CA1069725A - Clear air turbulence detector - Google Patents
Clear air turbulence detectorInfo
- Publication number
- CA1069725A CA1069725A CA278,630A CA278630A CA1069725A CA 1069725 A CA1069725 A CA 1069725A CA 278630 A CA278630 A CA 278630A CA 1069725 A CA1069725 A CA 1069725A
- Authority
- CA
- Canada
- Prior art keywords
- clear air
- air turbulence
- image dissector
- light
- laser
- 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
- 238000001228 spectrum Methods 0.000 claims abstract description 35
- 238000000034 method Methods 0.000 claims abstract description 20
- 238000001514 detection method Methods 0.000 claims abstract description 13
- 230000004044 response Effects 0.000 claims abstract description 4
- 230000035945 sensitivity Effects 0.000 claims abstract description 3
- 238000012545 processing Methods 0.000 claims description 6
- 230000005540 biological transmission Effects 0.000 claims description 2
- 238000004611 spectroscopical analysis Methods 0.000 claims 2
- 238000004458 analytical method Methods 0.000 abstract description 5
- 230000002596 correlated effect Effects 0.000 abstract 1
- 239000000443 aerosol Substances 0.000 description 6
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- 239000012298 atmosphere Substances 0.000 description 3
- 239000007789 gas Substances 0.000 description 3
- 238000012986 modification Methods 0.000 description 3
- 230000004048 modification Effects 0.000 description 3
- 238000010183 spectrum analysis Methods 0.000 description 3
- 239000003990 capacitor Substances 0.000 description 2
- 230000001427 coherent effect Effects 0.000 description 2
- 239000000470 constituent Substances 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 239000013618 particulate matter Substances 0.000 description 2
- 238000012935 Averaging Methods 0.000 description 1
- 230000003213 activating effect Effects 0.000 description 1
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- ZWIBGKZDAWNIFC-UHFFFAOYSA-N disuccinimidyl suberate Chemical compound O=C1CCC(=O)N1OC(=O)CCCCCCC(=O)ON1C(=O)CCC1=O ZWIBGKZDAWNIFC-UHFFFAOYSA-N 0.000 description 1
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- 239000002245 particle Substances 0.000 description 1
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- 229910001750 ruby Inorganic materials 0.000 description 1
- 238000010206 sensitivity analysis Methods 0.000 description 1
- 230000001960 triggered effect Effects 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P5/00—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft
- G01P5/26—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft by measuring the direct influence of the streaming fluid on the properties of a detecting optical wave
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/12—Generating the spectrum; Monochromators
- G01J3/26—Generating the spectrum; Monochromators using multiple reflection, e.g. Fabry-Perot interferometer, variable interference filters
-
- 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
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/95—Lidar systems specially adapted for specific applications for meteorological use
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01W—METEOROLOGY
- G01W1/00—Meteorology
- G01W2001/003—Clear air turbulence detection or forecasting, e.g. for aircrafts
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A90/00—Technologies having an indirect contribution to adaptation to climate change
- Y02A90/10—Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation
Abstract
Abstract of the Disclosure Method and apparatus is described for remote detection of location and intensity of clear air turbulence through the collection and analyses of back scattered laser light from a region where clear air turbulence may exist. The interference pattern of the collected light is determined through an ultra high resolution spectroscope and is analyzed with an image dissector. Information from the image dissector relating to the spectrum of back scattered light is correlated with the standard spectrum representing absence of clear air turbulence at the distance in question. By providing apparatus having high sen-sitivity and fast response, a device suitable for use in aircraft to detect clear air turbulence is provided.
Description
~0697Z5 BACKGROUND OF THE INVENTION
- This invention relates to a method and apparatus for detecting distance, direction and intensity of clear air turbulence. More partic-ularly, the invention relates to the use of laser beams, and the analy-sis of back scattered light from a region where clear air turbulence may exist, to detect the location and intensity of clear air turbulence.
It is known to use coherent, quasi-monochromatic light sources, such as those obtained from laser devices, to perform a spectral analysis of light scattered by atmospheric constituents. In letters to the Editor appearing in "Nature", Volume 203, page 508 (August 1, 1964) and Volume 203, page 1274 - 1275 (September 19, 1964), R.T.H. Collis and Myron G.H. Ligda describe the use of experimental pulsed ruby lasers to obtain back scattering from "clear" atmosphere.
They noted variations in signal intensity of the back scattered light, those variations fluctuating with time, and attributed the variations to variations in the particulate matter content, or aerosols, in the at-mosphere. Their observations were said to demonstrate the ability of laser "radar" (lidar) to detect discontinuities in the "clear" atmos-phere remotely by reference to back scattering from particulate matter, although they admitted that considerable technological development was necessary as well as a better understanding of the nature of clear air ~ -turbulence for practical application of their observations.
Subsequently, an article entitled "Frequency Spectrum of Laser Echoes from Atmospheric Constituents and Determination of the Aerosol Content of Air", by G. Fiocco and J.B. DeWolf, published in "Journal of the Atmospheric Sciences", Volume 25, May, 1968, at page 488, described laboratory experiments in which the spectrum of scattered radiation from lasers was analyzed in order to provide a measurement of the aerosol component of the air (at page 492 et. seq.). In the described experiments, scattered light from a laser .~
.. ~
iO6972~
, beam was observed with a receiving telescope located a particular distance and direction from a scattering volume. The collected light was collimated and then passed through a pressure-scanned Fabry-Perot interferometer. The free spectral range of the inter- -ferometer was determined to be 0.20~ at 6328A. A photo-multiplier was used to pick up the light passed by the interferometer and am-plify the same. The amplified photoelectron pulses were counted and printed. To compare the radiated and scattered spectra of the laser beam, a rotating wheel with portions of the surface cut away was placed at the center of the scattering volume so that the tele-scope was presented at alternate count intervals with scattering from the medium and scattering from the flat wheel surface. The spectra of light scattered from air containing natural aerosol par-ticles and air containing artificially produced dense fog were examined in this manner. Compared to the spectrum of light scatter-ed from the wheel, the spectrum of light scattered from the air and fog was shown to be broadened. This broadening effect was attribut-ed to a frequency shift which depends on the Doppler effect result-ing from the random motion of the scatterers having zero average displacement. It was suggested by the authors that by observing the width of the aerosol spectral peak, an indication of the exis-tence of clear air turbulence could be~obtained, although no fur-ther explanation of the basis of this thinking or the apparatus or method required to so analyze the width of the aerosol spectral peak to determine the existence of clear air turbulence was provid-ed. Indeed, an article by A.L. Cole, J.A. Jenney and G.M. McKee, DRB/DSIS, Accession No. 69-00549, dated January 16, 1969 indicates that clear air turbulence detection with lasers was beyond the state of the art as of August, 1968.
Finally, in a paper entitled "Normal Brillouin Scatter-ing in Compressed Gases", Journal of the Optical Society of America, Volume 56, No. 10, pages 1403-1405, October, 1966, the authors dis-, 106g7Z5 cuss Rayleigh scattering and Brillouin shifting of laser beam light from individual molecules of gases, producing a Doppler-broadened spectrum line. By passing light scattered at right angles to a laser beam illuminating the gas in a high pressure cell through a pressure-scanned Fabry-Perot interferometer and measuring the spectrum photoelectrically, a well-resolved Rayleigh-Brillouin triplet was obtained, each of the three components of the scattered light having the same half-width as the spectrum of the laser.
Collis and :Ligda in their letter in "Nature", VolUme 203, page 508, suggested that the intensity of the back scattered light showed discontinuities fluctuating with time. This suggests a time variation of the intensity of such back scattered light from a given volume in the atmosphere at a given distance, which means detection of clear air turbulence should be possible if the inten-sity distribution of the side bands of the back scattered laser light is examined and compared to the side bands of a normal atmos-pheric back scattered laser light. The side bands, generated by the Rayleigh-Brillouin scattering process, would have a significantly different intensity distribution produced by clear air turbulence. ~ -Heretofore, there has not been a device to permit remote detection of clear air turbulence used in association with aircraft~
It is an object of the present invention to provide apparatus and a method to permit remote detection of the location and intensity of clear air turbulence through analysis of the spectrum of scattered laser light. It is a further object of the invention to provide apparatus of a small enough size and fast enough response to be used in aircraft for detection of clear air turbulence.
SUMMARY OF THE INVENTION
In accordance with the invention applicant has provided a method for detecting clear air turbulence which comprises project-ing a pulsed laser beam in a volume where clear air turbulence may exist from which back scattered light is collected. The distance of '- 106g72~
the volume is determined by the time of observation after transmission of a specific laser pulse, similar to the manner in which a radar is used to determine the distance to an aircraft. The interference pattern of the collected light is analyzed to determine its spectrum and the spectrum of the collected light is compared with that of the known standard spectrum of light observed in the absence of clear air turb-ulence. Either Rayleigh-Brillouin scattering or Mie scattering assoc-iated with clear air turbulence may be so analyzed.
To detect the location and intensity of clear air turbulence using this method, a high peak power pulsed laser source such as a Nd :YAIG is used to direct a laser beam at a volume where clear air turbulence may exist. An ultra high resolution spectroscope preferably a Fabry-Perot interferomenter which creates a circularly symmetric interference pattern, is used to pick up and analyze the back scattered light from the laser and thereby provide an interference pattern. An image dissector, associated with the spectroscope, receives and dissects the interference pattern from the spectroscope. A programmer unit pro-vides a gating pulse to the image dissector so that the intensity of back scatter from a volume of air at a known distance is intermittently analyzed at the image dissector. This gating pulse is temporallY
coordinated with the laser pulse, to select a suitable distance of ob-servation. The image dissector is preferably a photomultiplier con-trolled by the gating pulse generated in the programmer unit. A
correlation computer receives the output from the image dissector and correlates the information of this output with the standard spectrum representing an absence of clear air turbulence. Indicator means are activated by the correlation computer when a significant departure from the standard spectrum exists, the indicator means showing the degree of departure from a standard spectrum.
- ` 1065~'725 -- BRIEF DESCRIPTION OF THE DRAWINGS
Other ojbects and advantages of the invention will become apparent upon reading the following detailed description and upon referring to the drawing in which:
Figure 1 is a schematic illustration of a clear air turbulence detection system according to the present invention;
Figure 2 is an illustration of a Fabry-Perot interferometer pattern of a type which might be obtained using apparatus according to the present invention;
Figure 3 is a graph showing the radial intensity distribution of the pattern according to Figure 2;
Figure 4 is a schematic side cross-section view of an image dissector tube;
Figure 5 is an analogue display of the observed spectrum using a device according to the present invention;
Figure 6 is a schematic view of digital data processing means according to the present invention.
While the invention will be described in connection with a preferred embodiment and procedure, it will be understood that it is not intended to limit the invention to that embodiment or procedure. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
Turning first to Figure 1, there is shown a pulsed laser 10 used as~light source. Laser 10 is a high peak power pulsed laser triggered by programmer unit 12. As previously indicated, a suitable laser source is a Nd3 :YAIG laser which can provide a high peak pulse power and appropriate pulse repitition rate suitable for application in a present invention (for background information regarding this type of laser, see the paper of Joseph E. Geusic et al, Coherent Optical Sources for Communications , Proc. IEEE, Volume 58, 697;~5 page 1434, October, 1970).
Back scattered light from volume of air 13 is directed into the collecting telescope 14 by mirror 16. Narrow band filter 18 blocks out ambient light.
Light from telescope 14 and narrow band filter 18 is passed through a Fabry-Perot interferometer 20 which creates a circular symmetric interference pattern, as shown in Figures 2 and 3. This pattern is projected onto image dissector 24 through lens 23. Image dissector 24 is prefereably a photomultiplier controlled by the programmer unit 12 by a gating pulse so that the intensity of back scatter from a volume of air at a known distance is inter-mittently analyzed. This permits determination of the distance of the volume of air for determining the location of the clear air turbulence. As an image disector, a photomultiplier tube comprised of a photocathode, "Channeltron" (trademark) electron multiplier array and multiple anode structure in the form of concentric rings is used to provide the necessary sensitivity and spectrum analysis capability by parallel signal processing. The design of the photomultiplier is based on the existing technology developed by Bendix Research Laboratories of the Bendix Corp., Electro-Optics Division (c.f. P.J. Korwek, "Circuitry and Operation of the Bendix Photon Counting Tube Model BX754, Technical Applications Note 6802, Bendix Research Laboratories, Southfield, Michigan, June 1968; W.G. Wolber, "The Choice of a Detector for an Airborne Laser Rangefinder", Technical Applications Note 6801, Bendix Research Laboratories, Southfield, Michigan, March 1968; "Bendix Chevron Microchannel Plate", data sheet;
Bendix Research Laboratories, Southfield, Michigan; "Model 6025, 6040, 6075 Channeltron Electron Multiplier Array", data sheet, Bendix Research Laboratories, Southfield, Michigan). Figure 4 shows a schematic diagram of the image dissector photomultiplier tube having concentric ring anodes 30, electron multiplier array ~ ~0697Zs 32, andphotorathode 34 enclosed within glass envelope 36. The concen-tric rings of the anode have sufficiently fine widths to resolve the Fabry-Perot interferometer concentric ring pattern when it is imposed upon the photocathode. Consequently, many rings are required to obtain a high resolution. The axis of the interference pattern and concentric anode ring structure must be coincident for proper operation of the spectrometer, otherwise loss of resolution will be experienced. The construction of the spectrometer therefore should be mechanically rigid and the image disector and Fabry-Perot interferometer must be fabricated into an integral unit. The conventional spectrum scanning technique of the Fabry-Perot is to make the interferometer ring pattern expand and contract by varying the interferometer etalon spacing (c.f.
"New Tropel Model 240", data sheet Tropel Inc., Fairport, N.Y.). A
pinhole blocks out all except a small portion of the optical signal power which is detected by a suitable photodetector. In such a serial scanning technique, most of the optical signal power is wasted because the pinhole blocks out most of the power. The invention presented here utilizesall of the optical signal power because the entire interference pattern is detected simultaneously and the resultant electrical signal is presented as an output at the multiple anodes in parallel at the same time. We can therefore expect shorter time requirements for spectrum analysis in comparison to the conventional serial scanning method by processing the data in parallel with suitable electronic computers such as microprocessors which employ digital methods.
The programmer provides appropriately timed trigger, gating and reference clock pulses to the laser, image dissector photomultiplier and correlation computer respectively. The timing relation of the trigger and gating pulses are such that the back scattered light from the desired volume of air at a given distance is detected. The reference clock pulses are used by the correlation computer to govern the rate of analysis for the detection of clear air turbulence.
---` 10~'7Z5 The image information from image dissector 24 is fed into a correlation computer 40 and comparea with input 42 of a standard spectrum which represents the absence of clear air turbulence.
The correlation computer may be adapted to detect either Mie scattering, which has a characteristic spectrum assoeiated with clear air turbulence, or the change in the Rayleigh-Brillouin scattered light spectrum associated with clear air turbulence. In Figure 2, a typical circularly symmetric Rayleigh-Brillouin spectrum from a Fabry-Perot interferometer is shown, with Brillouin doublets 52 beside a central Rayleigh peak 54.
As an example, a simple use of the image disector output as a clear air turbulence detector is described in Fig. 5. The outputs 56 from the multiple anodes 30 are connected to capacitors 58 which store the signal (electronic charge) until it is sampled by the eleetronic seanner 60 whieh may be a eommereial analogue multiplexer unit (e.f. for example "16 ehannel Analog Multiplexer Model MM16" (trade mark), data sheet, Datel Systems Inc., Canton, Mass.). Since the capacitors store the signal until sampled, full use of the received information is made. The serialized information which reproduces the radial intensity distribution of the inter-ferometer is displayed on a cathode ray tube 62 (CRT) an~- presented to an operator. With aetual flight experienee, the operator will learn to recognize dynamic variations in the displayed spectrum that represent the presence of elear air turbulenee. In effeet, the operator performs the function of the correlation computer ; which is programmed to reeognize signifieant patterns. The use of some eleetronie signal proeessingin between the seanner and the CRT
is not excluded from the scope of the present invention. The signal to noise ratio can be improved and the detection of clear air - turbulence enhanced by suitably eombining by eleetronic means the repeated groups of speetral line shapes, typical of Fabry-Perot ~0~9~7~5 interferometers, to obtain a single ayexaged group.
As a second example, Fig. 6 shows a block diagram of a digital data processing system for the correlation computer 40. - -Due to the nature of the photo-electric effect, the signal arriving at the anode is a series of pulses and the pulse rate is proportional to the light intensity. By connecting the output from the anodes to electronic pulse counters 70, we accomplish "photon-counting" and digitize the spectroscopic information at 71. The digital information is now readily sub-jected to analysis in a digital computer 72. Connection to the digital computer can be accomplished through an electronic scanner 74 which may be a commercial unit such as a high speed serializer (c.f. "High Speed Data Serializer MC2335" data sheets, Micro Consultants Ltd., England). Various programs can be provided to the computer at 76 to enhance the detection of clear air turbulence through indicator 77. The standard spectrum data for reference comparison is also provided through the program input 76. Averaging of the repeated groups of spectral lines, typical of Fabry-Perot interferometers, into a single group at 78 will improve the signal to noise ratio and enhance the detection of clear air turbulence. As a further improvement, an electronic shutter shown as an optical element 80 in Fig. 1 can be used to improve the signal to noise ratio of the spectrometer. By briefly closing the electronic shutterin between the brief times of observation of the backscattered light in order to block off the ambient light, and by activating the image dissector photo-multiplier at the same time with a gating pulse identical to that used for observing the backscattered light, we obtain digital information on the inherent noise generated within the spectrometer.
This digital data can then be subtracted from the following digital data on the backscattered light and thereby improve the signal to noise ratio. The procedure is in effect equivalent to phase _ 9 _ ~ ~069'7ZS
sensitive detecti~n, a method ~hich improves detectability of a signal manyfold. If the digital computer has parallel com putational capability, the electronic scanner can be bypassed and a fully parallel processing system constructed. Of course the software (program) can be changed to optimize the possibility of detecting clear air turbulence.
Thus, using apparatus of the type described with ultra high resolution spectra being achieved through the Fabry-Perot interferometer and pattern recognition by a correlation computer, the variation from normal of the Rayleigh-Brillouin or the Mie scattered light spectrum associated with clear air turbulence is readily determined. Any significant departure from the standard spectrum causes indica~or to be activated and provide approp-riate readings for clear air turbulence location and intensity.
Thus it is apparent there has been provided in accor-dance with the invention, a method and apparatus which will permit remote detection of clear air turbulence that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments there-2Q of, it is evident that many alternatives, modifications and varia-tions will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims. ~-.- - 10 -
- This invention relates to a method and apparatus for detecting distance, direction and intensity of clear air turbulence. More partic-ularly, the invention relates to the use of laser beams, and the analy-sis of back scattered light from a region where clear air turbulence may exist, to detect the location and intensity of clear air turbulence.
It is known to use coherent, quasi-monochromatic light sources, such as those obtained from laser devices, to perform a spectral analysis of light scattered by atmospheric constituents. In letters to the Editor appearing in "Nature", Volume 203, page 508 (August 1, 1964) and Volume 203, page 1274 - 1275 (September 19, 1964), R.T.H. Collis and Myron G.H. Ligda describe the use of experimental pulsed ruby lasers to obtain back scattering from "clear" atmosphere.
They noted variations in signal intensity of the back scattered light, those variations fluctuating with time, and attributed the variations to variations in the particulate matter content, or aerosols, in the at-mosphere. Their observations were said to demonstrate the ability of laser "radar" (lidar) to detect discontinuities in the "clear" atmos-phere remotely by reference to back scattering from particulate matter, although they admitted that considerable technological development was necessary as well as a better understanding of the nature of clear air ~ -turbulence for practical application of their observations.
Subsequently, an article entitled "Frequency Spectrum of Laser Echoes from Atmospheric Constituents and Determination of the Aerosol Content of Air", by G. Fiocco and J.B. DeWolf, published in "Journal of the Atmospheric Sciences", Volume 25, May, 1968, at page 488, described laboratory experiments in which the spectrum of scattered radiation from lasers was analyzed in order to provide a measurement of the aerosol component of the air (at page 492 et. seq.). In the described experiments, scattered light from a laser .~
.. ~
iO6972~
, beam was observed with a receiving telescope located a particular distance and direction from a scattering volume. The collected light was collimated and then passed through a pressure-scanned Fabry-Perot interferometer. The free spectral range of the inter- -ferometer was determined to be 0.20~ at 6328A. A photo-multiplier was used to pick up the light passed by the interferometer and am-plify the same. The amplified photoelectron pulses were counted and printed. To compare the radiated and scattered spectra of the laser beam, a rotating wheel with portions of the surface cut away was placed at the center of the scattering volume so that the tele-scope was presented at alternate count intervals with scattering from the medium and scattering from the flat wheel surface. The spectra of light scattered from air containing natural aerosol par-ticles and air containing artificially produced dense fog were examined in this manner. Compared to the spectrum of light scatter-ed from the wheel, the spectrum of light scattered from the air and fog was shown to be broadened. This broadening effect was attribut-ed to a frequency shift which depends on the Doppler effect result-ing from the random motion of the scatterers having zero average displacement. It was suggested by the authors that by observing the width of the aerosol spectral peak, an indication of the exis-tence of clear air turbulence could be~obtained, although no fur-ther explanation of the basis of this thinking or the apparatus or method required to so analyze the width of the aerosol spectral peak to determine the existence of clear air turbulence was provid-ed. Indeed, an article by A.L. Cole, J.A. Jenney and G.M. McKee, DRB/DSIS, Accession No. 69-00549, dated January 16, 1969 indicates that clear air turbulence detection with lasers was beyond the state of the art as of August, 1968.
Finally, in a paper entitled "Normal Brillouin Scatter-ing in Compressed Gases", Journal of the Optical Society of America, Volume 56, No. 10, pages 1403-1405, October, 1966, the authors dis-, 106g7Z5 cuss Rayleigh scattering and Brillouin shifting of laser beam light from individual molecules of gases, producing a Doppler-broadened spectrum line. By passing light scattered at right angles to a laser beam illuminating the gas in a high pressure cell through a pressure-scanned Fabry-Perot interferometer and measuring the spectrum photoelectrically, a well-resolved Rayleigh-Brillouin triplet was obtained, each of the three components of the scattered light having the same half-width as the spectrum of the laser.
Collis and :Ligda in their letter in "Nature", VolUme 203, page 508, suggested that the intensity of the back scattered light showed discontinuities fluctuating with time. This suggests a time variation of the intensity of such back scattered light from a given volume in the atmosphere at a given distance, which means detection of clear air turbulence should be possible if the inten-sity distribution of the side bands of the back scattered laser light is examined and compared to the side bands of a normal atmos-pheric back scattered laser light. The side bands, generated by the Rayleigh-Brillouin scattering process, would have a significantly different intensity distribution produced by clear air turbulence. ~ -Heretofore, there has not been a device to permit remote detection of clear air turbulence used in association with aircraft~
It is an object of the present invention to provide apparatus and a method to permit remote detection of the location and intensity of clear air turbulence through analysis of the spectrum of scattered laser light. It is a further object of the invention to provide apparatus of a small enough size and fast enough response to be used in aircraft for detection of clear air turbulence.
SUMMARY OF THE INVENTION
In accordance with the invention applicant has provided a method for detecting clear air turbulence which comprises project-ing a pulsed laser beam in a volume where clear air turbulence may exist from which back scattered light is collected. The distance of '- 106g72~
the volume is determined by the time of observation after transmission of a specific laser pulse, similar to the manner in which a radar is used to determine the distance to an aircraft. The interference pattern of the collected light is analyzed to determine its spectrum and the spectrum of the collected light is compared with that of the known standard spectrum of light observed in the absence of clear air turb-ulence. Either Rayleigh-Brillouin scattering or Mie scattering assoc-iated with clear air turbulence may be so analyzed.
To detect the location and intensity of clear air turbulence using this method, a high peak power pulsed laser source such as a Nd :YAIG is used to direct a laser beam at a volume where clear air turbulence may exist. An ultra high resolution spectroscope preferably a Fabry-Perot interferomenter which creates a circularly symmetric interference pattern, is used to pick up and analyze the back scattered light from the laser and thereby provide an interference pattern. An image dissector, associated with the spectroscope, receives and dissects the interference pattern from the spectroscope. A programmer unit pro-vides a gating pulse to the image dissector so that the intensity of back scatter from a volume of air at a known distance is intermittently analyzed at the image dissector. This gating pulse is temporallY
coordinated with the laser pulse, to select a suitable distance of ob-servation. The image dissector is preferably a photomultiplier con-trolled by the gating pulse generated in the programmer unit. A
correlation computer receives the output from the image dissector and correlates the information of this output with the standard spectrum representing an absence of clear air turbulence. Indicator means are activated by the correlation computer when a significant departure from the standard spectrum exists, the indicator means showing the degree of departure from a standard spectrum.
- ` 1065~'725 -- BRIEF DESCRIPTION OF THE DRAWINGS
Other ojbects and advantages of the invention will become apparent upon reading the following detailed description and upon referring to the drawing in which:
Figure 1 is a schematic illustration of a clear air turbulence detection system according to the present invention;
Figure 2 is an illustration of a Fabry-Perot interferometer pattern of a type which might be obtained using apparatus according to the present invention;
Figure 3 is a graph showing the radial intensity distribution of the pattern according to Figure 2;
Figure 4 is a schematic side cross-section view of an image dissector tube;
Figure 5 is an analogue display of the observed spectrum using a device according to the present invention;
Figure 6 is a schematic view of digital data processing means according to the present invention.
While the invention will be described in connection with a preferred embodiment and procedure, it will be understood that it is not intended to limit the invention to that embodiment or procedure. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
Turning first to Figure 1, there is shown a pulsed laser 10 used as~light source. Laser 10 is a high peak power pulsed laser triggered by programmer unit 12. As previously indicated, a suitable laser source is a Nd3 :YAIG laser which can provide a high peak pulse power and appropriate pulse repitition rate suitable for application in a present invention (for background information regarding this type of laser, see the paper of Joseph E. Geusic et al, Coherent Optical Sources for Communications , Proc. IEEE, Volume 58, 697;~5 page 1434, October, 1970).
Back scattered light from volume of air 13 is directed into the collecting telescope 14 by mirror 16. Narrow band filter 18 blocks out ambient light.
Light from telescope 14 and narrow band filter 18 is passed through a Fabry-Perot interferometer 20 which creates a circular symmetric interference pattern, as shown in Figures 2 and 3. This pattern is projected onto image dissector 24 through lens 23. Image dissector 24 is prefereably a photomultiplier controlled by the programmer unit 12 by a gating pulse so that the intensity of back scatter from a volume of air at a known distance is inter-mittently analyzed. This permits determination of the distance of the volume of air for determining the location of the clear air turbulence. As an image disector, a photomultiplier tube comprised of a photocathode, "Channeltron" (trademark) electron multiplier array and multiple anode structure in the form of concentric rings is used to provide the necessary sensitivity and spectrum analysis capability by parallel signal processing. The design of the photomultiplier is based on the existing technology developed by Bendix Research Laboratories of the Bendix Corp., Electro-Optics Division (c.f. P.J. Korwek, "Circuitry and Operation of the Bendix Photon Counting Tube Model BX754, Technical Applications Note 6802, Bendix Research Laboratories, Southfield, Michigan, June 1968; W.G. Wolber, "The Choice of a Detector for an Airborne Laser Rangefinder", Technical Applications Note 6801, Bendix Research Laboratories, Southfield, Michigan, March 1968; "Bendix Chevron Microchannel Plate", data sheet;
Bendix Research Laboratories, Southfield, Michigan; "Model 6025, 6040, 6075 Channeltron Electron Multiplier Array", data sheet, Bendix Research Laboratories, Southfield, Michigan). Figure 4 shows a schematic diagram of the image dissector photomultiplier tube having concentric ring anodes 30, electron multiplier array ~ ~0697Zs 32, andphotorathode 34 enclosed within glass envelope 36. The concen-tric rings of the anode have sufficiently fine widths to resolve the Fabry-Perot interferometer concentric ring pattern when it is imposed upon the photocathode. Consequently, many rings are required to obtain a high resolution. The axis of the interference pattern and concentric anode ring structure must be coincident for proper operation of the spectrometer, otherwise loss of resolution will be experienced. The construction of the spectrometer therefore should be mechanically rigid and the image disector and Fabry-Perot interferometer must be fabricated into an integral unit. The conventional spectrum scanning technique of the Fabry-Perot is to make the interferometer ring pattern expand and contract by varying the interferometer etalon spacing (c.f.
"New Tropel Model 240", data sheet Tropel Inc., Fairport, N.Y.). A
pinhole blocks out all except a small portion of the optical signal power which is detected by a suitable photodetector. In such a serial scanning technique, most of the optical signal power is wasted because the pinhole blocks out most of the power. The invention presented here utilizesall of the optical signal power because the entire interference pattern is detected simultaneously and the resultant electrical signal is presented as an output at the multiple anodes in parallel at the same time. We can therefore expect shorter time requirements for spectrum analysis in comparison to the conventional serial scanning method by processing the data in parallel with suitable electronic computers such as microprocessors which employ digital methods.
The programmer provides appropriately timed trigger, gating and reference clock pulses to the laser, image dissector photomultiplier and correlation computer respectively. The timing relation of the trigger and gating pulses are such that the back scattered light from the desired volume of air at a given distance is detected. The reference clock pulses are used by the correlation computer to govern the rate of analysis for the detection of clear air turbulence.
---` 10~'7Z5 The image information from image dissector 24 is fed into a correlation computer 40 and comparea with input 42 of a standard spectrum which represents the absence of clear air turbulence.
The correlation computer may be adapted to detect either Mie scattering, which has a characteristic spectrum assoeiated with clear air turbulence, or the change in the Rayleigh-Brillouin scattered light spectrum associated with clear air turbulence. In Figure 2, a typical circularly symmetric Rayleigh-Brillouin spectrum from a Fabry-Perot interferometer is shown, with Brillouin doublets 52 beside a central Rayleigh peak 54.
As an example, a simple use of the image disector output as a clear air turbulence detector is described in Fig. 5. The outputs 56 from the multiple anodes 30 are connected to capacitors 58 which store the signal (electronic charge) until it is sampled by the eleetronic seanner 60 whieh may be a eommereial analogue multiplexer unit (e.f. for example "16 ehannel Analog Multiplexer Model MM16" (trade mark), data sheet, Datel Systems Inc., Canton, Mass.). Since the capacitors store the signal until sampled, full use of the received information is made. The serialized information which reproduces the radial intensity distribution of the inter-ferometer is displayed on a cathode ray tube 62 (CRT) an~- presented to an operator. With aetual flight experienee, the operator will learn to recognize dynamic variations in the displayed spectrum that represent the presence of elear air turbulenee. In effeet, the operator performs the function of the correlation computer ; which is programmed to reeognize signifieant patterns. The use of some eleetronie signal proeessingin between the seanner and the CRT
is not excluded from the scope of the present invention. The signal to noise ratio can be improved and the detection of clear air - turbulence enhanced by suitably eombining by eleetronic means the repeated groups of speetral line shapes, typical of Fabry-Perot ~0~9~7~5 interferometers, to obtain a single ayexaged group.
As a second example, Fig. 6 shows a block diagram of a digital data processing system for the correlation computer 40. - -Due to the nature of the photo-electric effect, the signal arriving at the anode is a series of pulses and the pulse rate is proportional to the light intensity. By connecting the output from the anodes to electronic pulse counters 70, we accomplish "photon-counting" and digitize the spectroscopic information at 71. The digital information is now readily sub-jected to analysis in a digital computer 72. Connection to the digital computer can be accomplished through an electronic scanner 74 which may be a commercial unit such as a high speed serializer (c.f. "High Speed Data Serializer MC2335" data sheets, Micro Consultants Ltd., England). Various programs can be provided to the computer at 76 to enhance the detection of clear air turbulence through indicator 77. The standard spectrum data for reference comparison is also provided through the program input 76. Averaging of the repeated groups of spectral lines, typical of Fabry-Perot interferometers, into a single group at 78 will improve the signal to noise ratio and enhance the detection of clear air turbulence. As a further improvement, an electronic shutter shown as an optical element 80 in Fig. 1 can be used to improve the signal to noise ratio of the spectrometer. By briefly closing the electronic shutterin between the brief times of observation of the backscattered light in order to block off the ambient light, and by activating the image dissector photo-multiplier at the same time with a gating pulse identical to that used for observing the backscattered light, we obtain digital information on the inherent noise generated within the spectrometer.
This digital data can then be subtracted from the following digital data on the backscattered light and thereby improve the signal to noise ratio. The procedure is in effect equivalent to phase _ 9 _ ~ ~069'7ZS
sensitive detecti~n, a method ~hich improves detectability of a signal manyfold. If the digital computer has parallel com putational capability, the electronic scanner can be bypassed and a fully parallel processing system constructed. Of course the software (program) can be changed to optimize the possibility of detecting clear air turbulence.
Thus, using apparatus of the type described with ultra high resolution spectra being achieved through the Fabry-Perot interferometer and pattern recognition by a correlation computer, the variation from normal of the Rayleigh-Brillouin or the Mie scattered light spectrum associated with clear air turbulence is readily determined. Any significant departure from the standard spectrum causes indica~or to be activated and provide approp-riate readings for clear air turbulence location and intensity.
Thus it is apparent there has been provided in accor-dance with the invention, a method and apparatus which will permit remote detection of clear air turbulence that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments there-2Q of, it is evident that many alternatives, modifications and varia-tions will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims. ~-.- - 10 -
Claims (12)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for remote detection of location and intensity of clear air turbulence comprising:
(a) a high peak power pulsed laser source to direct a laser beam at a volume where clear air turbulence may exist;
(b) an ultra high resolution spectroscope to pick up and analyze the back scattered light from said laser for its interference pattern;
(c) an image dissector to receive and dissect the interference pattern from said spectroscope;
(d) a programmer unit to provide a gating pulse to said image dissector so that the intensity of back scatter from a volume of air at known distance is intermittently analyzed at the image dissector, the gating pulse being temporally coordinated with the laser pulse to select a suitable distance of observation;
(e) a correlation computer receiving the output from said image dissector to correlate it with the standard spectrum representing an absence of clear air turbulence;
(f) indicator means activated by said correlation computer when a significant departure from the standard spectrum exists, said indicator means showing the degree of departure from the standard spectrum.
(a) a high peak power pulsed laser source to direct a laser beam at a volume where clear air turbulence may exist;
(b) an ultra high resolution spectroscope to pick up and analyze the back scattered light from said laser for its interference pattern;
(c) an image dissector to receive and dissect the interference pattern from said spectroscope;
(d) a programmer unit to provide a gating pulse to said image dissector so that the intensity of back scatter from a volume of air at known distance is intermittently analyzed at the image dissector, the gating pulse being temporally coordinated with the laser pulse to select a suitable distance of observation;
(e) a correlation computer receiving the output from said image dissector to correlate it with the standard spectrum representing an absence of clear air turbulence;
(f) indicator means activated by said correlation computer when a significant departure from the standard spectrum exists, said indicator means showing the degree of departure from the standard spectrum.
2. Apparatus according to claim 1 wherein the laser source is a Nd+3 :YAIG laser.
3. Apparatus according to claim 1, wherein said ultra high resolution spectroscope is a Fabry-Perot interferometer which creates a circularly symmetric interference pattern, and wherein said image dissector is constructed for the spectroscopic analysis of a circularly symmetric interference pattern.
4. Apparatus according to claim 1, wherein said image dissector is a photomultiplier controlled by the gating pulse generated in the programmer unit.
5. Apparatus according to claim 1, wherein the back scattered light from said laser beam is directed to a collecting telescope before it passes to said spectroscope.
6. Apparatus according to claim 1, designed for fast response and use, for example, on jet aircraft, comprising a channel electron multiplier to improve the sensitivity of the image dissector, and having parallel data processing for fast response.
7. Apparatus according to claim 1, wherein photon counting instrumen-tation is associated with the image dissector.
8. Apparatus according to claim 1, wherein the back scattered light from the laser beam is directed to a collecting telescope before passing to the spectroscope, said ultra high resolution spectroscope is a Fabry-Perot inter-ferometer which creates a circularly symmetric interference pattern and wherein said image dissector is constructed for the spectroscopic analysis of circu-larly symmetric interference patterns, and said image dissector is a photo-multiplier controlled by the programmer unit by said gating pulse.
9. A method of detecting clear air turbulence comprising:
(a) projecting a pulsed laser beam in a volume where clear air turbulence may exist;
(b) collecting the back scattered light and creating an inter-ference pattern therefrom;
(c) determining the distance of the volume from which the back scattered light is received by observing this light at a specific time after the transmission of a laser pulse;
(d) analyzing the interference pattern of the collected light to determine its spectrum;
(e) comparing the spectrum of the collected light with that of the known standard spectrum of light observed in the absence of clear air turbulence.
(a) projecting a pulsed laser beam in a volume where clear air turbulence may exist;
(b) collecting the back scattered light and creating an inter-ference pattern therefrom;
(c) determining the distance of the volume from which the back scattered light is received by observing this light at a specific time after the transmission of a laser pulse;
(d) analyzing the interference pattern of the collected light to determine its spectrum;
(e) comparing the spectrum of the collected light with that of the known standard spectrum of light observed in the absence of clear air turbulence.
10. A method according to claim 9, wherein the variation in characteristic Rayleigh-Brillouin scattering associated with clear air turbulence is analyzed to detect clear air turbulence.
11. A method according to claim 9, wherein the variation in characteristic Mie scattering associated with clear air turbu-lence is analyzed to detect clear air turbulence.
12. A method according to claim 9, 10 or 11, wherein a cir-cularly symmetic interfernce pattern is obtained from the back scattered light and analyzed.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA278,630A CA1069725A (en) | 1977-05-17 | 1977-05-17 | Clear air turbulence detector |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA278,630A CA1069725A (en) | 1977-05-17 | 1977-05-17 | Clear air turbulence detector |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1069725A true CA1069725A (en) | 1980-01-15 |
Family
ID=4108686
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA278,630A Expired CA1069725A (en) | 1977-05-17 | 1977-05-17 | Clear air turbulence detector |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1069725A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1991015739A1 (en) * | 1990-04-09 | 1991-10-17 | Commonwealth Scientific And Industrial Research Organisation | A detection system for use in an aircraft |
| FR2663752A1 (en) * | 1990-06-25 | 1991-12-27 | Seso | METEOROLOGICAL PARAMETER MEASURING DEVICE. |
-
1977
- 1977-05-17 CA CA278,630A patent/CA1069725A/en not_active Expired
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1991015739A1 (en) * | 1990-04-09 | 1991-10-17 | Commonwealth Scientific And Industrial Research Organisation | A detection system for use in an aircraft |
| EP0525007B1 (en) * | 1990-04-09 | 1997-09-24 | Commonwealth Scientific And Industrial Research Organisation | A detection system for use in an aircraft |
| FR2663752A1 (en) * | 1990-06-25 | 1991-12-27 | Seso | METEOROLOGICAL PARAMETER MEASURING DEVICE. |
| EP0463914A1 (en) * | 1990-06-25 | 1992-01-02 | Societe Europeenne De Systemes Optiques S.E.S.O. | Devices for measuring meteorological parameters |
| US5214484A (en) * | 1990-06-25 | 1993-05-25 | Societe Europeenne De Systemes Optiques Seso | Apparatus for measuring meteorological parameters |
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