MIT Lincoln Laboratory continues the development of novel high-resolution 3D imaging laser radar technology and sensor systems. The sensor system described in detail here uses a passively Q-switched solid-state frequency-doubled Nd:YAG laser to transmit short laser pulses (~ 700 ps FWHM) at 532 nm wavelength and derive the range to target surface element by measuring the time-of-flight for each pixel. The single photoelectron detection efficiency has been measured to be > 20 % using these Silicon Geiger-mode APDs at room temperature. The pulse out of the detector is used to stop a > 500 MHz digital clock integrated within the focal-plane array. With appropriate optics, the 32x32 array of digital time values represents a 3D spatial image frame of the scene. Successive image frames from the multi-kilohertz pulse repetition rate laser pulses are accumulated into range histograms to provide 3D volume and intensity information. In this paper, we report on a prototype sensor system, which has recently been developed using new 32x32 arrays of Geiger-mode APDs with 0.35 μm CMOS digital timing circuits at each pixel. Here we describe the sensor system development and present recent measurements of laboratory test data and field imagery.

A 3D direct detection imaging laser radar was developed and tested to demonstrate the ability to image objects highly obscured by foliage or camouflage netting. The LADAR provides high-resolution imagery from a narrow pulse-width transmitter, high frequency receiver, and 3D visualization software for near-real-time data display. This work accomplished under DARPA contract number DAAD17-01-D0006/0002.

Two separate data collections using Arete Associates' FLASH lidar are presented. The hardware and the experimental arrangements are discussed. An airborne data collection over military targets in clear and obscuring camouflage environments provided high-resolution three-dimensional images for combat identification purposes. In the second field test, the sensor was suspended from a crane above the ocean surface to acquire FLASH imagery of anti-landing mines and obstacles in the highly turbid surf zone environment over a wide range of surf zone conditions.

We have developed a three-dimensional (3D) imaging ladar focal plane array (FPA) for military and commercial applications. The FPA provides snap-shot, direct detection, high-resolution range and range-sampled intensity imaging capability on a single chip. The FPA is made of a 64x64 element, 100-μm pixel pitch detector array that is directly bump bonded to a matched CMOS based silicon readout integrated circuit (ROIC) with parallel ladar signal processing at each pixel. A room temperature, SWIR InGaAs detector variant for imaging near 1.5-μm wavelengths and a cooled MWIR HgCdTe detector variant for imaging near 3-μm to 5-μm wavelengths have been fabricated. We have built a prototype SWIR FPA, integrated it to a compact, transportable SWIR flash ladar transceiver, and collected initial range images outdoors. We present the measured performances of the detector, the readout, and the image data collected with the focal plane array.

The Army Research Laboratory is researching a focal plane array (FPA) ladar architecture that is applicable for smart munitions, reconnaissance, face recognition, robotic navigation, etc.. Here we report on progress and test results attained over the past year related to the construction of a 32x32 pixel FPA ladar laboratory breadboard. The near-term objective of this effort is to evaluate and demonstrate an FPA ladar using chirped amplitude modulation; knowledge gained will then be used to build a field testable version with a larger array format. The ladar architecture achieves ranging based on a frequency modulation/continuous wave technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (rf) subcarrier that is linearly frequency modulated (chirped amplitude modulation). The diode's output is collected and projected to form an illumination field in the downrange image area. The returned signal is focused onto an array of optoelectronic mixing, metal-semiconductor-metal detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal resulting from the mixing process whose frequency is proportional to the target range. This IF signal is continuously sampled over a period of the rf modulation. Following this, a signal processor calculates the discrete fast Fourier transform over the IF waveform in each pixel to establish the ranges and amplitudes of all scatterers.

The U.S. Army Research Laboratory (ARL) is investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference ultra-high frequency (UHF) chirp with an optically detected, time-delayed UHF chirp scattered from a target. ARL is also investigating the use of metal-semiconductor-metal (MSM) detectors as unique self-mixing detectors, which have the ability to internally detect and down-convert the modulated optical signals. ARL has recently incorporated a 1x32 element linear MSM self-mixing detector array into a prototype FM/cw ladar system and performed a series of characterization and outdoor image collection experiments using this prototype. This paper discusses the basic performance of the prototype system and presents some fundamental measurements as well as ladar imagery taken on the ARL Adelphi campus.

Raytheon has been funded by DARPA to develop an FPA for single shot eyesafe ladar operation. The goal of the program is to develop new high speed imaging arrays to rapidly acquire high resolution, three dimensional images of tactical targets at ranges as long as 7 to 10 kilometers. This would provide precision strike, target identification from rapidly moving platforms, such as air-to-ground seekers, which would enhanced counter-counter measure (CCM) performance and the ability to lock-on after launch. Also a goal is to demonstrate the acquisition of hidden, camouflaged and partially obscured targets. Raytheon's approach consists of using HgCdTe APD arrays which offer unique advantages for high performance eyesafe LADAR sensors. In this paper we present the progress to date on the program. The detector array is coupled with a Readout Integrated Circuit, ROIC, that captures all the information required for accurate range determination. The two components encompass a hybrid imaging array consisting of two IC circuit chips vertically integrated via an array of indium metal “bumps.” The results of the first phase of the program are given, along with the second phase plans.

High-resolution three-dimensional flash ladar system technologies are under development that enables remote identification of vehicles and armament hidden by heavy tree canopies. We have developed a sensor architecture and design that employs a 3D flash ladar receiver to address this mission. The receiver captures 128×128×>30 three-dimensional images for each laser pulse fired. The voxel size of the image is 3”×3”×4” at the target location. A novel signal-processing algorithm has been developed that achieves sub-voxel (sub-inch) range precision estimates of target locations within each pixel. Polarization discrimination is implemented to augment the target-to-foliage contrast. When employed, this method improves the range resolution of the system beyond the classical limit (based on pulsewidth and detection bandwidth). Experiments were performed with a 6 ns long transmitter pulsewidth that demonstrate 1-inch range resolution of a tank-like target that is occluded by foliage and a range precision of 0.3” for unoccluded targets.

We propose and are in the process of progressively implementing an improved architecture for a laser based system to acquire intensity and range images of hard targets in real-time. The system design emphasizes the use of low power laser sources in conjunction with optical preamplification of target return signals to maintain eye safety without incurring the associated performance penalty. The design leverages advanced fiber optic component technology developed for the commercial market to achieve compactness and low power consumption without the high costs and long lead times associated with custom military devices. All important system parameters are designed to be configured in the field, by the user, in software, allowing for adaptive reconfiguration for different missions and targets. Recently we have started our transition from the initial test bed, using a laser in the visible wavelength, into the final system with a 1550nm diode laser. Currently we are able to acquire and display 3-D false-color and gray-scale images, in the laboratory, at moderate frame rates in real-time. Commercial off-the-shelf data acquisition and signal processing software on a desktop computer equipped with commercial acquisition hardware is utilized. Significant improvements in both range and spatial resolution are expected in the near future.

The Jigsaw program, sponsored by the Defense Advanced Research Projects Agency (DARPA), will demonstrate a multi-observation concept to identify obscured combat vehicles that cannot be discerned from a single aspect angle. Three-dimensional (3-D) laser radar (ladar) images of a nearly hidden target are collected from several observation points. Image pieces of the target taken from all the data sets are then assembled to obtain a more complete image that will allow identification by a human observer. In this effort a test bed ladar, constructed by the Night Vision and Electronic Sensors Directorate (NVESD), is used to provide three-dimensional (3-D) images in which the voxels have dimensions of the order of centimeters on each side. Ultimately a UAV born Jigsaw sensor will fly by a suspect location while collecting the multiple images. This paper will describe a simulated flight in which 800 images were taken of two targets obscured by foliage. The vehicle mounted laser radar used for the collection was moved in 0.076 meter steps along a 61 meter path. Survey data were collected for the sensor and target locations as well as for several unobscured fiducial markers near the targets, to aid in image reconstruction. As part of a separate DARPA contractual effort, target returns were extracted from individual images and assembled to form a final 3-D view of the vehicles for human identification. These results are reported separately. The laser radar employs a diode pumped, passively Q-switched, Nd:YAG, micro-chip laser. The transmitted 1.06 micron radiation was produced in six micro-joule pulses that occurred at a rate of 3 kHz and had a duration of 1.2 nanoseconds at the output of the detector electronics. An InGaAs avalanche photodiode/amplifier with a bandwidth of 0.5 GHz was used as the receiver and the signal was digitized at a rate of 2 GS/s. Details of the laser radar and sample imagery will be discussed and presented.

Flash lidars can produce high-resolution data in all three spatial dimensions. In addition, even low repetition rate lasers result in extensive data sets. The challenge presented by these systems is: “How do we reduce the inherently large sets of data to information that is useful to the human operator.” We discuss both sensor specific and general signal-processing tools developed to render 3D lidar data in a fashion that allows man in the loop identification of targets. Data collected during an airborne field test at Redstone Aresenal Test Area Three in Huntsville using Arete Associates FLASH lidar is used to present specific examples.

High-resolution 3D imaging ladar systems can penetrate foliage and camouflage to sample fragments of concealed surfaces of interest. Samples collected while the ladar moves can be integrated into a coherent object shape, provided that sensor poses are known. We detail a system for automatic data-driven registration of ladar frames, consisting of a coarse search stage, a pairwise fine registration stage using an iterated closest points algorithm, and a multi-view registration strategy. We evaluate this approach using simulated and field-collected ladar imagery of foliage-occluded objects. Even after alignment and aggregation, it is often difficult for human observers to find, assess, and recognize objects from a point cloud display. We survey and demonstrate basic display manipulations, surface fitting techniques, and clutter suppression to enhance visual exploitation of 3D imaging ladar data.

In the last years 3D-Vision systems based on the Time-Of-Flight (TOF) principle have gained more importance than Stereo Vision (SV). TOF offers a direct depth-data acquisition, whereas SV involves a great amount of computational power for a comparable 3D data set. Due to the enormous progress in TOF-techniques, nowadays 3D cameras can be manufactured and be used for many practical applications. Hence there is a great demand for new accurate algorithms for 3D object recognition and classification. This paper presents a new strategy and algorithm designed for a fast and solid object classification. A challenging example - accurate classification of a (half-) sphere - demonstrates the performance of the developed algorithm. Finally, the transition from a general model of the system to specific applications such as Intelligent Airbag Control and Robot Assistance in Surgery are introduced. The paper concludes with the current research results in the above mentioned fields.

Current treatments for vascular disorders in retina include laser surgery and laser therapy to eliminate the crucial influence of abnormal blood vessels. Three alternatives are in use: laser photocoagulation surgery, photodynamic therapy, and transpupillary laser thermotherapy. Even sophisticated laser-based devices and arrangements, in practice today, do not solve the problem of adequate dosage of the irradiation, that is an extremely critical matter. As an indirect source of information about the temperature and irradiation dosage, the changes of relief during tissue heating are proposed to be used. Temperature difference between treated and untreated zones achieves 10°C and more. The growth of temperature of the irradiated tissue leads to dimension changes, and thus to the changes of the topography of eye bottom. We studied a known phase difference double-beam interferometric technique modified in such a way, that a reference beam is kept in a reference position without scanning. A second, frequency-shifted beam is scanned along a trajectory crossing treated and untreated zones of the eye bottom. Changes in phase structure are analyzed to have a judgment on thermal processes in retina. Experimentally, eye model was used with a retina movable by means of a micro-screw. Sensitivity of the order of 0.1 μm was got. We recommend further studies on a live tissue, affected by laser radiation.

The Molecular Optical Air Data System (MOADS) is a compact optical instrument that can directly measure aircraft velocity, as well as the density of the air surrounding the aircraft. From these measurements, many air data products can be determined. Successful MOADS operation has been demonstrated in the laboratory using a wind tunnel. Recently, a MOADS prototype was designed and built in order to complete an upcoming flight experiment aboard a Beechcraft King Air 300. This flight program will be a significant milestone for direct detection lidar systems configured as an air data system aboard an aircraft. The background of the technology, ground experimentation summary of results, flight experiment approach, flight prototype design, and flight experiment planning are discussed.

Target recognition is an important issue on the military battlefield. Vibration signatures are robust and independent on target orientation. Hence, they are interesting to use for classification and identification of targets. Various sensors can be used to measure signatures induced by target vibrations, such as laser vibrometry and acoustic sensors. The output from both sensors can be presented as a frequency spectrum that represents the target vibrations. A field trial was conducted where some military targets were investigated. Simultaneous data were taken with two sensors: i) a laser vibrometry system consisting of a 1.55 μm eye-safe coherent laser radar; and ii) an acoustic data logging system with Bruel & Kjaer free field microphone and amplifier as the sensor part. The range to the targets was between 25 and 100 meters. Results from the field trial are reported and a comparison of the data from the sensors is presented.

A number of scientific, defense, and civilian/commercial applications of coherent laser radar require correction and/or deliberate generation of very large offset frequencies between the local oscillator (LO) reference frequency and the pulsed transmitter laser frequency. An example of this system requirement is the need for agile, stable, multi-GHz offset control between master and local oscillator (MO/LO) sources in space-based lidar applications, where platform motion must be compensated for in order to perform efficient heterodyne detection of much smaller Doppler shifts due to atmospheric winds, and to lower system signal processing bandwidth demands. Another example is generation of few-GHz MO/LO offsets to accurately resolve atmospheric absorption spectra and measure gas species concentration in coherent differential absorption lidar (DIAL) applications. In this paper, we describe the development of two generations of eyesafe, diode-pumped MO/LO laser technology and actively phase-locked control electronics, specific to the space-based Doppler platform compensation problem. The lasers are based on CTI's METEOR single frequency laser technology, using Tm,Ho:YLF (2.05 μm wavelength). Fast, programmable offset-locking of the two single-frequency lasers to as much as ± 10.0 GHz was consistently demonstrated using a custom-fabricated wideband (~ 4 GHz @ 3 dB down) 2 μm-sensitive heterodyne photoreceiver as the servo detection element. Offset-frequency step settling time, control accuracy, and the phase-sensitive servo system will be described in detail. Application of the technique and technology in atmospheric CO₂ DIAL measurement applications currently under way at CTI will also be discussed.

Many concepts for incoherent optical distance measurement, based on the time-of-flight (TOF) principle, are discussed in the past, but they differ in complexity and accuracy. The used modulation techniques and evaluations methods require different signal sources, which are controllable in frequency or phase delay in high precision. Development effort and outlay of TOF-systems will be reduced with the use of standard logic devices. The restrictions of these devices permit a limited number of phase or frequency steps, but the combination of standard logic devices and the principle of Phase-Shift Interferometry (PSI) offers the possibility to design a plain and precise system, at very low cost. Over the past 20 years many evaluation algorithms for PSI have been presented in different applications. The phase angle of an ideal interferogram is determinable with only three or four sampling values, but the usage of more sampling values will suppress emitter and detector non-linearities, phase shift errors and noise generally. This paper will present the design of the optimal phase-shift algorithm based on Fourier analysis of the complete recorded interferogram.

Since Sarajevo's sadly famous events (sniper alley), the military tried and hoped to detect snipers before they hit. The principle of the detection is based on the 'cat's eyes' effect according to which the light emitted by the system and incident on the sniper's sight reflects backward in the direction of the source. The system is thus composed of a laser emitter and a CCD array detector. Already existing equipment has been tested in operations and they present too low a probability of detection for the false alarm rate we want to reach. In order to specify equipment characteristics to industrials, it has been necessary to develop a sight laser detector model. The model presented here takes into account all the various elements of the system, from the laser emission to the CCD detection, and atmospheric propagation (ie attenuation and turbulence). The signal and noise probability density functions are calculated by combining the different elementary probability density functions encountered on the double-pass propagation. This Matlab coded model gives the probability of detection of the system for given geometrical (monostatic or bistatic) and electronic characteristics of the system and for a given probability of false alarms. In addition to this, measurements in the field made it possible to validate the budget link of the model and improve it. Those measurements also permitted to underline the importance of the target optical signature, namely its Laser Cross Section. The most significant parameters necessary to the validation of the model are measured. This study allows us to answer the question 'why is the probability of detection of existing systems too low and how could we increase it's efficiency?'

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.

Devon Island, in the Canadian High Arctic (75°22’N, 89°41’W), is the largest uninhabited island on the planet. The climate is that of a polar desert; it is cold, dry, dusty, rocky, and almost void of any vegetation. The eastern part of the island is still covered by an ice cap, a remnant of the Inuitian Ice Sheet system that covered the bulk of the area during the last Glacial Maximum 8 000-10 000 years ago.. The island is rich in well-preserved geology, relatively free of erosion. The feature of highest scientific interest on Devon Island is the ~23-million-year-old (Miocene), ~24 km diameter Haughton impact structure.. There are few other craters on this planet as well preserved and exposed as Haughton, mainly due to the unique climate that slows down erosion common on the rest of the planet.The NASA Haughton-Mars project is an international planetary analog research project headquartered at NASA Ames Research Centre and managed by the SETI Institute. The lidar work described in this work is a collaborative activity between the SETI Institute, the University of Guelph, the University of New Brunswick, Optech Inc., and the Canadian Space Agency. Field activities were conducted under the auspices of the NASA HMP and of the CSA. Specific sites of geological interest within Haughton impact structure were imaged using an Optech Ilris 3-d ground-surveying unit. This very high-resolution, 3-dimensional data allows for the field geologist to "re-visit" a field site well after the field season has finished. In this work, we will present the results of 3-dimensional scans of an ejecta block and of impact-generated rock formations that contribute to furthering our understanding of impact cratering, a fundamental and universal process of planetary formation and evolution, and to studies of the erosional history of Haughton Crater and surrounding terrain on Devon Island. We will demonstrate how using this tool in the field can increase safety and allow for precise measurements to be made after the field season is completed.

The optical wave would change its propagation path, as it encounters different dielectric media. The characteristic of the optical reflection would vary with different dielectric medium(n₂), while the optical beam is incident from air (n₁) into the medium (n₂). Generally, an acoustic wave would cause strain effect as it broadcasts in the medium (n₂) and leads to a change of refractive index Δn₂. The net refractive index of medium n₂ should be n₂+Δn₂. A plane sound wave would be adopted to be the acoustic source. As the result, the refractive index of medium n₂ should vary with the plane sound wave. We could evaluate the exist of acoustic wave underwater by accumulating the reflective optical signal. In this work, the typical ocean is taken to be the propagation medium(n₂) of plane sound wave. The simulation is to derive the data of optical reflection affected by different powers of plane sound wave. The result could be a considerable technology for optic-acoustic application.

The Gemini, Apollo, and Space Shuttle astronauts have accomplished rendezvous and docking using navigation sensor technologies that were state-of-the-art in their days. However, new applications require more advanced technologies and a more capable, autonomous relative navigation sensor will be important for future space operations. Potential benefits include reduced crew training, reduced reliance on ground systems, and more operational flexibility. Additionally, new sensor technologies enable uncrewed automated operations in low earth orbit and beyond. New sensors can reduce or eliminate the need to augment target spacecraft with cooperative devices and thus provide for greater flexibility and enhanced mission success. This paper identifies a set of specific sensor capabilities for future space operations and describes a 3-D imaging ladar sensor conceptual design to provide those capabilities.

Advanced Optical Systems, Inc. is developing the Autonomous Rendezvous and Docking Sensor Suite for Marshall Space Flight Center to provide real-time range and 6 Degree Of Freedom (DOF) information. This information facilitates the autonomous docking of two spacecraft. The sensor suite is comprised of the Advanced Video Guidance Sensor (AVGS) and the Wide Angle Laser Range Finder (WALRF). AVGS was developed under NASA's Demonstration of Autonomous Rendezvous Technology (DART) program for a cooperative target and is scheduled to fly in 2004. The prototype of the WALRF is being developed at AOS under a different program. The sensor suite can provide range and bearing data up to 5km and 6 DOF information up to 300m for the DART target configuration. Different target geometries can increase range detection and 6 DOF detection distance. The sensor suite is a laser-based optical system with a combined weight of less than 40lbs and a combined volume of less than 12”×10”×18”. The WALRF system employs a bistatic transceiver with an 8° field of view (FOV). This sensor is a time-of-flight range finder with a quad detector. The AVGS section of the suite is a monostatic transceiver with a 16° FOV and high-speed imager. This section of the suite uses a pattern recognition system that reduces imager data into 6 DOF information. In this paper we will outline in detail the AVGS and WLRF functionality as well as experimental range data and measurement accuracy.

Small target detection and tracking are important for laser radars in many applications such as Directed Infrared Countermeasures (DIRCM), fire control, target recognition and free space laser communications. The detection and tracking performance is depending on the mode of detection, signal to noise ratio, target signal statistics, beam jitter and turbulence induced intensity variations. We will show results of the rms tracking error vs SNR primarily for direct detection systems. For the general case of a certain signal and noise probability density functions (pdf) it is hard to obtain analytical solutions for the mean and variances of the estimates for the rms tracking error. We have therefore used numerical simulations to illustrate how the pdf and SNR will affect the tracking accuracy. A manifold of gamma functions and other pdf:s can be used to characterize the signal distributions to get a first hand on tracking performance. The results are presented as tracking errors vs the angular spot size of the laser beam in the tracking detector plane. We have also investigated the beam optimization problem for target detection and “power in bucket”, that is maximizing the laser energy at the target. We find that there are optimum beam sizes (w) vs. the rms jitter (σ) and that optimum w/σ (minimizing the false alarm rate for a given detection probability P_d) typically fall in the region 1-3 depending on probability of detection and the representative pdf for the application in mind.

In this paper, the performance of Geiger-mode avalanche photodiode (GAPD) receivers for range detection laser radar sensors is reported. The distribution of the non-linear avalanche detections is developed as a function of laser radar pulse width and energy for a given target and clutter range resolved cross-section with additive background noise. This distribution is then employed to design an efficient signal simulator, which was utilized to model performance and verify theory. Finally, an expression for the pulse energy that optimizes the probability of detection for partially obscured targets is given.

Lidar (extinction-to-backscatter) ratios are computed at 0.55, 1 and 10 μm, based upon a recently published summary of the physicochemical properties of climatically relevant aerosol species. The results agree very well with previously measured values in the literature, indicating that low Sa values for desert dust (15-30) and maritime (30-45) aerosols are clearly distinguishable from biomass burning (55-65) and urban/industrial pollution (55-80). The results show that most aerosol types can be discriminated by their absorption and scattering characteristics through use of spectral lidar ratios, except between biomass burning and pollution aerosols. Predictions of on- and off-axis scattering in the presence of these aerosol types illustrate the range of signal that may be expected in a bistatic lidar system in such cases, and indicate that bistatic lidar may be successfully used to detect a source lidar signal and discriminate the aerosol species present. These findings strongly suggest that a combination of passive and active remote sensing systems operating simultaneously (e.g., ground-based sky radiance and bistatic lidar), would be capable of directly measuring the absorption and scattering characteristics required to describe the optical behaviour of the aerosol with vertical resolution. This is expected to be of great utility to climate researchers or other communities interested in comprehensively measuring atmospheric optical properties.

In order to optimize their flight conditions, airborne platforms need to know precisely their true airspeed. In helicopters, measuring low air speeds is a severe issue because of the rotor flow. Optical air data sensors are therefore a good alternative to classical pneumatic probes. ONERA is involved for many years in simulation and design of coherent lidar and focuses its last research on eye-safe solid state lidars. This paper describes the study of performance of a reliable compact airborne system based on a 1.5 μm Erbium fiber laser and architecture. The average heterodyne current power is examined for the case of negligible turbulence and truncation effect. The spatial resolution of the measurement is deduced and its behavior versus transmitter beam parameters discussed.

Observing system simulation experiments (OSSE's) provide an effective means to evaluate the potential impact of a proposed observing system, as well as to determine tradeoffs in their design, and to evaluate data assimilation methodology. Great care must be taken to ensure realism of the OSSE's, and in the interpretation of OSSE results. All of the OSSE's that have been conducted to date have demonstrated tremendous potential for space-based wind profile data to improve atmospheric analyses, forecasts, and research. This has been true for differing data assimilation systems, analysis methodology, and model resolutions. OSSE's clearly show much greater potential for observations of the complete wind profile than for single-level wind data or observations of the boundary layer alone.

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.

Laser beams can be detected when propagating in the atmosphere, even if they are not pointed straight on the sensors. Mie scattering on aerosols allows to detect off-axis beams and to localize the source thanks to their geometric, spectral and temporal properties. In the recent years, Onera has conducted both theoretical and experimental studies to detect off-axis non cooperative pulsed laser beams in the lower atmosphere. Some simulations are presented, based on propagation and scattering physics. Experimental results are discussed.

The transportable scanning six-wavelength eleven-channel aerosol lidar of the Insitute for Tropospheric Research represents the most powerful tool for a comprehensive characterization of atmospheric particles with lidar. Particle backscatter coefficients are determined at 6 wavelengths between 355 and 1064 nm. Particle extinction coefficients are determined at 355 and 532 nm. The instrument makes use of the elastic backscatter, Raman lidar, and scanning lidar technique. The physical particle parameters including the single-scattering albedo are retrieved from the optical data with an inversion scheme based on Tikhonov's inversion with regularization. The optical and physical parameter allow to perform radiative impact studies on the basis of lidar observations. The system was successfully operated in the Aerosol Characterization Experiment 2 (ACE 2) and the Indian Ocean Experiment (INDOEX). A measurement example taken from the Lindenberg Aerosol Characterization Experiment 98 (LACE 98) exemplifies the potential of this instrument.

The NOAA (National Oceanic and Atmospheric Administration) Pacific Marine Environmental Laboratory's aerosol research program is focused on the impact of atmospheric particulates on climate and air quality. Our approach is to characterize relevant aerosol chemical, physical, and optical properties using a combination of in situ and remote instrumentation and optical models. The resulting data base includes long term measurements from northern hemisphere aerosol monitoring stations operated by NOAA and short term measurements from international intensive aerosol experiments. The data provide information on spatial and temporal means and variability in aerosol properties and on aerosol formation and transformation processes in the boundary layer. To demonstrate our in situ measurement and modeling capabilities, results are presented here from ACE Asia, an experiment we participated in during the spring of 2001.

A CCD based bistatic lidar (CLidar) system has been developed and constructed to measure scattering in the atmospheric boundary layer. The system used is based on a CCD camera, wide-angle optics and laser. Measuring near the ground with the standard monostatic lidar method is difficult due to the huge change in signal strength with altitude and the incomplete overlap between the laser and the telescope. High spatial (altitude) resolution is also desired near the ground for comparison with in-situ aerosol instruments. Imaging a vertical laser beam from the side with a CCD camera and wide-angle field of view optics overcomes both of these problems. While the molecular signal changes many orders of magnitude in the standard method, it only changes about one order with the CLidar method. In addition, the CLidar resolution near the ground is less than a meter. For perpendicular polarization, the molecular signal is nearly constant all the way to the ground. Other advantages of the CLidar method include low cost and simplicity. The signal is integrated on the CCD rather than with specialized electronics. With the bistatic CLidar method the scattering angle changes with altitude. The variation of scattering intensity with the scattering angle will be influenced by the aerosol size distribution and thus could help provide information on aerosol parameters of interest in the boundary layer.

In this paper, a 1.5 micron, 3-D scanning, portable and eyesafe aerosol lidar system is presented. The design, testing and field measurements of this lidar are introduced. An aerosol lidar model is used to evaluate lidar system's performance. At the end, the experimental and theoretic atmospheric detection results are presented and compared.

Analytic expressions for the mutual coherence function and the scintillation index of the lowest order Gaussian beam as a function of target roughness are developed for a bistatic configuration in weak and strong atmospheric turbulence. Results are based on Rytov theory and Kolmogorov spectrum model. The surface roughness is modeled by a thin complex phase screen with a Gaussian spectrum. The limiting cases of perfectly smooth and Lambertian targets are deduced. The particular cases of incident spherical and plane waves are also considered.

Interdigitated-finger metal-semiconductor-metal photodetectors (MSM-PDs) are widely used for high-speed optoelectronic applications. Recently, GaAs MSM-PDs have been utilized as optoelectronic mixers (OEMs) in an incoherent laser radar (LADAR) system. InGaAs MSM-PDs would allow LADAR operation at eye-safe wavelengths, mainly 1.55 μm. Unfortunately, the Schottky barrier height on InGaAs is quite low (~0.1-0.2eV) leading to high dark current and, hence, low signal-to-noise ratio. To reduce dark current, the Schottky barrier is typically “enhanced” by employing a high-band-gap lattice-matched Schottky enhancement layer (SEL). Detectors using SELs yield low dark current, high responsivity, and high bandwidths. In this paper we analyze the mixing effect in InAlAs Schottky-enhanced InGaAs-based MSM-PDs. We find that the measured frequency bandwidth of such a mixer is smaller than when used as a photodetector. Moreover, the mixing efficiency depends on the light modulation and mixed signal frequencies and decreases non-linearly with decrease in optical power. This is not observed in GaAs-based and non-Schottky-enhanced InGaAs MSM-PDs. We present a circuit model of the MSM-PD OEM to explain the experimental results.

Results of analytic comparison of potentialities of the CW-FM-ladar and the incoherent pulse lidar remote sensing are presented. Different heterodyning techniques applied to a CW-FM-ladar are considered. Comparative evaluations of the achievable signal-to-noise ratio, the minimum detected echo-signal, the operation range, and the range resolution, are carried out by taking into account some frequency-related parameters of both transmitting and receiving subsystems.

This paper reports the test results for a three-axis true airspeed measurement instrument that is based on the enhanced mode lidar (EML) concept. This instrument is a continuing Goodrich development effort that builds on the single-axis homodyne concept. Our system heterodynes a reference signal with the single particle lidar return from naturally occurring atmospheric aerosols. The reference signal's frequency is shifted by an acousto-optic modulator, thereby, enabling the measurement of the positive and negative Doppler frequencies. A scan head using a wedge to direct the beam at a 15-degree angle is rotated about the original line of site to trace a cone shape. The redirected beam plus the heterodyning enables the measurement of three-axis true airspeed. The instrument was evaluated using wind tunnel measurements and the results are reported.