GB2105137A

GB2105137A - Determining the angular deviation of a target

Info

Publication number: GB2105137A
Application number: GB08214865A
Authority: GB
Inventors: Alain Segalas
Original assignee: Societe Anonyme de Telecommunications SAT
Current assignee: Societe Anonyme de Telecommunications SAT
Priority date: 1981-05-26
Filing date: 1982-05-21
Publication date: 1983-03-16
Also published as: DE3219826A1; DE3219826C2; GB2105137B; FR2507042A1; FR2507042B1

Abstract

A device for determining the angular position of a target emitting radiation consists of a lens (1); an analyzing device (2) which preferably deflects the focus of the radiation in a circular path; a detection device (3) which comprises at least two sets of detectors (10,11, 12, 13 and 20, 21, 22, 23), the detectors in each set being perpendicular and the sets being interfaced; and a processing device (4) which calculates the angular position of the target. The use of the second set of detectors enables radiation from decoys to be recognised as such and rejected. <IMAGE>

Description

SPECIFICATION Device for determining the angular deviation of a target, particularly insensitive to decoys The present invention relates to a device for determining the angular deviation of a target, which is tracked for example, comprising a focusing lens, a device for analysing the field of observation, a detection device placed in the focal plane of the lens and comprising filiform detectors, an electronic processing circuit adapted to deliver, from the signals furnished by the detection device, signals indicative of the coordinates of the target, and therefore of its angular deviation, and a device for elaborating time windows for the signals furnished by the detection device.

Taking into account the time windows assotiated with the signals furnished by the detection device, in fact respectively associated with the various filiform detectors, on the one hand, and the analysis of the field of observation, on the other hand, space windows of the field of observation correspond to these time windows; in other words, space bands correspond, in the space concerned by the focusing lens, to the filiform detectors of the image plane of the focusing lens.

Due to such space windows, the field of observations is reduced solely to the field necessary for tracking the target, which already limits the possible decoy of this target to a small part of the field of observation.

Devices for determining the angular deviation of a source, particularly an infrared source, of this type are perfectly well known. Such devices include ones where the device for detecting the focal plane of the lens comprises four filiform detectors disposed at 90" with respect to one another in the manner of a cross, such as for example the one described in Applicants' French Patent 1 527867.

With such a detection device, it is possible to obtain the two Cartesian coordinates of a target-source, in a predetermined system.

To each of the filiform detectors, in the image plane, there corresponds in the object plane a space window formed by a band, or a bar, whose width is a function of the corresponding time window.

It will be readily appreciated that one of these filiform detectors cannot discriminate all the sources which would be found in its respective space window, because, during analysis, preferably effected by circular scanning, necessary for generating the information, all these sources, of the object space, would energize the detector, of the image plane, at the same time. If all the sources, among which the target whose angular deviation is to be determined, is included, are parasitic sources, this angular deviation measurement cannot be obtained precisely. It is then said that the target is decoyed. This phenomenon may occur whenever the target to be tracked and at least one parasatic source are "aligned" on a detection image line, provided, of course, that the analysis of the field of observation is rapid and that evolution of the sources is slow.

It is therefore an object of the present invention to limit the decoy of a target to be tracked, in the whole field of observation.

To this end, the present invention relates to a device for determining the angular deviation of a target, comprising - a focusing lens, - a device for analysing the field of observation, - a detection device placed in the focal plane of the lens and comprising a first network of four detectors disposed at 90" with respect to one another in the manner of a cross, - a processing circuit adapted to deliver, from the signals furnished by the detectors, signals indicative of the coordinates of the target, and therefore of the angular deviation thereof, and - a device for elaborating time windows for the signals furnished by the detectors, said angular deviation measuring device being characterised in that the device for detecting the focal plane of the lens comprises a second network of four detectors disposed at 90 with respect to one another in the manner of a cross, and respectively angularly inserted between the detectors of the first network, and means are provided for selecting that of the two networks of detectors from the angular deviation of the target is determined.

Due to the addition of this second network of detectors, real space windows are created.

In a preferred embodiment of the device of the invention, the detectors of the second network are offsett by 45" with respect to the detectors of the first network.

It will be noted that the device for analysing the field of observation is arranged preferably to rotate this field, i.e. so that the image of a source in the image plane of the focusing lens describes a circle.

When only one network of detectors is provided, two sources, one of which is located at the centre of the field and the other for example in the object space coresponding to the vertical detectors, provoke, for example at the output of the first vertical detector, only one pulse.

On the contrary, when two networke of detectors are provided, the same two sources mentioned above advantageously provoke at the output of the first detector of the second network, inclined at 45 with respect to the first vertical detector of the first network, two pulses spaced apart in time.

It is therefore from this detector of the second network, inclined at 45 with respect to the vertical detector of the first network, that one of the two magnitudes of the angular deviation of the target is determined, when the two pulses are not located in the same time window.

On the other hand, when these two pulses are located inside the same time window, as with the first vertical detector of the first network, it is from one or the other of the first horizontal detector of the first network and of the following detector at 45" of the second network that one of the two magnitudes of the angular deviation of the target is determined.

It is the selection means mentioned above which perform this function.

The invention will be more readily understood on reading the following description with reference to the accompanying drawings, in which: Figure lisa block diagram illustrating the principle of the device of the invention.

Figure 2 is a view of the detection device of the device of Figure 1.

Figure 3 is a schematic representation of the signals delivered by the detection device of Figure 2.

Figure 4 shows a first embodiment of the selection circuit of the device of the invention, and Figure 5 shows a second embodiment of the selection circuit of the device of the invention.

Referring now to the drawings, the device shown in Figure 1 and intended for determining the angular deviation of a target, comprises a focusing lens 1, a device 2 for analysing the field of observation, a detection device 3 and a processing and selection device 4.

The analyzing device 2, preferably located behind the lens 1, comprises for example a prism adapted to scan the field of observation and in particular to rotate it so that the image of a source of the object field in the image plane of the lens describes a circle.

The detection device 3, disposed in the image plane of the lens 1, comprises two networks of infrared detectors, which will be described in greater detail herein after, and delivering output signals to the device 4, from which the angular deviation of the target is measured.

The analyzing device 2 furnishes the device 4 with reference signals, as well as a clock signal, necessary for measuring the angular deviation. To this end, there is associated with the analyzing device 2 an angular coding device comprising for example an optical disc acting as angular coder and comprising a reference slot, making it possible to associate with each signal from a detector the angular position it of a source at the moment when its image is intercepted by this detector.

In addition, the device 4 comprises filtering means to allow the signals issuing from the detectors to pass only inside determined time windows.

All the devices which have just been mentioned are well known to the man skilled in the art and do not need to be described or illustrated further.

Let us now consider the detection device 3 which has been shown more clearly in Figure 2.

It comprises a first network R1 of four filiform detectors 10, 11, 12, 13 disposed at 900 with respect to one another in the manner of a cross, and a second network R2 of four otherfiliform detectors 20,21,22,23, also disposed at 90 with respect to one another in the manner of a cross and respectively offset by 45 with respect to the detectors of network R1.

Let us admit that the angular position of the sources mentioned above is considered with respect to the reference axis x'Ox extending along the horizontal detectors 11 and 13 of network R1, 0 being the centre of the device 3, i.e. the point of intersection of the axes passing through the eight detectors of the device. y'0y is the axis perpendicular to x'0x in the plane of the detectors.The image of a source located on the optical axis of the device, i.e. an axis perpendicular at point 0 to the plane of the detectors, describes, due to the analyzing device 2, a circle centred on 0, of radius Rand of equations:

x x = R cos e y y = RsinE The image of a source of Cartesian coordinates xO, yO, in the reference system x0y of the image plane of the lens 1, describes a circle of equations: :

x x = x0 + Rcosf 8 Y = yO + R sin e The equations of the axes passing through the detectors of the second network R2, in the same reference system defined by the first network, are written:

j x = y {x= For the source of coordinates x0, y0, the angular positions of its image on the first four bars (10, 11,20, 21) of the device are furnished by the system of equations::

It will be noted that the other four bars (12,22, 13, 23) either furnish redundant information, or allow a spectral separation. In this way, the detection system of the invention might comprise only a first pair of two filiform detectors at 90 with respect to each other and a second pair of two detectors offset angularly by 45" with respect to those of the first pair.

By way of example, two respective sources of coordinates

s x=o y=o et (X=O OU ( X = X, ou ty=o which could not be discriminated with the device with one network, may be with the device with two networks described hereinabove, as, further to the movement of scanning of the field of observation, the segment joining the two sources remains, in the image plane, parallel to itself. In other words, the instants of passage of the images of the two sources on the detectors of the second network are different.

The device described hereinabove therefore makes it possible to proceed with a separation in time of the information, as illustrated in Figure 3.

With respect to a reference pulse, already mentioned above, the pulses indicative of the angular positions '10 and 6"10 of interception by the two sources of the first detector 10 are identical, as illustrated in diagrams 3a, 3b, whilst the pulses '20 and "20 corresponding to the interception of the detector 20 (diagram 3c) are separated in time by a slot At t= T (e" ~ ≈) where T is the period of analysis. It will be noted that the energy furnished for position 10 is greater than that furnished for '20 or*"20.

If the two pulses '20 and "20 are not in the same time window, then the pulse found there is that of the source or of the tracked target.

If the two pulses '20 and "20 are in the same time window, the following detector will then be used, which will necessarily furnish two different angular positions, one inside and the other outside the new window.

In summary, if, for all the detectors, the corresponding windows do not comprise more than one pulse, it is that only the right target is correctly tracked.

If only one window associated with a detector comprises at least two pulses, it is an erroneous measurement or one falsified by noise.

If two windows corresponding to two detectors comprise at least two pulses, the measurement of the angular deviation may then be made with the other two detectors, the angular deviation being measured, as has been seen, over a half-period of analysis, due to the redundancy.

If at least three windows out of four comprise at least two pulses, a plurality of sources are present.

Measurement must than be effected by other known methods, such as for example by spectral separation, width discrimination, deconvolution.

The above comments may be schematized by the following flow-chart:

N : number of detectors emitting at least two pulses in their window YES NO OneSOL ]~=4 in the \ YES NO in the v zindow 4 t \1=1 > I YES ' NO I NO YES Imeasurement on the other two N=3or4 detectors a a plurality of sources r The selection of one or the other of the measurement detectors is effected by the selection means mentioned hereinabove.

It should be recalled that it is only if a parasitic source is more energetic than the target to be tracked that the angular deviation measurement of the target was vitiated with error with the device having only one network of detectors. In other words, two superposed sources, for a filiform detector, produce a signal corresponding to the sum of the energies of the two sources.

These selection means may be of analog or digital type, well known to the man skilled in the art. However, to render the present specification homogeneous, two examples will be given hereinbelow.

Analog selection Still considering only the half-periods of analysis, therefore the first two detectors (10, 1 1) and (20, 21) of the two networks R1 and R2, the output circuits of detectors 10 and 20 and those of detectors 11 and 21 are electronically associated, as being offset in two's by 45 , as shown in Figure 4.

The output of the detector 10 is connected to an amplifier 30 whose output is connected to one, 32, of the inputs of a threshold comparator 31. The other input 33 of the comparator 31 receives a signal of determined threshold but variable as a function of time and of the levels picked up on the other detectors. The output circuit of the detector 20 is analog and comprises an amplifier 40 and a comparator 41.

The analog output signals of the two comparators are converted into digital signals 1 if the level of the output signals of the amplifiers is greater than the level of the threshold signal, and 0 in the opposite case.

If the two output signals are 0, there is no parasitic source, and angular deviation can be measured either with detector 10 or with detector 20.

If the output signal of the circuit associated with the detector 10 is 0, and the other output signal 1, it is from detector 10 that measurement must be effected.

Inversely, if the output signal of the circuit associated with the detector 20 is 0, and the other output signal 1, it is from detector 20 that measurement must be effected.

If the two output signals of the two circuits associated with the two detectors 10 and 20 are 1, it is from the other two detectors 11 and 21 that measurement must be effected.

Digital selection From an established state where the angular deviation of the target is known at a given instant, the probable position of the target at the following instant may be determined, knowing the constants of the movement of the target and the sighting noises, due to algorithms of prediction. This predicted position gives the centre and the width, the latter by known factors which do not need to be described in the present context, of the tracking window.

It will therefore be seen that it is easy, under these conditions, sequentially to sample the signals delivered by the detectors, at instants and for periods of time which are programmable as a function of the predictions.

The signals issuing from the eight detectors are sent onto an analog multiplexer 60, via eight amplifiers 51-58 respectively (Figure 5).

The output of the multiplexer is connected to an analog-digital converter 80 via an amplifier 70 with programmable gain, intended to increase the dynamics of the converter 80. Each output signal of the detector, coded digitally, is then stored in a memory of a microprocessor 100, via a direct access channel 90.

Due to this, the data acquisition by the microprocessor may be continued during operation thereof, which is therefore not slowed down.

The program of the microprocessor is based on the flow-chart shown hereinabove and it is adapted to be able to reinitialize the variables, verify the data acquisition and, of course, furnish the angular deviation measurements.

Claims

1. In a device for determining the angular deviation of a target, comprising: - a focusing lens, - a device for analyzing the field of observation, - a detection device placed in the focal plane of the lens and comprising a first network of at least one pair oftwofiliform detectors at 90 with respect to one another, - a processing circuit adapted to deliver, from the signals furnished by the detectors, signals indicative of coordinates of the target, and therefore of its angular deviation, and - a device for elaborating time windows for the signals furnished by the detectors, the device for detecting the focal plane of the lens comprises a second network of at least one pair of two filiform detectors at 90 with respect to each other, respectively angularly inserted between the detectors of the first network, and means are provided for selecting that of the two networks of detectors from which the angular deviation of the target is measured.

2. The device of Claim 1, wherein the first network comprises four detectors disposed at 90 with respect to one another in the manner of a cross and the second network comprises four detectors disposed at 90 with respect to one another in the manner of a cross, and respectively angularly inserted between the detectors of the first network.

3. The device of one of Claims 1 and 2, wherein the detectors of the second network are offset by 45 with respect to the detectors of the first network, the filiform detectors of the first network and the filiform detectors of the second network, offset by 45 with respect to the first, possess output circuits which are respectively associated in two's.

4. The device of Claim 3, wherein the output circuit of a filiform detector comprises a threshold comparator.

5. A device for determining the angular deviation of a target substantially as hereinbefore described with reference to and as illustrated in Figures 1,2,3 and 4, or in Figures 1,2,3 and 5.

5. The device of one of Claims 1 and 2, wherein the filiform detectors are connected to an analog multiplexer connected to an analog-to-digital converter itself connected to a microprocessor.