Abstract:
A proximity fuzing system for missile warhead utilizes an electrostatic probe to detect the electric field inherently associated with an airborne target. The probe plates are oriented perpendicular to the missile longitudinal axis and are short circuit loaded to develop a probe current signal whose initial slope is analyzed to qualify the target being engaged on a near-miss trajectory as a valid target and whose first zero current crossing is utilized as a trigger point for detonating the missile warhead before the missiles reaches a position of minimum miss-distance relative to the target.

Description:
BACKGROUND OF THE INVENTION 
     Current missile fuzing systems typically utilize RF (radar) or optical (infrared) sensors to detect missile proximity to an airborne target and to detonate the missile warhead at the opportune moment in the missile trajectory to maximize the damage inflicted on the target. Unfortunately, these active proximity fuzing systems are susceptible to countermeasures effected by the target. RF sensors can be jammed electronically, and optical sensors can be confused by flares. The results are either no warhead detonation or detonation outside the target kill range. 
     It would of course be desirable that a proximity fuzing system not be susceptible to target countermeasures. To this end, serious consideration is being given to utilizing electrostatic sensors in proximity fuzing systems, see, for example, Ziemba et al. U.S. Pat. No. 4,291,627, issued Sept. 29, 1981. As is well known, the outer surface of any airborne target becomes electrostatically charged while in flight through the atmosphere due to the effects of air friction and engine ionization generation. Thus, detection of the electrostatic field closely surrounding an airborne target can provide the means for detecting the proximity of an attacking missile to the target. By appropriate processing of electrostatic sensor signals, the warhead can be detonated at a point in the missile trajectory proximate the target to maximize the possibility of target kill, see, for example, Krupen U.S. Pat. No. 4,183,303, issued Jan. 15, 1980. Since this inherent electrostatic field can not be readily recreated in disassociated relation to the target, engaging missiles equipped with electrostatic fuzing system sensors are not susceptible to being &#34;spoofed&#34; by any countermeasures a target can employ. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to provide an improved proximity fuzing system for missiles engaging airborne targets. 
     A further object is to provide an attacking missile fuzing system of the above-character, which is essentially immune to target countermeasures. 
     An additional object is to provide a proximity fuzing system of the above-character, wherein the potential for target kill by an attacking missile is maximized. 
     Other objects of the invention will in part be obvious and in part appear hereinafter. 
     Pursuant to the foregoing objectives, the present invention provides a passive proximity fuzing system for an attack missile, which utilizes an electrostatic probe to detect missile entry into the electric field inherently associated with an airborne target. The electrostatic probe is in the form of a pair of parallel spaced, conductive plates oriented perpendicular to the missile longitudinal axis, i.e., perpendicular to the missile trajectory path. The short circuit current signal response of the probe to entry into the target electric field is amplified and processed in accordance with a target algorithm to determine that the increasing initial slope of the probe signal waveform is within an established range of slope values characteristic of a valid target. If this criteria is satisfied, the missile warhead is detonated on the first zero crossing of the probe signal waveform, which corresponds to the most opportune point on a missile near-miss trajectory to inflict maximum damage on the target. 
     The invention according comprises the features of construction, combination of elements and arrangement of parts, all of which will be detailed below, and the scope of the invention will be indicated in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a full understanding of the nature and objects of the present invention, reference may be had to the following Detailed Description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a pictorial representation of a missile entering the electric field associated with an intended airborne target and equipped with an electrostatic proximity fuzing system in accordance with the present invention; 
     FIG. 2 is a circuit schematic, partially in block diagram form, of the electrostatic proximity fuzing system of the present invention; 
     FIG. 3 is a plot of the electrostatic probe short circuit current signal response to entry into a target electric field; and 
     FIG. 4 is a flow chart of the target algorithm for processing the probe signal waveform of FIG. 3. 
     Corresponding reference numerals refer to like parts throughout the several views of the drawings. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 portrays an airborne target 10, such as an airplane or helicopter, which in flight through the atmosphere has accumulated the indicated surface charges. These electrostatic charges create an electric field pattern represented by flux lines 12 radiating from the target and lines 14 of equal electrostatic potential encircling the target at various radial increments. It will be appreciated that the illustrated target electric field pattern is idealized since it does not reflect the disruption created by the surface charges accumulated on the surface of a missile 16 illustrated as having entered the target electric field on a target-engaging, near-miss trajectory 16a. 
     The body of missile 16 includes a nose section 18, a finned tail section 20 and intermediate warhead section 22. The nose section contains the electrical components of the proximity fuzing system of the present invention including an electrostatic probe, generally indicated at 24 and including a pair of parallel spaced, electrically conductive plates 26 and 8 oriented perpendicular to the missile longitudinal axis 17. 
     As seen in FIG. 2, plate 28 is grounded, while plate 26 is connected through a resistor R1 to the inverting input of an operational amplifier 30 in a manner to establish short circuit loading of electrostatic probe 24. The non-inventing input of amplifier 30 is referenced to a regulated voltage established at the junction between a resistor R2 and a zener diode D1 connected in series between positive supply voltage V s  and ground. Amplifier feedback is provided by resistor R3. 
     Amplifier 30 functions to convert the short circuit current signal response of electrostatic probe 24 to entry into the electric field of target 10 into a proportional signal voltage which is applied through a resistor R4 to the inverting input of a high gain operational amplifier 32. The non-inverting input of this amplifier is referenced to the regulated cathode voltage of zener diode D1 through a resistor R5. The parallel combination of resistor R6 and capacitor C1 provides high frequency roll-off for amplifier 32. The amplifier output is connected through a resistor R7 to a microprocessor 34. A filter capacitor C2 connects the microprocessor input to ground. The microprocessor converts the amplified analog probe signal received from amplifier 30 to a digital signal which is processed in accordance with a target algorithm to determine if target 10 is a valid target, and, if so, when during near-miss missile trajectory 16a to fire detonator 36 and explode the missile warhead so as to inflict maximum target damage. 
     By virtue of the illustrated longitudinal orientation of the electrostatic probe plates 26, 28, i.e., perpendicular to the missile longitudinal axis 17, and the short circuit loading of the probe, the probe current flowing in the input circuit of amplifier upon intercepting the electric field of an airborne target on near-miss trajectory 16a (FIG. 1) is of the waveform 38 seen in FIG. 3. The probe short circuit current flows with an initial increasing slope 38a which has been determined to vary as a function of the reciprocal of the range (R) of the probe to the target raised to the third power (1/R 3 ). While initial current flow and slope are illustrated as being negative, it will be appreciated that these signal characteristics may be positive or negative depending on the polarity of the charged target. As the target range closes, the short circuit current waveform suddenly reverses slope, crosses zero at point 38b and rises to a peak 38c of opposite polarity coincident with the point in the missile trajectory 16a of closest proximity to the target, indicated by dash line 39 in FIG. 2 and point 40 in FIG. 1. Beyond this point of minimum miss distance or range measured along dash line 41 (FIG. 1), the range-opening portion of the probe signal waveform is seen to be a mirror image of the range-closing waveform portion. It has been further determined that inflection point 38b or zero-crossing of the probe signal waveform occurs when probe 24 arrives at a position 42 where a dash line 43 intersects the missile trajectory 16a at an angle 44 of approximately 35° in front of dash line 41. Since zero-crossing inflection point 38b is a readily identifiable point on the probe signal waveform, and it occurs when probe 24 reaches position 42 in missile trajectory, these simultaneous events represent an ideal burst-point locus at which to detonate the missile warhead. By exploding the warhead before the missile reaches point 40 in its near-miss trajectory most proximate the target, missile body fragments are propelled by the combination of explosive and inertial forces more directly toward the target, thus inflicting maximum possible damage. It is seen that significantly less target damage is achieved if the warhead is detonated when probe 42 arrives at trajectory position 40 of minimum miss distance 
     The target algorithm by which microprocessor 34 processes probe signal waveform 38 to discriminate between valid and invalid targets and, if a valid target is identified, to detonate the missile warhead at F inflection point 38b is disclosed in the flow chart of FIG. 4. Digital data representing the probe signal waveform is sampled on a real-time basis at a rapid rate, e.g., every 0.5 ms., as indicated in step 50. After three consecutive data samplings, the next data point is predicted based on the three data points obtained from these previous samplings in step 51. This prediction is based on the probe signal waveform 38 having an initial slope 38a that varies as a function of the reciprocal of the range cubed (1/R 3 ). In step 52, the target algorithm determines whether the sampled data point, considered with the previous three data points as a data point set, is within a predetermined tolerance of the predicted data point and thus can be considered a valid data point. If not, steps 51 and 52 are repeated for the next sampled data point. As long as the condition of step 52 is satisfied on a running four consecutive data point basis, the number of valid data points in successive data point sets is counted in step 53. If an invalid data point is encountered, the steps are repeated until the number of consecutive valid data points reaches a predetermined minimum number, for example thirty, established in step 53. Once this condition is satisfied, the magnitude of the net sampled data point is inspected to determine if it has reached a valid target threshold established in step 54. This threshold is indicated at 54a in FIG. 3. If not, steps 50-53 are repeated for a new set of sampled data points. When the conditions of steps 52 and 53 are again satisfied, and the most recent data point magnitude reaches the valid target threshold 54a of step 54, step 55 is activated. Here the next data point is sampled, as indicated at 56, and is tested in step 57 to determine if its magnitude exceeds valid target threshold 54a. If not, the target algorithm starts over with new sets of data points. However, if this next data point exceeds the valid target threshold, the subroutine including steps 55-57 is repeated to see if a predetermined minimum number of consecutive data points are in excess of the valid target threshold magnitude. When this minimum number, for example five, is reached in step 55, the decision is made that a valid target is being engaged. At this point, the target algorithm repetitively samples next data points (step 58) looking for zero-crossing inflection point 38b (step 59) and, when detected, warhead detonator 36 is trigqered, as indicated in step 60. 
     It will be appreciated that the target algorithm of FIG. 4 could be truncated to accommodate exceptionally high target closure velocities. For example, a valid target recognition decision could be made based on satisfaction of the step 53 condition, and the target algorithm would go directly to step 58, skipping steps 54-57. Alternatively, steps 51-53 could be modified such as to simply look for a constantly increasing slop between a predetermined number of consecutive data points. 
     It is seen that the target algorithm of FIG. 4 is uniquely constructed to reliably identify from the probe short circuit current signal waveform that missile 16 is engaging a valid airborne target 10 and thereafter to detonate the missile warhead at the most opportune moment in a near-miss trajectory to maximize target kill potential. Since the target algorithm only processes data points on the signal waveform below the signal peaks, any clipping of the analog signal peaks does not affect valid target recognition and warhead detonation. Moreover, target recognition is independent of analog signal gain, and thus amplifier gain may be set as high as ambient noise conditions permit. 
     If missile 16 is on an impact trajectory with target 10, the zero-crossing inflection point 38b does not occur, and the missile warhead explodes upon target impact. 
     From the foregoing description it is seen that the objects set forth above, including those made apparent hereinabove, are efficiently attained, and, since certain changes may be made in the embodiment set forth without departing from the scope of the invention, it is intended that all matters of detail be taken as illustrative and not in a limiting sense.