BEAM STEERED LASER IFF SYSTEM

Abstract: A system (10) for friend-or-foe identification (IFF) comprises an interrogator unit (18) carried on a first platform (T1) for generating and transmitting a laser beam. A control unit (22) selectively directs the transmitted laser beam toward a second platform (T2). A coding/decoding unit (28) encodes an IFF interrogator message with which a portion of the transmitted laser beam is modulated. A laser reflector (30) carried on the second platform receives the coded laser beam. The reflector is a normally passive reflector which is activated by receipt of the transmitted laser beam. The reflector responds to the IFF message contained in the received laser beam by modulating the unmodulated portion of the transmitted laser beam. This newly modulated portion of the laser beam is reflected back toward the first platform. The coding/encoding unit decodes on the first platform, receives and decodes the response, and determines if it corresponds to a predetermined response which identifies the second platform as a "friend".

Description: BEAM STEERED LASER IFF SYSTEM Background of the Invention This invention relates to friend-or-foe identification systems (IFF) , and more particularly, to a beam steered laser IFF system for use in battlefield environments. With the advent of modern warfare, a battlefield is obviously a very dangerous place. Unfortunately, this is so both for one's comrades-in-arms as well as for the enemy. Given the amount of firepower deployed in a battle zone, the constant movement of men and materiel, the rapidity with which tanks, personnel carriers (APC's) , planes, helicopters move, and the inability to always know (regardless of the amount of effort employed) who is where, the chances of fratricidal harm being inflicted are probably higher than they have ever been. It has thus become an imperative to greatly limit, if not altogether eliminate, casualties resulting from "friendly" fire. One way of discerning who is a friend and who is not is by use of an IFF (Identification Friend-or-Foe) system. Various IFF systems are well-known in the art. These are typically radio frequency (RF) transmission systems, and while principally associated with aircraft, the same technology is applicable to land based vehicles, or ships. Certain RF systems, known as a co-operative systems, involve transmitting an inquiry signal to an unknown object (airplane, ship, tank) . If the object is a "friendly", it has some type of transponder for responding to the inquiry with an appropriate reply. Upon receipt of an appropriate reply, the object is designated as friendly. If the object does not provide the required response, it is designated a foe and may be attached. On drawback with co-operative systems is that it is always necessary for the object under inspection to have some mechanism for responding to an interrogation. A second drawback is that while cooperative IFF systems are the most positive types of identifications systems, and have been employed for a number of years in a variety of forms, they are not infallible. This is so for a number of reasons. For example, the response mechanism on the interrogated object may be inoperative. Or, because these type systems utilize codes, the code in the response mechanism may not be up-to-date. Consequently, the failure to respond to an interrogation signal cannot always be taken as an indication that the unknown object is hostile. As FR systems, they are vulnerable to jamming. Also, because of the power requirement of RF systems, they then to be large. A third factor which must be considered is battlefield density. Tanks, APC's, trucks, etc., have a high spatial density. In this era of non-linear battlefield, the possibility of friendly and enemy forces being close together is very high. Consequently, in using a co-operative system, care must be taken to avoid any ambiguities in an IFF identification. A spatially selective IFF system, using current RF technology, would require a small bandwidth and a correspondingly large antenna. At the same time, the system must have hemispheric coverage; which, in turn, requires electronic or electro-mechanical beam scanning. The result would be an expensive and impractical solution. What is required is a type of interrogation system which provides the required selectivity without use of a cumbersome scanning technique. Such a system would produce the needed area coverage, and would do so in a cost effective and space effective manner. It would also be advantageous if the system had minimum reliance on the operability, or up-to-date information carried by any queried targets. In this regard, laser technology may be employable in IFF systems. It is known in the art, for example, to transmit a laser beam at an unknown object and have the incident beam reflected back using a retroflector. See, for example, United States Patents 5,001,348 and 4,814,769. While useful, these laser techniques still do not solve the problems. The retro-reflector is a passive unit, and the laser beam directed at the target does not include intelligence to provoke a desired response. Summary of the Invention Among the several objects of the present invention may be noted to provision of an identification friend-or-foe (IFF) system for use in battlefield environments to quickly and accurately distinguish friendly from enemy forces; the provision of such a system which is readily integrated into existing communications and fire control systems; the provision of such an IFF system which is spatially selective in order to distinguish between friend or foe even on crowded battlefields in which enemy and friendly vehicles may be closely intermixed, and in dynamic battlefield situations in which the vehicles' relative positions rapidly change; the provision of such a system to be multi-mode so it can interrogate a single target or rapidly interrogate a series of targets to be queried; the provision of such a system in which the laser beam is steerable so as to specifically direct it at a particular target of inquiery; the provision of such a system to provide secure, covert IFF interrogations using a narrow bandwidth, coded transmission; the provision of such a system to not provide an increased cross-section, or visual or IR signal to an enemy; the provision of such a system for achieving spatial selectively using the power, rugged, and highly reliable modular solid state electronic and laser technology employing standard IFF coding techniques; the provision of such a system having no moving parts; the provision of such a system to employ an active transmitter and a normally passive receiver which is activated by receipt of an inquiry; the provision of such a system to be selectively mounted on a particular vehicle such as a lead tank; and, the provision of such a system to which is resistant to jamming. In accordance with the invention, generally stated, a. system for friend-or-foe identification (IFF) comprises an interrogator unit carried on a first platform for generating and transmitting a laser beam. A control unit selectively directs the transmitted laser beam toward a second platform. A coding/decoding unit encodes an IFF interrogator message with which a portion of the transmitted laser beam is modulated. A laser reflector carried on the second platform receives the laser beam. The reflector is a normally passive reflector which is only activated by the transmitted, coded laser beam. The reflector responds to the IFF message contained in the received laser beam by modulating the unmodulated portion of the transmitted laser beam. This newly modulated portion of the laser beam then becomes part of the reflected laser beam. The coding/encoding unit on the first platform receives and decodes the response message. It then determines if the decoded response corresponds to a predetermined response which identifies the second platform as a "friend". Other objects and features will be in part apparent and in part pointed out hereinafter. Brief Description of the Drawings Fig. 1 is a representation of a battlefield environment in which an identification friend-or-foe (IFF) system of the present invention may be employed; Fig. 2 is a block diagram of the IFF system; Fig. 3 is a perspective view of a communications unit carried on a target vehicle queried by the system; Fig. 4 is an elevational view of a retro-reflector unit of the system; Fig. 5a illustrates a grated surface emitting (GSE) laser interrogator unit and an acousto-optical modulator for steering a laser beam generated by the unit; Fig. 5b illustrates use of a Bragg cell for use as a steering unit with the laser interrogator unit; Fig. 6a represents a transmitted laser waveform modulated with an encoded query message; and Fig. 6b represents a reflected, return laser waveform, modulated with an encoded reply message. Corresponding reference characters indicated corresponding parts throughout the drawings. Description of a Preferred Embodiment Referring to the drawings, Fig. 1 represents a battlefield environment in which both friendly and hostile forces are present. The friendly forces include tanks Tl and T2, and personnel carriers PI and P2. The enemy forces are represented by tanks El and E2. Because of the fluid situation encountered on modern battlefields; i.e., where mobility is very important, the spatial separation between forces is constantly in flux. As a consequence, the friendly and enemy forces may often be in close proximity to one another. In Fig. 1, this is represented by the small spatial separation between enemy tank El and friendly personnel carrier PI, as contrasted with the spatial separation between enemy tank E2 and friendly tank T2. One purpose of friendly tanks Tl and T2 is to destroy enemy tank El. However, given the constant movement of the forces, the dust, noise, etc., it may not possible to either visually discern or verbally determine which vehicle as among tank El and carriers Pi and P2 is "friend", and which is "foe". And, unless this information is known, there is a possibility that a projectile fired by tank Tl or T2 may be aimed at one of the friendly vehicles rather than the enemy. To prevent this, it is important to be able to quickly and accurately distinguish between the two forces. Identification friend-or-foe systems have been previously used to discern friend from foe. While useful, these systems are basically radio frequency (RF) systems. As such they have a number of drawbacks. For example, they can be jammed. Also, the transponder or receiver which is used to answer any friend-or-foe query must always be operating. Since the transponder has active components, if these fail, the targets ability to answer a. query is lost, thus endangering the vehicle and its personnel. Referring to Fig. 2, a friend-or-foe identification system 10 of the present invention is shown. System 10 is installed on a platform such as tanks Tl and T2, or personnel carriers PI and P2. Each system includes both means for interrogating another vehicle, and means for responding to an inquiry. A platform such as the tank or personnel carrier does not necessarily have to carry both means. The interrogation and response portions of the system are implemented in three components. These are an operator/controller module 12, an electronics unit 14, and a communications module 16. In operation, an observer on a first platform such as tank Tl selects a target to interrogate. He may do this using, for example, a television camera, or a forward looking infrared receiver (FLIR) . Once a target has been selected, the observer activates an interrogation means 18 in communications module 16. Interrogation means 18 is a laser generator which generates and transmits a laser beam. As shown in Figs. 5a and 5b, the laser generator comprises a grated surface emitting laser generator which produces a laser beam steerable on one axis. The interrogation means is controlled through a laser electronics 20 contained in electronics unit 14. A control means 22 is used to direct the transmitted laser beam toward the selected target which is, in effect, a second platform. As shown in Fig. 5a, the control means includes an acousto-optical modulator 24 for steering the laser beam in a second axis. Or, as shown in Fig. 5b, the control means alternately includes a Bragg cell 26 for steering the laser beam in the second axis. Next, system 10 includes a coding means 28 for encoding an IFF interrogation message with which the transmitted laser beam is modulated. It will be understood that the coding means may be programmable so that each vehicle interrogated is interrogated with a different query. The particular message may then be selected by the operator when he selects the target. For example, one type message may be sent to a tank, and a second to a personnel carrier. Or, the message be selected at random in order to prevent an enemy from readily duplicating it. Other methods of selecting a transmitted inquiry may, of course, be employed. Regardless of the type of message selection process employed, the selected message is supplied through the operator module 12 to the laser electronics unit 20. Electronics unit 20 then modulates the transmitted laser beam with the inquiry message. As shown in Fig. 6a, only a portion of the transmitted beam is modulated. The remainder of the beam is, as shown, unmodulated. A reflector means 30 is carried on the second platform for receiving the laser beam and reflecting it back toward the first platform. Means 30 responds to the IFF interrogation message contained in the received laser beam by reflecting it back to its source. In doing so, it modulates the reflected laser beam, as shown in Fig. 6b, with a response message. The electronics unit on the first or transmitting platform includes a detector electronics section 32 which processes the return waveform. The output of section 32 is supplied to coding means 28. The decoding means next decodes the response message. It then determines if the message corresponds to a predetermined response which identifies the queried vehicle as a "friend". It is an important feature of the present invention that reflector means 30 be a passive reflector which is only activated when a transmitted laser beam is received. For this purpose, the reflector means comprises a corner cube retro-reflector 34 having total internal reflectance. It will be understood that in order for the retro-reflector to cover 360°, it may be made up of eight 45° sections, the location of the retro-reflector may, for example, be atop the tank or personnel carrier, or it may be in some other exposed area where it provides full coverage. Referring to Figs. 3 and 4, a base 36 is affixed to the side S of a vehicle, and the retro-reflector is mounted to this base. A plurality of reflector mounts 38 protrude outwardly from the base. The reflector means further includes positioning means 40 for positioning the retro-reflector 34 with respect to base 36. Means 40 includes a piezo-electronic positioner 42 for each reflector mount 38. As shown in Fig. 4, reflector means 30 includes a spoiler means 44 forming one side (the side connecting to base 36) of the retro-reflector. Means 44 comprises an optical energy absorber 46. The absorber rests upon the piezo-electric positioners 42. The positioners hold the energy absorber against or in close proximity to the retro-reflector. In this configuration, the reflector means is in its inactive, passive mode. When the modulated portion of the transmitted laser beam is received at the second platform, the absorber vibrates against the positioner. This vibratory motion, in turn, produces the the modulation of the previously unmodulated portion of the laser beam (see Fig. 6B) . Since this is reflected back to the transmitting vehicle, this modulation becomes the response message to the inquiry. If energy absorber 46 is within 1/4 wavelength of the incoming energy in a transmitted laser beam, then the reflector means remains passivated and there is no transmission from the retro-reflector back to the energy source. Consequently, the energy cross-section of the reflector means is at a minimum, expanding only when the modulated portion of an incident inquiry beam causes the reflector means to reflect a modulated response beam back to the transmitting platform. Only when the absorber is deflected one wavelength or more is the retro-reflector fully enabled to act as a corner reflector. The movement of the spoiler for the retro-reflector to go from fully active to passive is, for example, 1.3um. for a 1.54um. wavelength laser beam. It will be understood that system 10 may be useful for a communication system in addition to an IFF system. Thus, the same basic configuration of components would allow tank Tl, for example, so send preselected message to specific vehicles. In such situations, it allows the tank to selectively communicate with one selected vehicle even though that vehicle is in close proximity to a number of other vehicles. Operation of system 10 to perform as a communications system is similar to that above described and will not be set forth in detail. What has been described is an IFF and communications system which is useful in battlefield environments to prevent fratricidal situations where allied and enemy forces are in close proximity. System 10 is advantageous in that it employs a passive reflector which is only activated in the presence of a communication directed to it. Consequently there is not constant energy source which may allow the vehicle to otherwise be identified by enemy forces. The system can be implemented with state-of-the-art components. In view of the foregoing, it will be seen that the several objects of the invention are achieved and other advantageous results are obtained. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims: Claims : Having thus described the invention, what is claimed and desired to be secured by Letters Patent is: 1. A system of friend-or-foe identification (IFF) comprising: interrogator means carried on a first platform for generating and transmitting a laser beam; control means for selectively directing the transmitted laser beam toward a second platform; coding means for encoding an IFF interrogation message and modulating the transmitted laser beam with the message; and, reflector means carried on said second platform for receiving said laser beam and reflecting said laser beam back toward said second platform, said reflector means responding to the IFF interrogation message contained in said received laser beam by modulating said reflected laser beam with a response message, said decoding means decoding said response message and determining if said response message corresponds to a predetermined response which identifies the second platform as a "friend". 2. The system of claim 1 wherein the reflector means includes a normally passive reflector which is activated by said laser beam. 3. The system of claim 2 wherein the interrogator means includes a grated surface emitting laser generator for producing a laser beam steerable in one axis. 4. The system of claim 3 wherein the control means includes an acousto-optical modulator for steering the laser beam in a second axis. 5. The system of claim 2 wherein the coding means modulates one portion of the transmitted laser beam with the interrogation message, and said reflector is activated enabled upon receipt of said laser beam. 6. The system of claim 5 wherein said coding means only modulates a portion of said transmitted laser beam and said reflector is activated by said modulated portion to modulate another portion of said laser beam with said response message. 7. The system of claim 2 wherein said reflector is a corner cube retro-reflector having total internal reflectance. 8. The system of claim 7 in which said retro-reflector is mounted on a base attached to said second platform. 9. The system of claim 8 wherein said reflector means further includes positioning means for positioning the retro-reflector with respect to said base. 10. The system of claim 9 wherein said reflector means further includes spoiler means forming one side of said retro-reflector. 11. The system of claim 10 wherein said spoiler means includes an optical energy absorber mounted on a piezo-electric positioner comprising said positioning means, said energy absorber being held against said retro-reflector by said positioner, the retro-reflector being in a inactive, passive mode when said energy absorber is so positioned. 12. The system of claim 11 wherein said transmitted laser beam has a first modulated portion and a second unmodulated portion, said absorber being responsive to the modulated portion of said transmitted laser beam to vibrate against the positioner, the vibration of the absorber, in turn, modulating the second portion of the laser beam to create the response message reflected back to the first platform. 13. The system of claim 11 wherein said positioner means comprises a plurality of piezo-electric positioners. 14. The system of claim 13 wherein the spoiler means passivates the reflector means when the spoiler means is within 1/4 wavelength of the incoming energy in the transmitted laser beam. 15. The system of claim 4 wherein the control means steers the transmitted laser beam successively toward a plurality of second platforms. 16. The system of claim 15 wherein the control means automatically steers the transmitted laser beam toward the second platforms. 17. The system of claim 1 wherein the frequency of the transmitted laser beam and the frequency of the reflected laser beam are sufficiently far apart in the frequency spectrum so as to not interfere with one another. 18. The system of claim 3 wherein the control means includes a Bragg cell for steering the laser beam in a second axis. 19. A communications system for selectively communicating between an inquisitor and respondent comprising: signal generator means located at a transmission site for generating and transmitting an inquiry signal; control means for selectively directing the inquiry signal to one of a plurality of receiving cites; coder means for encoding an inquiry message and modulating the inquiry signal with the message; and, receiver means at a receiving site for receiving said inquiry signal and reflecting a portion of said inquiry signal back to said transmission site, said receiver means being a passive receiver which is only activated when an inquiry signal is received, said receiver further responding to receipt of an inquiry signal to modulate the reflected signal with a reply message, said coder means receiving said reflected signal and decoding said reply message to determine if the reply message is a proper response to the inquiry message. 20. The communication system of claim 19 wherein the signal generator means comprises means for generating a steerable laser beam. 21. The communications system of claim 20 wherein the signal generator means includes a grated surface emitting laser generator for generating and transmitting a laser beam steerable in one axis, and either of a Bragg cell or an acousto-optical modulator for steering the laser beam in a second axis. 22. The communications system of claim 21 wherein said receiver means includes a corner cube retro-reflector having total internal reflectance. 23. The communications system of claim22 in which said retro-reflector further includes an optical energy absorber comprising one side of the retro-reflector, and a plurality of piezo-electric positioners holding said energy absorber against said retro-reflector, the retro-reflector being in a inactive, passive mode when said energy absorber is so positioned. 24. The communications system of claim 23 wherein said transmitted laser beam has a first modulated portion and a second unmodulated portion, said absorber being responsive to the modulated portion of said transmitted laser beam to be activated by vibrating against the positioners, vibration of the absorber, in tun, modulating the second portion of the laser beam to create the reply message reflected back to said transmission site. 25. The communications system of claim 23 wherein said absorber passivates the retro-reflector when the absorber is within 1/4 wavelength of the incoming energy in the transmitted laser beam. 26. The communications system of claim 19 wherein the frequency of the transmitted signal and the frequency of the reflected signal are sufficiently far apart in the frequency spectrum so as to not interfere with one another. 27. A method of friend-or-foe identification (IFF) comprising: generating and transmitting a laser beam by an interrogator means carried on a first platform; selectively directing the transmitted laser beam toward a second platform; encoding an IFF interrogation message and modulating the transmitted laser beam with the message; receiving said laser beam by a reflector means carried on said second platform; and, reflecting said laser beam back toward said first platform including said reflector means responding to the IFF message contained in said received laser beam by modulating said reflected laser beam with a response message. 28. The method of claim 27 further including decoding of said response message, and determining if said response message corresponds to a predetermined response which identifies the second platform as a "friend". 29. The method of claim 27 further including steering said transmitted laser beam to one of a plurality of second platforms. 30. The method of claim 29 wherein the reflector means is a passive retro-reflector, and the method further includes activating the reflector means upon receipt by the retro-reflector of a transmitted laser beam. 31. A method of selectively communicating between an inquisitor and a respondent comprising: generating and transmitting an inquiry signal by a signal generator means located at a transmission site; selectively directing the inquiry signal to one of a plurality of receiving sites; encoding an inquiry message and modulating the inquiry signal with the message; and, receiving said transmitted inquiry signal at a selected receiving site and responding thereto by reflecting a portion of said inquiry signal back to said transmission site, responding to receipt of said inquiry signal including modulating the reflected signal with a reply message. 32. The method of claim 31 further including receiving said reflected signal at said transmission site, decoding said reply message, and determining if the reply message is a proper response to the inquiry message. 33. The method of claim 32 wherein receiving of said transmitted signal includes receiving said signal by a passive retro-reflector, and the method further includes activating said retro-reflector only upon receipt of the inquiry signal.

Inventors: HOLDER DAVID, WALDMAN GARY, WOOTTON JOHN R, HOLDER, DAVID, WALDMAN, GARY, WOOTTON, JOHN, R.
Applicants: ELECTRONICS & SPACE CORP, ELECTRONICS & SPACE CORPORATION
Application year: 1993
Main classification: G01S13
IPC codes: G01S17