predetermined number of frames ensures that accurate input measurements are taken before corrections to the UAV  26  are made. 
     In exemplary embodiments of the infrared sensor  28 , any suitable gimbal  46  may be used to mount the infrared sensor  28  to the UAV  26 . In other exemplary embodiments, the infrared sensor  28  may include a fix-post camera. The gimbal  46  may be mounted at a front or nose end of the UAV  26  that faces forward relative to the direction of travel T of the UAV  26 . After the initial detection for the predetermined number of frames, the processor  32  may then send a command to the mission computer  34  to adjust at least one of an elevation or azimuth of the gimbal  46  to put the reference infrared emitter  16  in boresight. The gimballed infrared sensor  28  may be configured to initially point at predetermined a latitude, longitude, and altitude (LLA) of the net  10 , such that an appropriate correction is then made to the gimbal  46 . Accordingly, the system  24  is configured to make both corrections to the angles of the gimbal  46 , and to the position and orientation of the UAV  26 . 
     During operation of the system  24 , the ingress angle B of the UAV  26  relative to the net  10  may be normal relative to a reference frame of the infrared emitters  16 ,  18  in the infrared emitter arrangement  14 , as shown in  FIG. 3 . If the ingress angle B is skewed, then the processor  32  may be configured to perform an affine transformation on the detected infrared emitters  16 ,  18 , based on the known distances between the infrared emitters  16 ,  18 , to derive an angle-angle-range that is normal to the net  10 . The affine transform occurs when the image of the net  10  is skewed, such that the projection of the net  10  is rotated or translated into a projection that is not skewed, e.g. when the infrared sensor  28  and the net  10  are parallel, using the predetermined dimensions of the net  10 . The rotation of the projection is the affine transformation and the output of the transformation includes the elevation and azimuth angles and slant range. The processor  32  may be configured to make corrections for the ingress angle B of the UAV  26  using the affine transformation and subsequent corrections to the UAV  26  may be made relative to the ingress angle B, such as pitch corrections. The affine transformation corrections made to the UAV  26  may be performed without using attitude information for the UAV  26 . 
     Alternatively, attitude information for the UAV  26  may be used to make corrections to the UAV  26 . For example, measurements from the gimbal  46  and the INS of the processor  32  may be used to make corrections for the UAV  26 . At the outset, the slant range SR of the UAV  26  for a predetermined pixel resolution R of the infrared sensor  28  may be determined using equation (1): 
     
       
         
           
             
               
                 
                   
                     S 
                     ⁢ 
                     R 
                   
                   = 
                   
                     d 
                     
                       R 
                       · 
                       IFOV 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     In equation (1), the slant range SR is determined based on the distance d between the infrared emitters  16 ,  18  and the instantaneous field-of-view FOV IFOV. The distance d is known. For example, the width W and height H between the infrared emitters  18  is shown in  FIG. 2 . The slant range SR is determined in every frame of the infrared sensor  28  where two of the infrared emitters  14  arranged along a common side of the net  10  are present in the field-of-view FOV of the infrared sensor  28 . If all of the infrared emitters  16 ,  18  are shown in the field-of-view FOV, the slant range SR may be used to adjust a pitch of the UAV  26 . The resolution is determined by pixel rows n multiplied by pixel columns m. 
     In an exemplary scenario in which all of the infrared emitters  14  are present in a first capture frame N and a second capture frame N+1, an elevation correction may be made for the gimbal  46  and from the processor  32  (INS) to the mission computer  34  to adjust the UAV  26 . For example, the correction may be a change in pitch of the UAV  26  performed by the mission computer  34 . 
     From frame N to frame N+1, a change or adjustment in position ΔN, ΔE, ΔD of the UAV  26  by the processor  32  may be made, and a change in orientation Δθ, Δϕ, Δψ of the UAV  26  by the processor  32  may be made. The changes are based on the center of the four corners of the net  10  being defined by x c ,y c . The image of the net  10  captured by the infrared sensor  28  may be moved by Δy c =y N+1 −y N  pixels from frame N to frame N+1. The correction for the UAV  26  received from the INS of the processor  32  may be determined using equation (2): 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                       y 
                       
                         i 
                         ⁢ 
                         a 
                       
                     
                   
                   = 
                   
                     
                       
                         n 
                         ⁢ 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               
                                 ϕ 
                                 
                                   N 
                                   + 
                                   1 
                                 
                               
                               + 
                               
                                 ϵ 
                                 ia 
                               
                             
                             ) 
                           
                         
                       
                       
                         2 
                         · 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               FOV 
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                     - 
                     
                       
                         n 
                         ⁢ 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               
                                 ϕ 
                                 N 
                               
                               + 
                               
                                 ϵ 
                                 ia 
                               
                             
                             ) 
                           
                         
                       
                       
                         2 
                         · 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               FOV 
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
     In equation (2), n corresponds to the pixel row for the resolution. If there is a change in the scene by elevation, then the row n space may be corrected. The correction in angle Δy ia  for the UAV  26  is also determined based on frame-to-frame angular error ∈ ia  from the INS, i.e. measurement noise. 
     In an exemplary embodiment in which the infrared sensor  28  is mounted to the UAV  26  by the gimbal  46 , from frame N to frame N+1, a change in gimbal angles Δα, Δβ, Δγ may be made. A correction for the gimbal  46  for the infrared sensor  28  may be determined using equation (3): 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                       y 
                       g 
                     
                   
                   = 
                   
                     
                       
                         n 
                         ⁢ 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               
                                 β 
                                 
                                   N 
                                   + 
                                   1 
                                 
                               
                               + 
                               
                                 ϵ 
                                 g 
                               
                             
                             ) 
                           
                         
                       
                       
                         2 
                         · 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               FOV 
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                     - 
                     
                       
                         n 
                         ⁢ 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               
                                 β 
                                 N 
                               
                               + 
                               
                                 ϵ 
                                 g 
                               
                             
                             ) 
                           
                         
                       
                       
                         2 
                         · 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               FOV 
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
     In equation (3), the gimbal correction Δy g  is determined based on frame-to-frame angular error ∈ 9  from the gimbal  46 , i.e. measurement noise. Accordingly, the entire elevation correction for the UAV  26  to obtain an image of the net  10  that is in the same position as in the focal plane in frame N may be determined by adding the change in image capture Δy c , the UAV correction Δy ia , and the gimbal correction Δy g , using equation (4): 
         el  correction=Δ y   c   +Δy   ia   +Δy   g   Equation (4):
 
     In another exemplary scenario in which all of the infrared emitters  18  are present in frame N and none of the infrared emitters  18  are present in frame N+1, an azimuth correction for the UAV  26  may be made. For example, the correction may be a change in yaw of the UAV  26  performed by the mission computer  34 . The correction in angle for the UAV  26  from the INS may be determined using equation (5): 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                       x 
                       
                         i 
                         ⁢ 
                         a 
                       
                     
                   
                   = 
                   
                     
                       
                         m 
                         ⁢ 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               
                                 ψ 
                                 
                                   N 
                                   + 
                                   1 
                                 
                               
                               + 
                               
                                 ϵ 
                                 ia 
                               
                             
                             ) 
                           
                         
                       
                       
                         2 
                         · 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               FOV 
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                     - 
                     
                       
                         m 
                         ⁢ 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               ψ 
                               + 
                               
                                 ϵ 
                                 ia 
                               
                             
                             ) 
                           
                         
                       
                       
                         2 
                         · 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               FOV 
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
     In equation (5), m corresponds to the pixel column for the resolution. If there is a change in the scene by elevation, then the column m space may be corrected. The correction in angle Δx ia  is also determined based on frame-to-frame angular error ∈ ia  from the INS. The correction in position for the UAV  26  from the INS may be determined using equation (6): 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                       x 
                       ip 
                     
                   
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         E 
                       
                       + 
                       
                         0.5 
                         ⁢ 
                         
                           ϵ 
                           ip 
                         
                       
                     
                     
                       IFOV 
                       · 
                       
                         ( 
                         
                           
                             SR 
                             N 
                           
                           + 
                           
                             v 
                             / 
                             FR 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     6 
                     ) 
                   
                 
               
             
           
         
       
     
     In equation (6), the correction in position Δx ip  is based on the slant range SR N  at frame N, the average velocity v of the UAV  26 , the frame rate FR, and the change in position ΔE of the UAV  26 , which is equal to E N+1 −E N . The change in position ΔE of the UAV  26  is defined by the UAV&#39;s change in position in the reference frame east direction relative to the normal direction of the net  10  from frame N to frame N+1. The azimuth correction for the gimbal  46  may be determined using equation (7): 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                       x 
                       g 
                     
                   
                   = 
                   
                     
                       
                         m 
                         ⁢ 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               
                                 γ 
                                 
                                   N 
                                   + 
                                   1 
                                 
                               
                               + 
                               
                                 ϵ 
                                 g 
                               
                             
                             ) 
                           
                         
                       
                       
                         2 
                         · 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               FOV 
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                     - 
                     
                       
                         m 
                         ⁢ 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               
                                 γ 
                                 N 
                               
                               + 
                               
                                 ϵ 
                                 g 
                               
                             
                             ) 
                           
                         
                       
                       
                         2 
                         · 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               FOV 
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     7 
                     ) 
                   
                 
               
             
           
         
       
     
     In equation (7), the gimbal correction Δx g  is determined based on frame-to-frame angular error ∈ 9  from the gimbal  46 . The total azimuth correction for the UAV  26  may be determined by adding the UAV angle correction Δx ip , the UAV position correction Δx ip , and the gimbal correction Δx g , using equation (8): 
         az  correction=Δ x   ia   +Δx   ip   +Δx   g   Equation (8):
 
     In still another exemplary scenario in which all of the infrared emitters  18  are present in frame N and only two of the infrared emitters  14  are present in frame N+1, an azimuth correction to the UAV  26  may be made by the processor  32 . For example, if Δx ia +Δx ip +Δx g  of equation (8) is positive, then the net  10  is moved to the right so that infrared emitters  18   a ,  18   b  on the right side of the net  10  (shown in  FIG. 2 ) are outside the field-of-view FOV of the infrared sensor  28 . Accordingly, equations (2) through (4) may be used to determine the difference Δx in pixels of the left lights  18   c ,  18   d  (shown in  FIG. 2 ) from frame N to frame N+1, via equation (9): 
         az  correction=Δ x+x   ia   +x   g   Equation (9):
 
     Equation (9) is used if the correction from the focal plane measurement Δx is more accurate than a measurement from the INS for the position correction Δx ip . In an exemplary operation, the system may first execute equations (2) through (4) pertaining to the elevation correction scenario. If equations (2) through (4) are not executed, equation (9) pertaining to the azimuth correction scenario is executed. If the elevation correction and azimuth correction equation (9) are not executed, then the other azimuth correction equations (5) through (8) are executed. Other scenarios may be possible and similar equations (algorithms) may be used to make the suitable corrections to the UAV  26  and the gimbal  46 . All of the algorithms may be carried out using the processor  32  in which the algorithms are stored. 
     Referring now to  FIG. 5 , a flowchart showing a method  50  for capturing an unmanned aerial vehicle in a net is shown. The method  50  may include the system  24  shown in  FIGS. 1-4 . Step  52  of the method  50  includes detecting an infrared emitter arrangement  14  arranged proximate the net  10  in a field-of-view FOV of an infrared sensor  28  arranged on the UAV  26 . Step  54  of the method  50  may include obfuscating the reference infrared emitter  16  of the infrared emitter arrangement  14  for predetermined intervals during detection to determine a position of the infrared emitter arrangement  14  in the field-of-view FOV. 
     Step  56  of the method  50  may include determining a slant range SR between the UAV  26  and the net  10  based on the field-of-view FOV of the infrared sensor  28  and the distances W, H between the plurality of infrared emitters  18 . Step  58  of the method  50  includes adjusting at least one of an azimuth or elevation of the UAV  26  based on the detected infrared emitter arrangement  14  in the field-of-view FOV. Step  58  may include adjusting at least one of a yaw, pitch, or roll of the UAV based on the determined slant range SR. Step  60  of the method  50  may include adjusting at least one of an elevation or azimuth of a gimbal  46  for the infrared sensor  28  to put the reference infrared emitter  16  in boresight after an initial detection of the infrared emitter arrangement  14 . 
     Although the disclosure shows and describes certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (external components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the disclosure. In addition, while a particular feature of the disclosure may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.