The well-known Langley extrapolation technique produces measurements of atmospheric optical depth (AOD) by collecting direct sun irradiance at multiple zenith angles. One common application of this technique is used by sun photometers such as in NASA’s AErosol Robotic Network (AERONET). This large, spatially distributed network collects time averaging data from across the globe and applying Beer’s Law, produces hourly estimates of AOD. While this technique has produced excellent data, the dependence on direct sun irradiance requires cloudless skies and line-ofsight to the sun. Atmospheric LIDARs, on the other hand, can operate in the presence of clouds and can also produce range-resolved measurements of AOD by applying the same Langley technique. For aerosol LIDARs, this technique requires that the LIDAR be capable of producing high quality waveforms within the atmospheric coherence time and also be capable of taking measurements off zenith. At least two unique angles are required to produce data, although 3+ are recommended. This paper will present an overview of the Langley technique applied with a 1064 nm atmospheric aerosol LIDAR, an overview of the LIDAR hardware and capabilities, sample data collected by the LIDAR, and challenges associated with this technique. It will be shown that while this technique is useful, it requires measurements at all three angles to be made when the atmosphere is reasonably horizontally homogenous. Furthermore, the system optics, alignment, and laser power must be kept constant (keeping the LIDAR’s system constant the same for all measurements) for the data to be useful in a Langley analysis.
The Georgia Tech Research Institute (GTRI) is developing a transportable multi-lidar instrument known as the Integrated Atmospheric Characterization System (IACS). The system will be housed in two shipping containers that will be transported to remote sites on a low-boy trailer. IACS will comprise three lidars: a 355 nm imaging lidar for profiling refractive turbulence, a 355 nm Raman lidar for profiling water vapor, and an aerosol lidar operating at 355 nm as well as 1.064 and 1.627 µm. All of the lidar transmit/receive optics will be on a common mount, pointable at any elevation angle from 10 degrees below horizontal to vertical. The entire system will be computer controlled to facilitate pointing and automatic data acquisition. The purpose of IACS is to characterize optical propagation paths during outdoor tests of electro-optical systems. The tests are anticipated to include ground-to-ground, air-to-ground, and ground-to-air scenarios, so the system must accommodate arbitrary slant paths through the atmosphere, with maximum measurement ranges of 5-10 km. Elevation angle scans will be used to determine atmospheric extinction profiles at the infrared wavelengths, and data from the three wavelengths will be used to determine the aerosol Angstrom coefficient, enabling interpolation of results to other wavelengths in the 355 nm to 1.627 µm region.
Many techniques have been proposed for active optical remote sensing of the strength of atmospheric refractive turbulence. The early techniques, based on degradation of laser beams by turbulence, were susceptible to artifacts. In 1999, we began investigating a new idea, based on differential image motion (DIM), which is inherently immune to artifacts. The new lidar technique can be seen as a combination of two astronomical instruments: a laser guide star transmitter/receiver and a DIM monitor. The technique was successfully demonstrated on a horizontal path, with a hard-target analog of a lidar, and then a true lidar was developed. Several investigations were carried out first, including an analysis to predict the system's performance; new hard-target field measurements in the vertical direction; development of a robust inversion technique; and wave optics simulations. A brassboard lidar was then constructed and operated in the field, along with instruments to acquire truth data. The tests revealed many problems and pitfalls that were all solvable with engineering changes, and the results served to verify the new lidar technique for profiling turbulence. The results also enabled accurate performance predictions for future versions of the lidar. A transportable turbulence lidar system is currently being developed to support field tests of high-energy lasers.
A Forward Looking Interferometer (FLI) sensor has the potential to be used as a means of detecting aviation hazards in
flight. One of these hazards is mountain wave turbulence. The results from a data acquisition activity at the University
of Colorado's Mountain Research Station will be presented here. Hyperspectral datacubes from a Telops Hyper-Cam
are being studied to determine if evidence of a turbulent event can be identified in the data. These data are then being
compared with D&P TurboFT data, which are collected at a much higher time resolution and broader spectrum.
The use of a hyperspectral imaging system for the detection of gases has been investigated, and algorithms have been
developed for various applications. Of particular interest here is the ability to use these algorithms in the detection of
the wake disturbances trailing an aircraft. A dataset of long wave infrared (LWIR) hyperspectral datacubes taken with a
Telops Hyper-Cam at Hartsfield-Jackson International Airport in Atlanta, Georgia is investigated. The methodology
presented here assumes that the aircraft engine exhaust gases will become entrained in wake vortices that develop;
therefore, if the exhaust can be detected upon exiting the engines, it can be followed through subsequent datacubes until
the vortex disturbance is detected. Gases known to exist in aircraft exhaust are modeled, and the Adaptive
Coherence/Cosine Estimator (ACE) is used to search for these gases. Although wake vortices have not been found in
the data, an unknown disturbance following the passage of the aircraft has been discovered.
The Georgia Tech Research Institute (GTRI) is developing a transportable multi-lidar system known as the Integrated
Atmospheric Characterization System (IACS). The system will comprise three lidars: an imaging lidar for profiling
refractive turbulence, a Raman lidar for profiling water vapor, and an aerosol lidar operating at 0.355, 1.064, and 1.625
microns for profiling aerosol extinction. All of the lidar transmit/receive optics will be co-aligned on a common mount,
pointable at any elevation angle from below horizontal to vertical. The entire system will be computer controlled to
facilitate pointing and automatic data acquisition.
The purpose of IACS is to characterize optical propagation paths during outdoor tests of electro-optical systems. The
tests are anticipated to include ground-to-ground, air-to-ground, and ground-to-air scenarios, so the system must
accommodate arbitrary slant paths through the atmosphere with maximum measurement ranges of 5-10 km.
Elevation angle scans will be used to calibrate the atmospheric extinction profiles and data from the three wavelengths
will be used to determine the aerosol Angstrom coefficient, enabling interpolation of results to other wavelengths in the
0.355 to 1.6 micron region. Some of the lidar engineering challenges and solutions are presented here.
The Georgia Tech Research Institute (GTRI) is developing a transportable multi-lidar instrument known as the
Integrated Atmospheric Characterization System (IACS). The system will be housed in standard shipping containers that
will be transported to remote sites by tractor-trailer. IACS will comprise three lidars: a 355 nm imaging lidar for
profiling refractive turbulence, a 355 nm Raman lidar for profiling water vapor, and an aerosol lidar operating at both
1.06 and 1.625 microns. All of the lidar transmit/receive optics will be co-aligned on a common mount, pointable at any
elevation angle from horizontal to vertical. The entire system will be computer controlled to facilitate pointing and
automatic data acquisition. The purpose of IACS is to characterize optical propagation paths during outdoor tests of
electro-optical systems. The tests are anticipated to include ground-to-ground, air-to-ground, and ground-to-air scenarios,
so the system must accommodate arbitrary slant paths through the atmosphere with maximum measurement ranges of
5-10 km. Elevation angle scans will be used to determine atmospheric extinction profiles at the infrared wavelengths, and
data from the three wavelengths will be used to determine the aerosol Angstrom coefficient, enabling interpolation of
results to other wavelengths in the 355 nm to 1.6 micron region. The imaging lidar for profiling refractive turbulence is
based on a previously-reported project known as Range Profiles of Turbulence.
The Georgia Tech Research Institute (GTRI) has developed a turbulence profiling lidar system based on the
differential image motion concept. The lidar measures a profile of mean square wave front tilt differences by
focusing a laser guide star at multiple ranges and then computing the differential image motion variance of guide
star images collected through multiple sub-apertures on the receiver. Direct inversion of the resulting integrals
suffers from high noise gain, so several different techniques were investigated to determine the refractive turbulence
profile. The best inversion method uses a non-linear fitting algorithm to fit a collection of functions to the
differential image motion profile. Each of the fitted functions then maps to a profile of refractive turbulence.
The Forward-Looking Interferometer (FLI) is a new instrument concept for obtaining the measurements required to alert
flight crews to potential weather hazards to safe flight. To meet the needs of the commercial fleet, such a sensor should
address multiple hazards to warrant the costs of development, certification, installation, training, and maintenance. The
FLI concept is based on high-resolution Infrared Fourier Transform Spectrometry (FTS) technologies that have been
developed for ground based, airborne, and satellite remote sensing. The FLI concept is being evaluated for its potential to
address multiple hazards including clear air turbulence (CAT), volcanic ash, wake vortices, low slant range visibility, dry
wind shear, and icing, during all phases of flight. This project has three major elements: further sensitivity studies and
applications of EOF (Empirical Orthogonal Function) Regression; development of algorithms to estimate the hazard
severity; and field measurements to provide an empirical demonstration of the FLI aviation hazard detection and display
capability. These theoretical and experimental studies will lead to a specification for a prototype airborne FLI instrument
for use in future in-flight validation. The research team includes the Georgia Tech Research Institute, Hampton
University, the University Corporation for Atmospheric Research, the Air Force Institute of Technology, and the
University of Wisconsin.
The Georgia Tech Research Institute (GTRI) has developed a new type of LIDAR system for monitoring profiles of
atmospheric refractive turbulence. The system makes real-time measurements by projecting a laser beam to form a laser
beacon at several successive altitudes. The beacon is observed with a multiple-aperture telescope and the motion of the
beacon images from each altitude is characterized as the differential image motion variance. An inversion algorithm has
been developed to retrieve the turbulence profile. GTRI built a brassboard version of the LIDAR instrument and tested
it in October and December 2007, with truth data from scintillometers and from balloon-borne microthermal probes. The
tests resulted in the first time-height diagram of the strength of turbulence ever recorded by a LIDAR.
We are developing a new type of lidar for measuring range profiles of atmospheric optical turbulence. The lidar is based on a measurement concept that is immune to artifacts caused by effects such as vibration or defocus. Four different types of analysis and experiment have all shown that a turbulence lidar that can be built from commercially available components will attain a demanding set of performance goals. The lidar is currently being built, with testing scheduled for summer 2007.
The Georgia Tech Research Institute and the University of New Mexico are developing a compact, rugged, eye safe lidar
(laser radar) to be used specifically for measuring atmospheric extinction in support of the second generation of the
CCD/Transit Instrument (CTI-II). The CTI-II is a 1.8 meter telescope that will be used to accomplish a precise timedomain
imaging photometric and astrometric survey at the McDonald Observatory in West Texas. The supporting lidar
will enable more precise photometry by providing real-time measurements of the amount of atmospheric extinction as
well as its cause, i.e. low-lying aerosols, dust or smoke in the free troposphere, or high cirrus. The goal of this project is
to develop reliable, cost-effective lidar technology for any observatory. The lidar data can be used to efficiently allocate
observatory time and to provide greater integrity for ground-based data. The design is described in this paper along with
estimates of the lidar's performance.
We are developing a new type of lidar for measuring range profiles of atmospheric optical turbulence. The lidar is based on a measurement concept that is immune to artifacts caused by effects such as vibration or defocus. Four different types of analysis and experiment have all shown that a turbulence lidar that can be built from commercially available components will attain a demanding set of performance goals. The lidar is currently being built, with testing scheduled for August 2006.
A new type of lidar is under development for measuring profiles of atmospheric optical turbulence. The principle of operation of the lidar is similar to the astronomical seeing instrument known as the Differential Image Motion Monitor, which views natural stars through two or more spatially separated apertures. A series of images is acquired, and the differential motion of the images (which is a measure of the difference in wavefront tilt between the two apertures) is analyzed statistically. The differential image motion variance is then used to find Fried's parameter r<sub>0</sub>. The lidar operates in a similar manner except that an artificial star is placed at a set of ranges, by focusing the laser beam and range-gating the imager. Sets of images are acquired at each range, and an inversion algorithm is then used to obtain the strength of optical turbulence as a function of range. In order to evaluate the technique in the field and to provide data for inversion algorithm development, a simplified version of the instrument was developed using a CW laser and a hard target carried to various altitudes by a tethered blimp. Truth data were simultaneously acquired with instruments suspended below the blimp. The tests were carried out on a test range at Eglin AFB in November 2004. Some of the resulting data have been analyzed to find the optimum frame rate for ground-based versions of the lidar instrument. Results are consistent with a theory that predicts a maximum rate for statistically independent samples of about 50 per second, for the instrument dimensions and winds speeds of the Eglin tests.
Unattended lidars operating in the mid-visible region for clouds and aerosols are currently deployed at tens of locations in the U.S. and in other countries. The micro-pulse lidar known as MPL is a very successful instrument in terms of numbers deployed, and it is also very sophisticated. In order to operate during daytime, micro-pulse lidars must have an extremely narrow field of view (FOV) and a very small optical bandpass. They are consequently not inexpensive, they tend to suffers from mechanical instability, and they are not field-serviceable when certain types of failures occur. In order to establish the optimum wavelength region for an unattended aerosol lidar, the spectral dependencies of eye safety standards, sky radiance, laser availability, detector performance, atmospheric optical properties, and optical materials are presented. In particular, eye safety standards allow a fluence of 1 J/cm^2 at 1.5 micron, which is 10^7 times the fluence allowed in the mid-visible. Pulse energies on the order of 10 mJ are sufficient to make daytime operation easy and low-cost. A conventional bistatic lidar configuration can then be used with a field of view on the order of milliradians, which eliminates the problem of mechanical instability, and the optical bandpass can be limited with an inexpensive interference filter. In addition, the InGaAs detectors used at 1.5 microns are much less susceptible to optical damage than the Geiger-mode silicon avalanche photodiodes (APDs) used in visible-light lidars.
We investigated an edge response of an extended object in a turbulent atmosphere using imagery data acquired with a double-waveband passive imaging system operating in the visible IR wavebands and an actively illuminated optical sensor. We made two findings. We found that the edge response of an extended object is independent of an exposure time, and an atmospheric tilt does not contribute to the image blur of an extended object. In addition, we found that turbulence-induced image blur for an extended object reduces, not increases, with the imager diameter. Therefore, one can reduce the turbulence-induced image blur for an extended object reduces, not increases, with the imager diameter. Therefore, one can reduce the turbulence-induced blur by increasing aperture diameter of an imaging lens. Both findings contradict the predictions of the conventional imaging theory, suggesting that the conventional theory is not applicable to extended anisoplanatic objects. We provided physical interpretation for the results obtained. In addition, we discussed the mitigation techniques that allow us to reduce both turbulence-induced image blur and edge waviness in optical images.
A dual-band imaging system with variable aperture diameter was constructed and horizontal and vertical atmospheric tilt components were measured on a 1-km near-the-ground horizontal path using discrete and extended visible and JR sources. The spatial and temporal tilt statistics were estimated from the recorded data. Tilt structure function, which also characterizes v ariance of the p ointing error caused by anisoplanatism of t he track point to the aim point in the 1 aser projection system, for small angular separation decreases inverse proportionally to the aperture diameter D1 . The tilt structure function is insensitive to sensor vibration. For a point ahead angle of 0.45 mrad the daytime rms pointing enor caused by tilt anisoplanatism is 12 prad for D= 6 cm, and it is 5 prad for D= 40 cm. The tilt power spectral density agrees well with theory. Jt has the "-2/3" power slope, and the ratio of the two knee frequencies is equal to the inverse ratio of the aperture diameters. The tilt temporal conelation increases with the aperture diameter. The temporal conelation scale is 0.25 sec for D=6 cm and it is 1 sec for D= 40 cm. The C measurements made with discrete JR sources and an optical imager agree well with the measurements made with a scintillometer. The structure function for the lateral (Y) tilt exceeds that for the longitudinal (X) tilt, which is inconsistent with the theoretical prediction. We believe that heat-induced turbulence from the JR sources and a wind component parallel to the optical path degraded the measurements of the vertical tilt. Three mitigation techniques were considered including an increase of the aperture diameter, integration of the image edge over the edge angular extent, and averaging of multiple frames. A multi-frame averaging technique is known to be efficient for mitigation of the effects of turbulence induced scintillation and laser speckle. We found that by averaging multiple image frames one can mitigate the effects of tilt anisoplanatism as well. We also found that the edge response for a multi frame averaged image and a single frame image is the same. This allows us to conclude that a multi frame averaging technique for an extended object does not affect the system angular resolution.
This paper describes a covert means of photographing the interiors of moving vehicles at all times of the day or night through clear or tinted windows. The system is called the Georgia Vehicle Occupancy System (GVOS). It utilizes an infrared (IR) strobe light to illuminate passenger and cargo compartments through side windows or the windshield. A high-speed, digital, infrared camera records the images and is capable of providing clear, stop-motion images of the interiors of vehicles moving at highway speeds. A human screener can view these images, or pattern recognition algorithms can be used to count the number of passengers, identify particular individuals, or screen the types and placement of cargo. Examples of vehicle interior images recorded at highway speeds are shown. For homeland security, such a system can be used to screen vehicles entering military bases or other sensitive sites or it can be implemented on highways for identifying and tracking suspicious individuals.
A laboratory prototype of the NEXLASER unattended aerosol and ozone LIDAR was operated in the Atlanta metropolitan area during the ozone season of 2002. An important aspect of an unattended LIDAR system is the ability to automatically assess system problems and correct for them. This paper details the set of tests that have been conducted to verify system performance, discusses how the tests have been incorporated into NEXLASER's operational software, and shows how aerosol and ozone data collected by the system compares to other measurements.
An experimental validation of the differential image motion (DIM) lidar concept for measuring <i>C<sub>n</sub></i><sup>2</sup> is reviewed. The field validation was performed by building a hard-target analog of the DIM lidar and testing it against a conventional scintillometer on a 300 <i>m</i> horizontal path, throughout a range of turbulence conditions. The test results supported the concept and confirmed that the structure characteristic <i>C<sub>n</sub></i><sup>2</sup> can be accurately measured with this method. A practical method is described for extending the validation technique to vertical profiles of <i>C<sub>n</sub></i><sup>2</sup>.
In the early nineties, James Spinhirne reported a revolutionary lidar concept: the Micro Pulse Lidar (MPL). His approach combined a large diameter, low pulse energy, high pulse repetition frequency transmitter with a narrow field, narrow optical bandwidth receiver to create an eye-safe visible lidar for cloud and aerosol studies. MPL systems present challenges because a significant amount of their operating range is within the overlap region, and the overlap function must be known to correctly interpret the data. Their photon-counting, Geiger-mode avalanche photodiodes are easily destroyed, the data must be corrected for count rate effects, and long averaging times are required for a reasonable signal-to-noise ratio. This paper examines a micro-pulse lidar approach using a receiver with long and short-range channels to avoid overlap corrections; photomultipliers and analog signal processing to avoid count rate effects; a significantly larger collecting aperture to decrease measurement time; a coaxial transmitter to minimize scattered light; and dual polarizations to increase the amount of information gathered on clouds and aerosols. Additional instrumentation to increase the amount of information that can be obtained from the lidar data is also examined.
An experimental validation of the differential image motion (DIM) lidar concept for measuring C<sub><i>n</i></sub><sup>2</sup> is reviewed. The field validation was performed by building a hard-target analog of the DIM lidar and testing it against a conventional scintillometers on a 300 <i>m</i> horizontal path, throughout a range of turbulence conditions. The test results supported the concept and confirmed that the structure characteristic C<sub><i>n</i></sub><sup>2</sup> can be accurately measured with this method. A practical method is described for extending the validation technique to vertical profiles of C<sub><i>n</i></sub><sup>2</sup>.
Accommodating the large dynamic range of lidar signals is always a challenge for optical engineers. Signals from low altitudes are much larger than signals from high altitudes because of their inverse-range-squared behavior, as well as atmospheric absorption and scattering. It is well known that the onset of received lidar signals with range can be controlled by adjusting the crossover of the laser beam into the receiver field of view. However, a careful analysis has shown that, in many lidar applications much of the system's dynamic range can be used up before the range where the crossover is complete. In addition, the analysis shows that defocus is the primary contributor to the geometrical overlap function in determining the range dependence of the signal, and that understanding defocus is necessary for the optical designer to optimize system performance. Examples are given to illustrate the improvements in dynamic range that can be achieved by optimizing the focus of a lidar receiver.
This paper describes the development of a laboratory prototype unattended LIDAR system to measure aerosol profiles to 10km and ozone profiles to 3km. One consideration in an unattended system is a robust, eye-safe optical design that can provide the necessary signal levels and dynamic range to produce profiles at required height, resolution, and accuracy. An equally important consideration is a set of algorithms to compute aerosol and ozone profiles under a range of atmospheric conditions. NEXLASER employs an atmospheric state model to help identify and adapt to the varied conditions it must encounter. The signal-to-noise requirements of the algorithms are demonstrated and related back to hardware design. Performance of the system is demonstrated with simulated atmospheric conditions.
Agnes Scott College and the Georgia Institute of Technology are jointly developing an eye safe atmospheric lidar as a unique hands-on research experience for undergraduates, primarily undergraduate women. Students from both institutions will construct the lidar under the supervision of Agnes Scott and Georgia Tech faculty members. The engineering challenges of making lidar accessible and appropriate for undergraduates are described. The project is intended to serve as a model for other schools.
We have experimentally validated the concept of a differential image motion (DIM) lidar for measuring vertical profiles of the refractive index structure characteristic C by building a hard-target analog of the DIM lidar and testing it against a conventional scintillometer on a 300 m horizontal path, throughout a range of turbulent conditions. The test results supported the concept and confirmed that the structure characteristic C can be accurately measured with this method. Analysis of the effect of scintillation on DIM lidar has been performed. It is shown that the lidar has a scintillation resistant capability. Turbulence and lidar calculations were performed. These calculations confirmed that the DIM lidar is practical.
We have investigated the feasibility of building an innovative optical remote sensing instrument to monitor the vertical profile of the refractive index structure characteristic C<SUB>n</SUB><SUP>2</SUP>. There is currently no active optical remote sensing instrument which is capable of doing this. Calculations have been performed for a system designed specifically to resolve a site survey question at the South Pole, where recent balloon soundings suggest that excellent astronomical seeing conditions could be obtained by mounting telescopes above a thin layer of atmospheric refractive turbulence near the surface. The new sensor considered here is essentially an imaging lidar which measures range- dependent laser beam wander, from which the vertical profile of C<SUB>n</SUB><SUP>2</SUP> can be derived. Calculations based on atmospheric characteristics and preliminary design parameters have been carried out for a practical system based on commercially available components. Design parameters include the choice of operating wavelength, elevation angle, transmitter and receiver diameters, and image scale. The calculations indicate that it is feasible to develop an optical remote sensor for monitoring vertical profiles of C<SUB>n</SUB><SUP>2</SUP> at the South Pole.
A single-ended, non-Doppler, laser wind sensor has been developed to measure path-integrated cross winds by viewing a distant target through a large telescope and observing the motion of a laser speckle pattern. The speed of the moving speckle pattern is determined by a cross-correlation between the signals from two detectors in the telescope focal plane. A prototype laser wind sensor was developed and tested. Results are shown for a laboratory test in a wind tunnel and for an outdoor test in a non-homogeneous wind field. Practical applications of the sensor are discussed, and possible modifications to measure two- or three-dimensional wind fields are described.
There is currently no active, single-ended optical technique for remotely sensing the refractive index structure characteristic C<SUB>n</SUB><SUP>2</SUP> in the turbulent atmosphere. A capability to remotely measure C<SUB>n</SUB><SUP>2</SUP> is needed in several areas. In astronomy, the vertical profile C<SUB>n</SUB><SUP>2</SUP>(h) is required in order to understand and improve the performance of adaptive optics systems, and measurements of C<SUB>n</SUB><SUP>2</SUP> in arbitrary directions above fixed points on the ground would be useful in site surveys. Researchers in basic atmospheric physics need an optical technique because it would be sensitive to temperature fluctuations only and not water vapor fluctuations, unlike the radar and acoustic sounders which are currently used. Understanding laser beam degradation, for communications, power beaming, or weapon system development, also requires a knowledge of C<SUB>n</SUB><SUP>2</SUP>. An optical remote sensor for C<SUB>n</SUB><SUP>2</SUP> could also be used for horizontal, path-averaged measurements, to infer fluxes of heat and momentum over land or sea surfaces. We have recently proposed three different lidar-type techniques for remote sensing of C<SUB>n</SUB><SUP>2</SUP>, based on the following phenomena: enhanced backscattering, residual turbulent scintillation, and image distortion. Each of these techniques is reviewed here in terms of its advantages and disadvantages for various applications, and some considerations for practical systems are also discussed.
A new volume-scanning crossed-path lidar technique for studying atmospheric intermittency in the surface and boundary layers is proposed. This technique provides a spatial resolution of 25 m and a temporal resolution of several seconds, and it permits us to study turbulent structures and processes in the atmospheric surface and boundary layers by performing a volumetric mapping of the optical refractive index structure parameter field. Unlike radar and acoustic sounders, this technique is not affected by humidity fluctuations. Lidar performance estimates show that the proposed technique is practical.
A new approach to the statistical analysis of fluctuating, photon-limited signals that permits us to accumulate and process the lidar returns without averaging of the reflected energy fluctuations is developed. This approach requires recording the photocounts for each pulse in a series of pulses and then determining photocount statistics. Based on the semiclassical theory of photodetection and Mandel's formula, a relationship has been obtained between the time-space cross correlation function and the cross spectrum of the lidar returns and corresponding photocount statistics. It is shown that the relative uncertainties of measuring the cross correlation or the cross spectrum of the lidar returns is determined by the general number of photocounts, but not by their mean value. A fast-scanning lidar system, which is based on a new photocounting analysis approach, is described for 3D wind field mapping in the atmosphere at altitudes up to 5 km. A program for the experimental verification of the new approach is presented.
The theory of a new lidar technique which exploits the residual turbulent scintillation (RTS) effect in order to remotely sense the structure parameter C<SUB>n</SUB><SUP>2</SUP> has recently been reported. In this paper, we describe the design considerations for a demonstration experiment. The primary objective of the demonstration is to collect and analyze a set of data which will demonstrate the RTS effect in the real atmosphere and relate it to C<SUB>n</SUB><SUP>2</SUP>. The second objective is to obtain detailed performance parameters which will permit us to design future RTS systems for routine C<SUB>n</SUB><SUP>2</SUP> profiling. The demonstration will require a transmitter based on a pulsed visible-light laser with a clean beam profile, and a receiver based on a gated imaging system with a digital readout. The receiver aperture must be large in order to collect as much light as possible. Specific design considerations are developed here for a demonstration based on an existing laser used in conjunction with the 1.5-meter telescope at the Starfire Optical Range in Albuquerque, New Mexico, and its associated optics and data recording equipment.
A new remote sensing technique is described here for monitoring the vertical distribution of the intensity of atmospheric turbulence. The technique has several applications, including improving the performance of adaptive optics systems, prediction of laser beam degradation on long distance propagation paths, and site surveys for astronomy. The physical phenomenon underlying this method is caused by phase fluctuations, and as a result, this method does not saturate with increasing refractive index structure characteristic C<SUB>n</SUB><SUP>2</SUP> or with distance, and it is not affected by variations in the inner scale of turbulence. This method permits us to measure both the vertical profile of C<SUB>n</SUB><SUP>2</SUP> and an anizotropy coefficient of the atmospheric turbulence. Estimates of expected measured quantities are obtained, and they show that the proposed technique could be realized with existing optical systems.
We have developed and operated an eyesafe lidar in support of an intensive set of air chemistry measurements in Atlanta, Georgia, which were part of the Southern Oxidants Research Program (SORP) during the summer of 1992. The lidar was used to monitor the thickness of the mixed layer by measuring the vertical distribution of boundary layer aerosols. The lidar system is based on a Raman-shifted Nd:YAG laser source at 1.54 microns wavelength with a pulse energy of 40 mJ and a pulse repetition frequency of 4 Hz. The receiver aperture was 46 mm in diameter and an InGaAs PIN diode was used as the detector. The lidar data was typically averaged over 1000 laser pulses, which required about 4 minutes. The lidar returns were range corrected to yield profiles of signal versus altitude in which the signal is proportional to the atmospheric backscatter coefficient. The profiles showed the vertical extent of boundary layer aerosols, and this was interpreted to find the mixed layer thickness. Data was acquired on nine days in July and August 1992. Measurements were typically made at 15-minute intervals from early morning until midafternoon. Mixed layer thicknesses provided by the lidar have been shown to be consistent with balloon sonde results, and they have proved to be useful in interpreting atmospheric chemistry results.
A new remote sensing technique is proposed for determining the turbulent parameters of the atmosphere using a single-ended lidar system. This technique is based on the enhanced backscattering effect and is insensitive to the scattering volume averaging effect on the intensity fluctuations of the reflected wave and the sounding beam. The corresponding measurements are independent of the turbulent scintillation spectrum and that permits the use of high power pulsed lasers with a relatively low repetition rate for determining the refractive index structure characteristic C<SUB>n</SUB><SUP>2</SUP>, its vertical profile C<SUB>n</SUB><SUP>2</SUP>(h) and inner scale of turbulence l<SUB>o</SUB> in the atmosphere. A theory of the method is developed, and the conditions are obtained for observing the backscattering amplification effect in the atmosphere with a laser beam scattered by aerosol. The signal-to-noise ratio and the sensitivity of the measured quantities to the inner scale of turbulence l<SUB>o</SUB> variations are estimated. A planned demonstration of this technique in the boundary layer of the atmosphere with an eyesafe lidar which has been developed at Georgia Tech is discussed.
A lidar system based on the 100 in. optical collimator at Wright- Patterson Air Force Base has been developed for middle atmosphere studies. The system has been demonstrated by recording Rayleigh backscatter returns from mesospheric air molecules at altitudes up to 90 km. These returns were then used to develop atmospheric density profiles. The design
of the system provided several unique engineering challenges due to the long focal length and size of the collimator used as the receiver telescope. Careful optical engineering in the receiver and an innovative, modular approach led to a design that eliminates potential problems due to defocus, detector nonuniformity, and detector saturation.
SC1242: Atmospheric Lidar Principles and Applications
This course provides a basic working knowledge of atmospheric lidar systems with discussions of the engineering parameters of the transmitter/receiver system and the data system, along with the interactions of the laser beam with the gases and particles that make up the air. The lidar equation, which is a model of received signal versus range, is introduced along with other factors that limit the signal-to-noise ratio, and measurement methodologies and signal inversion techniques are described.
Applications include chem-bio standoff detection, measuring transmittance versus range to support directed energy weapon system development, measuring concentrations of pollutants and greenhouse gases, and profiling temperature, winds, clouds, and aerosols. Example platforms include ground, airborne, and spaceborne systems.