Abstract:
Threat detection systems and methods are disclosed that employ position-sensing photodiodes to locate a munitions flash within a field of view of a collection optical system. The flash is then located on a map of the monitored terrain, which map can be displayed to a system user. Processing electronics determine whether the flash is actually munitions-based or is from another non-threatening light source.

Description:
FIELD 
       [0001]    The present invention relates generally to threat detection systems, and in particular to such systems that can detect the position of the threat based on the thermal flash from the threat. 
       BACKGROUND ART 
       [0002]    It is desirable to be able to identify and locate a munitions threat in a variety of hostile environments such as a battlefield. Examples of such threats include rifle and machine gun fire, artillery fire (e.g., from rockets and mortars), and various types of explosions. 
         [0003]    Current techniques for detecting a munitions-based threat and determining its location include acoustic sensing with microphones or optical imaging with an FPA (focal plane array). Acoustic threat detection systems typically employ triangulation based on sound arrival from two microphones. This method is low cost but also low range, and vehicle noise interference and firecrackers can cause a high false alarm rate. The pointing accuracy also depends on the separation of the microphones, which may not be able to be separated very far from each other. Air density variations between the two microphones also causes the pointing accuracy to decrease with range. Even filtered microphones receive not only sounds from the threat but also every other sound that falls in the non-unique acoustic range of the threat. 
         [0004]    Optical threat detection systems typically employ a visible or a cooled infrared camera to look at the visible or thermal flash from the discharge of the particular munition. The minimum frame rate required to detect such a threat should be in the 1000 frame per second range. However, this high frame rate requires that the focal plane be relatively small because of the nature of how the image electrons are clocked out. Consequently, focal plane arrays are limited to a small aperture and/or a narrow field of view (FOV). Another shortcoming of using focal plane arrays is that problem they receive not only the spectral information from the threat but also the spatial information as well. This is more information than necessary to make a threat determination. 
         [0005]    It would be desirable to have a threat detection system with the low cost of an acoustic system, the accuracy and large FOV of a large imaging focal plane array system and the high bandwidth of a small FPA, while only collect that information necessary to make an accurate assessment of the location of the threat. 
       SUMMARY 
       [0006]    An aspect of the invention is a threat-detection system with a collection optical system having first and second channels (i.e., channel A and channel B), with respective narrow-band pass filters in a single or dual aperture configuration. Channel A is configured to pass at least one and preferably both of the potassium doublet lines of 769.90 nm and 766.49 nm, while channel B includes a guard band nominally centered at a guard band wavelength of λ B =790 nm. Light associated with channels A and B is respectively imaged onto respective one-dimensional or two-dimensional position-sensing photodiodes (PSDs). These PSDs have the advantage over conventional imaging sensors in that they have a large area and fast response. 
         [0007]    When channel A receives a signal and channel B does not, and when the channel A signal is temporally appropriate for a munitions source, the position voltage from the PSD of channel A is used to form a lookup table that correlates the PSD voltage with FOV position of the collection optical system. An indicator as to the source (threat) location in the FOV can be overlaid on a map, such as a GPS map or video map, that corresponds to the threat location in the sensor FOV. Multiple systems can provide updates to a single threat location map. An example threat indicator would be a light or icon representative of the nature of the threat. 
         [0008]    When channel A and channel B receive comparable signals, the signal source would not be considered a threat because it would not have the proper spectral content indicative of a threat source but would more likely be a solar glint, bright light or other non-munitions-based light source. When channel A receives a much stronger signal than channel B but the temporal extent is much longer than a few milliseconds, then the detected light source would not be considered a threat but would more likely be a source other than a discharged munition, such as wood fire, search light, munitions on fire, etc. Using a pair of PSDs allows the threat detection to have a relatively low false alarm rate as compared to conventional threat-detection techniques. 
         [0009]    Additional features and advantages of the invention are set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description that follows, the claims, as well as the appended drawings. 
         [0010]    It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic diagram of a generalized embodiment of threat detection system of the present disclosure shown monitoring a section of terrain; 
           [0012]      FIG. 2A  is a more detailed view of the collection optical system shown in  FIG. 1 , with single-aperture collection optics; 
           [0013]      FIG. 2B  is similar to  FIG. 2A , except that the collection optical system includes dual-aperture collection optics; 
           [0014]      FIG. 3  is a face-on view of an example large-area position-sensing photodiode (PSD); 
           [0015]      FIG. 4  is a more detailed schematic diagram of the processing electronics of the threat detection system of  FIG. 1 ; and 
           [0016]      FIG. 5  is similar to  FIG. 4  and illustrates an example embodiment where the GPS unit and electronic compass are replaced by a video camera unit, and the processor unit includes a field-programmable gate array (FPGA). 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  is a schematic diagram of a generalized embodiment of threat detection system  10  according to the present disclosure. Threat detection system  10  includes a collection optical system  20  and processing electronics (“electronics”)  100 . Shown in  FIG. 1  is the field of view FOV of collection optical system  20  and the corresponding terrain section  12  of terrain  1   3  as covered by the FOV. Within the field of view FOV is a “flash”  14  caused by the discharge of a potassium-based munition  15 , which a threat source. Note that flash  14  need not be located on terrain section  12  per se, but can be anywhere in the space (volume)  16  covered by field of view and associated with the observation of the terrain section. The size of space  16  and the size of terrain section  12  depend on the field of view FOV and the distance of the collection optical system  20  to the ground, so that these sizes can vary widely and can cover a relatively large space associated with a battle field or other hostile environment. In various examples, threat detection system  10  is airborne or is ground-based. Further, multiple threat-detection systems  10  can be linked for redundancy and to obtain a more accurate threat assessment of the territory being monitored. 
       Collection Optical System 
       [0018]      FIG. 2A  is a more detailed view of the collection optical system  20  of threat detection system  10  as shown in  FIG. 1 . Collection optical system  20  includes along an optical axis A 1 , collection optics  22 , a fold mirror  24 , and a beamsplitter  30 . The example collection optical system  20  as shown in  FIG. 2A  is a single-aperture system that includes a beamsplitter  30  that defines a second optical axis A 2  at an angle (e.g., a right angle) to axis A 1 . Collection optical system  20  further includes along axis A 1  a narrow-band filter  36 A that passes light in the wavelength band Δλ A , a focusing lens  38 A, and a large-area position-sensing photodiode (LAPSD)  40 A. Likewise, collection optical system  20  further includes along axis A 2  a narrow-band filter  36 B that passes light in a wavelength band Δλ B , a focusing lens  38 B, and a LAPSD  40 B. An example bandwidth for wavelength bands Δλ A  and Δλ B  is about 10 nm. 
         [0019]      FIG. 2B  is similar to  FIG. 2A  and illustrates an example embodiment of collection optical system  20 , wherein the collection optical system is a dual-aperture system having two collection optics  22 A and  22 B. In the dual-aperture configuration of  FIG. 2B , each collection optics  22 A and  22 B has its own axis A 1  and A 2  so that beamsplitter  30  is eliminated. Collection optics  22 A and  22 B are sighted so that they image the same terrain section  12 . 
         [0020]    The portion of collection optical system  20  and the portion of processing electronics (discussed below) associated with axis A 1  and wavelength band Δλ A  is referred to herein as “channel A,” while the portion of collection optical system  20  and the portion of processing electronics  110  associated with axis A 1  and axis A 2  (in the single-aperture embodiment) and wavelength band Δλ B  is referred to herein as “channel B.” The optical portion of optical channel A is referred to as “optical path A,” while the optical portion of optical channel B is referred to as “optical path B.” Note that unlike the single aperture embodiment of  FIG. 2A , in the dual aperture embodiment of  FIG. 2B , channels A and B do not share a portion of optical axis A 1 . 
         [0021]    Wavelength band Δλ A  includes at least one of the well-known potassium doublet lines 769.896 nm and 766.490 nm associated with light from flash  14  as emitted by the discharge of potassium-based munitions. In an example embodiment, wavelength band Δλ A  includes both potassium doublet lines 769.896 nm and 766.490 nm. Wavelength band Δλ B  is a guard band centered at a wavelength some distance away from the potassium doublet lines. An example wavelength band Δλ B  includes a wavelength with a high atmospheric transmission but that is not associated with potassium-based discharges. An example center wavelength λ B  for wavelength band Δλ B  is λ B =790 nm. 
         [0022]    Collection optics  22  and focusing lenses  36 A and  36 B are configured to form respective focus spots  42 A and  42 B at LAPSDs  40 A and  40 B, which are located at respective focus planes FPA and FPB. 
         [0023]      FIG. 3  is a face-on view of an example prior art LAPSD  40 A or  40 B. The PSD has a photosensitive surface  41 A or  41 B. In an example, PSDs  40 A and  40 B are silicon-based photosensors that in an example include processing circuitry  43  configured with pre-amplifiers and sum/difference circuits to provide an X-Y voltage signal (signals SA and SB, respectively) representative of the X-Y locations of the centroid of the average light intensity of respective focus spot  42 A and  42 B formed on respective photosensor surfaces  41 A and  41 B. In an example, X-Y voltage signals SA and SB respectively comprise voltage signals SVXA, SVYA and SVXB and SVYB corresponding to the (x,y) output from the respective LAPSDs. 
         [0024]    For LAPSDs  40 A and  40 B, focus spots  42 A and  42 B have respective X-Y positions (X FA , Y FA ) and (X FB , Y FB ), which in an example correspond to the centroid of the detected light. LAPSDs  40 A and  40 B have a relatively large bandwidth of over 100 kHz, which is advantageous in detecting short flashes of light such as flash  14 . 
         [0025]    An example LAPSD for use in system  10  is available from SiTek Electro Optics, Partille, Sweden as “SiTek SPC-PSD (duolateral dual axis).” Example LASPDs have dimensions ranging from 4 mm×4 mm to 45 mm× 45  mm. Linear PSDs can be up to 72 mm long. 
         [0026]    In an example, collection optics  22  is or includes a telescope that provides collimated light  23  to focusing lenses  38 A and  38 B. Also in an example, collection optics  22  has a field size that corresponds to the dimensions of the LAPSDs  40 A and  40 B. 
         [0027]    In the operation collection optical system  20 , light  23  from flash  14  from potassium-based munition  15  is discharged within the field of view FOV is captured by collection optics  22  and relayed along optical axis A 1  to beamsplitter  30 . Beamsplitter  30  splits light  23  so that a portion  23 A (e.g., half) of light  23  travels along optical axis A 1  while the remaining portion  23 B (e.g. the other half) of light  23  travels along optical axis A 2 . Light portion  23 A passes through filter  36 A, which passes only light within the narrow wavelength band Δλ A . This filtered light  23 A is focused onto LAPSD  40 A by lens  38 A, forming a focus spot  42 A thereon. The X-Y location of the particular focus spot  42 A on the PSD  40 A corresponds to the location of flash  14  in the field of view FOV. PSD  40 A generates electrical signal SA representative of the centroid of the energy of focus spot  42 A incident on LASPD  40 A. This energy centroid is designated (X FA , Y FA ). 
         [0028]    Likewise, light portion  23 B is incident upon filter  36 B, which only passes light within the narrow wavelength band Δλ B . Since light portion  23 B is from a potassium-based munition, this light will not make it through filter  36 B and so no focus spot will be formed. However, if light  23 ′ from another light source  14 ′ that happens to emit light with wavelength band Δλ B  and within the FOV, then this light will pass through filter  36 B and be focused as a focus spot  42 B. PSD  40 B generates electrical signal SB representative of the centroid of the energy of focus spot  42 B incident on LASPD  40 B. This energy centroid is designated (X FB , Y FB ). 
         [0029]    Thus, when a potassium-based munition is fired, channel A receives a strong light signal while channel B will not, receive any light signal. This indicates a high probability of a threat being detected in the optical system FOV. If both channel A and optical channel B receive light signals of nearly the same intensity, then even if the signal is strong, the source of the signal is assumed to be spurious, e.g., a solar glint or from a non-threat light emitter. 
       Processing Electronics 
       [0030]      FIG. 4  is a more detailed schematic diagram of the electronics  100  of the threat detection system  10  of  FIG. 1 . Electronics  100  includes X-Y position electronics units  102 A and  102 B respectively electrically connected to PSDs  40 A and  40 B. X-Y position electronics units  102 A and  102 B are configured to respectively receive detector electronic signals SA and SB, calculate an X-Y position on each PSD and generate respective electronic signals S 1 A and S 1 B representative of the respective X-Y spot positions (X FA , Y FA ) and (X FB , Y FB ). In an example embodiment, X-Y position electronics units  102 A and  102 B are configured to calculate the centroids of the average light intensity of focus spots  42 A and  42 B in determining the X-Y spot positions (X FA , Y FA ) and (X FB , Y FB ). In an example embodiment, position electronics units  102 A and  1028  are incorporated into the respective PSDs in the aforementioned signal processing circuitry  43  therein (see  FIG. 3 ). 
         [0031]    Electronics  100  also include an orientation unit  104  configured to generate provide orientation information for threat detection system  10 . The orientation information is embodied in an electrical signal S 6 . In an example, orientation unit  104  includes an electronic compass  106  and a global positioning system electronics unit (“GPS unit”)  110 . Electronic compass  106  is configured to calculate a compass heading for a pointing direction corresponding to the direction in which collection optical system  22  is pointing. Electronic compass  106  generates an electrical compass signal S 2  representative of the pointing direction of threat detection system  10 . This pointing direction is referred to hereinbelow as the “system pointing direction.” GPS unit  110  generates a GPS signal S 3  representative of the GPS coordinates of ground portion  12  within the FOV, i.e., the GPS unit provides a GPS map of the terrain section  12  under surveillance based on a set of received GPS coordinates, as well as any map information stored in the GPS unit. 
         [0032]    Electronics  100  also includes a processor unit  120  that includes, for example, a microprocessor MP and a memory unit MU, or a field-programmable gate array (FPGA) (see  FIG. 5 ). Processor unit  120  is electrically connected to X-Y position electronics units  102 A and  102 B, to electronic compass  106 , and to GPS unit  110 . Processor unit  120  is configured to store and process information from these components. In particular, processor unit  120  includes an X-Y/FOV look-up table  124  created from the X-Y position data embodied in electronic signals S 1 A from X-Y position electronics units  102 A. 
         [0033]    Microprocessor MP of processor unit  120  is configured to perform a comparison of the X-Y positions from signals S 1 A and S 1 B of X-Y position electronics  102 A and  102 B and determine whether the X-Y position (X FA , Y FA ) corresponds to an actual detected threat or if the detected light is from a source other than a legitimate threat (i.e., other than a munitions-based discharge). Note that the look-up table is based only on the X-Y positions (X FA , Y FA ) associated with channel A since only channel A includes information about an actual threat. 
         [0034]    If signal S 1 A is received but not signal S 1 B, and if signal S 1 A is a short pulse (e.g., on the order of milliseconds), then detected flash  14  can be considered a threat. If signals S 1 A and S 1 B are comparable, the conclusion is that the light represents a non-threat because it came from a non-munitions flash (such as flash  14 ′), regardless of their temporal characteristics. An example non-munitions flashes is a solar glint, searchlight, or other bright light source. 
         [0035]    In addition, processor unit  120  is configured to perform a temporal comparison of the formation of focus spots  42 A as measured by its timing t A  to ensure that the detected focus spots are from an actual threat. In one example, processor unit  120  measures the timing of t A  to a timing threshold τ e.g., t A &lt;τ milliseconds This measurement of signals SA from respective channels A serves to minimize the false alarm rate (FAR). 
         [0036]    For each X-Y position determined to be a threat, a corresponding field of view (FOV) location is calculated and stored in the X-Y/FOV position look-up table. Processor unit  120  also includes a map  128  formed, for example, from GPS information from GPS signal S 3  and electronic compass information from electronic compass signal S 2 . Map  128  alternatively includes a video image, as discussed below in connection with  FIG. 5 . Thus the orientation information from orientation unit  104  is embodied in signals S 2  and S 3 . 
         [0037]    Electronics  100  further includes a display  134  electrically connected to processor unit  120  for displaying threat information as described below. 
         [0038]    When threat detection system  10  is arranged at a known altitude over ground area  12 , the X-Y position information from PSDs  40 A as stored in the X-Y position look-up table  124  is combined with the system pointing direction and the GPS map to indicate the location of a threat (flash)  14  as superimposed on the GPS map of the terrain section  12 . For a ground-based system, the location of flash  14  on the PSDs corresponds to a location in the system FOV, which translates to a known direction from which the threat came. 
         [0039]    Display  134  can be used, for example, to show the location of threat detection system  10  on the map, and also show coordinate lines that indicate the location of flash  14  on terrain section  12 . When two or more of threat systems  10  are combined (e.g., networked together), the exact location of flash  14  can be determined by triangulation and displayed on each display  134 . 
         [0040]    In one example of a ground-based threat detection system  10 , LAPSDs  40  are replaced by one-dimensional PSDs  40 . This is because for a ground-based threat detection system, the vertical extent is limited and one-dimensional PSDs can be made larger than LAPSDs. 
         [0041]      FIG. 5  is similar to  FIG. 4  and illustrates an example embodiment of threat detection system  10  wherein GPS unit  110  and electronic compass  106  of orientation unit  104  are replaced by a video camera unit  105  having a video camera lens  105 A and video electronics  1058  that generates orientation information as embodied in a video signal S 6 . Video camera lens  105 A is configured so that it views at least the terrain section  12  that is being monitored by collection optical system  20 . 
         [0042]    The threat position is then overlaid on top of a “video image” map in map  128  instead of overlaid on a GPS-based map. This approach works best for small FOVs, e.g., up to about 20°. Also, threat detection system  10  of  FIG. 5  has as processor unit  120  in the form of (or that includes) an FPGA. 
         [0043]    A major advantage of this technique for processing threat detection is the compactness, portability and field robustness of the approach. Threats can be identified with a pair of lenses, pair of PSD&#39;s, pair of PSD amplifiers, 6 A/D converters (one for the X output, out for the Y output and one for the “Sum” output for getting intensity) for each PSD), and a computer with data acquisition software. The simplicity of the processing also allows the entire unit to be very compact and the large detection areas available for the PSDs allow a reasonable F/# lens system for each one to be designed with a large FOV. This further enables the compactness of a system that can cover a large FOV. 
         [0044]    It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.