Abstract:
A two-color infrared warning device, having both wide-angle and narrow-angle search capabilities, and a directional countermeasure device are mounted on a gimbal pointing in the same direction. The warning device and gimbal are initially operated in a wide-angle, step-stare mode to search for threats. When a potential hostile target is found, the warning device changes to a narrow-angle mode to determine whether the potential target is hostile. When a hostile target is identified, the directional countermeasure device, which is pointed at the target by virtue of its mounting with the warning device on the gimbal, is activated.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the protection of aircraft against missile threats, and, more particularly, to a method and apparatus for locating, identifying, and taking action against hostile targets such as missiles. 
     Missiles fired at aircraft are usually guided either by a light seeker or by radar. Of the various types of seekers, infrared light seekers pose some of the greatest problems to aircraft defense. Unlike radar seekers, infrared seekers are passive and do not emit a detectable signal prior to the firing of the missile. Pilots therefore have little warning of their presence prior to the firing of the missile. Infrared-guided missile systems are relatively inexpensive, and human-portable units are widely available. 
     There has been a continuing effort to develop sensor systems and countermeasures that are carried on aircraft and are used to detect missile threats, particularly infrared-guided missiles, and take action against the missile threats. The sensor system must be effective to detect the infrared signature of a relatively small-sized missile at as great a distance as possible, in order to allow time for the countermeasure to be effective. In one approach, a wide-angle, two-color staring sensor system has been suggested to be particularly effective in detecting threats. This approach is limited by its low resolution and thence its ability to detect potential targets at great distances and susceptibility to smearing of the image, as well as the incomplete status of the detector technology. Additionally, the detection and warning components of the system must be integrated with the countermeasures components of the system. 
     There is an ongoing need for an improved approach to the protection of aircraft against missile threats and the like. The present invention fulfills this need, and further provides related advantages. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for the self-protection of aircraft against missile threats. The approach integrates a search, detection, and identification unit, combined with a directional countermeasure device. Searching is accomplished with high resolution over a large angular range to identify potential targets. Upon finding a potential target, narrow-angle viewing is performed to-determine whether the potential target is hostile. If the target is hostile, the directional countermeasure device, which is pointing at the hostile target by virtue of its being mounted on the gimbal of the searching apparatus, is utilized against the hostile target. The entire apparatus fits within a small form factor, and is fully automatic in its operation. 
     An aircraft self-protection system comprises a gimbal platform, and a gimbal-platform servo controller that receives an input signal from the gimbal platform and provides a stabilization command to the gimbal platform. A lens/detector unit is mounted on the gimbal platform. The lens/detector unit includes a two-color infrared detector having an output signal, a wide-angle lens system having a scene energy input from a pointing direction and a wide-angle-lens output beam focused onto the two-color infrared detector, and a narrow-angle lens system having the scene energy input from the pointing direction and a narrow-angle-lens output beam focused onto the two-color infrared detector. The lens/detector unit also includes a controllable optical switch having as inputs the wide-angle-lens output beam and the narrow angle-lens output beam, and an optical switch output beam, and a beam splitter (preferably a dichroic reflector) that splits the optical switch output beam into a first-color beam that falls onto a first-color region of the two-color infrared detector and a second-color beam that falls onto a second-color region of the two-color infrared detector. A directional countermeasure device (preferably a laser that upsets the seeker of an incoming missile threat) is mounted on the gimbal platform and is aimed in the pointing direction. An electronics unit mounted off the gimbal platform receives the output signal of the two-color infrared detector, analyzes the output signal, and provides command signals to the gimbal-platform servo controller, the two-color infrared detector, and the optical switch. The electronics unit desirably includes tracking logic which, in a searching mode of operation, commands the gimbal-platform servo controller to perform a repeating step-stare tracking mode of operation. 
     A method for protecting an aircraft uses a self-protection system mounted on a controllable gimbal. The self-protection system comprises a lens/detector unit having a wide-angle lens system, a narrow-angle lens system, a two-color separator that selectably receives scene energy from the wide-angle lens system or the narrow-angle lens system, and a two-color infrared detector that receives the scene energy in two colors from the two-color separator. The self-protection system also includes a directional countermeasure device. The apparatus described above is preferably employed as an integrated unit, for affordability. The method includes the step of searching for a target using the lens/detector unit by the repeated steps of viewing a viewed portion of a scene through the wide-angle lens system using the infrared detector in a staring mode, analyzing the viewed portion of the scene for the presence of a potential target, and, if no potential target is found, stepping the gimbal to a new portion of the scene, and thereafter repeating the steps of viewing, analyzing, and stepping until the potential target is located at a target location in the step of analyzing. Upon finding the potential target at the target location in the step of searching, the method includes the steps of identifying the target by switching the lens/detector unit to the narrow-angle lens system, so that the two-color infrared detector views the potential target at the target location through the narrow-angle lens system, and determining the nature of the potential target as either hostile or not hostile. Upon determining that the potential target is not hostile, the step of searching is repeated. Upon determining that the potential target is hostile, the directional countermeasure device aimed at the hostile target is activated. The aiming of the directional countermeasure device at the target is maintained in the narrow-angle mode as long as targeting is required. 
     This step-stare approach achieves a substantially greater resolution than possible with a staring system, because the lens/detector is stepped across the field of regard and also because the view may be changed from a wide angle to a narrow angle during the acquisition and identification process. The sensor is used in a wide-angle mode to make the initial acquisition of the potential target, and then switched to the narrow-angle mode for further identification and targeting. The present apparatus operates autonomously and does not require separate guidance by the aircraft itself. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an apparatus according to the invention; 
     FIG. 2 is a block flow diagram of a method for protecting an aircraft; and 
     FIG. 3 is a chart of a typical step-stare pattern. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts an aircraft self-protection system  20 , which is a preferred embodiment of the present invention. The self-protection system  20  includes a gimbal platform  22  upon which some of the other elements are mounted. The gimbal platform  22  may be of any operable type, such as a roll/nod, x-y, or elevation-over-azimuth gimbal. The positioning of the gimbal platform  22  is controlled with a closed-loop servo system. A gimbal-platform servo controller  24  receives an input signal  26  from the gimbal platform  22  and provides a stabilization command and control signal  28  to the gimbal platform  22 . 
     A lens/detector unit  30  is mounted on the gimbal platform  22 , so that it may be pointed by the movement of the gimbal platform  22 . The lens/detector unit  30  includes a two-color infrared detector  32  having an output signal  34  that carries the image of the scene. The detector  32  is preferably an infrared focal plane array (FPA) detector, which construction is known in the art. The term “two-color” means that the detector is able to process light of two different colors within an infrared wavelength band to which the detector is sensitive. It does not suggest that the detector must be able to produce separate output signals at two different specific wavelengths from broad spectrum light that impinges upon the detector. 
     The lens/detector unit  30  includes two different lens systems and a means to select between them for input to the detector  32 . A wide-angle (wide field of view, or WFOV) lens system  36  has a scene energy input beam  37  parallel to a pointing direction  38  and a wide-angle-lens output beam  40  focused onto the two-color infrared detector  32 . A narrow-angle (narrow field of view, or NFOV) lens system  42  has a scene energy input beam  43  from the pointing direction  38  and a narrow-angle-lens output beam  44  focused onto the two-color infrared detector  22 . 
     A controllable optical switch  46  has as an input the wide-angle-lens output beam  40  and the narrow-angle-lens output beam  44 , and an optical switch output beam  48 . The optical switch  46  is illustrated as a mirror  50  that may be moved into the beam path by a motor  52 . When the mirror  50  is in a retracted position, the wide-angle-lens output beam  40  is directed to the detector  32 . When the mirror  46  is moved to intercept the light path, the wide-angle-lens output beam  40  is blocked and the narrow-angle-lens output beam  44  is reflected to the detector  32 . 
     A beam splitter  54  splits the optical switch output beam  48  into a first-color beam  56  that falls onto a first-color region  58  of the two-color infrared detector  32 , and a second-color beam  60  that falls onto a second-color region  62  of the two-color infrared detector  32 . The beam splitter  54  may be of any operable type. It is preferably a dichroic beam splitter, but it could be a conventional beam splitter with an arrangement of mirrors and filters. 
     A directional countermeasure device  64  is mounted on the gimbal platform  22  and has an output beam  66  aimed in the pointing direction  38 . The output beam  66  of the directional countermeasure device  64  is thus aimed coaxially with the respective input light beams  37  and  43  to the lens systems  36  and  42 . In a practical device the maximum separation between the various beams  37 ,  43 , and  66  is at most a few inches, so they are essentially coaxial and collinear. The directional countermeasure device  64  may be of any operable type. It is preferably a laser that upsets the seeker detector of a missile. 
     An electronics unit  70  for the lens/detector unit  30  is mounted off the gimbal platform  22 . The electronics unit  70  includes a target discriminator  72  which receives the output signal  34  carrying the two different color images produced by the two-color infrared detector  32  and analyzes the output signal  34  to identify potential targets in the image. For example, the target discriminator removes clutter from the image by digital filtering, and identifies features that may be potential targets. Techniques for performing target discrimination are known in the art for other purposes and are not part of the present invention. The target discriminator  72  provides this information to an acquisition/tracking logic device  74 , which is also part of the electronics unit  70 . The acquisition/tracking logic device  74  directs the lens/detector unit  30  to aim more precisely at the potential target and switch to a mode of operation that allows more precise identification, as required. The acquisition/tracking logic device  74  provides a set-point command signal  76  to the servo controller  24  that causes the gimbal platform  22  to aim the pointing direction  38  at the potential target. The acquisition/tracking logic device  74  also sends a command signal  78  to the optical switch  46  to cause it to switch to the narrow-angle-lens output  44 . The potential target may be thereby better identified. 
     Based upon the additional information gained in the narrow-angle mode, the acquisition/tracking logic device  74  determines whether the target under examination is hostile. Such techniques are known in the art for other purposes, and are not part of the present invention. If it is judged to be hostile, the acquisition/tracking logic device  74  activates the directional countermeasure device  64  by a command signal  80 . 
     FIG. 2 illustrates in greater detail the method used to protect the aircraft. A self protection system mounted on a controllable gimbal is provided, numeral  90 . The self-protection system comprises a lens/detector unit having a wide-angle lens system, a narrow-angle lens system, a two-color separator that selectably receives scene energy from the wide-angle lens system or the narrow-angle lens system, and a two-color infrared detector that receives the scene energy in two colors from the two-color separator, and a directional countermeasure device. The self-protection system  20  described above is preferred, and reference will be made in the following discussion to its elements. 
     The apparatus searches for a target using the lens/detector unit  30  by the repeated steps of viewing a viewed portion of a scene through the wide-angle lens system  36  in the WFOV mode using the infrared detector  32  in a staring mode, numeral  92 . The viewed portion of the scene is analyzed for the presence of a potential target. If no potential target is found, the gimbal platform  22  is stepped to a new portion of the scene, numeral  94 . The steps of viewing and analyzing (step  92 ) and stepping (step  94 ) are repeated until the potential target is located at a target location in the step of analyzing. 
     The step-scan approach of steps  92  and  94  is particularly effective because it allows a very wide field of regard while a high spatial resolution is maintained. An example of a scanning raster is shown in FIG.  3 . The gimbal platform  22  is sequentially stepped in the pattern  1 ,  2 , etc. as shown, with the stepping halted for a sufficient time to perform a staring view of the scene in each of the stepped angular locations. The raster is repeated upon reaching the last position, here indicated as position  32 . In a design presently favored by the inventor, the detector  32  has a total view of 50×50 degrees, but is limited to a view of 50×25 degrees because it views two colors. Integration time during each staring step  92  is about 0.5 milliseconds. As the detector  32  is being read out, the gimbal platform steps to the next location on the raster. The image is moved 48 degrees in each step, so that there is a 2 degree overlap to account for any errors in gimbal pointing and the like. The result is that complete coverage over a 2.2π steradian field of regard is accomplished in about 0.5 seconds, unless a potential target is identified which causes the self-protection system to move to step  96 . With a 512×512 FPA array in the detector  32 , the pixel size is 1.4 milliradians. For an integration time of about 0.5 milliseconds, a detection range for a typical missile of about 22 kilometers is expected. 
     Upon finding the potential target at the target location in the step of searching, step  92 , the target is identified, numeral  96 , by switching the lens/detector unit  30  to the narrow-angle lens system  42  (NFOV mode) and aiming the pointing direction  38  toward the potential target location so that the input beam  43  is received from the potential target location. The two-color infrared detector  32  views the potential target at the target location through the narrow-angle lens system  42 . The electronics unit  70  is employed to evaluate the nature of the potential target as not hostile or hostile. This evaluation is by techniques known in the art for other applications, such as the infrared signature and emissions of the potential target, its size, and its pattern of movement. Upon determining that the potential target is not hostile, the step  92  of searching is repeated, numeral  98 . 
     Upon determining that the potential target is hostile, the directional countermeasure device  64  is activated, numeral  98 . The directional countermeasure device  64  is already aimed at the hostile target by virtue of the fact that its output beam  66  is aimed in the pointing direction  38 , the same direction from which the beam  43  is received. While the target being identified and the directional countermeasure device  64  is activated, the command  76  is used to cause the gimbal platform  22  to track the target so that the countermeasure pointing direction  66  remains locked onto the target. Upon disposition of the target, a new search is started, numeral  100 , by returning to step  92 . 
     Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.