Abstract:
The invention concerns a device for countering and tracking a threat with optical delay device in the form of a homing-head missile, comprising a homing head adapted to receive an incident coherent light beam and in deflecting same to produce a transmitted beam. The invention is characterized in that the homing head comprises a biprism dividing the transmitted beam into two sub-beams, the biprism being associated with an optical delay device introducing an optical path difference between the two sub-beams greater than the coherence length of the incident beam.

Description:
GENERAL TECHNICAL DOMAIN 
       [0001]    This present invention concerns an improved device for tracking a threat and deploying directed counter-measures. 
         [0002]    More precisely, it concerns a device for tracking and countering a threat in the form of a homing infrared missile (ADIR). 
       STATE OF THE ART 
       [0003]      FIG. 1  schematically represents a known pyrotechnic countermeasure device  3  protecting an aircraft  1 , for example, from a threat  2 . The threat  2  comes in the form of an infrared homing missile (ADIR). 
         [0004]    Device  3  is a pyrotechnic decoy that generates infrared rays when it is fired out of the aircraft  1 , when the latter has detected a threat. The infrared rays generated by the decoy  3 , which are more intense than the infrared rays generated by the aircraft  1 , cause a deviation in the trajectory of the threat. The threat homes in on the decoy  3  rather than on the aircraft  1 . 
         [0005]    However, these pyrotechnic decoy devices have a number of drawbacks. 
         [0006]    To begin with, they are difficult and expensive to design. In addition, they represent a fire risk inside the aircraft if they malfunction. Furthermore, in the event of a false alarm, they seriously compromise the stealth aspect of the aircraft  1 . Finally, they are consumable devices that have to be replaced regularly, and are highly specialised to one particular type of threat. 
         [0007]    As a consequence, current countermeasure devices are generally of the infrared illumination jammer type. 
         [0008]      FIG. 2  schematically represents a known Directional infrared Counter-Measure device (DIRCM). 
         [0009]    The aircraft  1  thus includes a missile departure detection device (or Missile Warning System—MWS) in the form of a multiplicity of detectors  6  mounted on the fuselage of the aircraft  1 . The detectors  6  detect the launch of a missile  2 , track the missile trajectory and identify it as a threat to the aircraft  1 . 
         [0010]    A device  7  for monitoring the launch detection device transmits the missile trajectory  2  to the control device  8  of a directional countermeasure system. 
         [0011]    The control system  8  then triggers the tracking device  4  which tracks the missile and determines its direction in space. The tracking device  4  detects the missile homing system  2  by its laser equivalent surface or “LES”, which is the quantitative value of the “cat&#39;s eye” effect. To this end, the tracking device  4  emits a tracking light beam  40  in the direction of the missile homing system  2  and measures the reflected echo in order to measure the LES of the “seek head”. 
         [0012]    The control device  8  also controls a jamming light beam  50  that has an angular opening (γ) produced by an infrared light source  5 . The light source  5  generally uses discharge-type infrared lamps, whose spectrum covers a broad spectrum of wavelengths, from the visible up to the far infrared. Once tracking has locked on correctly and the missile is securely within the beam of the light source  5 , the infrared illumination  5  is transmitted toward the missile homing system  2  in a specific sequence in order to cause jamming of the missile  2 , so that it no longer represents a threat to the aircraft  1 . 
         [0013]    These discharge light sources  5  also have many drawbacks however. 
         [0014]    Due to the coverage required from the light source, beam concentration requires a voluminous optical device (large aperture, long focal length, etc.) which is very bulky and heavy. The jammers based on discharge lamps are therefore difficult to accommodate on board aircraft. In addition, the illumination power available is rather limited, thus greatly limiting the effectiveness of such jammers using discharge lamps. 
         [0015]    Thus, with the evolution of the technology, it is now possible to create infrared laser sources that emit wavelengths of between 3 and 5 micrometres. This evolution of the technology allows us to achieve a significant improvement in tracking and countermeasure devices. In fact, with a laser source, it is possible to considerably increase the beam strength  50  and/or  40  with a far smaller source than the discharge type, and much closer to the diffraction limit. Because the laser source is a coherent source, all the beam energy is concentrated in a single wavelength. 
         [0016]    Increasing the beam strength over one or more specified wavelengths has certain advantages. 
         [0017]    A laser beam allows us to deposit a higher light level than that of the discharge lamps at the aperture entry of the homing device of the threat. 
         [0018]    The laser energy can be contained in several rays (typically two or three) so as to be able to effectively illuminate all types of missile homing device. 
         [0019]    Because of its high brightness (the beam is close to diffraction) a laser beam can be collimated with an optical device of small dimensions, rendering the latter easy to fit into aircraft while also providing a range of performance that is acceptable for the required functions of active tracking, of identification AD and jamming DIRCM. 
         [0020]    A laser beam can be reflected to a greater extent by the “seek head”, so that missile identification  2 —in particular by the modulation that it imparts to the reflected laser beam—is facilitated. A quality identification of the threat  2  allows us to transmit an effective jamming code, meaning a code that is truly designed for the missile type  2 . The source is very directional, thus enhancing the general stealth aspect of the aircraft. 
         [0021]    However, tracking and countermeasure devices using a laser source present a number of drawbacks. 
         [0022]    Because of the narrowness of the laser beam produced by the laser source (in general less than 1 milliradian), the tracking device  4  must be capable of very precise tracking of the missile infrared detector  2 . 
         [0023]    In addition, the coherence characteristic of the laser sources imposes the use of a beam shaping, orientation and stabilisation device which generates no interference that could have negative consequences for general system operation and particularly on the effective jamming of the missile homing system. 
         [0024]    It is therefore very difficult to design a tracking and countermeasure device that can rapidly and simply track the missile where an infrared laser source is used. 
       PRESENTATION OF THE INVENTION 
       [0025]    The invention proposes to overcome at least one of these drawbacks. 
         [0026]    To this end, the invention propose a device to counter and track a threat in the form of a missile with an infrared homing device, including a homing head designed to receive an incident coherent light beam and to re-direct the latter in order to produce a transmitted beam, characterised in that the homing head includes a double prism that divides the transmitted beam in two sub-beams, with the double prism being associated with an optical delay device that introduces an optical path difference between the two sub-beams that is greater than the coherence length of the incident beam. 
         [0027]    The invention is advantageously completed by the following characteristics, taken alone or in any of their technically possible combinations:
       the double prism is mounted to rotate in elevation around an axis perpendicular to its base and passing through its geometric centre;   the head is mounted to rotate in horizontal bearing around an axis parallel to the base of the double prism and passing through the geometric centre of the double prism;   the delay device includes an optical blade;   the blade is mounted on a mobile support, on a slide, with the device including movement resources that are designed to move the support of the blade on the slide as a function of a angular position of the double prism;   the movement resources include a bracket connected firstly to a link to a support of the double prism and secondly to a link to the support of the blade;   the movement resources include a drive motor for the blade support on the slide;   the blade is inclined on its support;   the blade and the movement resources are statically balanced so as to avoid angular destabilisation of the sighting line during the tracking phases by linear excitation of the beam homing head;   the double prism is mounted on a mobile support to rotate around the elevation rotation axis in a yoke;   a protective cover for the double prism is mounted to rotate in horizontal bearing;   the device includes an infrared laser source with a coherence length of the order of magnitude of one centimetre.       
 
         [0039]    The invention has many advantages. 
         [0040]    The tracking  4  and countermeasure  5  devices of the prior art can be merged in the invention. With a single laser source, it is possible to achieve both tracking and countermeasures by virtue of the beam strength. The light source is unique and of small dimension. A device according to the invention is very compact, which increases the stealth aspect of the device and of the assembly on which it is mounted. 
         [0041]    The invention solves the problem of interference due to the joint use of a coherent light source and of a homing head which includes a double prism that generates different optical paths for the rays of a laser beam that pass through it. The homing head is compatible with laser sources of small coherence length (typically a few centimetres). 
         [0042]    The homing head engenders very little loss of laser power. 
         [0043]    The homing head is very simple to create and to use. Little or no adjustment is necessary. 
         [0044]    It has no dead angle, and in particular allows zenithal sighting. It covers a homing space at least equal to 2π steradians. 
         [0045]    In addition, the mechanical placement of the homing head is symmetrical around its working axes. The centre of gravity of the head is therefore located on the working axes. The head can therefore be less sensitive to vibration from a structure on which is it mounted. The head itself does not generate any vibration or destabilisation of the sighting line. 
         [0046]    Finally, the displacement of the head is very rapid, because of its lower inertia and an appropriate motor drive. 
     
    
     
       PRESENTATION OF THE FIGURES 
         [0047]    Other characteristics, aims and advantages of the invention will emerge from the description that follows, which is purely illustrative and not limiting, and which should be read with reference to the appended drawings in which: 
           [0048]      FIG. 1 , already mentioned, schematically represents a known pyrotechnic countermeasure device; 
           [0049]      FIG. 2 , already mentioned, represents a device known from the prior art, that allows firstly tracking of, and secondly countermeasures against, a threat in the form of an infrared homing missile; 
           [0050]      FIGS. 3A and 3B  schematically represent the trajectory of optical rays inside a homing head according to the invention; 
           [0051]      FIG. 4  schematically represents the generation of a path difference between two optical rays passing through a homing head according to the invention; 
           [0052]      FIGS. 5A and 5B  represent the sub-apertures in the orientations of the head corresponding to  FIGS. 3A and 3B ; 
           [0053]      FIGS. 5C and 5D  respectively represent the output apertures in the orientations of the head corresponding to  FIGS. 3A and 3B  respectively; 
           [0054]      FIG. 6  schematically represents a view in section of one possible method of implementation of a homing head according to the invention; 
           [0055]      FIG. 7  schematically represents a view in perspective and an exploded view of one method of implementation of a homing head according to the invention; and 
           [0056]      FIG. 8  schematically represents the mounting of two tracking and countermeasure devices according to the invention on an aircraft  1 . 
       
    
    
       [0057]    On all of these figures, similar elements carry identical numerical references. 
       DETAILED DESCRIPTION 
       [0058]      FIGS. 3A and 3B  show that an improved threat tracking and countermeasure device according to the invention includes a homing head for an incident laser beam  50  obtained from a laser source  5 . 
         [0059]    In the remainder of this present description, for raisons of clarity, we present only a single direction of the trajectory of the light rays. According to the reverse light return principle, it can be seen that the rays can move in the reverse direction to that presented, and that the homing head can therefore be used both for tracking and for the countermeasures to the threat. 
         [0060]    The laser source  5  is preferably of the infrared laser type. Advantageously, the laser source  5  has a small coherence length (typically of the order of one centimetre). 
         [0061]    The homing head is designed to receive incident light beam  50  and to re-direct the latter in order to produce a transmit beam  60 . 
         [0062]    To this end, the head mainly includes two prisms  11 ,  12  forming a double prism  10 . The prisms  11 ,  12  are preferably of the rectangle and isosceles type. They are both attached to a face  112  represented by the hypotenuse of their base forming a triangle-rectangle. The double prism  10  therefore forms a cube. 
         [0063]    The face  112  separates the double prism  10  into two equal parts and forms a dioptre reflecting the rays of the incident laser beam  50  and then passing into the prisms  11 ,  12 . FIG.  3 A shows the trajectory of two incident rays  51 ,  52  in the double prism  10 . It will be observed that the rays  51 ,  52  are totally reflected by the face  112 . The rays  51 ,  52  are transmitted, and then references as  61 ,  62  respectively after the double prism  10 . 
         [0064]    The double prism  10  is mounted to rotate around an axis of rotation BB that is perpendicular to the base of the double prism, meaning perpendicular to the plane of  FIGS. 3A and 3B , and passing through the geometric centre of the double prism. Axis BB allows the rotation of the double prism  10  in the plane of  FIGS. 3A and 3B , and therefore the orientation of beam  60  in elevation. 
         [0065]      FIG. 3B  shows that a rotation by a angle beta of the double prism around axis BB allows a rotation of the transmitted beam  60  by a angle of 2β. To this end,  FIG. 3B  shows the trajectory of four rays, with the rays  61 ,  62 ,  63  and  64  being the transmitted rays. With a rotation of the double prism  10  by a value of at least 45° on either side of the optical axis  500 , the head can cover an angle in elevation of at least 180°. 
         [0066]      FIG. 4  shows that the rotation of the double prism  10  around axis BB results in a phase advance due to the optical path difference  13  between the two rays  61 ,  62  transmitted respectively by the prisms  11 ,  12 . The path difference  13  is measured in relation to the upstream phase reference  14  of the double prism  10  over the incident rays  51 ,  52 . 
         [0067]    Furthermore,  FIGS. 5A to 5D  show that the double prism divides the transmit beam  60  into two sub-beams. The countermeasure device output aperture is split into two parts, with one passing via prism  11  (aperture P 1 ), and the other passing via prism  12  (aperture P 2 ).  FIGS. 5A and 5C  correspond to the situation of  FIG. 3A , and  FIGS. 5B and 5D  correspond to the situation of  FIG. 3B .  FIGS. 5A and 5B  thus show that the two incoming sub-apertures, P 1  and P 2 , vary in particular as a function of the angle of orientation β of the double prism  10 .  FIGS. 5C and 5D  show that each prism has the effect of reversing and offsetting the aperture in a different manner for each angle of incidence leading to a change of direction of outgoing sub-apertures P′ 1  and P′ 2 . 
         [0068]    The path difference  13  results in interference between the rays  61 ,  62 . Such interference is very disadvantageous in relation to threat tracking and countermeasure activity. The dark zones of the interference patterns can in fact result in zones in which there is no detection of the threat and/or to zones in which there is no jamming. In addition, the fact that the path difference varies when using the head, in particular as a function of the angle of orientation of the double prism, again renders threat tracking and countermeasure activity more difficult. 
         [0069]    As a consequence, it is necessary to render the two sub-parts of the beam  60  mutually incoherent, so as to eliminate the interference between the rays exiting from the double prism. 
         [0070]    In this regard,  FIG. 6  shows that the homing head includes a delay device  130  that is used to render the two sub-parts of beam  60  mutually incoherent. 
         [0071]    Advantageously, device  130  includes an optical blade  131  in the path of one of the two sub-beams. The optical path delay introduced by the blade  131  is at least greater than the coherence length of the laser source  5  in all the working positions of the double prism. 
         [0072]    The delay blade  131  is chosen so as to satisfy several requirements. 
         [0073]    Firstly, the thickness of the blade must be sufficient to ensure the incoherence of the two sub-beams at all the working angles of the head. It inserts into a single sub-beam has an additional optical path that is greater than the sum firstly of the coherence length of the laser source and secondly of the optical path advance between the two apertures. The value of an additional optical path inserted by the blade  131  is greater than the said sum in the worst case of head operation. In this way, the incoherence of the two sub-beams exiting from the homing head is guaranteed, regardless of the angular position of the deflected transmitted beam. 
         [0074]    Secondly, the blade must guarantee low optical energy losses due to traversing material in the working spectral band of the laser source. The blade  131  must therefore be relatively thin. 
         [0075]    Thirdly, the blade must be anti-reflection treated. The anti-reflection treatment imparts a stealth aspect to the tracking and countermeasure device. It also allows the optical energy losses due to reflection at the walls of the blade at the input to the device to be limited. 
         [0076]    Fourthly, the blade must be mounted with a slight incline so as not to generate specular reflection in the incident direction, in order to prevent:
       firstly, parasitic reflections of the laser source inside the DIRCM device, which would reduce the effectiveness of tracking and jamming;   secondly, a potential for detection, through the “cat&#39;s eye” effect, by generating a non-negligible LES.       
 
         [0079]    Remember that it is preferably that the laser source should have a small coherence length, typically of just a few centimetres. 
         [0080]    As a consequence, if L is the thickness of the blade  131  and n is the refractive index of the material from which the blade is made, then it is necessary that: 
         [0000]      ( L×n−L )≧ Lc    
         [0081]    where Lc is the coherence length of the laser source. 
         [0082]    If we take materials with a high refractive index, like silicon for example, which has an index of the order of 3.4, then we get: 
         [0000]    
       
         
           
             L 
             ≥ 
             
               
                 L 
                 c 
               
               
                 2 
                 , 
                 4 
               
             
           
         
       
     
         [0083]    so that 1≧8.3 mm for a coherence length of the source  5  equal to 20 mm. It can be seen therefore that the blade is quite small. 
         [0084]    Other methods of implementation of the optical delay device  130  are also possible. In particular, these can have mirrors that introduce an optical path difference between the sub-beams. However, these implementation methods are more difficult to implement and are also less compact. 
         [0085]    The developments presented above describe the orientation of the head in elevation. In order to be able to cover an angular space of at least 2π steradians, it is necessary to have another axis of rotation of the head. 
         [0086]      FIGS. 3 and 6  show that, preferably, an input axis to the system is parallel to the base of the double prism  10 . The input axis is coincident with an axis of rotation AA passing through the geometric centre of the double prism  10 . Axis AA is the axis of orientation in horizontal bearing, and allows the prism to be rotated around the input axis for orientation of the transmitted beam  60  in horizontal bearing. For zenithal sighting, axis AA is parallel to the face  112 . The deflection angle (α) of the beam  50  in horizontal bearing is the same as the angle of rotation of the prism α in horizontal bearing around axis AA. 
         [0087]    In order to allow rotation around the two axes AA and BB, the double prism is mounted on two supports  19  covering the two faces corresponding to the base of the double prism  10 . Each support  19  is mounted to rotate in a yoke  15  with two arms  16 . A rotation and elevation drive motor is described later this present description. 
         [0088]    The arms  16  of the yoke  15  include a pivoting link with the supports  19  in order to allow rotation of the double prism  10  around axis BB between the arms of the yoke, as shown in  FIGS. 6 and 7 . The main axis of the yoke  15  is coincident with axis AA, and is therefore parallel to the base of the double prism  10 . A drive motor for rotation in horizontal bearing allows all of this mechanism to be driven around axis AA. 
         [0089]    In practise, the position of the blade  131  is controlled as a function of the angular position of the double prism  10  on the two axes AA and BB of rotation of the double prism  10 . For example, the blade  131  continuously covers the part of the aperture P 1  in front of prism  11 . When the double prism  10  rotates in elevation through an angle beta around axis BB, the blade  131  moves, and follows the central edge at the face  112  of the double prism  10 . To this end, the support  133  of the blade  131  is displaced in a linear movement CC on a slide  132 . 
         [0090]    Several methods of implementation of the means for movement of the blade support  131  are possible. 
         [0091]    A first method of implementation is such that when the double prism rotates so as to orientate the beam in elevation, the blade support  131  is displaced on its slide  132  by a drive bracket  14 . 
         [0092]    To this end, the bracket  14  is connected firstly to a support  19  of the double prism by link  141 , and secondly to the support  133  of the blade  131  by link  134 . Links  141  and  134  are of the pivoting link type. Link  134  is also mobile in translation in the support  133  in order to allow movement in direction CC on slide  132 . A mechanical cam system is also possible. 
         [0093]    In this method of implementation for movement of the blade  131 , the movement of the double prism, transmitted by a drive motor in elevation, located on axis BB or in link  141 , is also transmitted to the blade  131  by means of bracket  14 . Reciprocally, a movement transmitted to the blade  131  by a motor located close to the blade is transmitted to the double prism  10  by means of bracket  14 . 
         [0094]    A second method of implementation is such that the orientation of the double prism  10  and the movement of the blade  131  are effected by separate and synchronised motors. Bracket  14  is therefore no longer necessary, thus simplifying the mechanical mounting of the head. The mechanical parts have lower inertia and an appropriate motor drive to allow rapid movement of the double prism and of the blade. 
         [0095]    Advantageously, the blade and the movement resources are statically balanced by a static balancing device  135 . The static balancing device  135  is placed symmetrically to the blade for example, and its movement resources in relation to axis A-A. The balancing device allows us to avoid angular destabilisation of the sighting line during the tracking phases by linear excitation of the beam homing head. We thus eliminate the presence of any possible unbalance. 
         [0096]      FIGS. 6 and 7  show that the device includes a cover  17  for protection of the double prism and the rest of the device. The cover  17  is of more-or-less hemispherical shape. It mainly includes a material  18  that is transparent to infrared light emitted or received by tracking and countermeasure device. The material  18  covers a minimum angle of 180° in elevation on the cover  17  along a plane of symmetry of the cover. The cover  17  rotates in horizontal bearing in sympathy with the yoke  15 . All of the device is driven around axis AA so as to provide for scanning in the horizontal bearing plane. 
         [0097]    The device is designed firstly to effect the tracking of a missile and secondly to take countermeasures against a missile. The tracking phases can be successive or intermittent, according to the applications and the threats involved. 
         [0098]      FIG. 8  shows that in order to have full coverage of the space, two devices according to the invention can be mounted on the two opposite sides of an aircraft  1 . The devices of the invention are very compact and stealthy. They are mechanically balanced (this is the case in particular of the double prism and the head and blade) which means that they are not very sensitive to the vibration from a motor or the rotors of an aircraft. The head itself does not generate any vibration or any destabilisation of the sighting line. 
         [0099]    It can be seen that the developments described above apply to military aircraft, such as transport craft or attack and transport helicopters. The threats, for example, can be from ground-air missiles or air-air missiles in single combat. The case of multiple threats involving the firing of several missiles in a salvo is also envisaged. The developments described can also apply to civil aircraft, such as long-haul planes for example, against terrorist threats. It can also be seen that the developments described also apply to other types of vehicle, like tanks or trucks, and even to civil or military buildings or vessels threatened by missiles.