Patent Publication Number: US-6705230-B1

Title: Active optical side-looking fuze

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to the field of ammunition and explosive devices. More particularly, this invention pertains to the field of fuzes, primers and igniting devices. In still greater particularity this invention pertains to an active optical fuze. By way of further characterization, but not by way of limitation thereto, the device pertains to an active optical, side looking fuze employing two narrow beams, which never have the surface below in their field of view. Fog and cloud discrimination circuitry is also included. 
     2. Description of the Prior Art 
     Optical fuzing systems exist in which optical beams are projected in a variety of directions. One system produces overlapping hollow conical fields around the circumference of the missile. Such an arrangement is shown in U.S. Pat. No. 3,793,958 issued to John G. Holt et al. on Feb. 26, 1974. That device employs logic circuitry requiring successively increasing amplitude return pulses to trigger the warhead. In addition,a threshold detector and an integrator are used wherein a constant false alarm rate is determined such that only signals greater than the mean value of noise signals are passed through the threshold detector. While satisfactory for its intended purpose such a device, if used in a surface directed missile, would require surface discrimination logic. In addition, the backscatter discriminator would be ineffectual in situations where the backscatter reflections were of extremely uneven character and exceeded the threshold sensitivity for detecting a minimum reflecting target. 
     Another optical fuzing system is shown in U.S. Pat. No. 3,786,757 issued to Irving Goldstein et al. on Jan. 23, 1974. This device uses a number of hollow beams paired such that the ordnance is triggered only if a signal is received first by one, then by the other of the pair of beams. Surface reflections would be received simultaneously and thus false triggering is rendered unlikely. While satisfactory for its intended purpose, such a device would be less useful in situations, such as fog or other aerosols, where the backscatter reflections are uneven and exceed the threshold sensitivity for detecting a minimum reflecting target. 
     For surface targets it would be highly desirable to have a fuze which is both unaffected by surface clutter and also has the capability to discriminate backscatter reflection such as that from fog or other aerosols. Such a device would be suitable for all weather use. 
     SUMMARY OF THE INVENTION 
     The invention operates as a pulsed optical radar employing narrow optical beams. In a roll stabilized missile the beams are never directed toward the surface below so that clutter discrimination logic is not needed. Detonation occurs when a beam intersects any target structure. 
     Fog and aerosol backscatter discrimination capability is incorporated into the fuze. A frequency selecting means in the form of a high-pass filter can be incorporated into the triggering circuitry as discrimination means. Another arrangement uses the time gain response of a sensitivity time control amplifier connected to the triggering circuitry and the detecting apparatus. Both discrimination devices operate at optimum efficiency for optical pulse widths of 10 milliseconds or less. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the invention in use on a roll stabilized missile; 
     FIG. 2 is a block diagram of the invention; 
     FIGS.  3 -A-D are plots of the response of the laser radar fuze to a target in fogs of varying visibility; 
     FIG. 4 is a plot of the frequency spectrum of the laser radar fuze return in fog; 
     FIGS. 5A and B are plots showing the effect of sensitivity time control for 100 foot visibility fog. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a roll stabilized missile  9  is shown with optical beams  10  directed such that they never have surface  11  in view. Detonation of the warhead occurs when either of the beams  10  intersect any structure on a target  12 . 
     A block diagram of the active optical side-looking fuze is shown in FIG.  2 . The fuze has two identical channels (1 and 2). Each channel collimates the radiation from an injection laser  14  into a narrow pencil (less than 2°) transmitter beam  10  using a ½ inch simple refractive lens  15 . In the instant embodiment a 0.5° beam was used. Injection laser  14  is pulsed by transmitter modulator  20  at a pulse repetition frequency determined by a PRF generator  16 . A receiver beam  13  in each channel has a field-of view of 1°, using a 1.25 inch simple refractive lens  8 . The receiver beam width is set at twice the transmitter beam width to alleviate the optical alignment problems in production. 
     A photodiode with a 1 K ohm load is used as a detector  18  in each channel. Filter  17  which may include a spectral filter and/or a polarizing filter, is mounted in front of detector  18 . The signal plus noise of detector  18  is amplified in a video amplifier  19  which uses an AGC loop  21  to maintain a constant noise output at the threshold. The output of video amplifier  19  is fed into a sensitivity time control circuit  22  which provides fog discrimination. 
     A threshold amplifier  23 , operating as a OR gate generates a constant amplitude output pulse when the output of either of sensitivity time control amplifiers  22  exceed a threshold which is set above the noise level. This occurs when either of fuze sensory beams  10  intersects a target. 
     The output of threshold amplifier  23  drives a time coincidence detector  24  which is keyed to targets in a predetermined range by a range gate  25 . When target video pulses occur in range gate  25 , coincidence detector  24  gives a constant amplitude output pulse. This output pulse triggers a standard pulse generator  29  which produces a target pulse of constant amplitude and width. Target pulses are fed into a linear pulse integrator  26  which gives an output if ten target pulses occur in 0.5 milliseconds. The output of pulse, generator  26  triggers the firing circuit  27  and fires the warhead (not shown) by conventional means. 
     A frequency filter  28  is connected between threshold amplifier  23  and coincidence detector  24 . Frequency filter  28  is included to provide additional fog discrimination. Either sensitivity time control  22 , frequency filter  28 , or polarizing filter  17  or any combination thereof may be used for fog discrimination. 
     Mode of Operation 
     Referring to FIG. 1, narrow pencil beams  10  render the fuze insensitive to the size of target  12  over the range interval. Beams  10  may be pointed in a variety of directions as long as they are directed in or above a horizontal plane passing through missile  9 . In the embodiment shown in FIG. 1, beams  10  are pointed slightly upward. In another embodiment, beams  10  are pointed in the horizontal plane and directed toward the aft end of the missile. The choice of direction depends upon standard engineering considerations taking into account such factors as expected trajectories, missile velocity, etc. 
     Referring to FIG. 2, the beam range interval is set by range gate  25 . Sixty (60) feet has been found to be one suitable range setting. The optimum range for a particular application may be selected using standard engineering and design principles. Channels  1  and  2  operate independently of one another and detect the presence of target  12  when beam  10  intersects any structure on target  12  within the predetermined range. 
     Linear pulse integrator  26  outputs a pulse if ten target pulses occur in 0.5 milliseconds. The mean time between false alarms is therefore in excess of 10 4  seconds. For a probability of detection of 0.99, the decision logic requires a single pulse S-N of 7 db at the output of video amplifier  19 . 
     Because receiver beams  13  are narrow (1° in the present embodiment), the fuze is thermal noise limited and an avalanche silicon photodiode detector may be employed as photodiode  18 . Use of an avalanche diode enhances the effective maximum detectable target range in fog or other backscatter situations. An alternative is to reduce the effective maximum range or increase the peak power of laser source  14 . A 10 w GaAs injection laser is one suitable laser source  14  because it gives high peak power at room temperature. Different types of laser sources may be employed for different situations. 
     Plots of the instantaneous fog and target backscatter signal in Watts/ft 2  in the receiver field are displayed in FIG. 3 for clear sky and fog visibilities of 5000, 500, and 100 feet. These curves show the fog and target return in one sensory beam as a function of time after the initiation of a ten nanosecond, ten watt IR pulse with the target situated at a range of 50 feet immersed in fog of specified visibility. The sensory beam width considered was 1°. The transmitter field of view was 0.5°. The lens efficiency was 0.5. The dotted target return curve corresponds to the peak target return signal at a given round trip time delay after pulse initiation for a 0.2 reflectivity target. The return signal from a target at 50 feet is centered at 100 nanoseconds from time zero. The leading edge of the transmitted pulse is time zero. The peak fog return occurs approximately 15 nanoseconds after pulse initiation or, equivalently, at a range of 7.5 feet. 
     FIGS. 3 a-d  illustrate that the target return pulse is always greater than the fog signal over the time interval wherein the target return signal occurs. The fog back-scatter signal has a peak return greater than the target return during the time interval before the target return pulse arrives in fog with visible ranges less than 500feet. Therefore, no single threshold, such as is indicated by the horizontal dotted line in FIGS. 3 a-d , would be satisfactory as protection against fog. 
     In FIGS. 3 a-d  the fog signal rises exponentially in approximately 15 nanoseconds, then decays exponentially. The fog signal behaves like a critically damped exponential wave form with a time constant of 15 nanoseconds. Because of this characteristic a matched filter, incorporated into the fuze design, would reduce the fog return magnitude with respect to the target return. FIG. 4 shows a plot of the normalized frequency spectrum for the 15 nanosecond rise time fog return and a 10 nanosecond target return pulse. 
     Referring again to FIG. 2, some fog suppression may be obtained from a frequency filter  28  inserted between the detector  24  and the video amplifier  19 . The effect of frequency filter  28 , which may be a high pass filter, is shown in FIG.  4  and is based on a target return pulse being always greater than the fog return signal over the time interval when the target signal occurrs. Consequently, fog discrimination can be provided by decreasing the receiver sensitivity for near range signal return. 
     Referring again to FIG. 3, it is discerned that the peak target return signal is always greater than the fog return signal over the interval wherein the target signal occurs. Therefore, some form of receiver sensitivity versus time control could be incorporated into the fuze to suppress the early peak fog return. This fog discrimination scheme is illustrated in FIG.  5 . 
     FIG. 5A is a duplication of the 100 foot visibility fog return shown in FIG. 3 d . Superimposed on this plot is a sensitivity gain control curve which would be the time gain response of the sensitivity time control amplifier ( 22  in FIG.  2 ). The instantaneous time product of curves  1  and  2  in FIG. 5 a  is plotted in FIG. 5 b . Referring to FIG. 5 b , the early fog return may be suppressed by having a single threshold at the sensitivity time control amplifier output above 10 −6  Watts/ft 2 . This threshold gives fog protection over the 60 foot range interval for pulse widths of 10 nanoseconds or less. 
     The foregoing description, taken together with the appended claims, constitutes a disclosure such as to enable a person skilled in the electronic and mechanical engineering arts and having the benefit of the teachings contained therein to make and use the invention. Further, the structure herein described constitutes a meritorious advance in the art which is unobvious to such skilled workers not having the benefit of these teachings.