GB2305503A - Threat classification using passive infrared detection - Google Patents

Abstract

A threat classification method for distinguishing and ranking threats posed to an aircraft by missiles and which is discreet and reliable, with a very low false alarm rate, uses bispectral analysis of infrared radiation received from a surveillance field (using periodic panoramic scanning) and assesses threats on the basis of the ratio of attenuation of detected sources in two close, narrow spectral bands within the atmospheric transmission band. One of the detection bands preferably lies near the edge of the transmission band. The two-line analysis generates values of attenuation ratio representing in a significant manner the closeness and thus degree of threat of detected targets. Periodic panoramic detection makes it possible to follow the change in threats, and to classify them and generate a suitable counteraction. The surveillance device shown comprises scanning means O, B, M and two linear detection arrays B1, B2 which are each equipped with a cold filter F1, F2 in order to make a periodic measurement of the apparent intensity of a panoramic surveillance field with respect to two close detection lines, one of these lines lying at the edge of the transmission band, and means 11, 12; 21, 22; 30 for processing the array output signals in order to generate the said attenuation ratios and control a countermeasure device 40.

Description

"A THREAT CLASSIFICATION METHOD USING BISPHCTRAL INFRARED DETECTION AND A SURVEILLANCE DEVICE THEREFOR" The present invention relates to the self-protection of aircraft with respect to projectiles directed against them, such as propelled missiles.

The self-protection of an aircraft is conventionally performed by the use of a missile detection means, such as a Missile Launch Detector (abbreviated to MLD) or a Missile Approach Detector (abbreviated to MAD) depending on the flight phase to be detected, and means of counteracting or of countermeasures means (decoying, jamming) triggered by the detection.

Missile detectors may be classified into two categories depending on their spectral detection range electromagnetic devices and optronic devices.

The electromagnetic devices or radars act passively or actively: - passive-type devices generally perform a passive listening role with the aid of two spaced-apart antennas so as to locate the source angularly using goniometry by measuring the delay between the signals received by the two antennas; these devices have satisfactory discrimination but they are capable, however, of detecting only electromagnetic sources in the radar range and are therefore completely ineffective against infrared-guided missiles; - active devices use a wide-angle Doppler-type radar which detects the missiles by their approach speed; they have two major drawbacks: on the one hand, their angular precision is poor, and therefore their range is short, and, on the other hand, their discreetness is poor, which compromises the stealth of the platform.

Optronic devices are of the passive type and comprise two categories of detectors, depending on their spectral sensitivity band - ultraviolet (W) detectors and infrared (IR) detectors.

W detectors, which detect the motors of missiles, have a low false alarm rate (abbreviated to FAR) since missile motors, which are of a powder type, emit characteristic lines in the UV spectral band occulted by the ozone layer, and for which the scenery background is completely "black"; however, they are limited in terms of range since the UV band is very rapidly absorbed by the atmosphere and they do not have the capacity to detect missiles in the non-propelled phase (terminal flight phase).

IR detectors lie within an atmospheric transmission band (the 3-5 ym band II and the 8-12 ym band III, equally usable) which provides them with good detection ranges; they also have good angular discrimination but exhibit a non-negligible FAR since they detect many parasitic infrared sources in the sky (edges of clouds illuminated by the sky) or on the ground, because of their high sensitivity.

The invention aims to mitigate the drawbacks described hereinabove, by proposing an optimized selfprotection system, that is to say one which is operational over a spectral detection band including the infrared, as well as discreet and reliable, with, in particular, a low FAR despite the high speed of missiles and their ability to manoeuvre (which requires having a detection and tracking procedure which is very rapid, of the order of one second).

This objective is achieved according to the present invention by the use of a classification of the infrared sources detected, based on a panoramic analysis technique of the two-line type within an infrared transmission band of the atmosphere. The principle of two-line detection is already known, for example for assistance in the night flying of aircraft.

According to the invention, a rigorous classification (that is to say one without any risk of error) of the threats is possible without having to measure directly the absolute distance between these threats and the aircraft to be protected: since detection is performed by a periodic frame analysis, the classification is then made in real time depending on the source emission intensity detected in two narrow spectral bands lying within the said transmission band. This solution has the advantage of being discreet because it is passive, of being effective and of being simple to employ.

More precisely, the subject of the invention is a threat classification method using bispectral infrared detection, characterized in that, since the detection of sources emitting fluxes of light intensity within an atmospheric spectral transmission band is performed using a bispectral periodic analysis, the classification is achieved by comparing an attenuation ratio of the emission intensity of the detected sources, these ratios being established with respect to two narrow spectral bands lying within the said transmission band, and by re-updating these comparisons at each frame period.

Non-threatening sources, corresponding to objects on the ground, are the furthest away and have a fixed distance in an absolute reference frame and therefore a fixed attenuation ratio. They may be eliminated without any risk of increasing the FAR.

The invention also relates to an infrared surveillance sensor for implementing the said classification method, which includes scanning and detection means for simultaneously or sequentially measuring the apparent intensity of the infrared radiation sources present in a surveillance field, in two close spectral lines, preferably at the edge of the said transmission band for at least one of these narrow bands.

From fuller particulars, further characteristics and advantages of the invention will appear on reading the detailed description which follows, illustrated by the appended figures which represent respectively: - Figure 1, a functional flowchart of a classification method which embodies the invention; - Figures 2a, 2b and 2c, three graphs showing the variation in the transmission coefficient within the transmission bands of the atmosphere; - Figure 3, an illustrative embodiment of a surveillance device according to the invention.

The threat classification method in which the invention is embodied is based on exploitation of the fsct *it the attenuation by the atmosphere of the infrared radiation emanating from objects detected as infrared emission sources is a highly non-linear phenomenon, depending on the detection wavelength. Under these conditions, for a given atmosphere, the attenuation ratio R between the apparent intensities of this radiation in two particular detection bands, resulting in two-line detection, is directly representative of the distance of the object to the detection site and makes it possible to establish a classification of the detected objects depending on their threat.

In two-line detection, a first image is analysed in a first narrow detection band, defined as a spectral line of width ## equal to the resolution provided by the said detection and centred on a wavelength X1. Under these conditions, the apparent intensity IA of an object lying at a distance D from the aircraft to be protected, in an atmosphere defined by an extinction coefficient Cj which is an increasing function of the wavelength Xj, may be expressed, from the principles of light propagation, as: IA(X1) = Io(xl)e 1 1o being the intrinsic emission intensity of the object.

For a second image detected in a second spectral line centred on another wavelength X2 greater than X1, for which #2 is the extinction coefficient of the atmosphere, the apparent density is equal to: IA(A2) = IO(X2)e The attenuation ratio R, defined by IA(#1)/IA(#2), then is equal to: R = e( L-al)D when 1o(Xl) - Io(X2),that is to say when the two lines of wavelengths X1 and A2 are sufficiently close (it then being possible for the sources to be regarded as black bodies at temperatures close to the ambient temperature).

Thus, the value of the attenuation ratio R for a detected source is directly related to the distance D of this source, and a comparison, based on an increasing value between the values of this ratio for the various sources detected, provides a classification which corresponds to classification with respect to the distance between these sources and the aircraft to be protected.

By operation of the invention, the following functional steps, as illustrated in Figure 1, are thus carried out: - step 1 of detecting sources emitting light intensity fluxes within an atmospheric spectral transmission band by periodic frame analysis of the two-line type; - step 2 of classifying the threats by establishing, comparing and classifying, based on a decreasing value, the attenuation ratios R of the emission intensity of the detected sources with respect to two narrow spectral bands, called detection lines, lying within the said transmission band; - step 3 of re-updating these comparisons at each frame period; and - step 4 of counteracting, in order to trigger a counteraction which is appropriate in regard to the threats, set up from the previous steps, the closest (that is to say the most threatening) corresponding to the smallest attenuation ratio R (that is to say the one closest to 1).

In order for the ratio R to be a sufficiently sensitive measurement, that is to say sufficiently far from 1 depending on the distance, the value of the difference 2-1 is sufficiently far from 0. This is also the case, -in particular, when a line in the detection band lying with the atmospheric transmission band and the other detection line at the edge of this transmission band, at the point where the attenuation starts to become large, are chosen.

Moreover, in order to obtain a significant value of R, the line width tX is preferably just large enough for the flux detected by the detector to be distinctly greater than the read noise of the detector. To do this, low-temperature filtering, called "cold" filtering, defining the pass band EA, may be used so as not to generate re-emission noise outside this pass band, which noise, if detected, could mask the useful signal received in the transmission band.

The choice of the position of the detection lines also depends on the climatic conditions prevailing in the surveillance field. Curves C1 to C5 in Figures 2a, 2b and 2c represent the variation in the transmission coefficient T in atmospheric transmission windows at different emission distances: at 3, 10, 15, 20 and 30 km, for a different climate.The figures indicate examples of values of pairs of lines X1 and A2 which may be chosen in these conventional transmission windows: - the transmission window in Figure 2a is the 8-12 Zm band (band III) for a climate of the "Mid Latitude Summer" (abbreviated to MLS) type; the adopted pair of lines (X2 = 9 Zm and X1 = 8.25 ,nun) lies at the edge of the band; - the transmission window in Figure 2b is still band III, but for a climate of the "Mid Latitude Winter" (abbreviated to MLW) type; the example of the lines used (X2 = 9 m and X1 = 8.25 ym) lying at the edge of the band; - the transmission window in Figure 2c is band II (3-5 Zm) for a climate of the MLS type; the example of the lines used for the detection (A1 = 4 ssm and A2 = 4.6 Sm) lies near the transmission edges existing within the transmission band.

The curves in Figures 2a and 2b, calculated for the same transmission window, illustrate the fact that the transmission, and therefore the atmospheric attenuation, varies substantially with the type of ambient climate. In order to obtain the precise value of the distance D of the detected objects, it would therefore be necessary to know precisely the value of the absorption coefficients (a1 and a2) of the atmosphere corresponding to a given climate.

But if, as is reasonable to assume, the atmosphere remains uniform around the detection site, for example within a radius of a few kilometres (10 km in order to be more specific), it is pointless to determine the precise value of the distance D, that is to say to know that of the absorption coefficient of the detection line, in order to classify strictly the detected sources according to the threat which they represent. This is because, under these conditions of uniformity, a classification made directly according to their ratio R of apparent intensities is sufficient since it strictly corresponds, as shown previously, to a classification based on relative distances even if the atmosphere is not known.

An illustrative embodiment of an infrared surveillance device according to the invention is described hereinbelow with reference to Figure 3. It is based on the use of detectors made of a composite material of the mercury-cadmium- tellurium (MCT) type, the technology of which is known to those skilled in the art.

The infrared detector includes two linear arrays B1 and B2 which are composed of elementary sensors and of an integrated read circuit of the CMOS (IR-CMOS) type made of MCT material, or of the CCD (Charge Coupled Device) type. These two linear arrays are mounted in a cryostat C and cooled to the temperature necessary for having rated operation (typically 77 K).

Each linear array is equipped with a cold filter, respectively F1 and F2, also placed in the cryostat C, which gives it a narrow sensitivity spectral band, for example 0.5 corm, respectively centred on the chosen wavelength, X1 and X2. The background flux Fi conventionally passes through a front objective 0 arranged in a support frame B and is projected onto the linear detection arrays, B1 and B2, (these being fixed to the support frame), after having been reflected off the surface of a mirror M.

Periodic panoramic analysis scanning of the scene is achieved by rotating the support frame B about the axis Z'Z using known drive means (not shown), the refresh period of the analysis defining the frame period. The image formed by projection on the linear array is split up into pixels corresponding to the elementary sensors.

The luminance levels of these pixels are integrated in the sensors and then periodically read in the read circuits which deliver an output signal, respectively S1 and S2. The intensity of these signals reproduces periodically over time, for the entire space scanned, the variations in the level of apparent light intensity of the flux which emanates from the same spatial locations within this space and which is received by the linear arrays in their respective detection band.

The two linear arrays are spaced apart so that, during scanning, the time interval separating passage of the image of an infrared source successively over the two linear arrays is very short, of the order of a few microseconds. Under these conditions, the bispectral attenuation ratio R, which can be calculated from the signals coming from the two linear arrays, is not falsified by intensity fluctuations of the source or by atmospheric turbulence, which correspondingly reduces the FAR.

Next, the signals S1 and S2 are amplified in the amplifiers 11 and 12, digitized through the converters 21 and 22 which are respectively coupled to the amplifiers 11 and 12, and then compared by means of a digital processor 30. This processor calculates the attenuation ratios R at each analysis frame, sorts them in order of increasing values corresponding to a classification based on distances, in order to establish the classification of threats which was described previously. The result of this classification is sent to a countermeasures device 40 which will treat as a priority the most menacing source. Provision may also be made for there to be means 50 for displaying the image successively picked up.

Frame-by-frame surveillance of the attenuation ratio R of each infrared source detected may be performed by conventional programming of the processor 30, comparing the values of R taken successively over time for the same spatial location. This real-time classification monitoring increases the effectiveness of the counteraction since the higher the speed of a close infrared source, the more threatening it is. The FAR is then reduced to a virtually zero value.

Moreover, the processor can eliminate sources whose attenuation ratio is constant, without any risk of increasing the FAR, since these sources, for which the distance does not vary, are in fact fixed sources. The processing thereof is correspondingly simplified.

The invention is not limited to the illustrative embodiment described and shown. A detector well suited for the surveillance device according to the invention is the multiple quantum well (MQW) matrix detector obtained by deposition of ultrathin films of GaAs composite semiconductor materials. Such detectors are described, for example, in the article in the journal La Recherche No. 248, volume 23, page 1270 (1992) or in the article in the journal Applied Physics Letters, volume 60, page 895 (1992). These detectors were developed especially for assistance in the flying of aircraft at night.

An MQW matrix detector operating in sequential mode or in simultaneous mode may be employed by the person skilled in the art. For each pixel, the levels of apparent intensity in the detection lines at X1 and at X2 are processed as described previously in order to determine the relative threat level of the detected sources and thus to eliminate the false alarms.

Claims (12)

1. A threat classification method using bispectral infrared detection, in which, since the detection of sources emitting fluxes of light intensity within an atmospheric spectral transmission band is performed using a bispectral periodic analysis of a frame surveillance field, the classification is achieved by establishing, comparing and classifying , based on an increasing value, the attenuation ratios of the emission intensity of the detected sources, these ratios being established with respect to two close spectral lines lying within the said transmission band, and then by re-updating these comparisons at each frame period and in that a suitable counteraction results from this classification.

2. A classification method according to Claim 1, wherein the sources which have a constant attenuation ratio are eliminated from the said comparison.

3. A classification method according to one of the preceding claims, wherein the detection lines have a width limited by pass-band cold filtering so that the intensity of the detected flux is markedly greater than the detection read noise and so that re-emission noise outside the pass band delimited by the filtering is eliminated.

4. A classification method according to one of the preceding claims, wherein with the surveillance field having known climatic conditions, the choice of lines is optimized according to the climatic conditions.

5. An infrared surveillance device for implementing the method according to any one of the preceding claims, including scanning means and detection means for simultaneously or sequentially making a periodic measurement of the apparent intensity of the infrared radiation sources present in a panoramic surveillance field with respect to two close detection lines lying within the said atmospheric transmission band and signal processing means which are coupled to the measurement means, in order to calculate a light attenuation ratio of the measured intensities between the two lines and in order to send counteraction commands to countermeasures means

6. An infrared surveillance device according to Claim 5, wherein one of the said lines lies at the edge of the said transmission band.

7. An infrared surveillance device according to one of Claims 5 and 6, wherein the scanning means include a front objective , arranged in a support frame which can move rotationally about an axis in order to project an incident beam onto two linear detection arrays which are fixed to the support frame , after this beam has been reflected off the surface of a mirror , in that the detection means include the linear arrays of composite material which are mounted in a cryostat , each linear array being equipped with a cold filter and with a read circuit which delivers an output signal and in that the means for processing the output signals include amplifiers which are coupled to converters and then to a processor which calculates the attenuation ratios at each analysis frame by comparing the levels of the intensity of the output signals and classifies them in order of decreasing values, the classification established being sent to a countermeasures device

8. An infrared surveillance device according to Claim 7, wherein the processor compares the values of attenuation ratios taken successively over time for the same spatial location.

9. An infrared surveillance device according to Claim 7 or claim 8, wherein the processor eliminates the sources which have a constant attenuation ratio.

l0.An infrared surveillance device according to any one of claims 7 to 9, wherein the detection is performed with the aid of a multiple quantum well matrix detector.

11. A threat classification method substantially as described hereinbefore with reference to the accompanying drawings.

12. An infrared surveillance device for implementing the method according to claim 11, substantially as described hereinbefore with reference to the accompanying drawings and as illustrated in Figure 3 of those drawings.