Abstract:
An infrared countermeasures system is provided by ganging a plurality of modulators each of which modulates the output of a radiant source to generate at least one collimated beam of radiation. The modulators are so disposed with respect to each other that the beams generated the reby are staggered in angular phase. When the modulators are rotated together they will provide at a point in space remote therefrom a signal comprising a burst of pulses followed by a dead time when no signal is present.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to infrared countermeasures systems and more particularly to multipulse infrared countermeasures systems. 
     Modulated infrared sources are employed to countermeasure heat seeking missiles which home in on the heat generating portions of a target such as the engines of an airplane or helicopter. Certain of such systems provide a counter measuring signal to a heat seeking missile through spatial modulation by sweeping a beam in space. Reflective optics are rotated about a source of infrared radiation such that the missile receives a pulse of energy each time the beam passes the missile. 
     While illuminating the missile with single pulses of infrared radiation provides an effective countermeasure against the missile, it has been determined that better protection can be afforded by periodically illuminating the missile with bursts of pulses rather than single pulses. Prior to the present invention, the only method of obtaining pulse bursts was to provide concentric modulators driven at different speeds wherein a source is modulated by a first modulator and the output of the first modulator is further modulated by a second modulator. While this system performs more than adequately, it is limited in the amount of achievable gain, since the output from the first modulator is not reimaged prior to being applied to the second modulator. Furthermore, this system requires multiple drives since the modulators must be run at different speeds to achieve multipulse operation. This later system is disclosed in U.S. patent application Ser. No. 543, 299, filed by the inventors of this application on Jan. 20, 1975, and is assigned to the assignee of the present application. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an improved infrared countermeasures system. 
     It is another object of this invention to provide a high gain counter-measures system which illuminates a missile with bursts of pulses. 
     Briefly, in one embodiment a plurality of modulators is provided in which each collects and collimates the energy from a radiant source to form a beam. The modulators are ganged in such a fashion that the beams therefrom are angularly phased such that when the ganged modulators are rotated, the beams are swept past points in space to generate at such points a signal comprising a burst of pulses followed by a substantial dead time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a triaxial cavity modulator; 
     FIG. 2 is a drawing of a typical waveform obtained when the modulator of FIG. 1 is employed to modulate a source of radiant energy; and 
     FIG. 3 is a simplified schematic of an infrared countermeasures system employing the modulator of FIG.  1 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 1 of the drawings, there is illustrated thereby a modulator for an improved infrared countermeasures system. The object of this system is to provide bursts of pulses of infrared energy which when received by a heat seeking missile will cause the missile to avoid the target carrying the countermeasure system at which it is directed. 
     The output of such a countermeasures system will provide a multi-pulse (three in this example) signal typically as shown in FIG. 2 of the drawings. 
     Referring again to FIG. 1, the modulator includes three ganged modulators,  10 ,  12 , and  14 , each of which is made up of elements to properly shape the output from a source (not shown) which is disposed within the center of the modulators  10 ,  12  and  14  in the holes  16 ,  18 ,  20 , and  22 . 
     Looking at modulator  10 , it is seen that it includes three cavities,  24 ,  26 , and  28 . Cavity  24  is defined by a pair of cylindrical parabolas,  30 ,  32 ; cavity  26  is defined by a pair of cylindrical parabolas,  34 ,  36 ; and cavity  28  is defined by a pair of cylindrical parabolas,  38 ,  40 . 
     The cylindrical parabolas  30 ,  32 ;  34 ,  36 ; and  38 ,  40  collect and collimate the energy radiated from a source of radiation which would be disposed within the holes  16 - 22  such that a high intensity beam is formed. Accordingly, when the modulator  10  is rotated about the radiation source, spatial modulation will be provided at points in space remote from the source. Therefore, a single pulse of energy will be received at a point in space for each complete revolution of modulator  10 . If a pulse repetition frequency (PRF) N/min is desired, the modulator must be rotated at N RPM. 
     To reduce the speed of the modulator and yet achieve the same PRF, it is well known to provide additional reflecting optics on the same modulator to generate multiple beams. Thus, to reduce the RPM of modulator  10  by one-half N/2 RPM and still achieve N PRF, a second set of reflecting optics is arranged on the modulator  10 . This second set of reflecting optics is identical to the first set (cylindrical parabolas  30 ,  32 ;  34 ,  36 ; and  38 ,  40 ) but disposed on the back side of the modulator  10  (not shown) 180° from the first set shown. Therefore, two beams would be generated by modulator  10  displaced 180° apart such that if the modulator was rotated at a speed of N/2 RPM, a point in space would receive pulses at a PRF of N. 
     The modulator  10  as described is the subject matter of U.S. patent application Ser. No. 879, 541, filed Feb. 21, 1978, by the inventors of this application and assigned to the assignee of this application. While three cylindrical parabolas are shown to form each beam, more or less can be used, and the manner in which an individual beam is developed forms no part of the present invention. 
     As mentioned earlier, the object of this invention is to provide high intensity bursts of pulses at points in space rather than single pulses as described with respect to modulator  10 . The preferred manner of accomplishing this is to provide additional modulators  12  and  14  (for the case where bursts made up of three pulses are required.) 
     Modulators  12  and  14  are constructed identically to modulator  10 , and the modulators  10 ,  12 , and  14  are disposed to rotate together to modulate a source of radiation. 
     While the modulators  12  and  14  are constructed like modulator  10 , the arrangement of the reflecting optics forming the beams are angularly displaced from the reflecting optics forming the beams of modulator  10 . 
     One beam from modulator  12  is derived by collecting and collimating the output of a source contained in the center of the modulator by cylindrical parabolas  42  and  44  forming a cavity  46 ;  48  and  50  forming a cavity  52 ; and  54  and  56  forming a cavity  58 . The cylindrical parabola sections  60  and  62  of modulator  12  are employed to form a second beam 180° displaced from  5  the beam formed by parabolas  42 ,  44 ,  48 ,  50 ,  56 , and  58  in order to generate two beams by modulator  12  as mentioned earlier with respect to the description of modulator  10 . 
     The third modulator  14  is again constructed similarly to modulators  10  and  12 ; however, with the reflecting beam forming optics displaced with respect to those of the modulators  10  and  12 . One beam from modulator  14  is formed by cylindrical parabolas  64  and  66  forming a cavity  68 ,  70  and  72  forming a cavity  74 , and  76  and  78  forming a cavity  80 . The cylindrical parabola elements  82  and  84  form the second beam from modulator  14  in conjunction with other cylindrical parabola elements (not shown). 
     The cylindrical parabolas of the modulators  10 ,  12  and  14  are disposed between plates  86 ,  88 ,  90 , and  92 . Each of the elements of the cylindrical parabolas is preferably made of gold-plated stainless steel with polished optical surfaces to provide maximized reflective surface quality in the infrared portion of the spectrum. 
     The beam forming optics illustrated for the modulators  10 ,  12 , and  14  are phased so that the beams they provide will be likewise phased. Note that the center of one beam from modulator  10 , illustrated by center line  94  is angularly displaced from the center of one beam from modulator  12 , illustrated by center line  96 . In like fashion the center of one beam from modulator  14 , illustrated by center line  98 , is angularly displaced from the center of the beams from the modulators  10  and  12 . 
     The second beam forming optics located on each of the modulators  10 ,  12 , and  14  are spaced 180° from the illustrated beam forming optics such that the second beam forming optics will also form beams displaced from one another. 
     The output of the entire modulator will thus be a waveform as shown in FIG. 2, specifically a burst of three pulses  100 ,  102 , and  104  followed by a dead time  106 , followed by three more pulses  108 ,  110 , and  112  from the second sets of beamforming optics not illustrated in whole in FIG.  1 . 
     This burst is again followed by a dead time  114 . Thus in one 360° revolution of the modulator of FIG. 1 about a source, two bursts of pulses will be generated at points in space remote from the modulator. In one embodiment of the invention, the pulses in any three pulse burst are separated from their adjacent pulses by approximately 15°. 
     A simplified schematic of a mechanically modulated infrared radiation countermeasures system employing the modulator of FIG. 1 is illustrated in FIG. 3. A source of radiant energy  116  is disposed in the center of the modulators  10 ,  12 , and  141  The source  116  is preferably a rod, typically silicon carbide, heated electrically from a source  118 . The modulators  10 ,  12 , and  14  are constructed to rotate together and are typically driven by a drive motor  120 . When the modulators  10 ,  12 , and  14  are  20  rotated about the source  116 , the reflective optics of the modulators form be beams which at points in space remote from the countermeasures system produce a waveform like that shown in FIG. 2, specifically burst of pulses separated by dead time. 
     As is well known, the modulator may be encased in a window, and the window may have filtering properties to limit the output to a desired wavelength. 
     While the modulator described provides pulse bursts containing three pulses, it is contemplated that bursts of more or less than three pulses can be generated by stacking two or four or more modulators and spacing the reflective optics accordingly. Thus, it is to be understood that the embodiments shown are illustrative only, and that many variations and modifications may be made without departing from the principles of the invention herein disclosed and defined by the appended claims.