FR2681143A1 - IFF method of identification protected against interference and system for implementing it - Google Patents

Abstract

A method and system for identification protected from external interference and intended to identify an object located in the field of observation of a given station. The method involves carrying out two short interrogations: the first, after validation, puts on active alert the response circuits of a transponder located on board a friendly interrogated object; the second decisive one is transmitted only if the response to the first interrogation is of the passive type. The system for implementing the method comprises an interrogator (I) and a transponder (R) located on board the object to be identified. The interrogator (I) has a circuit (5) for transmitting pulsed laser beams, encoded and directed, and an encoding/decoding unit (6) consisting mainly of a set of processing circuits (11) and an infrared detector (8). The transponder has a detector (15), a decoding/overcoding unit (16) consisting mainly of a comparator (21) and a control unit (20), and a retroreflector (17), access to which is modulated by a circuit (18) controlled by the unit (16). Application to any carrier, airborne or not, of a weapons system.

Description

PROTECTED IFF IDENTIFICATION PROCESS
AGAINST INTRUSIONS AND THE BETTING SYSTEM
IN WORK
The present invention relates to the field of friend-enemy identification of an object or of a potential target in a field of observation and relates to a method and a friend-enemy identification system protected against intrusions.

 A friend-enemy identification system, abbreviated IFF (initials of "Identification of a Friend from a foe" in English terminology) must allow a given interrogator to identify, without ambiguity, an object in his field of observation as friend or enemy. The identification procedure consists in transmitting a coded signal from the interrogating station towards the object to be identified, then, in the case of a friend object, in receiving on the interrogating station, a response signal also coded from this object.

 IFF systems are conventionally composed of a transmitter and a receiver for the interrogator and of a receiver-re-transmitter means constituting a responder for the stations questioned, the coded communication signals being carried by radio waves, radar waves or light waves. Radio waves and radar waves have significant disadvantages due to insufficient directivity of their propagation in space, leading to possible interceptions, and due to their lack of "hardening", that is to say of their permeability to external intrusions and false echoes, making the system unreliable. Such systems can then facilitate the identification of carriers equipped with interrogators or answering machines operating with such waves.

 This is why more recent IFF systems use a very directive interrogator having laser emission means, of the pulse or modular type, and a responder comprising a modular retroreflector, such as a retro-reflecting device associated with a modulating screen. Under these conditions, the retroreflector returns the emission received according to a certain modulation to be decrypted by the interrogator.

Generally, modulation is of the all-or-nothing type, this type of modulation allowing operation at high peak power making it possible to extend the range of the transmission.

A solution of this type is described in the patent
FR 2,378,404. According to this patent, the laser wave is transmitted coded by the interrogator and is identified by the station interrogated by means of a detector associated with a processing circuit. This circuit emits a validation signal which controls an optoelectric modulating element, such as a tunable interference filter or an electrooptic switch, in order to authorize the retroreflection of the laser beam by means of a mirror.

Another solution, described in the patent
FR 2 602 346, implements a transponder comprising an acousto-optical modulation deflector, coupled to a retro-reflective retro-reflecting device, such as a reflective trihedron. The crossing of the acousto-optic deflector makes it possible to code, by a frequency shift, the back scattered laser beam which can then be validated on return by the interrogator. This validation is carried out in the form of a characteristic beat signal obtained, in an optical mixer, between the beam emitted and the beam received.

These two types of solution are not satisfactory in terms of protection, essentially because of the limited extent of the reception field, covered by the retroreflection systems used, and the permeability of the validation procedures used. Indeed
As far as the field is concerned, the range available for a retroreflector, for example of the trirectangle trihedron type, is of the order of 1 to 5 degrees. Under these conditions, the interrogation is only triggered after positioning the interrogating station in this field and the necessary positioning maneuvers can then be identified. To enlarge the angular field of measurement, it has been envisaged to scan the field at 'using a movable rotating mirror; but this type of adaptation requires a delicate rotational drive of the opto-electric modulator element or the addition of optical deflection elements, without however providing a sufficient instantaneous operational angular field. On the other hand, with regard to the validation means which operate both at the level of the answering machine and at the level of the interrogator, they require, in each of these cases, an extended initial laser emission in order to carry out the recognition procedures. so that when the object to be identified or the interrogator is an enemy, position and distance information can be collected.

In order to eliminate these drawbacks, the invention proposes a "hardened" interrogation procedure, that is to say highly protected from possible intruders, which can be triggered
- from any station located in a reception field covering the entire space surrounding the object questioned and therefore as soon as the object to be identified is located in the field of observation of the interrogating station.

- using two brief interrogations, materialized by two coded laser pulse trains; each interrogation is followed, in the case of an allied object, by a response, the nature of the first of these responses, revealed by its level of amplitude, conditioning the emission of the second interrogation,
More specifically, the subject of the invention is an IFF identification method, protected against intrusion, of an object spotted by an interrogating station having an interrogator emitting towards the object spotted a coded pulsed laser beam. and directive, receiving and analyzing in return a laser beam coming from an allied interrogated object, the allied object having a responder that can actively respond to the received beam in the form of a coded and directive pulsed laser beam in the direction of the interrogator, characterized in that it comprises the following successive stages
- carry out a first alert interrogation by the emission of a pulsed and coded laser beam from the interrogator towards the object to be identified, as soon as this object is spotted
- validate the laser beam received by the allied interrogated object and arm, after validation, the responder of the allied interrogated object which then passes from a passive state to an active state making it ready to respond actively
- analyze the nature, active or passive, of the response to the alert query from the responder and received by the interrogator
- carry out a second decisive interrogation by the emission of a pulsed and coded laser beam from the interrogator towards the object to be identified, if the first response is passive
- form an active response to the second interrogation using the armed responder in the direction of the interrogator
- analyze the response to the decisive interrogation from the answering machine and received by the interrogator.

The invention also relates to an IFF identification system for implementing this method. The interrogation station provided with locating means and, respectively, the allied interrogated object are equipped, for this implementation, with means for transmitting, respectively retroreflection, coded laser pulses combined with means for omnidirectional reception of these coded laser pulses, and to means for validating the laser pulses received, these validation means controlling the emission and, respectively, laser retroreflection means. The laser emission means are optically coupled to the tracking means of the interrogating station. The fact that the means for receiving the laser pulses are omnidirectional allows interrogation from any position of the interrogating station, as soon as the object is located. Other characteristics and advantages will appear on reading the description which follows , with reference to the appended figures which respectively represent
- Figure 1, a diagram of the main steps of the method according to the invention
- Figure 2, a block diagram of the opto-electronic organization of an interrogator and a responder of the identification system according to the invention
- Figure 3, an embodiment in vertical section of the responder of the identification system according to the invention.

 - Figure 4, an example of a large field retroreflector according to the invention.

 The block diagram shown in Figure 1 shows the main steps of the identification method according to the invention. The complete communication procedure between an interrogator placed on board the interrogating station and a responder placed on board the object to be identified, described below) corresponds to the case where the object to be identified is an allied object participating in the system of weapons of the interrogation station.

 The interrogator of the interrogating station transmits, in a first transmission step I, a first interrogation in the form of a coded pulsed laser beam and direction, in the direction of the object, as soon as it is identified by the interrogating station.

 This first interrogation is an alert interrogation for the object to be identified as an ally since it causes, at a reception and validation stage II, the "arming" of the responder of this object. The response provided by this object must be activation by the coded laser beam of the allied interrogated object. The answering machine is armed, after validation of the laser beam, when it is made ready to respond actively, at a later stage, to a new interrogation.

However, the triggering of a second interrogation is conditioned by the analysis, in step III, of the nature, active or passive, of the first response to the first interrogation received by the interrogating station.
- if the response is of the active type, that is to say, for example consisting of an overcoding of the first retoreflected interrogation, the procedure is stopped. Indeed, this type of response is not satisfactory because an allied object questioned is not yet armed at this stage, and therefore cannot actively respond to the first interrogation,
- if the response received is of passive type, that is to say at a weakened level of echo type, the object to be identified is indeed presented as an ally and the procedure can be continued.

 The continuation of the procedure is then carried out by the triggering of a second decisive interrogation at the emission stage IV. This second interrogation is presented, like the first, in the form of a pulsed, coded and directive laser beam. It is decisive in the sense that, the allied questioned object having been armed in step II, the response provided by this object must be active, that is to say having for example an overcoding of the response to the questioning. An active type response is formed in step V, for example obtained by retroreflection and over coding of the received laser beam, the over coding being carried out by suppressing some of the pulses from the retroreflected laser beam. The analysis, in step VI, of the response formed in step V and received by the interrogating station must confirm the conclusions of the first analysis.

for implementing the method according to the invention, comprising an interrogator I and a responder R.

 The interrogator I, of the interrogating station, essentially comprises three elements: a laser transmitter 5, pulse or continuous emission, preferably adjustable by all or nothing, an encoder / decoder validation unit 6 and an infrared radiation detector 8, whose receiving field axis is kept parallel to the axis of the laser beam emitted by the transmitter 5. The detector and the transmitter are oriented in the direction of the object to be identified, identified by a search system and target tracking, by any known suitable opto-mechanical scanning system, optically coupled to the target search and tracking system.

Depending on the wavelength of the laser beam used, for example 1.06 microns or 10.6 microns, the detector may comprise, in known manner, a photosensitive layer with infrared radiation, respectively in silicon or in a composite material Hg-Cd- Te (mercury- cadmium-tellurium)
The validation unit 6 makes it possible to code the laser beams emitted at 5 and to decode the received laser beam at 8. This unit performs the functions of the validation means 4 presented above.

 It comprises an amplifier 9, which amplifies the signal supplied by the detector 8, coupled to the input of a coded emission command 10 from the laser 5 through a set of signal processing circuits 11 delimited by a dotted line.

 The assembly includes a timer 12, the input of which is connected to the output of the amplifier 9 and the outputs of which are connected to the input of the control 10 and to the input of a comparator 13. A means display 14 receives information from comparator 13 and timing means 12.

After simultaneous triggering of the emission of the first interrogation in the form of a first pulse train coded T1 of the timing means 12 by the command 10, actuated by the operator during the first interrogation, this set manages the second interrogation in two phases, as follows
- in the absence, during a predetermined time interval To, of a significant coding or over-coding of the response to the first interrogation, coming from the detector 8, via the amplifier 9, and analyzed by the unit 6 , the delay means 12 automatically triggers a second pulse train coded T2. To do this, it applies an activation signal SA corresponding to the desired code, to the command 10. If, in the time interval To, a significantly coded or significantly coded return signal is received by the time delay means 12, the latter warns the operator of an active response to the first interrogation, by means of display means 14 and, on the other hand, delays or suppresses the triggering of the activation signal SA. The timing means is produced by a person skilled in the art in a known manner and can be constituted, for example, by a switch associated with a clock
- In response to the second interrogation, an infrared beam is received by the detector 8 which delivers a signal applied to the housing 6 for analysis; after amplification at 9, this signal is transmitted to comparator 13 via timer 12, in order to compare the code or overcoding information that it receives with that previously introduced into comparator 13. The result of this comparison is communicated to the operator using the display device 14.

 The comparator 13 can be a simple pulse counter and measurement of the time intervals between pulses, and the control 10 a signal generator associated with a voltage controlled oscillator.

 The electronic organization of an answering machine R according to the invention is also shown diagrammatically in FIG. 2 as an example of a basic embodiment, a more complete example of embodiment being presented below. It essentially comprises an infrared radiation detector 15 coupled, via a decoder / overcoder validation unit 16, to an omnidirectional retroreflector 17 associated with an optical modulator 18 of infrared radiation.

 The decoder / overcoder unit 16 is equipped with means for decrypting and validating the pulses received by the detector 15 and activating, if the received code is validated, the modulator 18 in order to overcode the retroreflected laser beam by deleting some of the pulses received. These means consist essentially of an amplifier 19, for amplifying the signal delivered by the detector 15, coupled to a control circuit 20 of the optical modulator 18 via a comparator 21, itself associated with a display device 22.

This box 16 allows, in the exemplary embodiment, to arm the responder during the first alert interrogation, and to provoke a suitable response during the second decisive interrogation. It works as follows
- the code contained in the first pulse train
Ti is decrypted and compared to the compliant code by accounting, at the level of the comparator 21, of the peaks of the signal supplied by the detector 15 and their frequency of appearance; the comparator 21 delivers the validation signal S1 to the control circuit 20 of the modulator 18 which opens, after a defined time interval from the end of the received laser emission, the access of the retroreflector 17 to the laser radiation of the second questioning
- After a predetermined time interval To, corresponding to the time interval separating the two interrogations, the comparator 21 triggers a second signal S2, of over coding, applied to the control circuit 20.

 The control circuit 20 comprises a code generator and a supply circuit (not shown), the code generator controlling the supply circuit according to an all-or-nothing operation in order to make the optical modulator 18 opaque or passing to radiation laser at the rate of overcoding. Overcoding consists in fact in removing the laser pulses from the second pulse train T2 by absorption in the optical modulator 18, the synchronism being ensured by the detection top of the first pulse.

For example, the first train of laser pulses
Ti can consist of three pulses distributed according to a time code, for example over 200 ms, the laser relaxation time during 100 ms. The second train T2 can comprise six pulses at the frequency of 50 Hz, that is to say for a total duration of 100 ms, of which the modulation means suppressing a determined number of pulses, distributed according to a pre-established scheme constitute the overcoding.

 An exemplary embodiment of the optical means of the responder R according to the invention is shown in vertical section in FIG. 3.

 In order to allow omnidirectional detection and retransmission of the laser beams, the responder R is, approximately, in the form of a cube 30 each of whose four faces is occupied, in part, by the same detector and the same retroreflector. There is a detection stage E1, comprising 4 detection sets, a reflection stage E2 comprising 4 retroreflection sets and an electronic stage E3.

On the detection stage E1, each detection assembly comprises a detector 15, or 15 ′, operating simply in direct detection, without optics other than a sun visor (not shown) and resting on a central support.
A at the top of reflecting surfaces 31 and 31 'having, for example, a cone shape.

 On the reflection stage E2, each retroreflection assembly comprises a retroreflective surface for the retroreflector 17 or 17 ′ also arranged at the top of cone-shaped surfaces SCI or 5C2. The surfaces of the retroreflectors 17 and 17 ′ are mirrors in the form of a spherical cap. This mirror shape coincides with the image surface of an associated converging lens L1 or L2. An exemplary embodiment of a large field retroreflector is shown in FIG. 4. The characteristics of the converging lens Li or L2 are chosen so that an incident beam F i is focused on the associated image surface 17 or 17 'when arriving most perpendicular to it for a wide range of angles of incidence. In this way, the retroreflector ensures a rigorous backscattering, by the so-called "cat's eye" effect, whatever the angle of incidence of its field. reception. This shape is preferable to that of the trirectangle, or cube corner, which has drawbacks of space, uneven distribution of the returned light energy and very high machining precision required.

 In FIG. 3, retroreflective surfaces 17 and 17 ′ are arranged at the focal points of wide field input optics, for example of lenses Li and L2, and are associated with the optical modulators 18 and 18 ′ interposed between the input optics and surfaces 17 and 17 '. The optical modulators are preferably constituted by liquid crystal screens. Other types of switchable electro-optical panels, such as PLZT ceramic plates, may also be suitable.

 The electronic decoder / overcoder unit 16 and a power supply 33, intended for all of the electronic elements, share the electronic stage E3.

The electrical circuit conveying the information received on stage El towards stage E3, and redistributed from stage
E3 on stage E2, appear schematically in the form of arrow lines in FIG. 3.

a steering assembly and a reflection assembly, to optimize coverage and facilitate independent access to the power supply and the electronic unit.

 The invention is not limited to the embodiments described and shown. In particular, it is within the reach of a person skilled in the art to produce simplified encoder / decoder or decoder / overcoder electronic boxes by eliminating the automated commands carried out by the comparators and by replacing them, for example, with displays coupled to commands integrated by a coding / decoding interface in the interrogator and a decoding / overcoding interface in the answering machine.

 On the other hand, it may be advantageous to use infrared telemetry or infrared imaging means pre-existing on the carriers to combine them with the means described, in order to constitute an IFF identification system falling within the framework defined by the invention. Conversely, it may be advantageous to use the means implemented in the invention to also use them in the context of telemetry or infrared imaging. Thus, the interrogator can receive, in return for the first interrogation, an echo type signal supplied by its detector making it possible to evaluate the distance from the telemetric responder and a measure of the average level of reflection of the target. The sending of the second pulse train can then be conditioned by the values of these parameters.

It is also within the reach of a person skilled in the art to merge the common structures of an interrogator and an answering machine according to the invention in order to constitute a single element which can serve, depending on the circumstances, as an interrogator or answering machine.
Finally, the responder of the identification system according to the invention can behave as a laser alert detector by the installation of known alarm means coupled to the described detection means.

Claims (6)

 1 - IFF identification method, protected against the intrusion of an object spotted by an interrogating station having an interrogator comprising means for emitting towards the object spotted a coded and directive pulsed laser beam, and means for receive and analyze in return a laser beam coming from an allied interrogated object, the allied object having a responder comprising means for actively responding to the received beam in the form of a coded and directive pulsed laser beam, in the direction of the interrogator, characterized in that it comprises the following stages  - carry out a first alert interrogation by the emission of a pulsed and coded laser beam from the interrogator towards the object to be identified, as soon as this object is spotted  - validate the laser beam received by the allied interrogated object and arm the responder of the allied interrogated object which then passes from a passive state to an active state making it ready to respond actively  - analyze the nature, active or passive, of the response to the alert query from the responder and received by the interrogator  - carry out a second decisive interrogation by the emission of a pulsed and coded laser beam from the interrogator towards the object to be identified, if the first response is passive  - perform an active response to the second interrogation using the armed responder in the direction of the interrogator  - analyze the response to the decisive interrogation from the answering machine and received by the interrogator.  2 - Method according to claim 1, characterized in that the armed responder actively responds to the second interrogation by retro-reflection and over-encoding, the over-encoding consisting in suppressing some of the pulses of the retroreflected laser beam.  3 - IFF identification system for implementing the method according to one of claims 1 or 2, comprising an interrogator (I) and a responder (R) communicating by laser7 radiation the interrogator having means of target search and for locating the object to be identified, means for emitting laser radiation (5) optically coupled to the means for searching and locating, means for detecting laser radiation (8) and means for validating (6) the radiation received, the responder (R) having means for detecting (15) the laser radiation emitted by the emission means (5) of the interrogator (I), validation means (16) and modular retroreflection means ( 17, 18) of this radiation in the direction of the interrogator, characterized in that  - the interrogator validation means are in the form of an encoder / decoder unit (6) comprising an amplifier (9), for amplifying the signal supplied by the detector (8), and a coded transmission command (10) for triggering the laser (5), the amplifier (9) and the control (10) being coupled through a set of signal processing circuits (11), comprising a timer means (12), a comparator (13) and a display means (14) intended to successively trigger the emission of two coded pulse trains (T1, T2) and to validate the signals detected in response  the means for validating the answering machine are in the form of a decoder / overcoder unit (16) comprising an amplifier (19), for amplifying the signal supplied by the detector (15), coupled to a control circuit (20 ) of an optical modulator (18) of retro-reflection means (17), via a comparator (21) associated with a display device (22), the comparator (21) decrypting the code contained in the first train T1, in accounting for the peaks of the signal supplied by the detector (15) and amplified by the amplifier (19), and applying to the control circuit (20) an arming signal (S1) during the first interrogation and an overcoding signal ( S2) during the second interrogation, after comparison with the corresponding compliant codes.  4 - Identification system according to claim 3, characterized in that the delay means (12) of the interrogator (I) consists of a multiple input / output switch coupled to a clock, in that the command d coded laser emission (10) from the interrogator (I) consists of a signal generator associated with a voltage-controlled oscillator, and in that the control circuit (20) of the optical modulator (18) comprises a generator code and a power circuit.  5 - Identification system according to claim 3, characterized in that the responder (R) is in the form of a cube (30) with three stages (El, E2, E3), each face of the cube giving access to a detection assembly comprising the detector on stage E1, and a reflection assembly comprising the retroreflective surface of the retroreflector (17, 17 ′) on stage E2, in that the retroreflective surfaces have the shape of a spherical cap and are arranged at the associated wide-field optical focal points (L1, L2) located on each of the faces of the cube (30), in that the detectors (15, 15 ') are arranged at the tops of reflecting surfaces (31, 31') of generally conical and reflecting shape, each optical modulator (18, 18 ') being interposed between the optics (L1, L2) and the retro-reflective surface (17, 17'), and in that the optical modulators (18, 18 ' ) are liquid crystal displays.  6 - Identification system according to claim 5, characterized in that the detection stages El and reflection E2 are divided into four identical modules each containing a detection assembly and a reflection assembly, the modules being arranged on the carrier so as to optimize the coverage of the reception field.