Patent Publication Number: US-6707052-B1

Title: Infrared deception countermeasure system

Description:
This invention relates to a countermeasures system for detecting and diverting an attacking unit. 
     When penetrating enemy territory under conditions of limited warfare, bombers suffer attack from enemy aircraft vectored by radar. These aircraft attack the bombers with air-to-air missiles which employ both microwave and infrared homing systems. Currently bombers of this class have increased their penetration capability by employing electronic countermeasures system to deny the attacking missiles accurate radar position data. In the past, infrared countermeasures systems have been employed in an attempt to deceive infrared homing systems by employing the use of flares or decoys, which provide the homing system with an incorrect angle of attack. These approaches suffer shortcomings such as having an insufficient power in the right portion of the spectrum and lacking a sufficient duration of burning time coupled with inherent break-away problems from the launching aircraft to be defended. Other problems have been encountered with the use of decoys because of their limited time of flight and the absence of an exact knowledge as when to launch, plus the over-all problem of carrying sufficient quantities of such decoys. 
     The purpose of this invention is to obviate the problems that have arisen in the prior art. This countermeasures systems is basically comprised of an enemy attack detection device which may be an active or passive system. The countermeasures system in responding to the presence of the attacking enemy produces a radiation as for example light which illuminates the attack unit, which, in turn, reflects a portion of the radiation. The reflected radiation is received by the countermeasures system, analyzed and information in signal form so received is used to control the characteristics of a radiation directed at the attack unit to thereby deceive the homing controls in the attack unit and divert the attack unit&#39;s direction. 
     Specifically, the basic function of the system in a preferred embodiment is the location of an attacking aircraft which is the missile carrier by passive electronics countermeasures or infrared techniques followed by illumination of the attacking aircraft with a continual laser beam. The next step requires the examination of the frequency pattern of the light reflected from spinning reticule or scanner in the missile head while the missile is on the plane and the last step requires the modulation of the laser beam with the appropriate frequency pattern and phase shift so that a false target is seen by the missile. The missile, accordingly, will attack this false target when it is launched. As soon as it has turned sufficiently to move the false target out of its field of view, the missile will also have lost the airplane. Since it cannot reacquire and has limited turning rates, the missile will wander and appear erratic thus aborting its mission. The attacking aircraft being unable to see the modulated infrared laser beam will conclude that the missile was defective. It is therefore seen that this new system is capable of acting as a continuously operable countermeasures system capable of denying angular information to infrared seekers employing spinning reticule direction finding techniques. 
     An object of this invention, therefore, is to deceive homing type attacking missiles by illuminating the missile with a false target signal. 
     Another object of this invention is to deceive an aircraft carrying a missile into believing there has been some malfunction in the missile by using a target error signal which is invisible to the aircraft&#39;s pilot. 
     Yet another object of this invention is to establish a compact countermeasures system incorporating a modulatable electromagnetic generator as a target error signal source. 
     Yet another object is to provide defense for aircraft against attacking missiles employing homing guidance as described that is completely automatic and does not require an operator. 
     Yet another object of this invention is to provide an efficient lightweight countermeasure system requiring relatively low power drain from the aircraft power supply uniquely adapting it for airborne use. 
     Yet another object of this invention is to provide angle deception for passive guidance systems of the type generally known to those skilled in the art as LORO (lobe on receive only). 
    
    
     Other objects, features and advantages will become apparent after consideration of the following detailed specification together with the appended drawings, in which 
     FIG. 1 illustrates an aircraft being pursued by aircraft carrying an attack missile. 
     FIG. 2 is a schematic of a multislit scanner. 
     FIG. 3 depicts a typical multislit scan element. 
     FIG. 4 shows a schematic of a countermeasures system in its preferred embodiment. 
     FIG. 5 represents a missile infrared video output. 
     FIG. 6 illustrates a missile integrated error signal from its infrared video output. 
     FIG. 7 shows a missile reference generator signal. 
     FIG. 8 depicts a countermeasures displaced oscillator signal. 
     FIG. 9 illustrates a laser modulator output. 
    
    
     Referring now to FIG. 1 where there is illustrated an aircraft  11  which is being pursued by an attacking aircraft  13  (partially shown), directly beneath the attacking aircraft  13  is illustrated a missile  14  of the air-to-air type which has just been launched from attacking aircraft  13 . Located in the rear of the defendant aircraft  11  is a deceptive infrared countermeasures system  12  which responds to the presence of the attacking aircraft  13  and emits a signal which is received by the missiles target signal generator  15  which causes the missile  14  to follow a doted path  10  into a diverted position  16 . 
     In order to obtain an understanding of the countermeasures system  12  and its effect on the target signal generator  15 , a study of a typical target signal generator will be made with reference now to FIG. 2, in which there is shown one form of a target signal generator. A complete and definitive description of this type of target signal generator may be had by a study of U.S. Pat. No. 3,034,405, titled “Multislit Scanner”. This type of target signal generator consists of a modified Cassegrain telescope having a spherical reflector  23  provided with an opening  24  in the center and a plane reflector  21  which is mounted on reflector support  31  by narrow supports  22 . Spherical reflector  23  is likewise secured to the reflector support  31 . A rotor  29  is supported for rotation about axle  28  by anti-friction bearings  32 . Axle  28  is mounted on reflector support  31  which in turn is mounted in a conventional manner. Scanner  26  which is alternately referred to a reticule is mounted on rotor  29  at the focal plane of the telescope and photosensitive detector  27  is mounted on axle  28 . The Cassegrain telescope comprising spherical reflector  23  and plane reflector  21 , scanner  26 , and photosensitive detector  27  constitute the target signal generator  15 . 
     Photosensitive detector  27  in a preferred example is formed of lead sulphide the resistance of which varies inversely with the intensity of incident radiation. The Cassegrain telescope focuses radiation from sources within the view of the telescope onto scanner  26 . The scanner  26  rotates with the rotor  29  at a spin frequency determined by a driving spin motor and reference generator  36 , which is illustrated as driving the rotor  29  via the drive shaft  34  and a drive member  33 . The incident radiant energy falling on scanner  26 , is chopped by the scanner in a manner to be described and variations in the intensity of the incident radiation falling on detector  27  are transmitted to amplifier  37 . 
     The infrared missile seeker  15  noted above is of the homing type and represents a serious threat because of its high accuracy to aircraft in moderately clear weather conditions. The homing mechanisms of these seekers operate near the region of near infrared and their detectors, e.g.,  27  are most sensitive at wavelengths of 1 to 3 microns. 
     Because of the short wavelengths used it is apparent from FIG. 2 that the optics of this system are small in size and offer high resolution accounting for the high performance obtained. The seeker described is purely passive and requires no transmitter because it homes on the exhaust heat of its target&#39;s aircraft, in this case, as shown in FIG. 1, defense aircraft  11 . As noted above when infrared energy is reflected by a primary reflector, namely, spherical reflector  23 , to a plane reflector  21 , it then passes through a central portion  24  of the spherical reflector. The reflected infrared energy simultaneously passes through a spinning scanner  26  such as that illustrated in FIG.  3 . This spinning reticule or scanner  26  affords discrimination against clutter such as clouds and sunlight and provides the basic direction finding information. The scanner  26  is spun by the spin motor reference generator  36  by the arrangement described above. After the infrared energy has passed through the scanner  26 , the energy has been chopped because of the scanner&#39;s structural configuration. This chopped energy strikes the photosensitive detector  27 . The key to the operation of this system is the spinning scanner  26  which is driven by the spin motor reference generator combination  36 . 
     Referring now to FIG. 3, where there is shown a typical scanner  26 , which consists of an infrared transparent material upon which is evaporated a metallic film pattern such as an opaque sector  42 . In the scanner  26  depicted it is seen to consist of two semi-circular sectors  41  and  46 . Sector  41  is for target sensing and is comprised of a plurality of slits  43 , each slit  43  consisting of a transparent sector  44  and an opaque sector  42 . Sector  46  is for indicating the phase of the signal generated by the target sensing sectors and is semi-transparent so as to permit one half of the radiant energy falling on the phasing sector to be transmitted. It should be noted that the pattern on the scanner  26  may take a variety of designs. In its operation at any instant of time, a true target will consist of a point lying wholly within one segment of the scanner  26  while clutter would intersect more than one segment of the scanner  26 . In this manner the seeker can discriminate between a true target and clutter. 
     The system is typically a null seeking system and when the target is in the center of the scanner  26  no chopped signal gets through. However, as the target moves further away from the center of the scanner an increasing chopped signal passes into the photosensitive detector  27  and to an amplifier  37  to yield an error signal. 
     As the scanner spins a cyclic chopped pattern is detected. The phase of the cyclic chopped pattern from the scanner depicted is compared by a phase comparator  38  when the phase of the reference generator signal to produce an angle correction voltage, in the same manner as the error signal is compared with a reference generator signal and a conical scanning radar. This angle correction voltage is fed to a control surface actuator  39  which steers the missile. In this manner the missile knows in which direction to correct its aiming error in order to hit the target. In this case, defense aircraft  11 . The relationship of the target signal generator and the signals produced therein with the signals sent from a countermeasures system  12  will be described more fully hereafter. 
     Referring now to FIG. 4 where there is schematically illustrated a countermeasures system that represents one embodiment of applicants&#39; invention. This system  12  is located in the rear of defense aircraft  11  and in order that this embodiment of the system be described a number of presumptions must be kept in mind throughout the study of the system, namely, that the attacking aircraft will be located somewhere within a solid angle θ, FIG. 1, centered dead astern. This, of course, does not preclude the location of another system of the same type in the forward part of the aircraft to detect missiles approaching from that direction or for that matter the system may be located at any of a number of positions on the defense aircraft  11 . It should also be kept in mind that in this embodiment the attacking aircraft  13  will be within some reasonable range, for example, 5,000 yards, when the enemy aircraft decides to launch the missile. At launch time the attacking aircraft will be emanating microwave radiation such as that from an aircraft ranging only radar or an airborne intercept radar. It is to be understood that while the system will be described in terms of a passive detection of enemy aircraft, the system may operate with the use of an active enemy detection system of either an infrared or microwave type. 
     In view of the foregoing examples, we can now turn to FIG. 4 in which the countermeasures system employs a small microwave parabolic antenna  51  which function is to receive both aircraft ranging only radar signals  50  and simultaneously receive infrared signals in a manner to be described more fully hereafter. As the attacking aircraft  13  approaches, the microwave parabolic antenna  51  receives the aircraft ranging only radar signals  50  and reflects them in a manner shown to a conically scanned element such as a triscanner  53 , which, in turn, permits their passage via radiator  54 , rotary joint  68  to a detector  73 . In order that microwave parabolic antenna  51  be capable of receiving both aircraft ranging only signals and infrared signals, there is supported on the boresight axis of the reflector an infrared lens with a microwave grating  56  supported by support rods  57 . The infrared lens with a microwave grating permits the passage infrared energy while reflecting the microwave energy back to the tri-scan element. These reflected radar signals  50  must pass through the rotating tri-scan element  53  which tri-scan element  53  receives its rotary drive from a spin motor and reference generator  62  via drive shaft  63  and drive elements  64 ,  66  and  67 . 
     The rotary joint  68  permits radiator  54  and its integrally attached tri-scan element  53  to rotate independently of microwave conduit  71  and the rest of the system. The microwave energy reflected from parabolic reflector  52  and microwave grating  56  passes through the radiator  54  and into a microwave detector  73 , which in turn feeds the information to scan video receiver  74 . The microwave parabolic antenna  51  is continually conically scanning and searching the aforementioned cone in the stern direction due to the rotary drive of tri-scan element  53  brought about by spin motor  62  whose operation was noted above. 
     In order that the antenna  51  continually search and track the output of the scan video receiver  74  is fed to phase comparators  76  and  77 , which are simultaneously receiving the output of the spin motor and reference generator  62 , the phase comparators  76  and  77  compare the phase of the error signal from the scan video receiver  74  with the phase of a signal from the reference generator which is directly coupled with the spin motor which conically scans the antenna. 
     The output of the phase comparators  76  and  77  are fed to an antenna servo search and track system  78  which has a search and track programmer and suitable amplifiers to increase the voltage from the phase comparators  76 ,  77  to control respectively the up-down slew motor  59  and the right-left slew motor  61 , which maintain the microwave antenna  51  in a continuous search and track path of the attacking aircraft  13 . It is therefore apparent that this arrangement will permit the system to accurately track the enemy aircraft in angle by tracking an aircraft ranging only signal. 
     Upon reception and tracking of this aircraft ranging only signal, this system would assume that the enemy was preparing to launch an infrared homing missile and the infrared deceptive jamming would be then initiated in the following manner. As soon as the microwave energy of the aircraft ranging only radar signal  50  is detected by microwave detector  73  and fed to the scan video receiver  74 , an output from the scan video receiver  74  would instantly activate laser switch control  81  whose output signal would pass through a normally closed switch  82  to activate a laser power supply, the output of which would activate a continually operable laser. 
     The desirability of using a laser light source resides in the fact that such lasers offer the property of emitting essentially monochromatic, phase coherent light energy in the near infrared portion of the spectrum. Monochromatic light output known as stimulated emission of radiation makes the infrared beam emerge from the laser with phase coherence so that a collimated beam is obtained without the use of auxiliary optics. Because the beam is essentially monochromatic and collimated, power densities per solid angle may be obtained which are many times higher than can be obtained with any other known type of optical frequency generator. A continually operable laser that may be used in the instant application relies on trivalent neodymium in calcium tungstate. The laser is fully described in the following publication: “Physical Review” May 15, 1962, Vol. 126, No. 4, by L. F. Johnson, on pages 1406 to 1409. A modulatable xenon lamp suitable for modulating the aforementioned laser is described in the September, 1962, issue of  Illuminating Engineering , at pages 589-591. Laser modulation techniques are further discussed in the publication,  Electronics  for Nov. 10, 1961, at pages 83-85. Other types of lasers which are modulatable to perform the function stated herein are the diode type laser as described in the publication,  Electronics  for Oct. 5, 1962, at pages 44-45. 
     It should be noted that while one laser light source is illustrated the system could function with two lasers. One laser to give continual operation and a second to give a modulated output. The discussion while directed to lasers as a light source is not meant to exclude other light sources of sufficient power and having frequency components at the correct wavelength. 
     The laser  84  in its now activated condition would emit a collimated beam of monochromatic infrared energy  85  aimed at the attacking aircraft and its infrared homing missile. The laser or laser beam director  84  is integrally attached by laser support member  86  to parabolic reflector  52 . Because of the integral physical relationship of the laser its beam will inherently follow the search and track function of the conically scanning microwave antenna  51 , and accordingly illuminate the attacking aircraft and missile simultaneously with the microwaves antenna tracking operation. Because of the early detection ranges of the microwave detector  73  this, of course, occurs prior to the attacking aircraft  13  launch of its air-to-air missile  14 . The infrared beam  85  emitted by the laser  84  is received by the target signal generator  15  in the missile  14  head. This beam is chopped and reflected by the spinning scanner  26 , recollimated by the spherical reflector  23  and transmitted back to the parabolic reflector  52  of the microwave antenna  51 . This collimated reflected and chopped beam of infrared energy is then reflected by the parabolic reflector  52  and detected by the photosensitive detector  58  mounted on support rods  57 . It is therefore apparent that the signal produced by the photosensitive detector  58  will represent the frequency of modulation of the infrared beam as reflected by the rotating scanner. This output signal from the photosensitive detector  58  is amplified by audio amplifier  87  and fed through a normally closed switch  88  to generator  89  which has a scan audio filter  91 . The scan audio filter may be a comb filter of resonant reeds in which the reed which is resonant at the scanners spin frequency gives an output from the scan audio filter  91  at the correct spin frequency which starts at a random initial phase with respect to the attacking missiles scanner phase. The scan audio filter  91 , in the example given, being of the comb filter type having resonant reeds in which the reed which is resonant at the scanner&#39;s spin frequency has the inherent characteristic of maintaining an output signal for a definite period of time after its input signal is removed. Digital and analog devices to determine frequency may also be used. This phase shifted scanner spin frequency signal is fed to a triggered oscillator  92 . This triggered oscillator  92 , for example, may be controlled by a sawtooth generator  93  and an amplitude control device  94 . The amplitude control device may be a Schmitt trigger, which has the property that an output of constant peak value is obtained for the time period that the input wave form exceeds a specific voltage. It is important for reasons to be explained hereafter that the output from the triggered oscillator  92  function for a distinct period of time, then cease its output for another distinct period of time to provide look-through period for a check of the scanners spin frequency, before repeating the signal. As mentioned above this is controlled by the sawtooth generator  93  and the amplitude control device  94 , which controls the oscillator  92  so it is turned ON and OFF for the proper intervals. This check of the scanners spin frequency is needed to determine any changes in the spin frequency and also to prevent ring-around between the laser  84  and the detector  58  or the system from locking up on its own modulation. This action takes place because the receiver is deactivated during transmission by the look-through process just described. The sawtooth generator  93  which is activated by the output from the scan audio filter produces a signal whose voltage increases with the passage of time until the Schmitt trigger of the amplitude control device  94  is activated at which time an output is noted from the amplitude control  94  which in turn triggers the oscillator  92  to pass the phase shifted scanner frequency detected by the scan audio filter  91 . The output from the oscillator  92  is illustrated in FIG.  8 . The output from the triggered oscillator  92  simultaneously actuates a laser modulation switch  96  and solenoid  90  which opens normally closed switches  82  and  88  which act to turn off the laser power supply  83  and the related laser  84 . It will be seen that as the circuit between laser switch control  81  and the laser power supply  83  is broken by the opening of switch  82 , the laser power supply is simultaneously activated by the actuation of laser modulation switch  96  which results in the emission of a modulated infrared beam  85  from laser  84 . Laser modulation switch produces a square wave shown in FIG. 9, which modulates the laser  84  at the spin rate of the missiles scanner. The laser modulation switch may include for example a rectifier to obtain only one polarity to be delivered to a power amplifier which controls a grid which in turn controls the laser modulation. This phase shifted modulated signal from the laser is now directed at the enemy&#39;s target signal generator  15  and brings about an angle deception by interchannel cross-coupling which will be discussed more fully hereafter. 
     The transmitted beam of monochromatic infrared energy  85  illuminates a volume of space much larger than the attacking aircraft. Energy will be reflected from portions of the airplane and from the reflected portions of the scanner in the target signal generator  15 . The energy reflected by the rotating scanner will be modulated at a rate determined by the number of reflected segments, their width and the spin rate of the spin motor  36 , FIG.  2 . Energy will also be reflected from the missile&#39;s detector  32  since it is coated to be nonreflected in the wavelength region of maximum detector performance and will consequently be more reflective than it otherwise would be at the wavelength of the laser beam. The difference in reflectivity between the detector and the scanner comprises the signal source of the ac signal received at the microwave antenna  51  in the defending aircraft. The photosensitive detector  58  in the countermeaures system will have incorporated therein a narrow band filter placed in front of it (not shown). Hence, because only a narrow wavelength region is used and because the signal to be detected from the missile is chopped, strong dc signals from clouds, sunlight, attacking aircraft itself and exhaust from the defending aircraft will be reduced to negligible portions. 
     The signal from detector  58  which contains the modulation components from both halves of the scanner  26 , FIG. 3, is passed to amplifier  87  which filters out the high frequency components from the upper half of the scanner  41 , leaving an error signal from the lower half of the scanner  46 , thus the output of the video amplifier  87  contains signal information directly related to the missile scanner rotational frequency. 
     As noted earlier there arises a relationship between an error signal in the target signal generator  15 , FIG. 2, and the spin motor reference generator  36 . 
     Referring now to FIGS. 5,  6 , and  7 , there is shown in FIG. 5, a typical missile infrared video output which is shown in its integrated form as a sine wave in FIG. 6, and represents an integrated error signal from the infrared video output. 
     FIG. 7 illustrates the missile reference generator signal from reference generator  36 . The reference generator  36  may either be a sine wave generator or an impulse generator. For purposes of convenience, a sine wave output has been shown in FIG.  7 . Since the infrared seeker compares the phases of FIG. 6 vs that shown in FIG. 7, and uses the output of its phase comparator  38  to activate the control surfaces of air-to-air missile  14  in order to give false information to such a system all that has to be done is to shift the phase of the error signal with respect to the reference generator signal. As described above, the laser beam  85  will be a square wave modulated at the scanner spin frequency and at some random phase with respect to the true target error signal phase of the spinning scanner. Since the laser beam  85  represents a strong signal which is displaced in time phase as compared to the target signal, the jamming signal represented by a square wave depicted in FIG. 9, will be shifted as shown in FIG. 9 with respect to the true error signal when compared with phase of the reference generator signal as shown in FIG.  7 . Hence false angle information is presented to the missile seeker system as a large error signal and will cause the missile threat to veer off from the true heading at some random false heading. 
     It should be clearly understood that the invention is not limited to the infrared portion of the electromagnetic spectrum, but is broadly applicable to any system using guidance systems employing spinning scanning direction finding techniques regardless of what portion of the electromagnetic spectrum is involved. 
     While there has been hereinbefore described what are considered preferred embodiments of the invention, it will be apparent that many and various changes and modifications may be made with respect to the embodiments illustrated, without departing from the spirit of the invention. It will be understood, therefore, that all changes and modifications as fall fairly within the scope of the present invention as defined in the appended claims are to be considered as part of the present invention.