Patent Application
20250012900
The invention relates to the field of detecting a target by means of a detector having a plurality of quadrants, the target preferably being illuminated. The invention has application, in particular, in the field of the semi-active laser guidance of an object toward the target, for automatic guidance of air-to-surface or surface-to-surface weapons.
A device for locating a target comprising a photodetector having four quadrants exploits a flux scattered by the target to be detected, in order to measure the direction of the target in its aiming mark. Such locating enables, in particular, tracking of a moving target such as an aeroplane, machine, missile, vehicle, tank etc. such a device can be mounted on a guidance system, for example a self-guided missile, or on a firing station, making it possible to track a beacon carried by the missile and a target.
The deviation between the sighting axis and the direction of the target is the angle error or angular deviation or angular loss of aim. In the case of guidance, this angle error is usually either reduced to zero or kept constant so that the guidance system comprising the locating device is correctly guided to the target.
The target may emit light naturally. However, in the context of a semi-active laser guidance, a high-power laser beam with low divergence emitted by a designator marks the target, and the laser flux scattered (backscattered) by the target is then used to measure the angle error. The designator is, for example, incorporated in the viewfinder or even carried by a third-party co-operative device.
In particular, two categories of guidance systems or sensors are known, which exploit such a laser flux scattered by the target to be located.
Laser spot trackers (LST) are pieces of equipment incorporated in a viewfinder, the function of which is to ensure that the laser beam emitted by the designator is well positioned on the target. An LST is more precisely a deviation indicator which determines the angular direction of the laser flux scattered by the target and this direction is compared to the direction of the target observed in a imager harmonised with the LST.
Homing devices or semi-active laser deviation indicators, designated by the term SALH, are incorporated in weapons and provide them with the ability to guide themselves with precision onto the laser spot positioned on the target which may be fixed, movable or mobile.
The main functions of these systems are as follows:
For location and guidance, a usual solution consists of using a multi-quadrant photodetector, in particular a four-quadrant photodetector, the photodetecting surface of which is in the form of a disc or square divided into four equal and independent photodetection surfaces. This photodetector is associated with a flux collecting optic enabling a more or less localised energy distribution, typically a homothetic disc of the illumination in the entry pupil of the optic, on the active surface of the photodetector. The position of this energy distribution depends on the angular direction of origin of the incident flux.
The level of energy collected by each of the quadrants thus varies according to this direction and it is this which enables the angle error to be determined.
Consider as an example a four-quadrant photodetector. In order to determine the angle error, the operations to be carried out with a four-quadrant detector consist of two comparisons of the signals received on the paired sectors: on the one hand, the sum of 2 quadrants at the top versus the sum of 2 quadrants at the bottom and, on the other hand, the sum of 2 quadrants on the left versus the sum of two 2 quadrants on the right. Two weightings of the detected signal are carried out along the vertical and horizontal axes. Then a tabulation previously performed in the factory is applied and gives the true angular positions in elevation and azimuth of the spot in relation to the reference frame of the detector, corresponding with the top-bottom and left-right weighting values.
This conversion step between weightings and angle error assumes that the energy distribution on the four-quadrant detector is identical in the means implemented at the factory and in the context of use.
However, this is not always the case.
The angle error precision is sometimes degraded, in particular in the presence of atmospheric turbulence or strong light phenomena.
More specifically, the laser signals generated by the four quadrants of the photodetector, corresponding to the illumination of the sensor originating from the designation of the target, may be disturbed:
Disturbances of type a) result from the problem described below.
Analogue and digital electronics systems have the function of filtering the signal captured by each channel or quadrant of the four-quadrant photodetector, for the purpose of detecting and recognising the temporal coding; and elaborating the information necessary for measuring the four energy portions of the laser pulses, for the purpose of locating the target. The elements of the electronic system must be able to guarantee good restitution of the laser angle errors, including in the case of multi-echoes with two pulses close together in time.
These requirements impose a compromise during design, between the sensitivity of the sensor, its instantaneous measurement dynamics, the effectiveness of the filtering of interference and the reliability of the restitution of the four pieces of energy information and therefore the precision of deviation measurement.
Distortion or saturation of one of the four useful signals can occasionally occur in the presence of intense parasitic modulations.
The signals coming from the four quadrants and used for carrying out the calculations of the two weightings, are then biased by the amputation of the laser signal on the distorted or saturated quadrant(s).
With regard to disturbances of type b), the problem is as follows.
The variations in the refractive index of the air on the path of the laser are manifest by an inhomogeneous illumination on the optical pupil of the sensor, and these inhomogeneities vary from the reception of one laser pulse to the next.
This non-uniformity, which varies over time, is reflected by modulations in the energy distribution on the photodetector and disturbs the two weightings.
For weak disturbances, the angle error noise is aggravated but does not jeopardise the success of the mission.
For strong disturbances, the dispersion of the angle error measurements delivered, hampers the location and guidance.
The invention proposes to overcome at least one of these disadvantages and, in particular, proposes diagnosing the two types of degraded operation: (a) saturation of the electronic system in the presence of strong background modulation and (b) inhomogeneity in illumination of the pupil in the presence of turbulence.
For this purpose, the invention relates to a method for locating a target by means of a detecting device comprising a photodetector comprising N×M quadrants positioned with respect to a sighting axis of the detecting device, the photodetector being configured to detect optical signals from the target to be detected, said method being implemented in a processing unit of said device and comprising the following steps, the target scattering an optical signal:
The invention is advantageously supplemented by the following features, taken individually or in any of the possible technical combinations thereof:
Y
1=(B−A)/(A+B), weighting comparing the top two signals;
Y
2=(D−C)/(C+D), weighting comparing the bottom two signals
Y=(B+D−A−C)/(A+B+C+D) weighting comparing the signals on either side of the vertical axis;
Z
1=(A−C)/(A+C), weighting comparing the two signals on the left
Z
2=(B−D)/(B+D), weighting comparing the two signals on the right
Z=(A+B−C−D)/(A+B+C+D) weighting comparing the signals on either side of the
The invention also relates to a device for detecting a target comprising a photodetector with a plurality of quadrants and a processing unit configured to implement a method according to the invention.
The invention also relates to a system for guiding a missile towards a moving target, said device including a detection device according to the invention.
The invention also relates to a computer program product comprising code instructions for implementing a method according to the invention when this is executed by a computer.
Thus, the invention enables a comparison between the measured signals coming from quadrants of the photodetector and levels which respect the theoretical energy distribution in order to validate the angle error measurements.
The invention makes it possible to identify erroneous angle error measurements caused by a degraded operation.
The invention makes it possible to increase the reliability of the deviation measurement function by the addition of a criterion on the geometries and radiometry of the spot on the photodetector.
Through the invention, an analysis of the coherence of the signals with respect to the theoretical distribution (typically a disc) makes it possible to determine that the uniformity of the illumination in the optical pupil is significantly degraded following scintillation phenomena on the paths of the laser from the designator to the target and/or from the target to the sensor or even degradation due to the electronic system as indicated above.
Thus, knowing how to identify a degraded operation, it is possible to implement corrective measures:
Other features, aims and advantages of the invention will emerge from the following description, which is given purely by way of illustration and not being limiting and which should be read with reference to the attached drawings, in which:
In all the figures, similar elements have identical reference signs.
This planar photodetector 2 is behind an optic 6 through which light rays from the target T to be located pass. Such an optic 6 is, for example, a convergent lens. In a complementary manner, the locating device comprises an emission unit 3 of a beam, known by the term designator, enabling illumination of the target T. In the case of semi-active laser guidance, the emitted beam is a laser beam. In another example, the beam is emitted by an emitter of a beacon carried by a remotely guided munition, of the LED emitter type or any other photonic emitter associated with a temporal modulation.
In the following, the case is considered in which the target returns on optical flux coming from the illumination by the designator 3.
The locating device 1 and the photodetector 2 are positioned with respect to a sighting axis AA which forms an angle error θ with the direction BB of the target T. This angle error θ is measured by means of the distribution of optical signals, in other words the optical flux backscattered by the target, on the quadrants of the photodetector 2.
In operation, the photodetector 2 being centred on the sighting axis AA, the optical signals detected by the photodetector 2 form a spot 21 centred, as illustrated in
The photodetector 2 is configured to detect optical signals scattered by the target. It is connected to a processing unit 4 which makes it possible, in particular, to measure the angle error and to validate this measurement according to a method which will be described below.
The processing unit 4 comprises an electronic system (not described or detailed because it is well known to a person skilled in the art) connected to each quadrant of the photodetector 2 in order to acquire the electronic signals corresponding to the optical flux detected by each quadrant and integrated by the photodetection surface of the quadrant.
As indicated in the introduction, the angle error may be false when the photodetector operates in degraded mode.
It is on the observation of this incoherence that the method of the invention is based; a method implemented in the processing unit 4 of the device described above.
Such a principle applies to any photodetector 2 comprising a plurality of quadrants, arranged in a matrix of size N×M quadrants, N×M being greater than or equal to four. Examples are shown here of quadrants conventionally arranged in a matrix, but this can be any arrangement, as illustrated, by way of example, in
Other arrangements can advantageously have a geometry which facilitates the connexion of output signals by having a proximity to the periphery for each quadrant. This would be the case, for example, for the arrangement of 4×4 quadrants consisting of triangles, illustrated in
In the context of a locating method described in relation to
The acquired signals correspond to the optical signals scattered by the target T to be located and detected by the photodetector 2. One signal is acquired per quadrant.
These signals are then processed in order to determine an angle error in elevation and in azimuth, respectively denoted S(t) and G(t) (step E2). This angle error is directly linked to the position of the spot on the photodetector. In particular, the angle error is obtained from a calculation of weightings, which corresponds to comparisons of the signal levels in different quadrants.
The angle error measured is obtained from the signals detected by each quadrant, as detailed more fully below.
Then, the measured angle error (step E2) is processed (step E4) in order to deduce indicators of the theoretical energy distribution corresponding to the optical signals, in other words of the light spot, in the quadrants. These indicators are simply designated as theoretical indicators.
In parallel, the acquired signals are processed (step E3) in order to calculate indicators of the energy distribution corresponding to the optical signals detected by the quadrants. These indicators are simply designated as measured indicators.
There are therefore, on the one hand, theoretical distribution indicators and, on the other hand, measured distribution indicators.
These theoretical and measured indicators are then compared (step E5) in order to verify their coherence and to validate or not validate the measured angle error.
The determination of the angle error in elevation S(t) and in azimuth G(t) comprises a calculation (step E22) of the weightings along these directions. These weightings exploit the detected signal levels in each quadrant and compare the signals on either side of the rows and columns of the matrix of quadrants in the directions along the elevation S(t) and azimuth G(t) axes. In other words, the weightings compare the signal levels of a plurality of quadrants.
According to an embodiment, a four-quadrant detector is considered, as in
The angle error is obtained on the basis of weightings in elevation and in azimuth. In the case of a matrix of 2×2 quadrants, the weighting in elevation is obtained by comparing, two-by-two, the signal levels detected by each quadrant along the axes of elevation and of azimuth. In particular, the weighting in elevation is given by PY=(SB+SD−SA−SC)/(SA+SB+SC+SD); it compares the signals on either side of the azimuth (vertical) axis and the weighting in azimuth is given by PZ=(SA+SB−SC−SD)/(SA+SB+SC+SD); it compares the signals on either side of the elevation (horizontal) axis.
According to an embodiment, in the case of a photodetector with N×M quadrants, with N×M greater than four, as in
Consequently, the method comprises a sub-step (step E21) of selecting a configuration of 2×2 quadrants for calculating weightings, the selected configuration being that for which the detected signal level is highest. In other words, it is that which will give the highest signal-to-noise ratio which will be maintained for the rest of the method.
On returning to the example of
According to another embodiment, a situation is considered in which the spot 23 occupies more than four quadrants on the detector with N×M quadrants. In this case, in order to improve the detection, it is useful to consider more than 2×2 quadrants. This is illustrated in
In
More generally, the quadrants detecting a signal define a matrix with Ni×Mi quadrants included in the N×M matrix of photodetector quadrants, Ni being less than or equal to N, Mi less than or equal to M, and the weightings in elevation and azimuth exploiting the signals detected in this matrix of Ni×Mi quadrants detecting signals, the weightings in elevation and in azimuth being respectively obtained by comparing the signals detected on adjacent quadrants taken along the rows of this Ni×Mi matrix on the one hand, and taken along the columns of this Ni×Mi matrix on the other hand. The Ni×Mi matrix is preferably the matrix of smallest dimensions containing quadrants reached by the spot 23, in other words detecting a signal.
Thus, the Ni×Mi matrix of quadrants detecting a signal is scanned in order to compare the signals along the rows and columns.
In this case, the sub-step E21 described above is not implemented and the calculation step of the weightings E22 takes into account more than 2×2 quadrants.
In parallel with the calculation of the weightings, a total signal-to-noise ratio RSBtot is calculated on the basis of the detected signals (step E23). Such a signal-to-noise ratio makes it possible to adapt the criterion used to verify the coherence of the distribution criteria with the dispersions over the signal levels in the quadrants, taking account of this signal-to-noise ratio. Here, it is the “measurement quality” that is adapted to.
Then, based on angle error tables, Tab1, stored in the locating device 1 an interpolation is implemented in order to associate with each weighting measurement, the values of angle errors G(t) and S(t) in azimuth and in elevation respectively (step E24). In an implementation of the invention, a bilinear interpolation is applied, which has the advantage of being simple to implement.
It is understood that the angle error tables depend directly on the geometry of the photodetector.
The tables of angle errors are produced at the factory during calibration of the photodetector on a discretisation of its angular operating range. For each point of the range, the signals leaving the quadrants are recorded. The “laboratory” environment of this characterisation at the factory guarantees the absence of disturbances of the laser signals and therefore makes it possible to constitute a model of the reference nominal operation.
This characterisation at the factory makes it possible to construct a theoretical mapping, consisting of the reference values expected for calculating a metric at any point in the field. This metric is used to verify the coherence of the measurements. This mapping dedicated to “validity” adds to the usual characterisation necessary for the needs of the “angle error” functionality which enables the establishment of correspondences between weightings and angle errors. The mapping of “validity” can therefore exploit the same source data and be formed from the same factory test protocol.
The performance of these mappings, generally proceeds as follows:
At the end of this step E2, the angle errors in azimuth G(t) and in elevation S(t) are obtained. These angle errors are used to locate the target T with respect to the sighting axis of the device, if they are validated in the later steps. This angle error is also used to track a target once located.
On the basis of the acquisition of the electronic signals on each of the channels (quadrant) of the photodetector, here the indicators of the distribution of the detected signals are determined (step E31).
The indicators of the measured distribution also comprise the weightings used for the angle error, of half-weightings. Half-weightings are also weightings, but only taking signals two-by-two.
In the four quadrant case, PY and PZ are the weightings which respectively compare the signals at the top and bottom, and in the four quadrant case PY1, PY2, PZ1, PZ2 are correctly speaking half-weightings since these weightings compare the signals two-by-two, for PY1 the two signals at the top, for PY2 the two signals at the bottom, for PZ1 the two signals on the left and for PZ2 the two signals on the right, and are given by the following expressions:
P
Y1=(SB−SA)/(SA+SB), weighting comparing the two signals at the top;
P
Y2=(SD−SC)/(SC+SD), weighting comparing the two signals at the bottom;
P
Y=(SB+SD−SA−SC)/(SA+SB+SC+SD) weighting comparing the signals on either side of the vertical axis;
P
Z1=(SA−SC)/(SA+SC), weighting comparing the two signals on the left;
P
Z2=(SB−SD)/(SB+SD), weighting comparing the two signals on the right;
P
Z=(SA+SB−SC−SD)/(SA+SB+SC+SD) weighting comparing the signals on either side of the horizontal axis.
By adding the calculation of the half-weightings to those of the weightings used in the estimation of the angle error, a redundancy of measured information is created.
More specifically, a unique weighting value is associated with an angle error (elevation or azimuth) (bijectivity), whereas the half-weightings can take several pairs of values according to the angle error on the other axis (azimuth or elevation, respectively). However, only one pair of values for the half-weightings verifies the geometric and radiometric coherence expected in theory for the luminous energy distribution on the different quadrants associated with this position in elevation and azimuth of the target.
These calculations can be transposed to any configurations with a plurality of quadrants.
In the case where there are more than four quadrants, the idea is to calculate moreover the weightings necessary for the angle error of other quantities associated with the distribution of the spot 23 on the photodetector. Thus, a person skilled in the art understands that a multitude of indicators can be calculated.
On the basis of these weightings, a metric is then determined (step E32) 2 on the basis of the measured indicators. The purpose of this metric 2 is to characterise the energy distribution of the laser spot 23 on the various quadrants of the photodetector. In an example, an easily applicable metric 2 is based on a maximum absolute deviation between the weightings and the half-weightings. Such a metric Q is, for example, given by the following expression in the case of four quadrants:
Ω=max([abs(PY1−PY2); abs(PZ1−PZ2); abs(PY1−PY); abs(PZ1−PZ); abs(PY2−PY), abs(PZ2−PZ)]).
Of course, in the case of configurations with more than four quadrants, this always involves calculating a metric based on the weightings and half-weightings.
Other metrics could be used, for example a metric based on calculations of distance by reference to a spot which respects the theoretical shape.
On the basis of the angle errors measured in step E2, by interpolation, for example bilinear interpolation, with theoretical mappings of weightings and half-weightings Tab2, theoretical indicators are determined for this measured angle error (step E41).
Here, this involves obtaining expected weightings and half-weightings for the angle error measured by symmetry with the obtaining of the theoretical indicators of the distribution of the acquired signals.
These indicators are denoted here as P′Y, P′Z, P′Y1, P′Y2, P′Z1, P′Z2. The mappings of the theoretical indicators Tab2 (here theoretical weightings and half-weightings) are obtained in the factory in a similar manner to the mappings Tab1.
These theoretical indicators have the same expressions as the measured indicators, with the difference that they have been calculated from electronic signals S′A, S′B, S′C, S′D measured at the factory on the quadrants.
It is then possible to characterise the energy distribution of the laser spot on the quadrants of the photodetector corresponding to these theoretical indicators (step E42). A metric Q′ is applied which has the same definition as the metric 22 applied in step E32.
Returning to the preceding example, the metric Q′ is thus based on a maximum absolute deviation between the theoretical weightings and the theoretical half-weightings, and given by the following function:
Ω″=max([abs(P′Y1−P′Y2); abs(P′Z1−P′Z2); abs(P′Y1−P′Y); abs(P′Z1−PZ); abs(P′Y2−P′Y), abs(P′Z2−P′Z)]).
The verifying of the coherence between the indicators of the theoretical and measured distributions is advantageously implemented by comparison (step E51) of the metrics Q and 2″ and comprises calculating a comparison criterion between these metrics.
Such a criterion is given by the following formula: Crit=abs(Ω-Ω″) namely an absolute difference of the two metrics.
This criterion is then compared to a threshold (step E52).
Two cases can be distinguished:
According to an embodiment, the threshold S is arbitrarily fixed and advantageously results in a compromise between
In a complementary manner (step E53), it may be necessary, depending on the compromise sought, to add one or more dependencies to the threshold such as
In the first case, the signal-to-noise ratio RSBtot of the measured signals is used to define the applicable threshold, by interpolation of Tab3 data established for a few angular positions in elevation and azimuth, and based on a few tabulated levels of the RSBtot. Therefore, the threshold depends on the measured angle error.
In the second case, an interpolation is performed in a mapping of the threshold according to the measured (current) angle error by using the Tab3 data.
At the end of the comparison with the threshold, a validity status of the measured angle error is obtained.
The method has been described in the context of a photodetector having a matrix with a plurality of quadrants.
However, various alternatives are possible and they result from the same principle which consists of exploiting the a priori knowledge of the energy distribution of the spot on a multi-quadrant photodetector.
The spot can have a different geometry from a uniformed disc. Other shapes are sometimes preferred, in order to simplify or compact the optical element upstream of the photodetector and/or as a consequence of incorporation in a device combining a plurality of optronics channels, for example in the case of a piece of equipment (with central shutter) and/or to favour certain specifications of the deviation measurement response, for example, a reduced dependence on the size of the spot on the target and/or on the distance between the target and the SAL sensor, whether it be of LST or SALH type.
These other spot geometries on the photodetector can be, in particular: a ring type shape, a “Gaussian” type shape, a “square” type shape, a shape based on those mentioned above and having a spatial modulation resulting from optics elements for reducing scintillation effects.
The multi-quadrant photodetector can be of the four-quadrant type, as taken more particularly in the previous example; this is the usual geometry; but it can have any other geometry which, combined with the theoretical size and shape of the laser spot, enables an anomaly to be identified in the energy distribution over the quadrants. In particular, as described above, a first family of extensions covers photodetectors with N×M elements, each element being square or rectangular. Another family of extensions groups the photodetectors with geometries having a number of angular sectors greater than 4.
Laser deviation measurement using a multi-element photodetector is applied in many fields.
In the context of semi-active weapon guidance or laser target pointing, a target intercepts the beam and scatters part of the energy to the SAL sensor.
In other contexts, multi-element photodetectors are combined with lasers in direct configurations. In these direct configurations, the laser is directed towards the sensor equipped with a multi-element photodetector and the sensor develops an angle error in order to assist the pointing and/or relative movement of a carrier module of the laser and of a carrier module of the sensor. This type of direct configuration instrumentation is used in multiple fields, such as the space field, the field of industrial machines, the field of public works and the field of autonomous vehicle guidance, in particular in cooperative contexts. In the last two fields, disruptive phenomena can be encountered, to which the invention provides a response: the problem of non-homogeneity caused by atmospheric scintillation, and the problem of parasitic flux.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2112328 | Nov 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2022/052165 | 11/23/2022 | WO |
