GB2265273A

GB2265273A - Device for spatial analysis with a laser wave, in particular for a missile homing head

Info

Publication number: GB2265273A
Application number: GB8805359A
Authority: GB
Inventors: Bertrand Remy; Jean-Louis Meyzonnette
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1987-03-09
Filing date: 1988-03-07
Publication date: 1993-09-22
Anticipated expiration: 2008-03-07
Also published as: GB8805359D0; FR2688317B1; GB2265273B; DE3807733A1; FR2688317A1

Description

2265273 Device for spatial analysis with a laser wave, in particular for a

missile homing head The present invention relates to a device for spatial analysis with a laser wave, intended in particular for the gui- dance of missiles.

Current infrared devices used for the guidance of missiles, called "infrared homing head" or, more commonly, "infra- red seekers" make most often use of passive infrared imaging techniques. These techniques allow the detection of the target being tracked thanks to the radiation emitted by the target itself. The angular deviation between the direction of the target and the direction followed by the misssile is measured by the device. The simplest devices measure the position of the target by means of a so-called four-quadrant detector and the most sophisticated reconstruct an image by means of a system of spatial analysis of the field of view, associated with a linear array of phot odetectors or by means of a two-dimensional array, or "mosaic", of such detectors. The target is brought back into the field by action on the missile control surfaces, which allows it to home on target.

These systems have the disadvantage that they can be jammed rather easily by means of an infrared flare simulating the target. In addition, they do not allow to perform in a simple manner the autonomous acquisition of the target in the Case where the latter is seen on a complex background.

It is possible to remedy these deficiencies through the implementation of laser imaging techniques for the phases of target acquisition, recognition and tracking. These techniques allow to construct an image of the target and of the background scene. This image may represent either the distance between each point of the scene and the device, or the Doppler velocity of each point (i.e., the projection of the velocity on the seeker-scene line) and these two types of image can -ing modes.

be obtained simultaneously or in two separate operat The target can thus be displayed either through its motion over the landscape, or through its relief on the background. The use oil processing techniques such as, for example, correlation between the distance image 2nd a target relief model, or detection of moving objects in the velocity image permits a target extraction and tracking much more performing than the types of processing performed on passive infrared images, these being not so rich in information as active images.

Laser active imaging uses a laser beam with low divergence (less than one milliradian) for illuminating the scene point by point. The transmitted beam is in general made up of a sequence of frequency-modulated pulses, the Optical wave being used as 2 carrier wave. The transmitted pulse is backscattered by the scene in the half-space and a portion of the backscattered flux is picked up by a receiving system within which it is made to interfere with a reference laser wave called "local oscillator". The measurement of the go-and-return time of the pulse allows to compute the distance of each point of the scene, and the measurement of the frequency shift due to the Doppler effect permits to compute the velocity by means of the formula:

f D = 2V R /z where f D is the frequency shift,. V R the projection of velocity on the line joining the iriager and the target, and)\ is the i.;avelength of the laser radiation.

A system of angular scanning allows in addition a sequen- tial scanning of all points of the scene.

An object of the present invention.is a device for spatial analysis with a laser wave, in particular for a missile homing head, including means of illumination in a sighting direction, associated with means for sequential scanning of the analyzed region of space, means for receiving the beam reflected back by the analyzed region of space in the successive directions of the transmitted beam, and means for processing said back reflected beam in order to reconstruct an image of the analy zed region of space, said means of sequential scanning of the analyzed region of space including means for producing a cir cular scanning combined with an angular deviation of the trans mitted beam, said receiving means including a fixed linear array of photodetectors towards which is successively directed the back-reflected beam corresponding to the successive angular deflections of the corresponding transmitted beam, preceded by the same scanning means as for transmission, and said processing means being common to the various photodetec tors, a multiplexer being provided between said processing means and said linear array of photodetectors.

-ion will Other objects and features of the present invent become apparent from the following detailed description of - 11 X - preferred embodiments given with reference to the accompanying drawings, in which:

- Figure 1 is a block diagram of a preferred eiribodime-xit of a device according to the present invention; - Figures 2 and 3 illustrate the principle of FM-CW pro cessing; - Figure 4 is a schematic diagra-im showing the effect of go and-return ti-ne of the laser light in cirenlar scanning; and - Fioure 5 illustrates the scanning process in the wide-field acquisition mode.

Figure 1 shows the block diagram of a device for spatial analysis with a laser wave. One can see a laser illuminator i whose role is to produce a series of pulses modulated in fre quency. By means of a deflector 5, these pulses are directed successively in the various directions in space, and de- tected, after back-reflection by the scene, by an optics that focuses the beam on various elemental photodetectors 18-1, 18-2,..., etc. of a receiving linear array 18 corresponding to the successive sighting directions, respectively.

The continuous laser beam emitted by a laser 2 is modulated in frequency by a modulator 3 in the form of a sequence of adjacent pulses. Depending an the mode of operation of the homing head, the modulation frequency may be constant or vary linearly for the duration of the pulse. The modulator can be outside the laser cavity (acousto-optic modulator) or, preferably, located in the laser cavity (intracavity electro-optic modulator).

The deflector 5 can be electro-optical, which allows a discontinuous scanning. The laser beam at the output of the deflector has then a fixed direction for the full duration of the pulse (for example, about 5 to 10 microseconds), then is switched to the next direction, the difference between the two directions corresponding to the difference between the sighting directions of two adjacent detectors. The deflector can also use opto-mechanical methods of continuous scanning or methods applied inside the laser cavity.

The local oscillator wave is tapped after the frequency modulation, for example by means of a semi-transparent optical plate 9.

Reference 10 denotes a system for separating the trans mitted beam from the received beam, constructed according to known methods using the polarization properties of the beams.

This system 10 allows to direct towards the scene the quasi totality of the illuminating flux and to direct towards the detector the backscattered flux, which makes it possible to use of a common transmitting/receiving optics (shown schema tically at 11) that allows to obtain a transmitted beam with the desired diameter and divergence.

A system of so-called circular scanning 12, common to the transmission and reception channels, is combined with the sys tem of angular deflection in order to achieve analysis of a circular field, the number of image elements (also called "pic ture elements" or "pixels") resolved along the diameter being the double of the number of photosensitive elements of the linear array 18. This scanning system is supplemented by 2 device 13 for orienting the sighting line in elevation and in bearing. This device 13 can be made up of two moving mir rors or be a gimballed platform.

This scanning can also be used within the same device for obtaining a complementary passive image with a field possibly different from that of the active image. To this end, a di chroic mirror 29 separates spectrally the laser wave passing through it. from the passive wave with a wide spectrum (for example with wavelengths from 8 to 11 microns) that is reflec led towards a linear array 30 of passive photodetectors. This array permits to reconstruct a thermal image of the scene, independently of the active image. This line2r array 30 can also be placed in one and the same cryostat as the linear ar ray 18 in order to simplify the device.

The laser pulse reflected back by the target or by a point of the scene goes back through the elements 13, 12, 29 and 11 and is directed by the separator 10 towards a mirror 15.

An Optical mixer 16 constituted, for example, by a semi-trans parent plate, combines the received wave with the local oscil lator wave. A lens '17 focuses both waves on one of the photo detectors of the linear array 18. Figure 1 shows, as a non limitative example, the case of a linear array with six ele ments 18-1 to 18-6, the flux being focused on tke element 18-2.

The electrical signals delivered by each of the detectors 18-1 to 18-6 are amplified by amplifiers numbered 20-1 to 20-6.

A multiplexer 21 selects the reception channel capable of receiving the echo and directs the corresponding signal to an electronic mixer 22 that ensures a centering of the fre quency of the heterodyne signal in the acceptance band of the processing that follows. An electronic local oscillator 23 supplies mixer 22 with a sinusoidal signal whose frequency varies with the distance of the target and with the Doppler frequency shift. This shift is given roughly by the knowledge of the flight kinematics of the missile in the case where the velocity of the target is low or zero. The signal resulting form the mixing process is analyzed by a spectrum analyzer 24, for example a surface acoustic wave (SAW) analyzer. Depen ding on the mode selected by control circuits 27 (Doppler mode or distance mode), logic circuits 25 carry out the compu tation of the Doppler velocity or of the distance of each point and reconstruct an image of the scene taking into account the position of the circular scanning 12 and the number of the detector given by a sequencer circuit 28 that controls the deflector 5. These logic circuits 25 may include an image me mory allowing the storage of the information as it is avail able. Circuits 26, for example making up a computer, perform the extraction of the target in the field and generate error signals permitting to correct the trajectory of the missile through action on it's control surfaces.

The control of the device is assumed by circuits 27 that carry out in particular the computation of the optimum interval T between the laser pulses as a function of the dis- tance of the target; T must be both a submultiple of the go-and-return time of the pulse and as small as possible while including as a minimum the beat duration necessary for a cor rect measurement of the frequency by the spectrum analyzer 24, and the switching time of the deflector 5. The circuits 27 21SO generate the control signal for the electronic oscil lator 23 as a function of the value of the Doppler frequency shift and of the distance of the target. The sequencing cir cuits 28 supply the frequency modulation Start pulses intended for the control cirz:jiL 4 of the modulator 3 and the switching pulses intended for the control circuit 6 of the deflector 5.

Two modes of operations are possible and capable of being used alternately: the Doppler imaging mode and the distance imaging mode. In the Doppler mode, the modulator 3 is not excited and the laser wave is consequently not modulated in frequency. The measurement, by the spectrum analyzer 24, of the frequency resulting from the mixing at 22 of the heterodyne beat signal and of the signal with a known frequency generated at 23 allows the direct comoutation of the Doppler frequency shift of each point of the scene. This mode permits the imme- di2te extraction of any moving target, as its sensitivity can be very high (50 cm/sec for t=10.6 m and a spectrum analyzer with a 100-kHz resolution).

The distance mode uses the so-called FM-CW modulation technique whose principle is illustrated in Figure 2. We first consider Figure 2 showing the frequency/time diagrams of the waves transmitted and received after back-reflection on a target in the case of a go-and-return time of the wave shorter than the duration of the pulse. The laser pulse being modulated linearly in frequency, the heterodyne mixing of the backreflected wave and.the local oscillator (a replica of the transmitted wave) produces a fixed beat frequencyU related to the time shift of the frequency ramps. This beat frequency is, therefore, proportional to the go-and-return time of the flux and is given by the formula:

A F = K x 2D/C where K is the modulation frequency/time slope and 2D/C is the go-and-return time of the pulse (D being the target dis tance and C the velocity of light). In the presence of a Dop pler frequency shift, it appears a dist2nce/Doppler ambiguity, for we have then:

&F = K x 2D/C + F Doppler and the measurement ofAF does not allow to compute both D and F Doppler' This ambiguity is not necessarily a penalty in the case where all points of the target have the same velocity, for then the relief of the target is not disturbed in the image and its extraction is still possible.

In the case where the target is blended into the background because of the false distance shift related to the Doppler effect, the inversion of the sign of the frequency/time characteristic allows to make it appear again. The distance/

Doppler ambiguity can also be removed in the conventional way through successive transmission, towards the same point, of two pulses with opposite slopes K and V.

In order to transmit pulses whose duration T1 is shorter than the go-andreturn time of the flux, typically 20 ps for a target located at 3 km, and consequently in order to increa- _se the rate of the transmitted pulses, the heterodyne mixing will be performed not between a back-reflected pulse and the corresponding transmitted pulse, but between a back-reflected pulse and the pulse transmitted a time NT' after the corres- ponding transmitted pulse, with.AT = NT'+ Lt where A.T is equal to 2D/C and denotes the go-and-return time of the pulse and,&t is equal to AF/K (with AF denoting the beat frequency me2sured in this case and K the frequency/time modulation slope).

This is shown in Figure 3 where the delayAT corresponding to the go-and-return time of the pulse has been chosen, as an example, equal to 3T'+,At.

This allows to ensure a time overlap between the pulses reflected back by the scene and the local oscillator pulses, and consequently allows to these two pulses to be focused si- multaneously on the linear array 18.

- io - The computation of the quantity NTI is carried out after a preliminary measurement of the distance D of the scene assuming the case of Figure 2a, i.e., by using a long pulse, with a duration longer than the go-and- return time T. These preliminary measurement of the distance D of the target constitutes the so-called acquisition phase for this target; the next phase, during which the rate of the transmitted pulses is increased, constitutes the so-called tracking phase.

When the distance of the scene decreases, for example due to the displacement of the missile, the period T' decreases to its minimum possible value (for a given N value) equal to the sum of the beat minimum duration necessary for a measure ment by the spectrum analyzer 24 and of the duration of angu lar switching of the deflector 5. The period T' increases then as to have 2D/C = (N-1)T'. This cycle repeats it- suddenly so.self up to N=0 (close target) As mentioned common to above, the system of circular scanning is the transmission and reception channels.

Under these conditions, due to the time spent by the pulse for the go-and-return trip and to the progression of the cir cular scanning during this time, the circular scanning applied to a back-reflected pulse is generally not identical to the circular scanning that has been applied to the corresponding transmitted pulse, and the more so as scanning is more rapid and the target is farther; the back-reflected pulse runs the risk of being focused not on the linear array of 18 but outside of it then photo detectors as this is shown in dashed lines in Figure 4. More precisely, the angle of rotation 69 of the circular scanning during the go-and-return time of the flux, which is a function of the scanning angular velocity -CLand of the go-and-return time 2D/C is written as 1g = 2D/C xII.

- il - When LQ is too high the shift Al = fAgc (where o angle formed by the sighting line of the and the sighting line of the central of the optics is such that the pulse is detector, that supplies no longer any signal. can see that the shift Al at the focus of the as a function of the rank of the detector in (is proportional to the angleoO. It is equal center (corresponding to the axis Of scanning to the f ield.

In this device, this problem is solved by providing means compensating for the possible differences of circular scanning applied to the transmitted beam and to the back-reflected beam. This compensation Can be achieved by determining, in a first phase, the distance D of the target and by applying, in a second phase, to the deflector 5 a rotation by a polar angle equal to -bg = -(2D/C)S1. This allows to obtain an optimum reception of the pulses. To this end, a motor 7 rotates the deflector 5 as a function of the approXiM2te distance of the target computed by the computing circuits 25 and of the rotation speed of the circular scanning measured by a sensor (not shown).

The preliminary measurement of the distance is made by transmission of a pulse in a sighting direction negligibly affected by the circular scanning. It is in particular possible to choose as sighting direction that corresponding to a reception on the central detector.

This preliminary measurement of the distance will advantageously be combined with the measurement carried out during the detector in question detector) at the focus focused beside the In Figure 4 one optics increases the linear array to zero in the and, therefore, center of the field) and is maximum at the edge of the is 2 the acquisition phase described above and preliminary to the phase of tracking or increasing the pulse rate. The pulse transmitted in the central sighting direction during this preliminary phase will then be a long pulse.

Furthermore, another problem due to the go-and-return time of the laser wave could appear if the sequential addressing of the points of the field corresponding to the directions seen by each elemental photodetector of the linear array was chosen to be common to the transmission and local oscillator channels, i.e., if the local oscillator wave was tapped after an angular deflection.

As a matter of fact, as the optical elements 11 and 17 put in correspondence with a given position of this addressing a photodetector with a given rank in the linear array 18, due to the progression of this addressing during the go-and-return time of a pulse, there might be no spatial overlap between the back-reflected pulse and the local oscillator wave at the linear array of photodetectors 18.

In this device,, to avoid this problem the local oscillator beam is tapped by the plate 9 at the il- luminator, before the deflector 5, and an optical component 31 produces from this fixed beam several sub-beams permitting the simultaneaous illumination of the detectors 18-1 to 18-6 of the linear array. The component 31 is advantageously cons tructed by means of known holographic techniques.

Figure 5 shows a variant of the scanning process in the acquisition phase in the case where the field analyzed by the circular scanning 12 is not sufficient for ensuring the acqui sition of the target. The circular scanning is then stopped and the device 13 for orienting the sighting line (made up 13 - of two moving mirrors or a gimballed platform, for example) performs a sequence of scans (for example, lines with fast retrace) shifted along the perpendicular direction, in order to analyze a wide total field in several strips. The linear array 18 is oriented (by means of the circular scanning 12) perpendicularly to the direction of the line scanning. As in the normal mode, the deflector 5 allows to address the detectors, as described above. If it is desired to increase the iM2,,e rate, the deflector 5 can address only a reduced number of detectors of the linear array 18 in order to achieve a "meshed" coverage of the field, for example one detector out of two or three.

The system does not form 2 full image of the field but, if the meshes are sufficiently small, the acquisition of a target (for example, in the Doppler acquisition mode) is ensured. Furthermore, the sequencer 28 controls the multiplexer 21 so as to connect the detectors in question to the processing circuits.

Fig ure 5 shows the succession of the pulses obtained in the field and 2 potential target.

Claims

1. A device for spatial analysis with a laser wave, in particular for a missile homing head, including means of illumination in a sighting direction, associated with means for sequential scanning of the analyzed region of space, means for receiving the beam reflected back by the analyzed region of space in the successive directions of the transmitted beam, and means for processing said back-reflected beam in order to reconstruct an image of said analyzed region of space, said means for sequential scanning of said analyzed region of space including means for performing 2 circular scanning combined with an angular deflection of the transmitted beam, said means of reception including a fixed linear array of photodetectors towards which is successively directed said backreflected beam corresponding to the successive angular deflections of the corresponding transmitted beam, preceded by the same means of circular scanning as for transmission, and said processing means being common to the various photodetectors, a multiplexer being provided between said processing means and said linear array of photodetectors.

2. A device according to claim 1, including in addition means compensating for the possible differences of said circular scanning applied to said transmitted beam and to said"corresponding back-reflected beam, due to a relatively rapid circular scanning and a relatively long go-and-return time of the light wave.

:L5 -

3. A device according to any of claims 1 and 2, wherein said processing means include means for tapping a local oscillator wave from said transmitted beam, means for directing this local oscillator wave towards the one of said photodetectors that receives said back-reflected wave at the same instant of time, while compensating for the possible differences of deflection between said local oscillator wave tapped at a given time and said back-reflected wave at the same instant of time due to a relatively rapid angular deflection and a relatively long go-and-returne time of said light wave, and means for producing a beat between these two waves.

4. A device according to claim 3, wherein said means compensating for the possible differences of deflection between said local oscillator wave and said back-reflected wave includes means for tapping said local oscillator wave from said transmission beam located before the angular deflection of said beam and means for directing this local oscillator wave simultaneously towards all said photodetectors.

5. A device according to claim 2, wherein said means compensating for the possible differences in the circular scanning applied to said transmitted beam and to said corresponding back-reflected beam include means for, in a first phase, de- termining the distance D of said target in the corresponding sighting direction and means for, in a second phase, rotating said angular deflector by a polar angle -12(2D/C) (where -Q denotes the angular velocity of said circular scanning).

6. A device according to claim 5, wherein said means for de- termining in a first phase the distance of the target include means for transmitting in a so-called central sighting direction which is a sighting direction of said angular deflector negligibly affected by said circular scanning.

7. A device according to claim 6, wherein said transmitted wave is in the form of continuous pulses modulated linearly in frequency, said processing means including means for producing a beat between said back- reilected wave and a local oscillator wave tapped from said transmitted wave in order to extract the distance of said target, and wherein at least one relatively long pulse, compatible with the distance of the farthest target, is transmitted in the central sighting direction.

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8. A device according to clailu 7, wherein said phase of preliminary determination of the distance of the target constitutes a phase of acquisition of the target, this acquisition phase being followed by a tracking phase during which are trans- mitted pulses with a duration T' relatively short and equal to the quotient of the go-and-return time determined during the acquisition phase by an integer N, said distance of the target being then expressed as a function of the beat frequency determined during this tracking phase and of the quantity NTI.

9. A device according to any of claims 7 and 8, wherein, when the target is moving, two frequency ramps with opposite slopes are substituted for each pulse, or frequency ramp, in order to determine simultaneously its distance and its velocity.

10. A device according to any of claims 7 and 8, wherein, when the target is moving, a pulse with constant-frequency modulation is substituted for each pulse modulated linearly in frequency in order to determine the velocity of the target.

11. A device according to claim 8, wherein during sai acquisition phase, said circular scanning is stopped, a device for orienting said sighting line performs a sequence of lines with rapid retrace in order to analyze several strips, and in each strip said angular deflector addresses a number of points that can be lower than the number of said detectors in order to obtain a meshed field.

12. A device for spatial analysis ivith a laser wave substantially as hereinbefore described ivith reference to the accompanying draiTings.

12. A device for spatial analysis ivith a laser iTave substantially as hereinbefore described ivith reference to the ac--oLi-nanyiiig drai,,, ings.

h 11 Amendments to the claims have been filed as follows 1. A device for spatial analysis with a laser wave, in particular for a missile homing head, including means of illumination in a sighting direction, associated with means for sequential scanning of the analyzed region of space, the illumination means directing the laser wave in successive directions in space, means for receiving the beam reflected back by the analyzed region of space in the successive directions of the transmitted beam, and means for processing said back-reflected beam in order to reconstruct an image of said analyzed region of space, said means for sequential scanning of said analyzed region of space including means for performing a circular scanning combined with an angular deflection of the transmitted beam in order to achieve analysis of a circular field, said means of reception including a fi_red linear array of photodetectors towards which is successively directed said back- reflected beam corresponding to the successive angular deflections of the corresponding transmitted beam, preceded by the same means of circular scanning as for transmission, and a radial scan, effected by said linear array of detectors, being made at each position of the circular scan, and said processing means being common to the various photodetectors and receiving signals from the individual photodeVtors of the array, and a multiplexer being provided between said processing means and said linear array of photodetectors.

2. A device according to claim 1, including in addition means compensating for the possible differences of said circular scanning applied to said transmitted beam and to said corresponding bac'..-- reflected beam,, due to a relatively 1 rapid circular scanning and a relatively long go-andreturn time of the light wave.

IR 3. A device according to any of claims 1 and 2, wherein said processing means include means for tapping a local oscillator wave from said transmitted beam, means for directing this local oscillator wave towards the one of said photodetectors that receives said backreFlected wave at the same instant of time, while compensating for the possible differences of deflection between said local oscillator wave tapped at a given time and said back-reflected wave at the same instant of time due to a relatively rapid angular deflection and a relatively long go-and-returne time of said light wave, and means for producing a beat between these two waves.

4. A device according to claim 3, wherein said means compen sating for the possible differences of deflection between said local oscillator wave and said back-reFlected wave includes means for tapping said local oscillator wave from said transmission beam located before the angular deflection of said beam and means for directing this local oscillator wave simultaneously towards all said photodetectors.

5. A device according to claim 2, wherein said means compensating for the possible differences in the circular scanning applied to said transmitted beam and to said corresponding back-reflected beam include means for, in a first phase, determining the distance D of said target in the corresponding sighting direction and means for, in a second phase, rotating said angular deflector by a polar angle -M2D/C) (where J2 denotes the angular velocity of said circular scanning).

6. A device according to claim 5, wherein said means for de- o termining in a first phase the distance of the target include means for transmitting in a so-called central sighting direction which is a sighting direction of said angular deflector negligibly affected by said circular scanning.

7. A device according to claim 6, wherein said transmitted wave is in the form of continuous pulses modulated linearly in frequency, said processing means including means for producing a beat between said back- reflected wave and a local oscillator wave tapped from said transmitted wave in order to extract the distance of said target, and wherein at least one relatively long pulse, compatible with the distance of the farthest target, is transmitted in the central sighting direction.

8. A device according to clail,17, wherein said phase of preliminary determination of the distance of the target constitutes a phase of acquisition of the target, this acquisition phase being followed by a tracking phase during which are trans- mitted pulses with a duration T' relatively short and equal to the quotient of the go-and-return time determined during the acquisition phase by an integer N, said distance of the target being then expressed as a function of the beat frequency determined during this tracking phase and of the quantity NTI.

c- 1 10. A device according to any of claims 7 and 8, wherein, when the target is moving, a pulse with constant-Frequency modulation is substituted For each pulse modulated linearly in frequency in order to determine the velocity of the target.

11. A device according to claim 8, wherein during said acquisition phase, said circular scanning is stopped, 2 device for orienting said sighting line performs a sequence of lines with rapid retrace in order to analyze several strips, and in each strip said angular deflector addresses a number of points that can be lower than the number of said detectors in order to obtain a meshed field.