The favored scheme for the remote monitoring of atmosphere pollutants is based on the differential absorption of radiations by the target species. The sources of these radiations are the Rayleigh or Mie scattering from the distributed atmospheric aerosols at to slightly different wavelengths. This technique is known as deferential absorption lidar (DIAL) and offers the best range capability compared to other remote sensing techniques. However, the DIALs are complex and one system is dedicated to monitor only one species. The accuracy in DIAL measurements are also critically dependent on environment and instrumental parameter. The other promising technique for remote pollution monitoring is the Raman lidar. This is based on the detection of Raman back-scattered signals from the pollutant species and allows detection of several species simultaneously. However, Raman lidars do not allow long range capabilities. The application of lidars for atmosphere pollutant mapping is ultimately tied up with trade-offs between required or desired sensitivity, range capability and the data averaging time. The state-of-the-art of the two most promising lidar technologies, their limitations and recent advances in their applications are reviewed in this paper.

The Mobile Lidar Trailer (MLT) was developed and operated to characterize launch vehicle exhaust plume and its effects on the environment. Two recent applications of this facility are discussed in this paper. In the first application, the MLT was used to characterize plumes in the stratosphere up to 45 km in support of the Air Force Space and Missile Center's Rocket Impact on Stratospheric Ozone program. Solid rocket motors used by Titan IV and other heavy launch vehicles release large quantities of gaseous hydrochloric acid in the exhaust and cause concerns about a possible depletion of the ozone layer. The MLT was deployed to Cape Canaveral Air Station since October 1995 to monitor ozone and to investigate plume dynamics and properties. Six campaigns have been conducted and more are planned to provide unique data with the objective of addressing the environmental issues. The plume was observed to disperse rapidly into horizontally extended yet surprisingly thin layer with thickness recorded in over 700 lidar profiles to be less than 250 meters. MLT operates with the laser wavelengths of 532, 355 and 308 nm and a scanning receiving telescope. Data on particle backscattering at the three wavelengths suggest a consistent growth of particle size in the 2-3 hour observation sessions following the launch. In the second type of application, the MLT was used as a remote sensor of nitrogen dioxide, a caustic gaseous by-product of common liquid propellant oxidizer. Two campaigns were conducted at the Sol Se Mete Canyon test site in New Mexico in December 1996 an January 1997 to study the dispersion of nitrogen dioxide and rocket plume.

A ground-based lidar system with an accurate scanning capability has been built up for detailed investigations of jet airplane condensation trails. The scanning of contrails including their small scale structures requires a complex and fast system control In particular the positioning of the scanning mount is time critical. A CCD camera is therefore integrated with the lidar system for the definitive capture of the target. The whole system is controlled by a software package, which has been developed with the PC based operating system Microsoft Windows. By this means a complete contrail scan can be performed within 20 to 30 sec, which is fast enough to investigate contrails even in the vortex regime. The combination of lidar measurements of a contrail cross section with data acquired simultaneously by the CCD camera allows the determination of the spatial spread of contrails and to extrapolate the optical depth determined by lidar within the 40 degree viewing angle of the CCD camera. This technique will be used to validate and possibly improve existing algorithm which determine contrails in AVHRR satellite images.

A lidar system capable of simultaneous aerosol, temperature and line-of-sight wind measurements based on a narrowband transmitter at 532 nm and four iodine resonance vapor filters, one used to lock the laser at a precise transmitting frequency and three used in the receiver as frequency discriminator/analyzer, is proposed.

Direct detection Doppler lidar systems for tropospheric wind speed profiling are considered, concentrating on the spaceflight application. A precisely developed model for the fringe imaging technique is extended to the case of the aerosol signal with a significant Rayleigh background. An analytic model for the Doppler precision of an edge technique analyzer with dual filters, offset in frequency, is developed, also with provision for the Rayleigh background signal. The two models are compared to recent modeling results from McGill at NASA Goddard Space Flight Center, and good agreement found. The less rigorous analytic models developed here offer insights into the design and limitations of fringe imaging and edge detection Doppler analyzers. In particular, a requirement here offer insights into the design and limitations of fringe imaging and edge detection Doppler analyzers. In particular, a requirement for wide wind speed dynamic range implies low sensitivity for he edge detection technique, but not for the fringe imaging technique. The fringe imaging and edge detection techniques are compare for relative precision, for lidar signals of specified amplitude. A detailed conceptual design of a spaceflight Doppler wind lidar system, employing a very large optical collector in order to obtain adequate backscatter signals, shows that the fringe imaging analyzer is a factor 10 superior to the dual-etalon edge detector.

A transportable ground-based differential absorption lidar specifically designed for ozone and aerosol profiling in the lower troposphere was developed at the National Oceanic and Atmospheric Administration/Environmental Technology Laboratory (NOAA/ETL). The NOAA/ETL ozone lidar has the unique capability of measuring vertical profiles of ozone concentration from near the surface up to 3 km, and measuring vertical profiles of aerosol from the surface to about 10 km. The innovative hardware design and improved signal processing techniques make the system efficient, compact, and easily transportable. A recently implemented 2D scanning system provides the capability of measuring ozone concentrations and aerosol in a vertical plane. The lidar has been deployed in seven field experiments in California, Illinois, and Boulder, Colorado since summer 1993. Lidar observations of vertical profiles of ozone concentrations and ozone advection fluxes in Southern California during high ozone season revealed interesting structures of ozone distributions in the Los Angeles urban area, and near the Cajon Pass which is a major corridor of ozone transport from Los Angeles to the Mojave Desert.

Progress in understanding and developing methodologies to do low frequency, wideband FM-coding on a variety of laser sources is presented. Several practical FM-coding formats have been studied and compared for use with large, 'slow' IR detectors. The objective of this work is to establish practical methods of doing long baseline absorbance measurements in the lower troposphere that could be degraded by large integrated effects of optical turbulence. A comparative study of producing high index modulation on laser diodes and other practical laser sources is presented, along with numerical simulations and experimental result of detected signal strength. These results are presented with the intent of illustrating useful laser sources and FM- coding schemes that could allow effective detection of trace atmospherically broadened absorbance signatures. Results of field measurement will be discussed in order to suggest further improvement for this technique. Signal-to-noise issues and effects of phase distortion will be outlined with the view of suggesting correct optimization of laser transmitter and detector/preamp subsystems for these types of FM signal recovery. Finally, efforts at developing thermo-optically stable external electro-optic modulators will also be discussed.

NASA Langley has an active water vapor differential absorption lidar program taking measurements from both C-130 and ER-2 aircraft. A research effort has started to increase the signal-to-noise ratio in the DIAL receiver by 1) evaluating new very low noise avalanche photo didoes (APD), 2) designing an optics system that will focus the return light signal to the APD efficiently and 3) constructing a 10-MHz waveform digitizer board that will be small enough to be placed at the APD and telescope. With these advances we anticipate improving the signal-to-noise ratio by a factor of ten over the current receiver system.

In this paper, we describe the temporal laser pulse dynamics of the 1.645 micrometers Er:YAG laser pumped by a 1.532 micrometers Er:glass laser. It can be shown that controllable double pulsing of the 1.645 micrometers laser action can be obtained by gain-switched type operation and a variable cavity loss. The rate equation dynamics model can predict this behavior, and thereby provide a design platform for generating controllable double pulsing outputs at high repetition rates. Applications to lidar techniques are discussed. Some innovative applications include polarization discrimination techniques in target detection technology and mitigating atmospheric turbulence effects in measurements.

The problem of laser selection for spaceflight DIAL or Doppler lidar is considered. Spaceflight lidar requires tens of watts of laser output, and the low efficiency of lasers imposes costly burdens on the spacecraft platform. DIAL requires a tunable laser, and Doppler an ultraviolet laser, so the high efficiency of the Nd:YAG laser is compromised. The alexandrite laser can in principle provide higher systems efficiency for DIAL or Doppler than the Nd:YAG, being intrinsically tunable, and capable of reaching the ultraviolet with frequency doubling. High power 680 nm laser diodes are now available with sufficient power to pump alexandrite to the necessary power levels. A Q-switched laser configuration is modeled to obtain a projection laser efficiency of 13 percent. A more conservative estimate is 3.5 percent, well below the 9 percent achieved with Nd:YAG. Considering the energy savings through intrinsic tunability, frequency doubling to the ultraviolet, and extremely narrow spectral linewidth, a Doppler wind lidar system based on the alexandrite laser would have four to nine times the efficiency of the Nd:YAG alternative.

We compare the efficiency of a classifier based on probabilistic neural networks and the general least squares method. Both methods must accommodate noise due to uncertainty in the measured spectrum. The evaluation of both methods is based on a simulated transmittance spectrum, in which the received signal is supplemented by an additive admixture of noise. To obtain a realistic description of the noise mode, we generate several hundred laser pulses for each wavelength under consideration. These pulses have a predetermined correlation matrix for different wavelengths; furthermore, they are composed of three components accounting for the randomness of the observed spectrum. The first component is the correlated 1/f noise; the second component is due to uncorrelated 1/f noise; the third one is the uncorrelated white noise. The probabilistic neural network fails to retrieve the species concentration correctly for large noise levels; on the other hand, its predictions being confined to a fixed number of concentration bins, the network produces relatively small variances. To a large extent, the general least square method avoids the false alarms. It reproduces the average concentrations correctly; however, the concentration variances can be large.

Chemical absorption signatures in lidar data can be difficult to identify when the signal to noise ratio is small. For lidar interrogation of unknown chemical mixtures it is advantageous to sample with many different wavelengths, covering the largest possible absorption bandwidth. A more effective DIAL measurement can be made if one known a priori which wavelengths will be absorbed by the unknown chemical(s). An algorithm has been developed which quickly identifies the absorbed laser lines by examining the temporal cross-correlation between wavelengths. Once this determination has been made the remote chemical mixture can be re-sampled with fewer wavelengths resulting in higher data rats at the sensitive wavelengths. This algorithm was shown to be successful with actual DIAL measurements of remote chemical mixtures. A second detection algorithm will also be presented that uses the temporal autocorrelation at a single wavelength to detect the presence of a time dependent chemical absorption, e.g. form a chemical plume, in a noisy time series. The overall DIAL sensitivity using these algorithms will be compared with standard methods.

This paper reviews recent progress in methods for optimal detection and estimation of vapor concentration using data from frequency-agile lidar. Following a summary of the likelihood ratio statistical method for constructing optimal tests, the paper shows how the basic approach derived in an earlier paper can be generalized to include the use of transmitter pulse energy measurements for reducing the estimation variance due to shot-to-shot fluctuations in the pulse energy. The transmitter normalization method developed here is compared with the usual ratioing approach on simulated and actual lidar data. Finally, the paper extends the earlier fixed-size data sample likelihood ratio approach to include the time series aspect of data collection. Modeling the path-integrated concentration vector as a simple random walk process in time, the earlier maximum likelihood (ML) estimates are replaced by Kalman filter estimates. The Kalman filter estimates are compared to the unfiltered ML estimates on vapor chamber data.

Issues related to the development of direct detection, long- range CO₂ DIAL systems for chemical detection and identification are presented and discussed including: data handling and display techniques for large, multi-(lambda) data sets, turbulence effects, slant path propagation, and speckle averaging. Data examples from various field campaigns and CO₂ lidar platforms are used to illustrate the issues.

The Geophysics Directorate of Phillips Laboratory has recently completed redesign of a heterodyne CO₂ differential absorption lidar which can simultaneously measure range resolved radial velocity, aerosol backscatter, and differential absorption. The transportable system utilizes two CO₂ transversely excited atmospheric (TEA) lasers which can be discretely tuned to many of the rotational lines compromising the 00 degree 1 to 10 degrees 0 vibrational bands of CO₂. These lines span a spectral region from about 9.2 to 10.8 micrometers and allow for the DIAL measurement of some minor atmospheric molecular constituents as well as many anthropogenic organic species which have absorption bands in this spectral region. Transmission and reception is coaxial via a single shared 12 inch telescope and hemispherical scanner. Complete spectral processing of the heterodyne signals provides not only backscatter and differential absorption information but also radial wind velocity. Each TEA laser produces a line dependent pulse energy of 20-80 mJ at up to 150 Hz. Presently, the system is processor limited to a net pulse rate of 140 Hz. Results shown will include time-height cross-sections of cirrus backscatter, comparisons of CO₂ DIAL-derived water vapor profiles with simultaneous surface and radiosonde in-situ measurements, and wind velocity profiles in the troposphere.

To determine the presence of a pollutant cloud int he atmosphere, France and the United States have collaborated on the development of a DIAL and DISC LIDAR. This system called MIRELA, is financed by the DGA and ERDEC. It was developed in cooperation with the CILAS company and uses a frequency agile CO₂ laser designed and manufactured by the Hughes Aircraft Company. Before using a LIDAR for the remote detection of atmospheric pollutants, the optical characteristics of the products to be detected must be known. This basic characterization is used to define the parameters of the system and select the detection technologies and algorithms. A simulation with the HITRAN data base provides a set of expected measurements. Comparison with the real results is excellent. The tests were run on realistic clouds. The backscattered signal received from the aerosols at the front of the cloud was detected as well as the return from a target placed beyond the cloud thus a transmission measurements was taken simultaneously with the backscattering measurement. These measurements show that the backscattering signals are characteristic of the cloud and will be used to detect and identify the products.

The effects of flight geometry, signal averaging and time- lag correlation coefficient on airborne CO₂ dial lidar measurements are shown in simulations and field measurements. These factors have implications for multi- vapor measurements and also for measuring a shingle vapor with a wide absorption spectra for which one would like to make DIAL measurements at many wavelengths across the absorption spectra of the gas. Thus it is of interest to know how many wavelengths and how many groups of wavelengths can be used effectively in DIAL measurements. Our data indicate that for our lidar about 80 wavelengths can be used for DIAL measurements of a stationary vapor. The lidar signal is composed of fluctuations with three time scales: a very short time scale due to system noise which is faster than the data acquisition sampling rate of the receiver, a medium time scale due to atmospheric turbulence, and a long time scale due to slow atmospheric transmission drift from aerosol in homogeneities. The decorrelation time scale of fluctuations for airborne lidar measurements depends on the flight geometry.

Atmospheric NO_x distribution in an urban area of Japan in winter season were measured by a tunable solid-state lidar system based on a Ti:sapphire laser and a Nd:YAG laser. We show the possibility of long-time air pollution monitoring. We also discuss the reason of the change of NO_x concentration by weather conditions.

A flashlamp pumped Ti:sapphire laser has been constructed which could be used to make atmospheric DIAL measurements of ozone from aircraft. A 9-mm diameter by 15-cm long rod is pumped by four flashlamps, two lamps fired in series at a time with 300 microsecond time separation between firings to produce the 'on' and 'off' line DIAL laser pulses. The laser cavity has tow arms, one lasing at 867-nm and the other at 897-nm. The Q-switched output is doubled and tripled with a LBO and BBO crystal respectively to achieve DIAL pulses at 289-nm and 299-nm. Line narrowing is achieved with the use of three SF-10 prisms. Such a system could be used on an unpiloted atmospheric vehicle.

This paper reports on recent progress made in developing rapidly tunable MWIR lidar system for the detection and identification of multiple trace atmospheric molecules. The lidar systems and multiline DIAL approach are described in detail. The unique advantages of the lidar system and multiline DIAL measurement technique are demonstrated with results from field measurements.

LLNL has utilized optical parametric oscillator technology to develop and field a rapidly-tunable mid-wave IR DIAL system. The system can be tuned at up to 1 KHz over the 3.3- 3.8 micron spectral region, where hydrogen-bond stretching modes provide spectroscopic signatures for a wide variety of chemicals. We have fielded the DIAL system on the LLNL site on range, turbulence, and receiver aperture size. In this paper we describe the interplay of turbulence and speckle to produce the observed nose fluctuations at short range.

Differential absorption LIDAR (DIAL) with multiple wavelengths provides capabilities for separately identifying and quantifying chemicals in mixtures that are impossible for conventional two-line DIAL. It also permits several choices about how to convert individual laser pulse returns into estimates of gas concentrations. These choices concern both averaging techniques and procedures for fitting averaged data to library spectra of possible gases. The purpose of this paper is to compare several analysis options using real data taken with 8 or 10 mid-wave IR wavelengths in field test. The options fall naturally into two groups, as implied above. The first group comprises ways to combine data averaging with ratioing. The second group comprises various maximum-likelihood estimators and least-squares fits of the averaged data. Several options arise in the second group because data is taken at multiple wavelengths; for two-line DIAL there would be only a single option in this second group. This paper compares the result of field data analysis for these two groups of options. The properties of the data acquired by the multi-line DIAL system are first described. Then the three averaging/ratioing techniques are discussed. The various options for extracting concentration estimates from averaged data are compared. Finally, the implications for remote sensing data analysis are discussed.

The US Air Force Phillips Laboratory is evaluating the feasibility of long-standoff-range remote sensing of gaseous species present in trace amounts in the atmosphere. Extensive system integration in the laboratory and an airborne test are leading to remote sensing ground test and airborne missions within the next year. This paper describes the design, external interfaces. and initial performance of the Laser Airborne Remote Sensing acquisition, processing, and control system to be deployed on the Phillips Laboratory NC-135 research aircraft for differential absorption lidar system performance tests. The dual-CPU VME-based real-time computer system synchronizes experiment timing and pulsed CO₂ laser operation up to 30 Hz while controlling optical subsystem components such as a laser grating, receiver gain, mirror alignment, and laser shutters. This real-time system acquires high rate detector signals from the outgoing and return laser pulses as well as a low rate health and status signals form the optical bench and the aircraft. Laser pulse and status data are processed and displayed in real time on one of four graphical user interfaces: one devoted to system control, one to remote mirror alignment, and two other interfaces for real-time data analysis and diagnostics. The dual-CPU and multi- layered software decouple time critical and non-critical tasks allowing great flexibility in flight-time display and processing.

Various efforts are underway to improve meteorological sounding measurements. One such effort is the development of the GOES High Resolution Interferometer Sounder instrument. This instrument has the potential to enhance these measurements by providing higher spectral resolution and simultaneous broadband coverage. This instrument consists of a Fourier Transform Spectrometer (FTS). The heart of this FTS is a Michelson interferometer. The degree to which the sounding performance is enhanced depends crucially on the performance of this interferometer. A system performance analysis of this interferometer is presented. The analysis is based on the MIT Lincoln Labs design of the GHIS instrument. In the analysis, the interferometric modulation as a function of key system parameters is calculated. These parameters include the following: finite field of view, wavefront shear, tilt of interferometer elements, and multiple reflections in the beamsplitter. The impact of modulation changes on the spectral performance is computed by taking an appropriate fourier transform.

An airborne fourier transform interferometric sounder is being developed to perform atmospheric measurements for the National Polar-orbiting Operational Environmental Satellite System. The interferometer is designed to provide high spectral resolution, low noise data from the NASA ER-2 aircraft suitable for synthesizing and comparing data of potential future satellite-borne sounding instruments, such as AIRS, IMAS, ITS or IASI. The collection and scanning optics provide a 7.5 degree field of view over a cross-track field of regard of +/- 48.2 degrees. The interferometer operates with +/- 2.0 cm optical path difference (OPD) over the spectral range from 3.6-16.1 micrometers . Dynamic alignment is performed using a concentric HeNe laser. Three separate filter/lens/detector assemblies are cooled to 65K using integral rotary stirling coolers. Most of the optical instrumentation is contained within a pressurized N2 enclosure to minimize the effects of descent condensation. The instrument processor/controller is based on a 133 MHz Pentium CPU supporting a dedicated digital signal processor for real-time x16 data decimation. Noise performance referred to a 250 K scene is estimated to be 0.10 K at 14.9 micrometers , 0.15 K at 8.7 micrometers and 0.2 K at 4.7 micrometers .

We report aircraft-based scintillometry measurements for visible light propagation over near-horizontal paths of 50 to 120 km, at altitudes around 40,000 ft. We present time series and histograms of normalized irradiance variance and path-averaged C_n² values. We also present temporal power spectra and covariance functions of irradiance, and derive turbulence inner scale values from the shape of the normalized covariance functions.

It has become clear that in order for lidar technologies to gain wider acceptance outside the research community, they must be smaller, less expensive, and more autonomous. ERIM International has conducted a design study to determine the minimum package size for a fieldable tropospheric ozone lidar. After considering several different wavelength pairs for the differential absorption lidar measurement, a design was selected based on Raman shifting the 4th harmonic of an Nd:YAG laser from 266 nm to 289 nm using deuterium and from 266 nm to 299 nm using hydrogen. Model results indicate that the three wavelengths used in concert will allow measurements of ozone out to a range of nearly 5 km with an accuracy of 5 ppb or better with a one hour integration time. The overall system design consists of a sensor head mounted inside a small shipping crate with the laser, Raman shift tubes, receiving telescope, and detectors and separate data collection/control module in a rugged case. It is anticipated that the system could be built in a combined package occupying less than 2 m³.

Results of spatial-angular LIDAR modeling based on an efficiency criterion introduced are represented. Their analysis shows that a low spatial-angular efficiency of traditional VIS and NIR systems is a main cause of a low S/BR ratio at the photodetector input. It determines the considerable measurements errors and the following low accuracy of atmospheric optical parameters retrieval. As we have shown, the most effective protection against intensive sky background radiation for ground-based biaxial LIDAR's consist in forming of their angular field according to spatial-angular efficiency criterion G. Some effective approaches to high G-parameter value achievement to achieve the receiving system optimization are discussed.

Laser reception autodyne lidars harness the physical phenomenon of specific nonlinear response of the laser as a self-oscillating circuit to a weak external optical signal. The paper reports the original results of investigations in this field obtained in the Laboratory headed by the author.