CA1075341A - Cw-laser illuminator detector - Google Patents
Cw-laser illuminator detectorInfo
- Publication number
- CA1075341A CA1075341A CA216,949A CA216949A CA1075341A CA 1075341 A CA1075341 A CA 1075341A CA 216949 A CA216949 A CA 216949A CA 1075341 A CA1075341 A CA 1075341A
- Authority
- CA
- Canada
- Prior art keywords
- digital
- scanning
- successive
- averaged
- digital signals
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/26—Measuring arrangements characterised by the use of optical techniques for measuring angles or tapers; for testing the alignment of axes
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S3/00—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received
- G01S3/78—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received using electromagnetic waves other than radio waves
- G01S3/782—Systems for determining direction or deviation from predetermined direction
- G01S3/789—Systems for determining direction or deviation from predetermined direction using rotating or oscillating beam systems, e.g. using mirrors, prisms
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Photometry And Measurement Of Optical Pulse Characteristics (AREA)
- Optical Radar Systems And Details Thereof (AREA)
Abstract
ABSTRACT
The present invention relates to a incident laser detection apparatus. The apparatus comprises scanning means for scanning a field of view and for responding to radiation of a predetermined range of wavelengths to produce an analog signal which corresponds to the intensity of the radiation. An analog-digital conversion means is provided for converting the analog signal to a series of digital signals. An averaging means provided for deriving from each successive series of the digital signals for successive scans of the field of view a series of averaged digital signals. A digital memory is provided for storing successive series of averaged digital signals for successive scans of the field of view. A comparator is provided which is responsive to the digital signals and the averaged digital signals for successive scanning positions whereby an instantaneous digital value for a given scanning position of a given scan is compared with the stored averaged digital value for the same given scanning position of a selected previous scan. Finally, means are provided for signalling the presence of a dis-crepancy exceeding a predetermined threshold sensed by the comparator between the stored averaged digital value and the instantaneous digital value for any scanning position.
The present invention relates to a incident laser detection apparatus. The apparatus comprises scanning means for scanning a field of view and for responding to radiation of a predetermined range of wavelengths to produce an analog signal which corresponds to the intensity of the radiation. An analog-digital conversion means is provided for converting the analog signal to a series of digital signals. An averaging means provided for deriving from each successive series of the digital signals for successive scans of the field of view a series of averaged digital signals. A digital memory is provided for storing successive series of averaged digital signals for successive scans of the field of view. A comparator is provided which is responsive to the digital signals and the averaged digital signals for successive scanning positions whereby an instantaneous digital value for a given scanning position of a given scan is compared with the stored averaged digital value for the same given scanning position of a selected previous scan. Finally, means are provided for signalling the presence of a dis-crepancy exceeding a predetermined threshold sensed by the comparator between the stored averaged digital value and the instantaneous digital value for any scanning position.
Description
This invention relates to a laser beam detector for detecting in-cident laser beam radiation and determining the angle of incidence thereof.
The need to detect incident laser radiation is important with the advent of so-called "smart bombs" and other optically guided devices a~tracted towards a target illumina~ed by a laser beam. Early detection and identi-fication of laser illumination is essential to the deployment of counter-measures such as smdke screensl flares, or counter weapons.
The present invention provides a scanning detector for scanning a predetermined field of view, and having sensitivity to laser radiation of the anticipated wavelength or wavelengths. The detector provides an output analog signal which is converted to a digital signal and integrated or averaged over one or more complete scans of the field of view. Each corresponding sampling point is averaged over many scans using an appropriate weighting function to smooth out fluctuations in the trace used for comparison. Another way of achieving this objective is to add N consecutive sampling points and divide the sum by N, this process being carried on continuously while scanning around - the value of N usually being 2 or 3 at most. This process is sometimes called "box convolution" since it implies the product of the digital data by a rectangular function of amplitude 1 and width "~". The integrated digital signal is stored in digital form in a push-through memory and in one embodi-ment retained for a predetermined number of successive scans (say, ten scans).
Por each of the predetermined number of successive scans, the output of the analog-digital converter is compared with the stored integrated digital values in corresponding sequence, and if a discrepancy exceeding some threshold dis-crepancy between the stored integrated digital value and the immediate digital value obtained from ~he analog-digital converter is detected at no more than a small number, say two, consecutive scanning angle positions, a suitable display or alarm is actuated identifying the existence and the angular position of the discrepancy, which may be presumed to be caused by incident laser radiation.
Further refinements of the apparatus include the use of a wide angle reflector and associated detector and logic apparatus and a narrow angle reflector and associated detector and logic apparatus, so that once the angle of ~he input .
~37S341 laser radiation is established coarsely, a more precise identification of the angle can be obtained using the narrow-angle system. The comparator logic may optionally provide an inhibit action to prevent a large anomalous signal from markedly modifying the background reference value at any given point.
This inhibit action prevents the laser beam signal from being integrated along with the other radiation, which would tend to reduce the discrepancy between the output of the analog-digital converter and the stored signal.
In the said embodiment, after the predetermined number of scans, the output of the integrator is once again fed into the push-through memory so as to update the memory without interruption of the repeating scanning and comparison cycle.
The scan-to-scan averaging of the digital signal tends to reduce the effect of noise signals and provide a more reliable steady state stored signal against which the immediate output of the analog-digital converter may be compared. The actuation of the alarm, display, etc. only when a significant discrepancy is noted at no more than two (say) successive points is intended to prevent false alarms resulting, for example, from a large and sudden luminous flux, as from flare burst or the sudden illumination of the field of view by a searchlight, since these la~ter would be expected if sufficiently bright) to produce a discrepancy at more than two successive scanning posi-tions.
In an alternative embodiment, the memory is updated on every scan and the spatial and temporal averaging are achieved by employing a low pass digital filter.
The invention will now be described with reference to the accom-panying drawings, in which:
Figure 1 is a schematic diagram of an embodiment of laser beam detection apparatus according to the invention;
Figure 2 is a schematic diagram of an alternative arrangement of photodetectors for use with laser beam detection apparatus according to the invention;
Figure 3 is a schematic diagram illustrating a refinement of the ~7S3~L
laser beam detection apparatus according to the invention involving the use of a wide angle scanning system in conjunction with a narrow angle scanning system;
Figure 4 is a schematic block diagram illustrating a low pass digital filter which can be substituted for the averaging and skip logic of Figure l; and Figure 5 is a schematic block diagram of an alternative embodiment of the inventive system incorporating the low pass digital filter arrangement of Figure ~ and a preferred comparator logic network.
Referring to Figure 1, a reflector 15 within a housing 11 having a transparent cover 13 is mounted upon a suitably supported rotatable base 17 having a central aperture therethrough. The reflector 15 is rotated by means of a suitable driving mechanism; for example, the base 17 may be in the form of a ring gear having external teeth meshing with a drive gear 19 which pene-trates into the housing 11 and is driven by an external motor 21. Incident radiation striking the rotating reflector 15 is reflected onto a photodetector 23 within the housing 11. The photodetector 23 is chosen to be sensitive to the anticipated laser wavelength. The electrical analog output of the photo-detector 23 is amplified by an amplifier 25 and received hy an analog-digital converter 279 which samples the analog signal at successive intervals in response to a reflector position sensor 29 whose output is also received by the converter 27. The position sensor 29 may be directly associated with the reflector 15, or, as shown in Figure l, may instead be associated with the drive mechanism for the rotating reflector. It may for example be in the form of a microswitch actuated by one or more depressions or protrusions on the periphery of the drive gear 19, or may be an electromagnetic sensor responsive to one or more peripheral magnets or magnetized regions on the drive gear 19, may be an optical sensor responding to one or more predetermined indexing marks on the periphery of the gear 19, or may be any other suitable device.
The digital output of the analog-digital converter 27 is in the form of a series of digital signals which are fed to a digital integrator 31 and thence via skip logic 33 to a digital filter and comparator logic Ullit 37.
~l~7~i34~L
The output of the digital integrator 31 is fed via skip logic unit 33 to a push-through memory 35 which stores the integrated or averaged digital signal for a complete scan in response to a synchronizing signal obtained from the position sensor 29, so that the sequence of digital values in the push-through memory 35 bears a constant relationship to the integral of the series of ana-log values taken at a predetermined sequence of scanning angles of the rotat-ing reflector 15. The integrated digital output of the integrator 31 for a complete scan remains stored in the push-through memory 35 for a predetermined number of following scans of the rotating reflector 15, and is then updated by a new integrated signal corresponding to the next following complete scan of the rotating reflector 15. For example, the stored signal in the push-through memory 35 may be updated on every tenth scan. During intermediate scans, the integrated digital values stored in the push-through memory 35 are read out, ; recycled via skip logic 33 to be immediately read in again, and at the same time passed to the digital filter and comparator logic unit 37 for comparison with the output of the analog-digital converter 27. In response to the signal from the position sensor 29, the output of the push-through memory 35 is synchronized with the output of the analog-digital converter 27 so that the stored and immediate digital values corresponding to each successive scanning angle of the rotating reflector 15 at which the analog signal output is samp-led, are compared at the same time.
The unit 37 detects discrepancies exceeding a predetermined thres-hold discrepancy between the detected signal ~after conversion to a digital signal) and the stored signal. The threshold level to be exceeded will depend upon expected background noise, but laser signal to averaged noise signal ratios of 100 or better can be anticipated,so that the threshold level can be at least 5 or 10 times expected noise level. The digital filter also distinguishes discrepancies between the stored signal and the immediate signal exceeding the threshold at no more than two (say) successive sampling angles, from those discrepancies exceeding the threshold associated with three or more successive sampling angles. Since a laser beam is expected to be a very narrow angle beam, this digital filter affords spatial discrimination insuring 10~S34~
that only a sharp direct illumination actuates the alarm or registers on the display. For example, if the sampling interval for each sample of digit}zed input information is of the order of 1 mrad, then a laser beam at a distance of more than 100 meters can be expected to appear at the detector in no more than two successive sampling positions of the detector, since a typical laser beam is only of the order of 1 - 2 cm. wide, or up to about 10 cm. with a beam expander. However, if the width of the incident laser beam were expec-ted to be as wide as the difference between two or more sampling angles, the digital filter could be selected to distinguish discrepancies over no more than three (or some higher number) of sampling angles from broader-angle discrepancies. Flare flashes~ the switching on and off of search lights, etc.
and other broad angle illumination will thus not trigger the alarm or be presented on the display.
The scanning rate should be fast enough to permit several succes-sive scans to occur within the anticipated duration of laser radiation. If this is done, "false alarm" signals occupying a relatively narrow angle can be discriminated against by providing an inhibit function in the skip logic.
The inhibit function operates whenever a large discrepancy appears between no more than two (say) successive sampling positions, and prevents updating of the push-through memory by the integrator 31 so that the anomaly con-tinues to be compared wlth the stored steady-state information. The alarm or display may be sent to register only i~ upon two, ~hree or more succes-sive scans, the anomaly continues to register. This will insure that quick flashes are discriminated against and only steady illumination is observed.
By discriminating both in time and in space through the inhibit function and the digital filter respectively, any resulting display of narrow-angle steady-state illumination can be presumed to be the result of an incident laser beam.
If laser illumination at more than one wavelength or band of wave-lengths is anticipated, and if a suitable broad band photodetector catmot be obtained, it is possible to use a multiplicity of photodetectors as illus-trated for example in Figure 2. The input illumination is reflected by the S3~
rotating reflector 15 as previously, but instead of passing only to a single photodetector 23, a portion of the illumination is reflected by sequential dichroic mirrors 41, 43 ~more than two could be provided if necessary) and re-ceived by suitable photodetectors ~5J ~7 each having appropriate differing wavelength sensitivities. Following amplification, the output signals of the three photodetectors 23, 45 and 47 may be added together in an adder 48 for transmission to the analog-digital converter ~not shown in Figure 2); these signals could be processed separately if desired.
Given a suitable display device 39, the scanning angle at which incident laser beam radiation is detected can be viewed and registered. A
separate control signal may be provided for the control of associated radar apparatus, countermeasure deployment apparatus, etc. Ilowever, depending upon the capacity of the push-through memory 35 and upon the optical and mechanical characteristics of the rotating reflector 15 and its associated drive mechan-ism, the angle may initially be determined more imprecisely than desired. If very accurate bearings are required, a narrow angle scanning system substan-tially identical to that of Figure 1 can be connected in tandem with the wide angle scanning detector. The wide angle reflector will be positioned to lead the narrow angle reflector by a small lead angle. If an anomaly is detected on the wide angle reflector, it will be determined within an angle correspond-ing to no more than three (say) successive sampling positions. This deter-mination can be used, through appropriate interlocking logic, to trigger the narrow angle reflector to scan the angle of view corresponding ta the three successive sampling positions of the wide angle reflector, and a separate detection, memory and logic unit 53 can be utilized to determine, within the limits of resolution of the narrow angle apparatus, the precise angle within the three successive scanning angle positions of the wide angle reflector at which the incident radiation is arriving. The precise angle can then be dis~
; played in a display unit 55 and again a control signal can be derived to actuate suitable countermeasure apparatus.
The detection apparatus thus far described determines only the azimuth angle of incident radiation. In many cases it is anticipated that the elevation of the source will not be required. However, a separa~e scanning 7S3~
device could be provided to determine the angle of declination of radiation.
Figure 4 illustrates a ~ow pass digital filter which can be used in conjunction with the system according to the invention in substitution for the averaging function or the skip function or both. The low pass digital filter gives a decreasing weight to every consecutive point, or else to points in con-secutive scans, the latest point ~or scan) being the most important. This smo-oths out fast fluctuations; to this extent the low pass digital filter provides a substitute for the averaging function. Furthermore, for each value corres-ponding to the sampling positions in a scan, the low pass aigital filter adds the corresponding value in the consecutive scans with again a decreasing weight value which tends to smooth out the fluctuations for each value with time.
The block diagram of Figure 4 is for a low pass digital filter de-fined by the formula y M = ~ M (K-l)Y
x,y y~=O X,Y-YI y~l This formula is valid for scan-to-scan averaging and establishes the averaged background level M y at the position 'x' after the data Mx y is obtained from the scan 'y'. (A similar unit and formula could be devised for smoothing at successive locations within a scan.) The diagram of Figure 4 corresponds di-rectly to the implementation of the recursive relation Mx y = K Mx~y + (1 K) x,y-l-The input values for the digi~al filter of Figure ~ are the digital value Mx y obtained from the analog-digital converter at given points during successive scans and the filter time constant K. These are shown as inputs to the system at positions 61 and 63. Time constant K is inverted at position 65 and then multiplied by M y at multiplying stage 67. The inverted time con-stant is also used to derive the function l-l/K at stage 69. This function is multiplied at stage 71 by the averaged background level value M y 1 obtained at position 73 from the previous scan (and thus from push-through memory 35).
The output of multiplier 71 is added by adder 75 to the output of multiplier 67 to obtain a new averaged background level value at position 77 and is also applied as an input to the push-through memory 35.
To give a more explicit description of the filter, consider what happens when M y is entered. The filter has available at position 73 from _ 7 _ push-through memory 35 the averaged background level Mx y 1 from the previous scan y-l. This is multiplied by the factor 1 K in multiplier 71 to which 1 is entered from position 69. The result is then added in the adder 75 to K M y obtained in a similar way. The output 77 of the filter is now M y which is stored in the "PUSH THROUGH MEMORY" replacing M y 1 ready for the next cycle.
Referring to Figure 5 the analog-digital converter 27 supplies its output as an input 61 to the low pass digital filter 62 corresponding essen-tially to that shown in Figure 4 and to comparator unit 79. The interaction of the digital data input and the low pass digital filter 62 with the push-through memory 35 need not be further described; further discussion will be given only to the operation of the comparator 79 and the other output logic.
In the comparator 79 a delayed pulse actuator 81 receives as an input signal the output of the position sensor 29. The adder 83 receives as inputs the output of the delayed pulse actuator 81, the output of the analog-digital converter 27 and the output of the negative of the current digital value at the time stored in the push-through memory 35, which is obtained by a conventional means at position 85. The output of the adder 83 is applied to absolute value converter 87 whose output is in turn applied as one input to the other adder 89. The adder 89 receives as a second input the negative of the threshold level set value applied from input position 93 to sign con-verter 91 and thence to the adder 89. The output of the adder 89 is applied to discriminator 95. If the output is greater than zero, the light emitting diode array 97 (receiving as a position input the output of position sensor 29) is actuated to display the location of the suspected input laser radiation and also alarm 99 are actuated. If the output of the adder 89 is less than zero no output activity is generated.
To follow what happens to the signal using the same nomenclature as used in the description of Figure 4, during any scan y, the digital value Mx y l~ stored in push-through memory 35, is subtracted for each scanning position via positions 85, 83 from the current value M . The absolute x,y value of the difference is then taken and the threshold level is subtracted ~ i~7534~
from this absolute value. If thP absolute value exceeds the threshold level then the discriminator 95 actuates the alarm system and causes a display of the location of the radiation which has triggered the alarm.
. .
The need to detect incident laser radiation is important with the advent of so-called "smart bombs" and other optically guided devices a~tracted towards a target illumina~ed by a laser beam. Early detection and identi-fication of laser illumination is essential to the deployment of counter-measures such as smdke screensl flares, or counter weapons.
The present invention provides a scanning detector for scanning a predetermined field of view, and having sensitivity to laser radiation of the anticipated wavelength or wavelengths. The detector provides an output analog signal which is converted to a digital signal and integrated or averaged over one or more complete scans of the field of view. Each corresponding sampling point is averaged over many scans using an appropriate weighting function to smooth out fluctuations in the trace used for comparison. Another way of achieving this objective is to add N consecutive sampling points and divide the sum by N, this process being carried on continuously while scanning around - the value of N usually being 2 or 3 at most. This process is sometimes called "box convolution" since it implies the product of the digital data by a rectangular function of amplitude 1 and width "~". The integrated digital signal is stored in digital form in a push-through memory and in one embodi-ment retained for a predetermined number of successive scans (say, ten scans).
Por each of the predetermined number of successive scans, the output of the analog-digital converter is compared with the stored integrated digital values in corresponding sequence, and if a discrepancy exceeding some threshold dis-crepancy between the stored integrated digital value and the immediate digital value obtained from ~he analog-digital converter is detected at no more than a small number, say two, consecutive scanning angle positions, a suitable display or alarm is actuated identifying the existence and the angular position of the discrepancy, which may be presumed to be caused by incident laser radiation.
Further refinements of the apparatus include the use of a wide angle reflector and associated detector and logic apparatus and a narrow angle reflector and associated detector and logic apparatus, so that once the angle of ~he input .
~37S341 laser radiation is established coarsely, a more precise identification of the angle can be obtained using the narrow-angle system. The comparator logic may optionally provide an inhibit action to prevent a large anomalous signal from markedly modifying the background reference value at any given point.
This inhibit action prevents the laser beam signal from being integrated along with the other radiation, which would tend to reduce the discrepancy between the output of the analog-digital converter and the stored signal.
In the said embodiment, after the predetermined number of scans, the output of the integrator is once again fed into the push-through memory so as to update the memory without interruption of the repeating scanning and comparison cycle.
The scan-to-scan averaging of the digital signal tends to reduce the effect of noise signals and provide a more reliable steady state stored signal against which the immediate output of the analog-digital converter may be compared. The actuation of the alarm, display, etc. only when a significant discrepancy is noted at no more than two (say) successive points is intended to prevent false alarms resulting, for example, from a large and sudden luminous flux, as from flare burst or the sudden illumination of the field of view by a searchlight, since these la~ter would be expected if sufficiently bright) to produce a discrepancy at more than two successive scanning posi-tions.
In an alternative embodiment, the memory is updated on every scan and the spatial and temporal averaging are achieved by employing a low pass digital filter.
The invention will now be described with reference to the accom-panying drawings, in which:
Figure 1 is a schematic diagram of an embodiment of laser beam detection apparatus according to the invention;
Figure 2 is a schematic diagram of an alternative arrangement of photodetectors for use with laser beam detection apparatus according to the invention;
Figure 3 is a schematic diagram illustrating a refinement of the ~7S3~L
laser beam detection apparatus according to the invention involving the use of a wide angle scanning system in conjunction with a narrow angle scanning system;
Figure 4 is a schematic block diagram illustrating a low pass digital filter which can be substituted for the averaging and skip logic of Figure l; and Figure 5 is a schematic block diagram of an alternative embodiment of the inventive system incorporating the low pass digital filter arrangement of Figure ~ and a preferred comparator logic network.
Referring to Figure 1, a reflector 15 within a housing 11 having a transparent cover 13 is mounted upon a suitably supported rotatable base 17 having a central aperture therethrough. The reflector 15 is rotated by means of a suitable driving mechanism; for example, the base 17 may be in the form of a ring gear having external teeth meshing with a drive gear 19 which pene-trates into the housing 11 and is driven by an external motor 21. Incident radiation striking the rotating reflector 15 is reflected onto a photodetector 23 within the housing 11. The photodetector 23 is chosen to be sensitive to the anticipated laser wavelength. The electrical analog output of the photo-detector 23 is amplified by an amplifier 25 and received hy an analog-digital converter 279 which samples the analog signal at successive intervals in response to a reflector position sensor 29 whose output is also received by the converter 27. The position sensor 29 may be directly associated with the reflector 15, or, as shown in Figure l, may instead be associated with the drive mechanism for the rotating reflector. It may for example be in the form of a microswitch actuated by one or more depressions or protrusions on the periphery of the drive gear 19, or may be an electromagnetic sensor responsive to one or more peripheral magnets or magnetized regions on the drive gear 19, may be an optical sensor responding to one or more predetermined indexing marks on the periphery of the gear 19, or may be any other suitable device.
The digital output of the analog-digital converter 27 is in the form of a series of digital signals which are fed to a digital integrator 31 and thence via skip logic 33 to a digital filter and comparator logic Ullit 37.
~l~7~i34~L
The output of the digital integrator 31 is fed via skip logic unit 33 to a push-through memory 35 which stores the integrated or averaged digital signal for a complete scan in response to a synchronizing signal obtained from the position sensor 29, so that the sequence of digital values in the push-through memory 35 bears a constant relationship to the integral of the series of ana-log values taken at a predetermined sequence of scanning angles of the rotat-ing reflector 15. The integrated digital output of the integrator 31 for a complete scan remains stored in the push-through memory 35 for a predetermined number of following scans of the rotating reflector 15, and is then updated by a new integrated signal corresponding to the next following complete scan of the rotating reflector 15. For example, the stored signal in the push-through memory 35 may be updated on every tenth scan. During intermediate scans, the integrated digital values stored in the push-through memory 35 are read out, ; recycled via skip logic 33 to be immediately read in again, and at the same time passed to the digital filter and comparator logic unit 37 for comparison with the output of the analog-digital converter 27. In response to the signal from the position sensor 29, the output of the push-through memory 35 is synchronized with the output of the analog-digital converter 27 so that the stored and immediate digital values corresponding to each successive scanning angle of the rotating reflector 15 at which the analog signal output is samp-led, are compared at the same time.
The unit 37 detects discrepancies exceeding a predetermined thres-hold discrepancy between the detected signal ~after conversion to a digital signal) and the stored signal. The threshold level to be exceeded will depend upon expected background noise, but laser signal to averaged noise signal ratios of 100 or better can be anticipated,so that the threshold level can be at least 5 or 10 times expected noise level. The digital filter also distinguishes discrepancies between the stored signal and the immediate signal exceeding the threshold at no more than two (say) successive sampling angles, from those discrepancies exceeding the threshold associated with three or more successive sampling angles. Since a laser beam is expected to be a very narrow angle beam, this digital filter affords spatial discrimination insuring 10~S34~
that only a sharp direct illumination actuates the alarm or registers on the display. For example, if the sampling interval for each sample of digit}zed input information is of the order of 1 mrad, then a laser beam at a distance of more than 100 meters can be expected to appear at the detector in no more than two successive sampling positions of the detector, since a typical laser beam is only of the order of 1 - 2 cm. wide, or up to about 10 cm. with a beam expander. However, if the width of the incident laser beam were expec-ted to be as wide as the difference between two or more sampling angles, the digital filter could be selected to distinguish discrepancies over no more than three (or some higher number) of sampling angles from broader-angle discrepancies. Flare flashes~ the switching on and off of search lights, etc.
and other broad angle illumination will thus not trigger the alarm or be presented on the display.
The scanning rate should be fast enough to permit several succes-sive scans to occur within the anticipated duration of laser radiation. If this is done, "false alarm" signals occupying a relatively narrow angle can be discriminated against by providing an inhibit function in the skip logic.
The inhibit function operates whenever a large discrepancy appears between no more than two (say) successive sampling positions, and prevents updating of the push-through memory by the integrator 31 so that the anomaly con-tinues to be compared wlth the stored steady-state information. The alarm or display may be sent to register only i~ upon two, ~hree or more succes-sive scans, the anomaly continues to register. This will insure that quick flashes are discriminated against and only steady illumination is observed.
By discriminating both in time and in space through the inhibit function and the digital filter respectively, any resulting display of narrow-angle steady-state illumination can be presumed to be the result of an incident laser beam.
If laser illumination at more than one wavelength or band of wave-lengths is anticipated, and if a suitable broad band photodetector catmot be obtained, it is possible to use a multiplicity of photodetectors as illus-trated for example in Figure 2. The input illumination is reflected by the S3~
rotating reflector 15 as previously, but instead of passing only to a single photodetector 23, a portion of the illumination is reflected by sequential dichroic mirrors 41, 43 ~more than two could be provided if necessary) and re-ceived by suitable photodetectors ~5J ~7 each having appropriate differing wavelength sensitivities. Following amplification, the output signals of the three photodetectors 23, 45 and 47 may be added together in an adder 48 for transmission to the analog-digital converter ~not shown in Figure 2); these signals could be processed separately if desired.
Given a suitable display device 39, the scanning angle at which incident laser beam radiation is detected can be viewed and registered. A
separate control signal may be provided for the control of associated radar apparatus, countermeasure deployment apparatus, etc. Ilowever, depending upon the capacity of the push-through memory 35 and upon the optical and mechanical characteristics of the rotating reflector 15 and its associated drive mechan-ism, the angle may initially be determined more imprecisely than desired. If very accurate bearings are required, a narrow angle scanning system substan-tially identical to that of Figure 1 can be connected in tandem with the wide angle scanning detector. The wide angle reflector will be positioned to lead the narrow angle reflector by a small lead angle. If an anomaly is detected on the wide angle reflector, it will be determined within an angle correspond-ing to no more than three (say) successive sampling positions. This deter-mination can be used, through appropriate interlocking logic, to trigger the narrow angle reflector to scan the angle of view corresponding ta the three successive sampling positions of the wide angle reflector, and a separate detection, memory and logic unit 53 can be utilized to determine, within the limits of resolution of the narrow angle apparatus, the precise angle within the three successive scanning angle positions of the wide angle reflector at which the incident radiation is arriving. The precise angle can then be dis~
; played in a display unit 55 and again a control signal can be derived to actuate suitable countermeasure apparatus.
The detection apparatus thus far described determines only the azimuth angle of incident radiation. In many cases it is anticipated that the elevation of the source will not be required. However, a separa~e scanning 7S3~
device could be provided to determine the angle of declination of radiation.
Figure 4 illustrates a ~ow pass digital filter which can be used in conjunction with the system according to the invention in substitution for the averaging function or the skip function or both. The low pass digital filter gives a decreasing weight to every consecutive point, or else to points in con-secutive scans, the latest point ~or scan) being the most important. This smo-oths out fast fluctuations; to this extent the low pass digital filter provides a substitute for the averaging function. Furthermore, for each value corres-ponding to the sampling positions in a scan, the low pass aigital filter adds the corresponding value in the consecutive scans with again a decreasing weight value which tends to smooth out the fluctuations for each value with time.
The block diagram of Figure 4 is for a low pass digital filter de-fined by the formula y M = ~ M (K-l)Y
x,y y~=O X,Y-YI y~l This formula is valid for scan-to-scan averaging and establishes the averaged background level M y at the position 'x' after the data Mx y is obtained from the scan 'y'. (A similar unit and formula could be devised for smoothing at successive locations within a scan.) The diagram of Figure 4 corresponds di-rectly to the implementation of the recursive relation Mx y = K Mx~y + (1 K) x,y-l-The input values for the digi~al filter of Figure ~ are the digital value Mx y obtained from the analog-digital converter at given points during successive scans and the filter time constant K. These are shown as inputs to the system at positions 61 and 63. Time constant K is inverted at position 65 and then multiplied by M y at multiplying stage 67. The inverted time con-stant is also used to derive the function l-l/K at stage 69. This function is multiplied at stage 71 by the averaged background level value M y 1 obtained at position 73 from the previous scan (and thus from push-through memory 35).
The output of multiplier 71 is added by adder 75 to the output of multiplier 67 to obtain a new averaged background level value at position 77 and is also applied as an input to the push-through memory 35.
To give a more explicit description of the filter, consider what happens when M y is entered. The filter has available at position 73 from _ 7 _ push-through memory 35 the averaged background level Mx y 1 from the previous scan y-l. This is multiplied by the factor 1 K in multiplier 71 to which 1 is entered from position 69. The result is then added in the adder 75 to K M y obtained in a similar way. The output 77 of the filter is now M y which is stored in the "PUSH THROUGH MEMORY" replacing M y 1 ready for the next cycle.
Referring to Figure 5 the analog-digital converter 27 supplies its output as an input 61 to the low pass digital filter 62 corresponding essen-tially to that shown in Figure 4 and to comparator unit 79. The interaction of the digital data input and the low pass digital filter 62 with the push-through memory 35 need not be further described; further discussion will be given only to the operation of the comparator 79 and the other output logic.
In the comparator 79 a delayed pulse actuator 81 receives as an input signal the output of the position sensor 29. The adder 83 receives as inputs the output of the delayed pulse actuator 81, the output of the analog-digital converter 27 and the output of the negative of the current digital value at the time stored in the push-through memory 35, which is obtained by a conventional means at position 85. The output of the adder 83 is applied to absolute value converter 87 whose output is in turn applied as one input to the other adder 89. The adder 89 receives as a second input the negative of the threshold level set value applied from input position 93 to sign con-verter 91 and thence to the adder 89. The output of the adder 89 is applied to discriminator 95. If the output is greater than zero, the light emitting diode array 97 (receiving as a position input the output of position sensor 29) is actuated to display the location of the suspected input laser radiation and also alarm 99 are actuated. If the output of the adder 89 is less than zero no output activity is generated.
To follow what happens to the signal using the same nomenclature as used in the description of Figure 4, during any scan y, the digital value Mx y l~ stored in push-through memory 35, is subtracted for each scanning position via positions 85, 83 from the current value M . The absolute x,y value of the difference is then taken and the threshold level is subtracted ~ i~7534~
from this absolute value. If thP absolute value exceeds the threshold level then the discriminator 95 actuates the alarm system and causes a display of the location of the radiation which has triggered the alarm.
. .
Claims (7)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Incident laser beam detection apparatus comprising scanning means for scanning a field of view and responding to radiation of a predetermined range of wavelengths to produce an analog signal corresponding to the intensity of said radiation;
analog-digital conversion means for converting the said analog signal to a series of digital signals;
averaging means for deriving from each successive series of said digital signals for successive scans of the field of view a series of averaged digital signals;
a digital memory for storing successive series of averaged digital signals for successive scans of the field of view;
a comparator responsive to the said digital signals and the said averaged digital signals for successive scanning positions whereby an instan-taneous digital value for a given scanning position of a given scan is com-pared with the stored averaged digital value for the same given scanning position of a selected previous scan; and means for signalling the presence of a discrepancy exceeding a predetermined threshold sensed by the comparator between the stored averaged digital value and the instantaneous digital value for any scanning position.
analog-digital conversion means for converting the said analog signal to a series of digital signals;
averaging means for deriving from each successive series of said digital signals for successive scans of the field of view a series of averaged digital signals;
a digital memory for storing successive series of averaged digital signals for successive scans of the field of view;
a comparator responsive to the said digital signals and the said averaged digital signals for successive scanning positions whereby an instan-taneous digital value for a given scanning position of a given scan is com-pared with the stored averaged digital value for the same given scanning position of a selected previous scan; and means for signalling the presence of a discrepancy exceeding a predetermined threshold sensed by the comparator between the stored averaged digital value and the instantaneous digital value for any scanning position.
2. Apparatus as defined in claim 1, wherein said averaging means comprises a low-pass digital filter receiving the output of the analog-digital converter.
3. Apparatus as defined in claim 1, wherein said averaging means comprises a digital integrator.
4. Apparatus as defined in claim 3, including means for refreshing the digital memory by replacing the averaged digital signal stored therein by a further set of averaged digital signals obtained as a result of a later scan after a predetermined number of successive scanning cycles.
5. Apparatus as defined in claim 2, 3 or 4, wherein said averaging means, for any given scanning position, effects averaging over two or three successive scanning positions including said last mentioned given scanning position.
6. Apparatus as defined in claim 2, 3 or 4, additionally comprising a position sensor for sensing the scanning position of the scanning means, and synchronizing means associated with the position sensor for synchronizing the operation of the analog-digital converter with that of the digital memory whereby corresponding scanning position digital values are compared by the comparator.
7. Apparatus as defined in claim 2, 3 or 4, wherein said scanning means comprises a detector for detecting incident electromagnetic radiation of said predetermined range of wavelengths and producing said analog signal, a reflector for capturing incident electromagnetic radiation and directing such radiation to the detector, and means for rotating the reflector through said field of view whereby the radiation emanating from the field of view is scanned by the detector.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA216,949A CA1075341A (en) | 1974-12-27 | 1974-12-27 | Cw-laser illuminator detector |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA216,949A CA1075341A (en) | 1974-12-27 | 1974-12-27 | Cw-laser illuminator detector |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1075341A true CA1075341A (en) | 1980-04-08 |
Family
ID=4101945
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA216,949A Expired CA1075341A (en) | 1974-12-27 | 1974-12-27 | Cw-laser illuminator detector |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1075341A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0269142A1 (en) * | 1986-10-16 | 1988-06-01 | NobelTech Systems Aktiebolag | An apparatus for determining the path of a pulsed light beam |
-
1974
- 1974-12-27 CA CA216,949A patent/CA1075341A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0269142A1 (en) * | 1986-10-16 | 1988-06-01 | NobelTech Systems Aktiebolag | An apparatus for determining the path of a pulsed light beam |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA3017819C (en) | Lidar based 3-d imaging with varying illumination intensity | |
| US5168528A (en) | Differential electronic imaging system | |
| US4949074A (en) | Method of intrusion detection | |
| US4903009A (en) | Intrusion detection device | |
| CA2451782C (en) | Method and device for suppressing electromagnetic background radiation in an image | |
| JPS5451556A (en) | Distance measuring apparatus | |
| EP3433633A1 (en) | Lidar based 3-d imaging with varying pulse repetition | |
| CA2071580C (en) | Very high angular resolution laser beam rider detector | |
| JP2000266540A (en) | Electronic level | |
| JPH02504441A (en) | scanning intrusion detection device | |
| JPH03501181A (en) | Wide area intrusion detection method | |
| EP0394888A3 (en) | Object detection apparatus of the photoelectric reflection type | |
| EP0630510A1 (en) | Infrared intrusion sensor | |
| JPH09501253A (en) | Infrared type intrusion detector | |
| IL227118A (en) | Method for the detection of pulsed laser radiation and imaging laser alerter | |
| CA1327842C (en) | Light pulse detecting system with highly reduced false alarm rate, usable for laser detection | |
| CA1075341A (en) | Cw-laser illuminator detector | |
| WO1998021617A1 (en) | Laser system for the detection of optoelectronic objects (oeo) | |
| US6433330B1 (en) | Sun optical limitation illumination detector (SOLID) | |
| JP3263311B2 (en) | Object detection device, object detection method, and object monitoring system | |
| US4385817A (en) | Focusing detecting device | |
| EP3011256B1 (en) | Low power laser sensing | |
| CA1086399A (en) | Intrusion alarm | |
| JPH01277724A (en) | Target detector | |
| JP2005315585A (en) | Optical sensing system |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |