Abstract:
A single receiving aperture used, for example, in an airborne seeker system collects energy for three discrete energy sensors/receivers including a laser spot tracker, an RF (millimeter wave) transmitter/receiver, and an infrared detector. The RF transmitter/receiver is located at the focus of a primary reflector located on a gimbal assembly. A selectively coated dichroic element is located in the path of the millimeter wave energy which reflects infrared energy from the primary reflector to an optical system which re-images the infrared energy on the infrared detector. The outer edge or rim of the primary reflector is deformed so that the incoming laser energy focuses to a location beyond the RF transmitter/receiver. The laser sensor is positioned adjacent the RF transmitter/receiver at this location in a back-to-back orientation. The laser energy is then detected using a secondary reflector and an optical system which directs the laser energy from the secondary reflector to a laser detector.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to antenna sensors and more particularly to a millimeter wave, infrared and laser sensor employing a common receiving aperture. 
     2. Description of Related Art 
     Single mode sensors used, for example, in missile seekers are known to exhibit degraded performance because of false target acquisitions. In order to overcome this inherent deficiency, a dual-mode seeker including millimeter wave (MMW) and infrared (IR) sensors has been developed. One such system which substantially improves false alarm rate is shown and described in U.S. Pat. No. 5,214,435 entitled, “Millimeter Wave and Infrared Sensor In A Common Receiving Aperture”, issued to T. C. Brusgard et al. on May 25, 1993, the details of which are incorporated herein by reference. There an integrated millimeter wave (MMW) and an infrared (IR) common aperture sensor includes a common primary reflector for infrared and millimeter wave energy. An active transmitter/receiver millimeter wave horn assembly located at the focus of the primary mirror transmits and receives millimeter wave signals off of the primary reflector. A selectively coated dichroic element is located in the path of the millimeter wave energy on the axis between the feed and the primary reflector. The dichroic element reflects infrared energy from the primary mirror to a focal point and at the same time transmits and focuses millimeter wave energy to a transmitter/receiver. An optical system relays the infrared energy to a focal plane behind the primary mirror. The dichroic element transmits and focuses millimeter wave energy without significant attenuation such that optical and millimeter wave energy may be employed on a common boresight. Weather conditions and the time of day may adversely affect the ability of an infrared sensor to acquire the target while not affecting millimeter wave energy. Infrared has better resolution at closer ranges and the two can complement each other in target acquisition and rejection of countermeasures. 
     Although a single mode laser type missile seeker is also known, it relies on an external laser designator to pick out a target which further ensures reliable target acquisition. However, the laser designation may not be on the sweetest part of the target and the infrared can determine the optimum terminal aimpoint. Weather also affects the range performance of a laser designator system. 
     As processing has gotten faster and sensors have gotten smaller, more capability can be put into the same volume. This is particularly true of missile seekers and their capability to autonomously find and negate targets. 
     SUMMARY 
     It is an object of the present invention, therefore, to provide an improvement in multi-mode sensors. 
     It is another object of the present invention to provide an assembly of multi-mode sensors located in a common receiving aperture. 
     It is still another object of the invention to provide a seeker having multi-mode sensors located in a common receiving aperture so as to minimize platform size and weight while providing essentially no increase in volume. 
     These and other objects are achieved by an improvement in the system taught in the above referenced Brusgand et al. patent by the inclusion of a laser spot tracker in the system. In the present invention, a single receiving aperture collects energy from a millimeter wave (MMW) RF sensor, an infrared (IR) sensor and a semi-active laser energy sensor aligned on a common central axis. An active millimeter wave transmitter/receiver is located at the focus of a primary mirror aperture. A selectively coated dichroic element is located in the path of millimeter wave energy and feeds the energy to an RF detector while reflecting infrared energy from the primary reflector to an optics system which re-images the IR energy on an infrared detector. The edge of the primary mirror is furthermore bent so that laser energy focuses beyond the RF receiver and thus produces no detrimental affects on either the RF or the IR energy received. The laser energy is captured using a second lens system located in front of the dichroic element and the RF receiver. 
     Since weather affects the performance of both IR and laser type sensor/receivers, an RF sensor can locate a target until the laser and infrared receiver can get close enough to see through any adverse weather. 
     Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific examples, while disclosing the preferred embodiments of the invention, it is given by way of illustration only, since various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood when considered in conjunction with the accompanying drawings which are provided by way of illustration only, and thus are not meant to be limitative of the invention, and wherein: 
     FIG. 1 is a partially cutaway perspective view of a first embodiment of a tri-mode seeker in accordance with the subject invention; 
     FIG. 2 is a longitudinal central cross-section of the embodiment of the invention shown in FIG. 1; 
     FIG. 3 is a front planar view of the primary mirror included in the embodiment of the invention shown in FIG. 2; 
     FIG. 4 is a central cross-sectional view of the laser sensor of the tri-mode seeker shown in FIG. 2; 
     FIGS. 5 and 6 are illustrative of front and back exploded perspective views of the laser sensor shown in FIG. 4; 
     FIG. 7 is a perspective view of a second embodiment of the subject invention; 
     FIG. 8 is a central longitudinal cross-sectional view of the second embodiment of the invention shown in FIG. 7; 
     FIG. 9 is a central longitudinal sectional view of the lens system shown in FIG. 8; 
     FIG. 10 is an exploded perspective view of the laser sensor of the second embodiment shown in FIGS. 7 and 8; 
     FIG. 11 is an electrical block diagram of one embodiment of the receiver portion of the laser sensor included in the embodiment of the subject invention shown in FIGS. 1 and 2; and 
     FIG. 12 is an electrical block diagram of a variation of the receiver portion of a laser sensor shown in FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention is directed to a common aperture for three receivers/sensors of millimeter wave (MMW,) infrared (IR) and laser energy which are coaxially aligned on a common boresight or central longitudinal axis (CL) of seeker apparatus used, for example, in an airborne platform such as a missile. 
     Referring now to the drawings wherein like reference numerals refer to like components throughout, reference is first made to FIGS. 1-6 wherein a tri-mode seeker  10 , is depicted in accordance with a first embodiment of the invention. Reference numeral  11  in FIGS. 1 and 2 denotes an annular base member to which is secured a collar  12 , for supporting a gimbal assembly  14  on which is mounted the components of the seeker. Reference numeral  16  denotes a primary mirror  16  assembly which includes a parabolic reflecting surface  15  and central opening  18  through which passes an infrared (IR) sensor/receiver  20  which is secured to a rear base portion  22 . 
     Located in front of the IR receiver  20  is a millimeter wave (MMW) transmitter/receiver  24  which both transmits RF energy to a target and receives the RF energy reflected therefrom. A dichroic mirror assembly  26  is mounted on the outside of the MMW transmitter/receiver  24  facing the IR receiver  20 . The dichroic mirror assembly  26  allows RF energy to pass therethrough to the transmitter/receiver  24  while reflecting IR energy to an optical system  28  which forms part of the IR receiver  20 . 
     A laser spot tracker  30  is mounted on the back side of the MMW transmitter/receiver assembly  24  and includes a semi-active laser sensor assembly  32  including an immersion lens system  34 . In this embodiment of the invention, the inversion lens system  34  includes a secondary parabolic mirror  36 , an optical bandpass filter  37 , and a convex focusing lens  38 . A spheroidal radome  40  having a rounded front end is fitted over the outside of the secondary mirror  36  and is affixed to the collar 12 . 
     As further shown in FIG. 2, received RF energy  42  passing through the radome  40  is focused to a patch antenna element  44  of the RF transmitter/receiver assembly  24 . Incident IR energy  46 , however, is reflected off the face  15  of the primary mirror  16  where it is reflected again by the surface of the dichroic mirror  26  to the optical system  28  where it is fed to a cryogenically cooled IR detector element  48  located in the focal plane of the optical system  28 . 
     The outer peripheral edge  50  of the primary mirror  16 , as shown in FIG. 3, is slightly bent so that laser energy  52  is reflected to the surface of the secondary mirror  36  where it is again reflected through the lens  38  to the laser sensor  32 , the components of which are shown in FIGS. 4,  5  and  6 . 
     Referring now to FIGS. 5 and 6, in addition to the lens  38  which is mounted on an annular front cover member  54 , the laser sensor assembly  32  additionally includes a flat circular component support member  56  called a circuit card assembly (CCA) member  56  having a quadrant avalanche photodiode detector (QAPD)  58  or equivalent laser sensor mounted on the front side thereof, and an application specific integrated circuit (ASIC) package  60  is mounted on the back side thereof. An avalanche photodiode device is a well known circuit element and comprises a solid state device manufactured and supplied by Perkin Elmer Optoelectronics, which is a company located in Vaudreuil, Quebec, Canada. An ASIC is a circuit element well known in the semiconductor electronics and is designed to perform a specific function. Further, the CCA member  56  is adapted to be mounted on the face  61  (FIG. 5) of a front chassis member  62 . 
     A pair of CCAs  63  and  64  are located between the rear surface  65  (FIG. 6) of the front chassis member  62  and face  66  of a rear chassis member  67 . An MMW transmitter in the form of a “HOTLink”™ digital communication link  68  and an analog-to-digital A/D converter  70  are mounted on the front of the CCAs  63  and  64  as shown in FIG. 5. A fourth CCA  72 , on which is mounted an oscillator (OSC) package  74  and a field programmable gate array (FPGA)  76 , a device having a plurality of devices which can be selectively configured on demand, is fitted to the rear surface  78  of the rear chassis member  67  as shown in FIG. 6. A set of connector pins  80  is located on an outer peripheral portion  84  of the rear chassis. Finally, a flat rear cover  82  is secured to the rear chassis member  67 . 
     The combination of the secondary lens  36 , the immersion lens  38 , and the electronic components described above form a compact package which makes the laser sensor  30  compatible with the system shown and described, for example, in the above referenced Brusgard et al. patent, U.S. Pat. No. 5,214,438. 
     Referring now to FIGS. 7-10, shown thereat is a second embodiment of the invention and comprises a modification of the embodiment shown and described with respect to FIGS. 1-6. The second embodiment comprises a tri-mode seeker  10 ′ also having three sensor/receivers of MMW, IR and laser energy integrated into a single composite assembly. 
     As shown, for example, in FIGS. 7 and 8, the seeker  10 ′ includes, among other things, a primary mirror assembly  16 , including a reflector  15 ′, IR sensor/receiver  20  and a MMW transmitter and receiver  24  substantially as before; however, the laser sensor  30  (FIG. 2) is modified as shown in FIGS. 7 and 8 by reference numeral  30 ′ as well as certain relatively minor structural changes, for example, to the gimbal system  14 ′ which need not be considered in any significant detail as it relates to the inventive subject matter of the present invention. 
     Whereas the first embodiment utilized a separate secondary mirror element  36  and immersion lens  38 , the second embodiment as shown, for example in FIG. 8, now eliminates the discrete mirror element  36  in favor of an immersion lens assembly  90  which integrates the secondary mirror function therein. As shown in FIG. 9, the lens assembly  90  is comprised of two lens components  92  and  94 , which are fitted together as a composite lens assembly for mounting on a chassis member  96  shown in FIG. 10 along with a bandpass light filter  37 ′ in the form of a ring being located on the back side of the lens  92  around the periphery of the flat lens element  94 . What is significant about the immersion lens configuration  90  is that the secondary mirror function now embodied in the concave surface area  100  having a reflective coating  101  applied to the outer surface thereof. As shown in FIG. 8, laser energy reflected off of the outer edge  50 ′ of the mirror surface  15 ′ passes through the ring filter  37 ′ to the surface  100  of the immersion lens  90  where it is re-reflected back to the photodetector  58 . 
     Referring now to FIG. 10, the chassis  96  includes a set of pin connectors  102  for feeding signals generated by the circuit omponents  104  mounted on a CCA board  105  to external circuitry, not shown. The components include, for example, the quadrant avalanche photodiode detector (QAPD)  58  affixed to the flat circular CCA to the front side of CCA board  105 . On the back side of the CCA board  105  are located some, if not all, of the electrical components shown in FIGS. 5 and 6 as well and become evident when the electrical block diagrams of FIGS. 11 and 12 are considered hereinafter. As further shown in FIG. 10, a circular rear cover member  106  is adapted to be fitted to the chassis  96  so as to protect the components mounted on the CCA board  105 . FIG. 8 shows two cable assemblies  108  and  110  which are adapted to feed signals from both the laser receiver  34 ′ and the RF transmitter/receiver  24  to the rear of the seeker assembly  10 ′ for further processing. 
     Referring now to FIG. 11, shown thereat is an electrical block diagram of the electronics components shown in FIGS. 5 and 6 and associated with, but not necessarily with one embodiment of the semi-active laser receiver in the subject invention. As shown, laser energy is bandpass filtered by the filter  37  where it enters the immersion lens system  34  and from there it is directed to an optical detector which is comprised of an avalanche photodiode quadrature detector  58  implemented in solid state material such as silicon (Si). The APD quadrature detector  58  receives a bias voltage via a QAPD HV BIAS circuit lead  57  from an external source, not shown. A temperature signal indicative of the QAPD temperature is fed to external circuitry, not shown, for use in external signal processing circuitry, also not shown. A common ground lead  71  is shown being applied to the laser sensor assembly  32 . Additionally, two input signals identified as “lastnotfirst” and MLO are fed to the FPTA  80  via circuit leads  77  and  79 . 
     The output of the QAPD detector  58  is fed to the analog ASIC  60  where it is next fed to an analog to digital (A/D) converter  70 . Analog signals from the ASCIC  60  and a digital output from the A/D converter  70  are fed to the FPGA  76  which operates in conjunction with a 30 MHz crystal oscillator  74  to control the HOT-link™ transmitter  68 . 
     It should be noted with respect to the analog ASIC  60  that it is configured to include four (4) matched transimpedance pre-amplifiers, each with four selectable matched gain steps; a high pass filter at the output of each pre-amplifier; four matched gain post amplifiers; two (2) banks of eight (8) matched sample-and-hold circuits for each pre-amp and post-amp output; peak detection circuitry of the sum of the sample-and-hold circuits for event recognition; pulse shaping and target ambiguity circuitry for discrimination; and high-gain and low-gain circuits for providing data sets which are fed to the A/D converter  70 . 
     A second embodiment of the semi-active laser receiver provides an analog output instead of a digital output and is shown in FIG. 12 where the HOT-link™ transmitter  68 , the control FPGA  76  along with the analog ASIC  60  and A/D converter  70  are moved off the sensor portion  32  of the laser spot tracker  30  to another location and consists simply of four matched transimpedance amplifiers  108  which are coupled to the analog output of the QAPD  58 . The transimpedance amplifiers have gain adjustment applied thereto via circuit lead  109 . The output of the transimpedance amplifiers are fed to four (4) 50 ohm unbuffered amplifiers  110  whose respective outputs A, B, C and D are fed to external circuitry, not shown, via shielded cables  112 ,  114 ,  116  and  118 . 
     In an exemplary tri-mode seeker in accordance with the subject invention, mounted in a platform such as a missile, laser radiation return from a target strikes the edge  50  of the primary mirror  16  about 0.3 inches around the periphery of the mirror which is tilted. 
     Because the reflection is around the edge of the mirror, the laser collecting area of approximately 27 sq.cm. on a five inch diameter mirror  16  is significant. Yet the combination of the objects, primary mirror and secondary mirror and immersion lens produces a 0.006 inch focus spot on a 0.19 inch laser quadrant detector  58 . The primary mirror edge is bent so that the longer wavelength millimeter wave energy will still be passed onto the RF receiver. The primary mirror along with the relay infrared optics  28  is a F/2.5 optical system, which means that the infrared energy does not use the full aperture and is not distorted by the 0.3 inch bend in the primary mirror  16 . Since the edge  50  of the primary mirror  16  is used for the laser receiver, the light rays are essentially parallel and a filter coating can be placed on the immersion lens that can be very narrow. A light filter with only 14 nm. bandpass can be used to reject other light energy besides the laser energy. Rejecting other light energy significantly reduces the background noise that can be generated in the detector and further enhances signal to noise ratio and therefore target range detection. 
     As the missile closes in on the laser spot source, the signal intensity at the laser spot receiver goes up proportional to the range squared. The dynamic range of the received signal can be in the order of 125 db. Switching the gain of the transimpedance amplifiers along with a 12 bit A/D  70  converter, placing four channels of switching amplifiers in an application specific integrated circuit or ASIC  60  saves considerable space for making a compact laser receiver that will fit on the front of the gimbal assembly  14 . 
     The foregoing detailed description merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.