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.
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 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.
The Georgia Tech Research Institute is currently developing a device to demonstrate a hands-free focus technology for
head-mounted night vision sensors. The demonstrator device will integrate a computational imaging technique that
increases depth of field with a digital night vision sensor. The goal of the demonstrator is to serve as a test bed for
evaluating the critical performance/operational parameters necessary for the hands-free focus technology to support
future tactical night vision concepts of operation. This paper will provide an overview of the technology studies and
design analyses that have been performed to date as well as the current state of the demonstrator design.
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.
Although existing night vision equipment provides a significant improvement in target detection in low light conditions,
there are several limitations that limit their effectiveness. Focus is a significant problem for night vision equipment due
to the low f-number optics required to obtain sufficient sensitivity as well as the dynamic nature of night vision
applications, which requires frequent focus adjustments. The Georgia Tech Research Institute has developed a prototype
next-generation night vision device called the Improved Night Vision Demonstrator (INVD) in order to address these
shortfalls. This paper will describe the design of the INVD system as well as an analysis of its performance.
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.
The present study investigates whether photoactivated attachment of cartilage can provide a viable method for more effective repair of damaged articular surfaces by providing an alternative to sutures, barbs, or fibrin glues for initial fixation. Unlike artificial materials, biological constructs do not possess the initial strength for press-fitting and are instead sutured or pinned in place, typically inducing even more tissue trauma. A possible alternative involves the application of a photosensitive material, which is then photoactivated with a laser source to attach the implant and host tissues together in either a photothermal or photochemical process. The photothermal version of this method shows potential, but has been almost entirely applied to vascularized tissues. Cartilage, however, exhibits several characteristics that produce appreciable differences between applying and refining these techniques when compared to previous efforts involving vascularized tissues. Preliminary investigations involving photochemical photosensitizers based on singlet oxygen and electron transfer mechanisms are discussed, and characterization of the photodynamic effects on bulk collagen gels as a simplified model system using FTIR is performed. Previous efforts using photothermal welding applied to cartilaginous tissues are reviewed.
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>.
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.
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.
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.
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.
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.
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.
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.
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 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.