Abstract:
A combat IFF system, for use in a combat exercise or on the battlefield, including a helmet-mounted passive IFF response unit and a weapon-mounted IFF interrogatory unit for each soldier. Infrared (IR) signals are employed for both challenge and response. The IR response signal is a very narrowly-targeted reflection of the relatively narrow IR transmit signal, thereby minimizing interception opportunities. The transmit and response signals are encoded in a transaction that cannot be compromised even when either or both signals are intercepted and decoded by the enemy. The combat IFF system includes biometric anti-spoofing features that prevent any use by an enemy in possession of captured units. Military radio-frequency (RF) spectrum is not required so there are no bandwidth limitations on simultaneous IFF transactions in the battlefield. A combat IFF transaction is completed in milliseconds.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to combat identification systems for the dismounted soldier and more particularly to a secure covert identification as friend or foe (IFF) system for interrogating a dismounted soldier with a coded infrared (IR) signal that is selectively retroreflected and encoded by the target when recognized as a valid challenge.  
           [0003]    2. Description of the Related Art  
           [0004]    Dismounted Armed Forces have an interest in the remote and secure identification of a person as friend or foe, during combat training exercises and in armed conflicts. Identification as friend or foe (IFF) systems are well-known in the art for military aircraft and other weapons systems. Such systems are useful for preventing action against friendly forces. The military platform commanders on a modern battlefield must accurately identify potential targets as friend-or-foe (IFF) when detected within range of available weapon systems. Such target IFF presents a difficult decision for a military platform commander, who must decide whether to engage a detected target while avoiding accidental fratricide. This problem is even more difficult for the dismounted soldier who may be moving covertly through an unknown combat zone at night with limited visibility. Simple visual assessments of other dismounted soldiers is not a reliable IFF method for military platforms or dismounted infantry.  
           [0005]    The art is replete with proposals for IFF systems for military platforms in modern land battlefields. But commanders often still rely on low-resolution visual and infrared images to identify detected targets. Commanders often must operate under radio silence to avoid detection by an enemy. With infrared (IR) imagers alone, the identification of individual dismounted soldiers is not feasible, although the IR signatures of land vehicles may have some use. IFF systems that require one or more radio signals are limited in channel-capacity and must bear the overhead of selecting and/or awaiting an available battlefield channel before completing the IFF task. Active-response systems require the emission of a signal by the unknown respondent in response to a verified challenge, which may compromise the security of both interrogator and respondent. Active transponders are subject to capture and may be used for spoofing by the enemy in a battlefield or a combat training environment. Passive response systems rely on the return of an echo (reflection) of a challenge signal to the interrogator, but simple reflection schemes are easily compromised and more elaborate passive reflection schemes are still subject to intercept, compromise or capture for use by the enemy in spoofing the interrogator.  
           [0006]    As described in U.S. Pat. No. 4,851,849 by Otto Albersdoerfer, a typical active IFF technique for vehicles is to equip a military vehicle with a transponder that emits a coded return signal when an interrogating radar pulse is detected by its receiver. As described in U.S. Pat. No. 5,686,722 by Dobois et al., a more sophisticated active IFF technique for vehicles uses a selective wavelength optical coding system with tunable optical beacons mounted on each vehicle. By spreading the optical broadcast energy into frequency in a precise manner, the beacon identifies the host vehicle to friendly receivers while remaining covert to the enemy.  
           [0007]    As described in U.S. Pat. No. 4,694,297 by Alan Sewards, a typical passive IFF technique for vehicles is to equip a military vehicle with a passive antenna that reflects an interrogatory radar beam while adding a distinctive modulation by varying the antenna termination impedance responsive to evaluation of the interrogatory beam. A more sophisticated passive electro-optical IFF system for vehicles is described in U.S. Pat. No. 5,274,379 by R. Carbonneau et al. wherein each friendly vehicle is provided with a narrow-beam laser transmitter and a panoramic detector. If a vehicle detects a coded interrogator laser beam and identifies the code as friendly, it opens a blocked rotating retro-reflector to clear a reflection path back to the source, where it can be identified by another narrow field-of-view detector. A further modulation is also added to the reflected beam to identify the reflecting vehicle as friendly. If an improperly coded beam is detected, the transmission path is not cleared, thereby preventing reflection of that beam and warning is sent to the vehicle commander of an unfriendly laser transmission. Others have proposed similar passive optical IFF systems for vehicles, including Wooton et al. in U.S. Pat. No. 5,459,470 and Sun et al. in U.S. Pat. No. 5,819,164.  
           [0008]    The art is less populated with IFF proposals for the lone dismounted soldier (the infantryman on foot). Whether in actual combat or in a training exercise, the dismounted soldier operates with severe weight limits and little onboard electrical power. The friendly foot soldier has no distinctive acoustic, thermal or radar cross-section that may be used to assist in distinguishing friendlies from enemies. But some practitioners have proposing IFF solutions for the dismounted soldier, both active and passive. For example, in U.S. Pat. No. 6,097,330, Kiser proposes an active IFF system for identifying concentrations of ground troops (or individuals) from an aircraft by interrogating a (heavy) human-mounted radio transmitter carried by one of the group with a narrow-cast optical signal. As another example, in U.S. Pat. No. 5,299,277, Rose proposes a compact active IFF system to be carried by each individual dismounted soldier for use in combat exercises or on the battlefield. The system includes a clip-on beacon and a hand-held (flashlight-style) or weapon-mounted detector. The beacon radiates a spread-spectrum low-probability-of-intercept (LPI) signal at optical frequencies that are selected to be invisible to the usual detectors present in the battlefield. Rose doesn&#39;t consider the problem of spoofing with captured devices. As yet another example, in U.S. Pat. No. 5,648,862, Owen proposes an active IFF system implemented by adding provisions for coded two-way transmissions to the night-vision systems often worn by dismounted soldiers. As a final example, in U.S. Pat. No. 5,966,226, Gerber proposes an active combat IFF system for each dismounted soldier that includes a weapon-mounted laser projector for interrogating suspected targets and a harness including means for receiving the interrogatory signal and means for responding with an encoded radio, acoustic or optical signal. But these proposals do not resolve the spoofing problem (through capture of a beacon or harness, for example). and are not particularly covert because the responding target generally broadcasts an active signal either continuously or in response to interrogation. Any IFF proposal employing broadcast signals also faces a battlefield channel capacity (or channel availability delay) problem as well.  
           [0009]    There is still a clearly-felt need in the art for an IFF system for the dismounted soldier that provides true passive covertness and that cannot be spoofed under any battlefield conditions. The desired IFF system requires little power and is adapted to prevent any use of captured equipment or intercepted signal codes. Finally, the system should be inexpensive enough to permit equipping every soldier with the necessary interrogation and response equipment for combat exercises or actual battlefield conditions. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.  
         SUMMARY OF THE INVENTION  
         [0010]    This invention solves the above-described problems by providing a system, for use in either a combat exercise or on the battlefield, that includes a passive helmet-mounted identification as friend or foe (IFF) response unit and a weapon-mounted IFF interrogatory unit for each soldier. Infrared (IR) signals are employed for both challenge and response. The IR response signal is a very narrowly-targeted reflection of the relatively narrow IR transmit signal, thereby minimizing interception opportunities. The transmit and response signals are encoded in a transaction that cannot be compromised even when either or both signals are intercepted and decoded by the enemy. The combat IFF system includes biometric security (anti-spoofing) features that prevent any use by an enemy soldier in possession of captured units. No military radio-frequency (RF) spectrum is used so there are no limitations on simultaneous IFF transactions in the battlefield. A combat IFF transaction is very brief and may be completed within the typical human reaction time interval.  
           [0011]    It is a purpose of this invention to provide a secure covert combat IFF system adapted for use either in a combat exercise or on the battlefield. It is an advantage of this invention that the IFF transaction employs line-of-sight IR signals that are not likely to be surreptitiously intercepted. It is a feature of this invention that both transmit and response signals are encoded for security in a manner that prevents compromise even if one or both are intercepted and decoded by an enemy. It is another advantage of this invention that the response unit is entirely passive and cannot radiate any energy that may be intercepted.  
           [0012]    It is another purpose of this invention to provide a combat IFF system having a reduced cost and weight without using any additional military RF spectrum. It is a feature of this invention that the response unit is integrated into the helmet of the dismounted soldier, adding little weight thereto. It is an advantage of this invention that the interrogatory unit and the response unit generally include commercial off-the-shelf (COTS) components available in quantity at relatively low cost and the response unit may be powered by a small lithium-ion battery for up to three years. It is another feature of this invention that the entire IFF transaction occurs in the optical spectrum, using no military RF spectrum.  
           [0013]    It is yet another purpose of this invention to provide a combat IFF system that cannot be compromised by the enemy, whether by interception, spoofing or capture. It is a feature of this invention that the helmet-mounted response unit is automatically disabled upon doffing of the helmet and may be reactivated only by means of selected biometric data unique to the soldier fitted with the helmet; and similar provisions may be made for the interrogatory unit. It is an advantage of this invention that an IFF transaction cannot be compromised or spoofed even by an enemy in possession of both decoded signals together with captured interrogatory and response units.  
           [0014]    In one aspect, the invention is a method for identifying as friend or foe a combat response unit having a helmet-mounted challenge receiver and a retroreflector obturator, including the steps of projecting an IR transmit signal including a transmitted code of the day (TCOD) onto the combat response unit from a combat interrogatory unit, receiving the IR transmit signal and TCOD at the challenge receiver, selectively reflecting the IR transmit signal by opening and closing the retroreflector obturator according to a response code of the day (RCOD), receiving the reflected IR transmit signal and RCOD at the combat interrogatory unit, and combining the received RCOD with the TCOD to identify the combat response unit as friend or foe.  
           [0015]    In a preferred embodiment, the invention is a system for combat IFF communications including a combat interrogatory unit having projector means for projecting an IR transmit signal including a TCOD, receiver means for receiving a reflected IR transmit signal including a RCOD, and means for combining the received RCOD with the TCOD to identify the source of the reflected IR transmit signal as friend or foe; and a helmet-mounted combat response unit having sensor means for receiving a projected IR transmit signal including the TCOD, retroreflector means for reflecting an incoming IR transmit signal generally back along the incoming path thereof, obturator means for obstructing the retroreflector means to prevent reflection thereby, and means for opening and closing the obturator means according to the RCOD.  
           [0016]    The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    For a more complete understanding of this invention, reference is now made to the following detailed description of the embodiments as illustrated in the accompanying drawing, in which like reference designations represent like features throughout the several views and wherein:  
         [0018]    [0018]FIG. 1 is a sketch illustrating the operation of the combat identification as friend or foe (IFF) system of this invention;  
         [0019]    [0019]FIG. 2 is a sketch of the helmet-mounted combat response unit of this invention illustrating the IR sensor and retroreflector/obturator array;  
         [0020]    [0020]FIG. 3 is a sketch of the combat interrogatory unit of this invention illustrating a preferred weapon-mounted disposition thereof;  
         [0021]    [0021]FIG. 4 is a schematic diagram illustrating the optical operation of the combat response unit of FIG. 2;  
         [0022]    [0022]FIG. 5 is a schematic block diagram illustrating the detailed operation of the combat response unit of FIG. 2;  
         [0023]    [0023]FIG. 6 is a schematic block diagram illustrating the detailed operation of the combat interrogatory unit of FIG. 3;  
         [0024]    [0024]FIG. 7 is a chart illustrating the relationship between examples of the code of the day (COD), the transmitted code of the day (TCOD) and the response code of the day (RCOD) according to the method of this invention;  
         [0025]    FIGS.  8 ( a )-( d ) are sketches illustrating several examples of the means for accepting biometric data employed in the combat response unit of FIG. 2; and  
         [0026]    [0026]FIG. 9 is a block diagram of a flow chart illustrating the IFF method of this invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    [0027]FIG. 1 illustrates the operation of the combat identification as friend or foe (IFF) system of this invention. The combat interrogatory unit  20  of this invention is shown in a preferred weapon-mounted disposition where the challenging soldier may target the helmet-mounted combat response unit  22  of this invention worn by a soldier in a combat simulation exercise or in actual combat. An infrared (IR) transmit signal  24  is projected by unit  20  upon operator command. Transmit signal  20  radiates outward along a narrow beam, eventually illuminating response unit  22 . For example, transmit signal  24  may be embodied as a 6 milliradian beam of 1540 nm IR light, which then illuminates an area of about 6 meters on a side at a typical weapon range limit of 1000 meters. Upon being received, detected and verified at response unit  22 , transmit signal  24  is then retroreflected back to interrogatory unit  20  as the response signal  26 . For a 6 milliradian transmit signal  24 , response signal  26  includes a reflection of, for example, a 1.5 centimeter portion of the 6 meter transmit beam  24 . This 1.5 cm reflected portion includes about 0.002 percent (−47 dB) of the initial energy of transmit signal  24 , which is generally reflected back to interrogatory unit  20  by a precision retroreflector (see FIGS. 2 and 5). Response signal  26  is received at interrogatory unit  20  reduced by an additional −8 dB, which leaves sufficient power for IFF detection and processing at interrogatory unit  20 . The 1540 nm signal wavelength is preferred because it is eye-safe and has relatively low absorption and scattering loss in the usual battlefield smoke and haze.  
         [0028]    [0028]FIG. 2 illustrates helmet-mounted response unit  22  in some additional detail, illustrating the IR sensor and retroreflector array  28  consisting of an IR sensor  30  and a retroreflector/obturator  32  in alternating disposition. Retroreflector/obturator  32  provides the precision return along the incoming path of any ray arriving from a direction within a 90 degree cone of coverage. Array  28  may include twelve sensors  30  and retroreflector/obturators  32 , alternated to dispose one of each for every 50-degrees of azimuth, thereby providing the desired 360-degree hemispherical coverage from 90-degrees above the horizon to 30-degrees below the horizon. Array  28  is fixed to the response unit  22  by any useful means, such as, for example, rivets, cement or stitching (not shown), and may be oriented asymmetrically with respect to the vertical axis to enhance the desired hemispherical coverage above the horizon and down to 30 degrees below the horizon. The entire helmet-mounted response unit  22  weighs about 230 grams and requires no power except for the low-power electronic logic components described below in connection with FIGS.  4 - 5 , which may be powered for up to 3 years by a small lithium-ion battery. All elements described in connection with the exemplary embodiment of response unit  22  are available as COTS products, including the necessary battery.  
         [0029]    Retroreflector/obturator  32  may employ any useful retroreflector device known in the art, such as, for example, one of the line of Tech Spec™ Corner Cube Retroreflectors (Trihedral Prisms) available from Edmond Industrial Optics, Barrington, N.J. The obturator portion of retroreflector/obturator  32  may include a mechanical shutter device capable of cycling open and closed within a few milliseconds, or more preferably, a liquid crystal device (LCD) disposed over the retroreflector portion, such as the LCD-CDS92106 available from Cubic Defense Systems, San Diego, Calif.  
         [0030]    [0030]FIG. 3 illustrates interrogatory unit  20  in a preferred weapon-mounted disposition. Interrogatory unit  20  is removably fixed by a clamp  34  to a weapon barrel  36 , which in use is removably fixed to a weapon carried by a dismounted soldier (FIG. 1), and includes an IR projector  38  for projecting IR transmit signal  24  and an IR receiver  40  for receiving response signal  26 , which is a reflected portion of IR transmit signal  24 . IR projector  38  may include, for example, a 1540 nm laser diode suitable for sharing with a laser range finder system such as the 750-Series laser rangefinder receivers available from Analog Modules, Inc., Longwood, Fla. IR projector  38  is preferably factory-aligned with IR receiver  40  for ease of boresighting and operation in the field. Interrogatory unit  20  may also include an IFF indicator  42  for indicating the results of an IFF challenge transaction, and a mode switch  44 , for changing selected operating parameters, such as the IFF encoding scheme, signal timing, power on/off or IR projector wavelength, for example. Interrogatory unit  20  also includes a remote interrogation switch (not shown) that may be removably fixed to any convenient spot proximate the weapon trigger area (not shown) for convenient access during use for interrogation of unidentified response units. This remote interrogation switch may also include means for accepting biometric data (not shown, see FIG. 8) representing an operator fingerprint (or iris pattern or the like) for security purposes such as, for example, disabling (by a time-out timer) a captured unit to prevent unauthorized use by an enemy soldier. Provision may be made for either local battery or vehicle-cable power and/or for coupling to digital vehicle-systems for any useful purpose.  
         [0031]    The two-way signal-to-noise budget for an IFF transaction may be appreciated with reference to the following example. A +44 dBm IR transmit signal level may be provided at 1540 nm by a commercial off-the-shelf(COTS) class-i laser (25 watts). At an exemplary 1000 meter range limit, IR transmit signal  24  arrives at response unit  22  with a power of −8 dBm, having lost, for example, 52 dB en route through spreading, absorption and scattering losses. IR transmit signal  24  is then reflected by the appropriate retroreflector/obturator  32  and returned to IR receiver  40  with a −24 dB spreading loss (less than simple spherical spreading because of the retroreflector properties), a −3 dB obturator insertion loss, and scattering and absorption losses of less than −1 dB, leaving about −36 dBm at IR receiver  40 , which is perhaps 8-10 dB above the noise threshold of an exemplary COTS laser detector, such as the Analog Modules No. 754 available from Analog Modules, Inc., Longwood, Fla. Even in a battlefield smoke and haze condition, the 1540 nm laser signal suffers only an additional −7.4 dB of scattering loss over the 1000 meter two-way path, leaving perhaps 1-3 dB of signal-to-noise ratio (SNR) margin in the IFF transaction of this example. The inventors have calculated link margins showing that IFF transactions are 99.5% successful in clear air at 1540 nm with a 5 dB margin (8 dB is needed for smoky haze).  
         [0032]    [0032]FIG. 4 is a schematic diagram illustrating the optical operation of response unit  22  (FIG. 2). As described in detail hereinbelow in connection with FIG. 7, transmit signal  24  (FIG. 1) includes a transmit code of the day (TCOD) including a frame-synchronization preamble (not shown) followed by a TCOD  24 ( a ) followed by a TCOD interrogation pulse stream  24 ( b ). In operation, TCOD  24 ( a ) is received by one or more of the plurality of IR sensors  30  and presented to the challenge receiver  46  for verification. When TCOD  24 ( a ) is verified, challenge receiver  46  produces a response code of the day (RCOD) signal  48 , which includes a logical combination of selected information from TCOD  24 ( a ) and from the local memory of challenge receiver  46  (not shown) that is described in detail hereinbelow. RCOD signal  48  is presented to the obturator driver  50 , which produces an obturator signal  52  for opening and closing (making translucent and opaque) the obturator  54  in each of the plurality of retroreflector/obturators  32 . A biometric identification (1D) signal  56  is also presented to obturator driver  50  to enable or disable operation thereof based on the verification of a scanned thumbprint input by the dismounted soldier in possession of helmet-mounted response unit  22 , which is further described hereinbelow in connection with FIGS. 5 and 8( a )-( d ). RCOD signal  48  includes a delay and a response pulse stream  26 ( a ). The RCOD delay is sufficient to permit TCOD interrogation pulse stream  24 ( b ) to arrive at reflector/obturator  32 . Obturator  54  is then cycled open and closed in accordance with obturator signal  52  to produce response pulse stream  26 ( a ) by reflecting selected elements of interrogation pulse stream  24 ( b ) from the retroreflector  58  in at least one of the plurality of retroreflector/obturators  32 .  
         [0033]    [0033]FIG. 5 is a schematic diagram illustrating the logical operation of response unit  22  (FIG. 2). The elements of FIG. 5 are arranged in three functional categories for clarity; (a) helmet sensors, (b) helmet logic and (c) helmet memory. In operation, helmet-mounted response unit  22  is first programmed with certain biometric data unique to the user and with a code of the day (COD), which includes a challenge code (CC) followed by a response code (RC). The helmet is then donned by the user according to a predetermined anti-spoofing protocol. The donning soldier first passes his/her thumb over a biometric thumbprint sensor  60  in a manner (known only to the soldier) for which response unit  22  has previously been programmed. Biometric thumbprint sensor  60  produces the coded biometric data stream  62 , which is immediately evaluated by the biometric identification logic  64 . This evaluation involves comparing data stream  62  to the earlier-stored thumbprint scan data  66  for the donning soldier to produce an enable/disable decision. If logic  64  decides that the thumbprint scan fails, the soldier may be afforded, for example, two additional opportunities to enable the helmet-mounted response unit by means of an audible or visual indicator (not shown). After two additional attempts, logic  64  decides that the biometric ID is invalid, and sets biometric ID signal  56  to “disable” obturator driver  50 , permanently shutting obturator  54  to prevent any retroreflection. Moreover, biometric ID signal  56  may also be presented to a self-destruct timer  67 , which begins a logic and memory self-destruction count-down. When completed, this countdown triggers the erasure of all logic and data stored in the helmet memory, thereby making a captured helmet-mounted response unit worthless to an enemy. The helmet electronics may easily be reprogrammed by the friendly forces for use in another combat exercise or the like.  
         [0034]    After successfully completing a thumb scan and donning the helmet, biometric ID signal  56  is set to “enable” and obturator driver  50  is enabled so the soldier is thereafter equipped to passively respond to valid incoming interrogatory signals in the manner now described. When the soldier again doffs the helmet, the helmet-doffing sensor  68  immediately signals logic  64 , which resets biometric ID signal  56  to “disable” and obturator driver  50  is again disabled, permanently shutting obturator  54  to prevent any retroreflection.  
         [0035]    In use, response unit  22  first begins the IFF transaction upon the arrival of IR transmit signal  24  from interrogatory unit  20  (FIGS. 1 and 6). TCOD  24 ( a ) is received by at least one IR sensor  30 , which produces a signal  70  that is presented to the signal verification and decoding logic  72 . The relative location on the helmet of the particular IR sensor  30  receiving the signal is identified in logic  72  to determine the arrival quadrant (Front, Right, Rear, or Left) of IR transmit signal  24 . This arrival quadrant information may be used in logic  72 , for example, to bias other IF processing parameters or to signal the helmeted soldier, by audible or visual means, of the arrival quadrant so that appropriate action may be undertaken (such as, for example, skeptical evasion of IFF interrogatories arriving from behind enemy lines). TCOD  24 ( a ) is validated by examining it to verify that it includes a valid CC followed by a randomly-generated generated number (RGN). This is accomplished by comparing the received CC with the CC portion of the COD  74  stored in the helmet and accepting the RGN if the incoming CC matches the stored CC. The entire stored COD  74  and the incoming RGN are then passed as a data signal  76  to the response code of the day (RCOD) generation logic  78 , which creates the RCOD  80  by combining the CC and RC portions of the stored COD with the PRN in a predetermined manner. After generation, RCOD  80  is passed to obturator driver  50  for use in opening and shutting obturator  54  accordingly. The first portion of RCOD  80  is a closed-shutter interval that is computed as a combination of the stored RC and the incoming RGN. The second portion of RCOD  80  is an open-shutter interval that is determined by some portion of the CC. RCOD  80  is synchronized to begin at a predetermined interval (such as 10 ms) after the end of the incoming TCOD so that the interrogator may evaluate RCOD  80  to verify that the reflecting response unit is a friend and not a foe, thereby completing the IFF transaction.  
         [0036]    [0036]FIG. 6 is a schematic diagram illustrating the logical operation of interrogatory unit  20  (FIG. 3). The elements of FIG. 6 are arranged in three functional categories for clarity; (a) interrogator sensors/emitters, (b) interrogator logic and (c) interrogator memory. In use, interrogatory unit  20  first begins the IFF transaction when the trigger sensor  82  detects a command from the operator to interrogate a target. Trigger sensor  82  produces a signal  84  that is presented to the TCOD generation logic  86 , which responsively retrieves a locally-stored copy of the COD  88  and a RGN  90  from a pseudorandom number (PRN) generator  92 . TCOD generation logic  86  then combines the CC portion of COD  88  with RGN  90  to produce the TCOD portion of TCOD  94 , which is then followed by the TCOD interrogation pulse stream. TCOD  94  is then amplified as necessary to drive the TCOD projector  96 , which produces the IR transmit signal  24 , initiated with the frame-synchronization preamble (not shown), and directs it to the target. TCOD is also presented to the signal decoder and verifier logic  98  for use in processing any incoming IR signals received at the IR sensor  100 . When the TCOD portion of TCOD  94  is transmitted (see FIG. 7), logic  86  notifies logic  98  of the start of the response interval. Any incoming response signal  26  at RCOD sensor  100  produces a RCOD signal  102  that is presented to logic  98  for evaluation. Logic  98  reverses the encoding process discussed above in connection with FIG. 5 and below in connection with FIG. 7. RGN  90  and the RC from COD  88  are used to compute the proper closed-shutter interval, which is compared to the delay interval between completion of the transmission of TCOD  94  and the beginning of response pulse stream  26 ( a ). The appropriate elements of COD  88  are then used to compute the proper open shutter interval, which is then compared to the actual length of the incoming response pulse stream  26 ( a ). If the two comparisons are not successful, logic  98  does nothing; leaving the IFF display  104  inactive. When the comparisons are both successful, logic  98  produces an IFF indicator signal  106 , which activates IFF display  104 , thereby completing the IFF transaction. IFF display  104  may be a simple light-emitting diode (LED) indicator or an audible signal or any other useful indicator consistent with requirements for security and covertness.  
         [0037]    As described above in connection with FIG. 5, interrogatory unit  20  may also include anti-spoofing means to prevent use by an enemy of a captured interrogatory unit. A biometric sensor  108  may be used to accept thumbprint or fingerprint input or retinal image input or any other useful biometric data unique to the authorized operator. Upon activation, biometric sensor  108  presents the incoming biometric data  110  to the biometric ID logic  112  for comparison to the biometric ID data  114  stored previously by the same soldier. If, at some point, logic  112  decides that the biometric ID is invalid, a biometric ID signal  116  is set to “disable” and TCOD generator logic  86  is disabled, permanently shutting down TCOD projector  96 . Moreover, biometric ID signal  116  may also be presented to a self-destruct timer  118 , which begins a logic and memory self-destruction count-down. When completed, this countdown triggers the erasure of all logic and data stored in the interrogator memory, thereby making a captured weapon-mounted interrogatory unit worthless to an enemy. The interrogator electronics may easily be reprogrammed by the friendly forces for use in another combat exercise or the like.  
         [0038]    After successfully completing a biometric scan, biometric ID signal  116  is set to “enable” and TCOD generator logic  86  is enabled so the soldier is thereafter equipped to actively interrogate targets in the manner described above. After a predetermined period of inactivity, established by, for example, a simple timer (not shown), logic  116  resets biometric ID signal  116  to “disable” and TCOD generator logic  86  is again disabled, permanently shutting down TCOD projector  96 .  
         [0039]    [0039]FIG. 7 is a chart illustrating the relationship between exemplary embodiments of the COD  120 , the TCOD  122  and the RCOD  124  of this invention. Each of the six symbols of COD  120  and TCOD  122  are 7-bit values (ranging from 0-127) encoded as pulse positions within a 36 microsecond frame interval, which is established and synchronized for each IFF transaction by a three-pulse TCOD frame-synchronization preamble (not shown). Each pulse is 66.67 ns in duration, which allows the use of an inexpensive 15-MHz microprocessor (not shown) for implementing the various logic described herein. The position of a pulse within a frame represents the corresponding 7-bit symbol. COD  120  is changed daily and all interrogatory units and response units in an IFF system must be updated daily with the new COD  120 . COD  120  includes a 4-symbol CC  126  and a 2-symbol RC  128 . TCOD  122  includes CC  126  and a 2-symbol RGN  130  followed by a 10-ms buffer interval followed by TCOD interrogation pulse stream  132  consisting of a 50 ms stream of 1-ms spaced pulses. RCOD  124  includes a delay  134  during the closed-shutter interval followed by a response pulse stream  136  during the open shutter interval. Delay  134  is computed by dividing the absolute value of the difference between RGN  130  and RC  128  by ten and adding 10 ms, providing a range of values from 10-20 ms in 1 ms steps. This coding method is both secure and covert because RGN  130  is shared only by the interrogator and target even though COD  120  is known by all friendlies. Although interception of RGN  130  is unlikely because of the narrow TCOD beam (3 milliradians), any such interception is useless without prior knowledge of COD  120  because RC  128  is not transmitted and both RC  128  and RGN  130  are required to compute delay  134 . The open shutter interval that reflects response pulse stream  136  is computed in milliseconds by adding three times the fourth digit of CC to five, giving a range of values from 5 to 32 ms in 3 ms steps.  
         [0040]    Thus, the duration of response pulse stream  136  cannot be spoofed without prior knowledge of COD  120  or interception of TCOD  122  and the duration of delay  134  cannot be spoofed without having both the prior knowledge of COD  120  and the successful interception of TCOD  122 . Such spoofing also presumes prior knowledge of the decoding algorithms described above. Each IFF transaction is completed within 60 ms. An interrogator may repeat an interrogation as desired to reduce the probability of false negatives. Assuming that interrogation is repeated up to four times before accepting a negative (partial response) result, the entire IFF transaction is completed within 250 ms. This method of obtaining a response is necessary only under extreme scintillation and range conditions, at the limits of link performance.  
         [0041]    FIGS.  8 ( a )-( d ) show several examples of suitable means for accepting biometric data for helmet-mounted response unit  22  described above in connection with FIGS. 2 and 5. One suitable device for accepting biometric fingerprint or thumbprint data is the FCD4B14 FingerChip™ available from AtMel Corporation, San Jose, Calif. As depicted in FIG. 8( a ), the print sensor  138  is disposed such that a finger  140  may be drawn across print sensor  138  while in continuous contact therewith. Continuous contact is important and is facilitated by providing a curved surface  142  generally as shown. As finger  140  moves over print sensor  138 , a stream of digital biometric data is produced by print sensor  138  and presented on a data bus (not shown) for manipulation by microprocessor-controlled logic (not shown). Clearly the exact biometric data stream depends not only on the fingerprint but on the direction and manner in which finger  140  is drawn across print sensor  138 . This is an important feature of this invention because it adds considerable security to helmet-mounted response unit  22  in that the availability of the proper finger or thumb alone is insufficient to satisfy the biometric security requirement; the user must also recall and repeat with reasonable accuracy the exact manner in which the thumb or finger was drawn across print sensor  138  when first storing the user&#39;s biometric ID data in response unit  22 .  
         [0042]    [0042]FIG. 8( b ) shows another mounting arrangement suitable for print sensor  138 , although the curved surface  142  is preferable. FIG. 8( c ) shows yet another example employing the curved surface  144 , which importantly permits continuous contact between finger  140  and print sensor  138  while moving thereover.  
         [0043]    [0043]FIG. 8( d ) demonstrates an exemplary arrangement for placing print sensor  138  within helmet-mounted response unit  22  so that the user may draw the thumb  146  over print sensor  138  while grasping helmet-mounted response unit  22  preparatory to donning same. Any useful audio and/or visual indicator means (not shown) may be provided to inform the user that helmet-mounted response unit  22  has been successfully activated, thereby affording the opportunity to retry activation by repeating the movement of thumb  146  across print sensor  138 . If desired, a the repeated attempts may be accumulated against a limited number followed by self-destruction of all logic and data stored in helmet-mounted response unit  22 .  
         [0044]    In an alternative embodiment of helmet-mounted response unit  22 , the biometric data necessary to identify several members of a combat unit may be stored in biometric ID storage  66  (FIG. 5) so that any combat unit member may activate helmet-mounted response unit  22 . This permits a member of the combat unit to “borrow” the helmet and its response unit  22  (FIG. 2) from another member of the same combat unit and successfully activate it for use in the battlefield. The size of the group of authorized users is limited only by the memory available in biometric ID storage  66 , which may be loaded by mass-transfer of digital data collected from the thumb-scans of all members of the combat unit. Similarly, in an alternative embodiment of interrogatory unit  20 , the biometric data necessary to identify several members of a combat unit may be stored in biometric ID storage  114  (FIG. 6) thereby permitting a member of the combat unit to “borrow” the weapon and response unit  22  (FIG. 2) from another member of the same combat unit and successfully activate it for use in the battlefield.  
         [0045]    [0045]FIG. 9 is a block diagram showing a flow chart exemplifying the IFF transaction method of this invention. This process begins with the step  148  where the interrogating soldier triggers an interrogation command at interrogatory unit  20  (FIGS. 1, 3 and  6 ). In the step  150 , a TCOD is created by accepting the RGN produced in the step  152  and the COD retrieved from local memory in the step  154 . In the step  156 , an IR transmit signal encoding the TCOD is projected to the targeted response unit. In the step  158 , the TCOD is received at the targeted helmet-mounted response unit. More precisely, the 3-pulse frame-synch preamble and the first six symbols of the TCOD are first received and decoded in step  158 , which may also include an arrival quadrant indication step (not shown) to notify the helmet wearer by some useful means of the quadrant (Front, Right, Rear, or Left) from which the received TCOD has arrived. In the step  160 , the received TCOD is validated by first verifying that the biometric security has been satisfied in the step  162  and then retrieving from local storage the COD in the step  164 . As described above in connection with FIG. 7, the CC (the first four symbols from the COD) is compared with the CC from the received TCOD and, if matched, the next two symbols of the received TCOD are decoded as the RGN in the step  166 . In the step  168 , the RGN from step  166  and the RC (the fifth and sixth symbols of the COD) are used to compute the closed shutter interval (the delay interval) of the RCOD and the fourth symbol of the COD is used to compute the open shutter interval (the response pulse stream) of the RCOD. In the step  170 , the RCOD from step  168  is used to cycle the obturator, thereby retroreflecting the interrogatory pulse stream portion of the TCOD according to the RCOD.  
         [0046]    In the step  172 , the RCOD is received and decoded at the interrogatory unit and validated in the step  174 , which uses the locally-stored RGN and COD retrieved in the step  176  to duplicate the computations used to create the RCOD at the response unit and to compare the received RCOD with the locally-computed RCOD. In the step  178 , the results of step  174  are evaluated to make a friend or foe decision, completing the IFF transaction initiated in step  148 . If Friend, then the step  180  signals the interrogatory unit user in some useful manner, such as lighting up an LED for a short time. If Foe, then the step  182  initiates a repetition of the IFF transaction (with a new RGN) or does nothing, thereby indicating that the IFF transaction has failed to identify a friend.  
         [0047]    Clearly, other embodiments and modifications of this invention may occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawing.  
       We claim: