Abstract:
A method for boresighting of a designation system, including a tracker responsive to a detector with reference to an indicator, including the step of directing a beam of light at a target, using a light source, so that the beam of light is reflected from a spot on the target while a temperature of the spot remains substantially constant. The method further includes focusing at least part of the reflected light as an image on the detector and determining a misalignment of the indicator and the image.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to an in-action boresight for laser designation systems. 
     Modem weapon systems, which employ laser-guided bombs and missiles, require highly accurate alignment of their designation systems in order to achieve a high probability of target acquisition. Traditional methods of achieving this involve ground-based pre-flight calibration of detectors with their corresponding designator, commonly known as boresighting. Ground-based boresight systems are typically robust, heavy and bulky. After ground-based boresighting has been conducted, however, misalignments can develop between the detectors and designators due to environmental conditions, i.e. mechanical and thermal loads including vibrations, shocks and temperature variation. These misalignments can significantly degrade the performance of the designation systems. 
     To overcome the misalignment problem, in-flight boresight systems have been developed which can be operated a short time prior to weapon operation. Thus, the misalignments that could normally have occurred from boresighting to designator operation are significantly reduced. These systems, however, are typically made up of a large number of optical components which have the potential for introducing further thermo-optical errors and are prone to in-flight misalignment. Furthermore, current methods rely on local heating of specific types of targets, such as ceramics, using laser radiation in order to generate hot-spots, which are then detected by sensor systems. These methods have number of drawbacks, which are discussed below. 
     As an example, consider FIG. 1 which shows a target  500  where a laser beam (not shown) is incident on the target surface  502 , thereby generating laser spot  504 . Heat is conducted by target  500  and this results in a temperature distribution on target surface  502 . Concentric closed loops  506 ,  508  and  510  are isotherms (lines of constant temperature on target surface  502 ) and indicate a typical temperature distribution caused by laser spot  504 . The temperature is highest at laser spot  504  and decreases with radial distance. It will be readily appreciated that isotherms  506 ,  508  and  510  are in general non-circular and non-symmetric around laser spot  504 . This is due to asymmetric conduction within the material that makes up target  500 . Thus, a sensor (not shown) that is operative to detect the local heating which results from laser spot  504 , will incorrectly detect a center  512  for example, instead of the correct center  501  of laser spot  504 . 
     The above description illustrates a number of major drawbacks of current boresight systems. Firstly, a period of time, which is non-negligible when compared with the time required for boresighting, is required to heat target surface  502  at the center  501  of laser spot  504  to a temperature that allows sensor detection (typically 25 degrees Celsius above target surface temperature). Secondly, a specific target type is required, such as certain ceramics, which has the particular conductive properties required for generating thermally detectable laser spot. Thirdly, asymmetric conduction on the target surface, as depicted graphically in FIG. 1, can result in incorrect detection of the laser spot center, thereby degrading the accuracy of the system. Fourthly, in order to effect thermal detection, a large number of additional optical components must be added to the designation system. As mentioned above, these additional optical components increase the probability of in-flight misalignment and reduce accuracy. 
     There is therefore a need for an accurate and rapid in-action boresight which has a minimum of additional optical components. The system should not rely on laser heating of specific targets, but should rather detect an optical laser spot. This would both increase the system accuracy and eliminate the time required for heating a target, thereby reducing the overall boresighting time. Furthermore, the system should not be limited to a specific target type, but should allow boresighting on a variety of targets. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for in-action boresighting of designation systems. 
     According to the teachings of the present invention there is provided, a method for boresighting of a designation system, including a tracker responsive to a detector with reference to an indicator, comprising the steps of (a) directing a beam of light at a target, using a light source, so that the beam of light is reflected from a spot on the target while the spot temperature remains substantially constant; (b) focusing at least part of the reflected light as an image on the detector; and (c) determining a misalignment of the indicator and image. 
     There is furthermore provided, in a boresighting system for aligning an indicator with an image of a spot on a target, a method of displaying the alignment, comprising the steps of providing a video monitor; and displaying a representation of the indicator together with a representation of the image on the video monitor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a schematic depiction of a target with a laser spot incident on its surface (prior art); 
     FIG. 2 is a schematic depiction of a designation system constructed and operative according to the teachings of the present invention; 
     FIG. 3 is a schematic depiction of a video image before boresighting; 
     FIG. 4A is a schematic depiction of a video display after boresighting by moving a cross-hair; and 
     FIG. 4B is a schematic depiction of a video display after boresighting by moving displayed pixels. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principles and operation of the in-action boresight according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Referring again to the drawings, FIG. 2 shows the designation system  10 , which is made up of a laser designator  14 , receiving optics  28  and a detector  16 , which are all mounted on a rigid gimbaled base  12 . Rigid gimbaled base  12  is required for the mounting of all components so as to minimize the possibility of misalignment between the various components. A synchronization line  13  synchronizes the operation between laser designator  14  and detector  16 . A tracker line  17  connects detector  16  to a tracker  11 . Preferably, tracker  11  is connected to a video monitor  21  via a video line  19 . Designation system  10  is positioned at a distance R from a target  22 , where R is referred to as the range-to-target. Target  22  is usually remote, relative to designation system  10 , such that R is typically greater than 1500 meters. 
     In brief, the objective of boresighting is to align an indicator, such as a cross-hair (not shown), encoded in tracker  11 , with a laser spot image (not shown). After boresighting is complete, typically a cross-hair indicates the location of a laser spot center on target  22 . The indicator and laser spot image may be simultaneously represented as a video image. In a preferred embodiment of the present invention, a cross-hair and laser spot image are displayed simultaneously on video monitor  21 . Boresighting of designation system  10  is achieved according to four main stages, namely: stage I—designation; stage II—laser-spot detection; stage III—signal processing; and stage IV—misalignment correction. These stages must be carried out sequentially, starting with stage I and ending with stage IV. The features of each of the stages, as well as their interrelation, are described in detail below. 
     In Stage I, the purpose of laser designator  14  is designating, i.e. creating a laser spot  26  on target  22 . As a preferred embodiment, laser spot  26  is formed on the surface  24  of target  22 . If target  22  is a diffuse body, such as a cloud, water droplets or even pollution, laser spot  26  can also be formed on particles within target  22 . Laser designator  14  is typically a pulsed infra-red or visible-light laser which can be pulsed at a wide range of frequencies (alternatively pulses per second, PPS). Laser designator  14  is activated in external triggering mode by detector  16  via synchronization line  13 , thereby producing laser beam  20 . Laser beam  20  is directed towards target  22  and is incident on the target surface  24 . Incident laser beam  20  creates an optical laser spot  26  on target surface  24 , which is reflected from surface  24  and produces a reflected beam which is referred to herein as the laser echo  27 . Optical laser spot  26  is “optical” in the sense that laser beam  20  is merely reflected from surface  24  and does not appreciably change the temperature at the location of target  22  where it is incident. Thus, laser echo  27  can include visible, infra-red or near infra-red wavelengths. In general, target surface  24  may be composed of any partially reflective substance: even certain atmospheric conditions or clouds constitute suitably reflective surfaces. It should be emphasized that the purpose of laser beam  20  is not to cause local heating of target surface  24 , but rather to generate an optical laser spot  26 . 
     In stage II, target detection, laser echo  27  from optical laser spot  26  is incident on receiving optics  28 . Laser echo  27  is focused by means of receiving optics  28  resulting in focused beam  29  which is incident on detector  16 . To effect detection of laser echo  27 , detector  16  incorporates a sensor  15  of some kind. Typical examples of sensor  15  include Forward-Looking Infra-Red (FLIR) sensors or Charge-Coupled Device (CCD) such as GICCD and EBCCD sensors, for example. Detector  16  triggers and synchronizes laser designator  14 . This means that a laser pulse is initiated by detector  16  and then the detector integration time is set to a time-frame window on which laser echo  27  is expected to be received. This window corresponds to any reasonable range to target R. A range gate is employed to eliminate spurious light signals from short ranges (typically less than 1500 meters). Thus parallax errors, which could cause misalignment, are eliminated. The focusing of beam  29 , which is incident on detector  16 , results in the formation of a laser spot image  23  on the surface  18  of sensor  15 . Background light (not shown), from the target for example, is also incident on sensor surface  18 . All light signals incident on sensor surface  18  are received by detector  16  and transferred via tracker line  17  to tracker  11 . 
     Part of the function of tracker  11  is to distinguish between the coordinates of laser spot image  23  and background light that is incident on sensor surface  18 . (The preferred method employed to achieve this is discussed later in detail.) Coordinates of the center (not shown) of laser spot image  23  and background light, which are stored as successive video frames in tracker  11 , can be converted into a video image  40  (see FIG. 3) and transferred via video line  19  to video monitor  21  where these coordinates are visually displayed. It is pointed out that video image  40  can be stored or displayed in a variety of virtual or physical forms, such as random-access memory, magnetic tape, etc. 
     FIG. 3 is a schematic depiction of a video image  40 , showing a laser spot image  46 , background light  49  and a cross-hair  45 . Laser spot image  46  is located with its center at a spot image center  47  and cross-hair  45  is located with its center at a cross-hair center  48 . Cross-hair  45  may be synthetically generated on video image  40  with its coordinates encoded in tracker  11  (see FIG.  2 ). Thus, video image  40  simultaneously represents laser spot image  46 , cross-hair  45  and background light  49 . In general, laser spot image  46  and cross-hair  45  are not initially coincidental (if laser spot image  46  and cross-hair  45  are coincidental, then the system is boresighted). The misalignment, between spot image center  47  and cross-hair center  48  is designated M in the figure. 
     The primary purpose of stage III, Signal Processing, is to determine misalignment M. This function is performed by tracker  11 , which computes the misalignment M between spot image center  47  and cross-hair center  48 . The signal-to-noise-ratio (SNR) of laser spot image  46  is proportional to the reflectivity of target surface  24  and inversely proportional to the range-to-target R. Thus, when a combination of low target reflectivity and range-to-target R results in a low SNR, the tracker  11  must integrate several (e.g. 20 to 40) video image frames in order to accurately detect spot image center  47 . A preferred method for achieving this is discussed below. 
     Coordinates of laser spot image  23  and cross-hair  45 , which are encoded in tracker  11 , can be transferred via video line  19  to video monitor  21 , for visual display, much like that shown in FIG.  3 . Cross-hair  45  may be synthetically generated on video display  44  with its coordinates encoded in tracker  11  (see FIG.  2 ). In general, a video display image processed by tracker  11  contains laser spot image  46  as well as background light  49 . 
     In general, a video frame processed by tracker  11  contains laser spot image  46  as well as background light  49 . Laser designator  14  is limited in that it can only operate at a maximum frequency of approximately 15 pulses per second (PPS). Thus, a video format is selected which is some multiple of laser designator  14  operating frequency. For example, in order to detect only laser spot image  46 , laser designator  14  is triggered at one half of the video frame rate of video monitor  21 . Thus, if the video frame rate is 30 Hz, such as in RS170 format, laser designator  14  is triggered at 15 pulses per second (PPS) which is half the RS170 format frame-rate. Alternatively, if the video frame rate is 25 Hz, such as in CCIR format, laser designator  14  is triggered at 12.5 PPS. This results in the reception of a laser spot image on every even video frame and an image with no laser spot on every odd video frame, or vice versa. Tracker  11  then integrates the even frames in a first memory bank  32  and the odd frames in a second memory bank  34 . In this manner, tracker  11  processes laser spot image  46  in first memory bank  32  and simply discards background light  49 , from second memory bank  34 , simultaneously. 
     Due to the short integration time, only laser spot image  46  is stored in first memory bank  32 , because background light  49  data does not exceed inherent tracker  11  noise levels. In this manner tracker  11  accurately determines spot image center  47 . At this point, tracker  11  contains the coordinates of both spot image center  47  and cross-hair center  48 . Thus, tracker  11  computes a misalignment M between spot image center  47  and cross-hair center  48 . 
     In stage IV, Misalignment Correction, boresighting is completed in tracker  11 , by aligning spot image center  47  and cross-hair center  48 . For visual display, it is desirable to keep cross-hair  45  as close as possible to the center of video display  44 . Two preferred methods are employed to achieve this. The first method is described with respect to FIG.  4 A and the second method is described with respect to FIG.  4 B. 
     The first method is often employed when spot image center  47  of laser spot image  46  is sufficiently close to the center of video display  44  as depicted in FIG.  4 A. In this instance, boresighting is achieved by moving cross-hair  45  from a first cross-hair center  48 ′ to a second cross-hair center that is coincidental with first spot image center  47 , which corresponds to misalignment M′. Thus, after boresighting, the center of cross-hair  45 ′ is coincidental with first spot image center  47  and is close to the center of video display  44 . 
     The second method is often employed when a first spot image center  47 ′ of laser spot image  46  is not sufficiently close to the center of video display  44  as depicted in FIG.  4 B. Here, the misalignment between first spot image center  47 ′ and cross-hair center  48  is M″. In this instance, boresighting is achieved by moving the entire video display  44 , excluding cross-hair  45 , to a new matrix of pixels. In general, the display of the correction of misalignment M″ is achieved by utilizing vertical columns of synthetic pixels  50  on the side of video display  44  and horizontal rows of synthetic pixels  52  at the top (or bottom) of video display  44 . For example, if the display is moved towards the left-hand side such that vertical columns of synthetic pixels  50  are added to video display  44 , then corresponding columns of pixels (not shown) on the right-hand side of video display  44  are removed from video display  44 . Thus video display  44  maintains its original size. In this manner the entire video display  44  is moved laterally and longitudinally such that a second spot image center of laser spot image  46 ′ is coincidental with cross-hair center  48 , and is thus close to the center of video display  44 . 
     It will be appreciated that the above invention fulfills the need for an accurate and rapid in-action boresight which has a minimum of additional optical components. Boresighting is based on the detection of an optical laser spot and, as such, eliminates the need for targets heating. Thus accuracy is increased and the additional time required for heating a target is eliminated. Furthermore, boresighting can be performed on a variety of targets, thereby increasing flexibility and versatility. 
     It will be further appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the spirit and the scope of the present invention.