Signal processing for pulsed Doppler lidars utilizing distributed returns from naturally occurring aerosols is discussed. The various issues associated with both the real- time and off-line processing modes are presented and the practical solution adopted is outlined. Examples of data processed with these various schemes are presented and the relative merits of each are discussed.

The Multicenter Airborne Coherent Atmospheric Wind Sensor instrument is an airborne coherent Doppler laser radar (Lidar) capable of measuring atmospheric wind fields and aerosol structure. Since the first demonstration flights onboard the NASA DC-8 research aircraft in September 1995, two additional science flights have been completed. Several system upgrades have also bee implemented. In this paper we discuss the system upgrades and present several case studies which demonstrate the various capabilities of the system.

Solid-state coherent Doppler lidar sensors operating at eyesafe wavelengths have broad application to a variety of wind measurement scenarios. We have developed a modular, autonomous, high PRF, diode-pumped coherent lidar sensor that is appropriate for wind profiling at high temporal resolution. This paper describes the design of the sensor, provides examples of high-resolution wind data, and compares the performance with modeling.

Doppler lidar sensors provide a unique capability to generate high resolution 3D distributions of wind and aerosol data. Appropriately processed, these data can yield useful detection, tracking and short-term prediction information relating to the extent, density and location of potentially dangerous isolated aerosol plumes. The aerosol data are analyzed to detect above-threshold inhomogeneities and the wind and turbulence data are used to provide short- term prediction of plume propagation. Broadcast of these data to plume dispersion models can enable robust prediction of dispersion and propagation over longer time periods. Performance predictions are given for both wind and aerosol measurements. Sample processed field data results are presented for Doppler lidars operating at the eyesafe 2 micron wavelength.

The measurement of winds from a space borne platform is of significant scientific importance to both weather prediction and climate research. One of the key technologies embodied in coherent detection of winds from space is the use of large aperture, compact, lightweight, high-quality wavefront, photon-efficiency optics. This paper discusses the optical design, the mechanical design, material preference, diamond turning issues, polishing requirements, and coating selections for the primary mirror of a 25X afocal beam expander intended for use in space-based coherent lidar systems.

The Mini-Raman Lidar System (MRLS) is a `proof-of-principle' chemical sensor that combines the spectral fingerprinting of solar-blind UV Raman spectroscopy with the principles of lidar to open a new venue of short-range (meters to tens of meters), non-contact detection and identification of unknown substances on surfaces. The device has potential application to `first responders' at the site of a chemical spill. The MRLS is portable and has been used both in the lab and in the field. Theoretical estimates and actual laboratory data suggest the possibility of detecting contaminants with a surface coverage of < 1 g/m² at a distance of three meters for one second of signal integration. Increasing the optical throughput efficiency, integrating pattern recognition software, and incorporating a laser with a wavelength near 250 nm are the primary goals for the development of a prototype system.

Measurement of smoke particle emission and plume dispersion from biomass fires using a traditional integrating nephelometers technique is not so convenient because of highly variable emission rates, dispersion and the varying elevations of the smoke plume above the terrain. Eye-safe compact diode laser can measure the value and spatial profile of the optical backscattering coefficient due to smoke particles along the laser beam at the different direction and from different moving platforms. The lidar backscattering return signal can be averaging not only in time but in space with desirable resolution from minimum step to maximum lidar range. By using both active and passive lidar operation mode the dynamic of the plume albedo and atmospheric optical depth inside the narrow cone and bandwidth would be estimated too.

Wider use of lidar systems requires that they continue to become smaller, less expensive, and more reliable. To address this need, ERIM International has developed the M10 portable atmospheric lidar to measure aerosol profiles and cloud optical and physical properties. The system is based on a compact Nd:YAG laser and operates at either 532, 1064, or 1574 nm. The 1574 nm wavelength was added recently to address eye safety concerns. The system has been ruggedized for unattended field use and has been on several data collection campaigns recently. Date and analyses are presented from the Program for Regional Oxidants: Photochemistry, Emissions, and Transport Summer 1997 field study in Pellston, Michigan, and also from the Hyperspectral Day/Night Radiometry Assessment field study at Eglin AFB in Florida.

Current results from laboratory testing of an eye-safe, ground-based ozone lidar instrument specialized for ozone differential absorption lidar measurements in the troposphere are presented. This compact prototype instrument is intended to be a prototype for operation at remote field sites and to serve as the basic unit for future monitoring projects requiring multi-instrument networks. In order for the lidar to be widely deployed, it must be fairly easy to use and maintain as well as being cost-competitive with a ground station launching ozone sondes several times a week. To achieve these goals, the system incorporates (1) an all- solid state compact OPO transmitter, (2) a highly efficient, narrow bandpass grating-based receiver, (3) dual analog and photon-counting detector channels, and (4) a PC-based data acquisition system.

Standoff sensors for rapid remote detection of chemical emissions from either clandestine chemical production sites, chemical and biological warfare agents, concealed internal combustion engine emissions or rocket propellants from missiles are required for several DoD applications. The differential absorption lidar (DIAL) operating in the infrared wavelengths has established itself as a very effective tool for rapidly detecting many of the chemicals, with sufficient sensitivity with a range of several kilometers. The wavelengths required for this task lie within the atmospheric window regions 3 to 5 micrometers and 8 to 12 micrometers . We are currently developing a differential absorption lidar (DIAL) tunable in the 3 to 5 micrometers range for detecting low concentrations of chemical species with high sensitivity (5 ppb) and accuracy (error < 10%) measurements for greater than 5 km range. We have successfully established the feasibility of an innovative frequency agile laser source which is the crucial component of the infrared DIAL. A diode-pumped ytterbium YAG laser was built for pumping and rapidly tuning an optical parametric oscillator (OPO) over the mid-infra red region. Good performance (5 mJ/pulse) of the laser and low threshold wide infra red tuning of OPO (2.2 - 3.1 micrometers ) were demonstrated. The simulated performance of the topographical IR-DIAL showed that 5 ppb concentration can be measured at 5 km range with a 35 cm telescope.

The development of a ground-based, eye-safe lidar system for differential absorption lidar measurements of trace gases such as methane in the mid-IR wavelength region and for aerosol measurements at 1.5 micrometers is described. This prototype lidar system will be used for urban ambient trace gases and aerosol detection. An optical parametric oscillator pumped by an injection-seeded frequency-doubled Nd:YAG laser is employed as radiation source. The expected minimum detectable range of the system is about 1000 meters for range-resolved measurements of methane and several kilometers for aerosol measurements. The system performance is being tested through measurements of sources of atmospheric methane and aerosols.

This paper presents a multi-wavelength algorithm that utilizes the weighted mean of all possible DIAL pairs. The weights are a function of the differential absorption coefficient between the DIAL pairs. The algorithm is shown to have greater sensitivity and robustness than two- wavelength DIAL. In addition, an algorithm for estimating the column content (CL) in the presence of multiple vapors is described. The algorithm iteratively fits the lidar equation to the data by adjusting the CL of each vapor of interest. Results are shown using topographic chamber test data collected in Dugway Proving Ground in 1996 using our frequency-agile lidar sensor.

The Air Force Research Laboratory (AFRL) Active Remote Sensing Branch has developed the Laser Airborne Remote Sensing (LARS) system for chemical detection using the differential absorption lidar technique. The system is based on a high-power CO₂ laser which can use either the standard ¹²C¹⁶O₂ or the ¹³C¹⁶O₂ carbon dioxide isotopes as the lasing medium, and has output energies of up to 5 J on the stronger laser transitions. The lidar system is mounted on a flight-qualified optical breadboard designed for installation into the AFRL Argus C- 135E optical testbed aircraft. The Phase I ground tests were conducted at Kirtland AFB in 1997, prior to the LARS flight tests performed in September 1997 at Kirtland AFB and the Idaho National Engineering and Environmental Laboratory. The Phase II ground tests were conducted in 1998 to determine the optimum performance of the LARS systems, after the incorporation of modifications and improvements suggested by the flight test results. This paper will present some of the chemical detection and radiometric results obtained during the Phase II ground tests. Following the presentation of the direct detection results, a summary of current work on a heterodyne DIAL system is given.

A novel method of performing DIAL (Differential Absorption Lidar) measurements of airborne chemicals is presented. The technique, called RaDIAL, utilizes the Raman returns from atmospheric nitrogen and oxygen as the `on' and `off' wavelengths for a particular chemical species. Both laboratory and field tests of RaDIAL for molecules with absorption bands in the UV solar blind demonstrate the utility of this detection scheme. The advantages of RaDIAL for range-resolved chemical species detection/monitoring include insensitivity of the measurement to laser pulse-to- pulse energy fluctuations and variations in aerosol burden. The RaDIAL technique offers the desired high sensitivity associated with DIAL while keeping the data reduction simple and free of complex approximations.

Many trace atmospheric gas constituents have optical absorption bands in the 2 - 5 micrometers atmospheric transmission window. Remote sensing of these compounds is possible with an appropriate laser source. We use stimulated Raman scattering in hydrogen to shift pulsed, Cr:LiSAF laser emission from the near infrared to this mid-infrared band. Injection seeding the oscillator with a spectrally narrow, low-power diode laser produces a tunable, spectroscopic grade source. We have combined this laser source with transmitting and receiving optics in order to make double- ended, long-path DIAL measurements. For example, we are able to detect ambient levels of water and methane and trace levels of ethane over a two mile, round-trip path. Spectral control is critical for making these measurements for several reasons. First, the DIAL technique requires a spectrally narrow source to tune across the narrow absorption bands of molecules with absorption features in the 2 - 5 micrometers band. Second, good spectral control allows species-specific detection when there are target species with closely spaced absorption features. Third, strong water vapor and CO₂ absorption bands are common throughout the 2 - 5 micrometers band causing large fluctuations in the background transmission. Good spectral control enables species specific detection within this highly variable transmission background.

A high sensitivity, CO₂ lidar detector, based on recent advances in ultra-low noise, readout integrated circuits (ROIC), is being developed. This detector will combine a high speed, low noise focal plane array with a dispersive grating spectrometer. The spectrometer will filter the large background flux, thereby reducing the limiting background photon shot noise. In order to achieve the desired low noise levels, the HgCdTe FPA will be cooled to approximately 50 K. High speed, short pulse operation of the lidar system should enable the detector to operate with the order of a few noise electrons in the combined detector/ROIC output. Current receiver design concepts will be presented, along with their expected noise performance.

Lidar receivers perform time and/or space averaging to decrease the variance of the optical power estimates. In this paper we study an Avalanche PhotoDiode based receiver. The number samples to reach a given minimum variance depends on the receiver transfer function. Herein, we review the linear receiver and derive the number of samples for the logarithmic pre-amplifier. Comparing the two receivers, we show that the signal variance for the logarithmic case is degraded by a factor that vanishes as the receiver aperture increases. These results can be readily applied to the problem of estimating log-power returns in the context of Differential Absorption lidar systems. As an application example, we study two different log-power estimators and compare their performance.

In previous work, we presented a methodology for optimally processing data from lidar with frequency-agile wavelength capability using techniques of multivariate statistics. Among the applications considered was the case of range- resolved lidar with short (delta function) transmitter pulses. This paper extends that analysis by deriving a method for estimating range-dependent vapor concentration for arbitrary pulse shapes. A Bayesian statistical approach leads to a MAP (maximum a posteriori) estimator for C(z), the concentration at range z. The estimates are computed iteratively for a given set of multiwavelength lidar return data using an approximation to the Gauss-Newton method. The concentration estimates are then used as the basis for a detection algorithm for the leading edge of the vapor plume based on the CUSUM approach. The detection and estimation approaches are illustrated on a combination of synthetic and field test data collected by SBCCOM at the Idaho National Engineering and Environmental Laboratory test site.

A new technique combining active and passive remote sensing instruments for the estimation of surface latent heat flux over the ocean is presented. This synergistic method utilizes aerosol lidar backscatter data, multi-channel infrared radiometer data and microwave scatterometer data acquired onboard the NASA P-3B research aircraft during an extended field campaign over the Atlantic ocean in support of the Lidar In-space Technology Experiment in September of 1994. The 10 meter wind speed derived from the scatterometers and the lidar-radiometer inferred near- surface moisture are used to obtain an estimate of the surface flux of moisture via bulk aerodynamic formulae. The results are compared with the Special Sensor Microwave Imager (SSM/I) daily average latent heat flux and show reasonable agreement. However, the SSM/I values are biased high by about 30 W/m². In addition, the MABL height, entrainment zone thickness and integrated lidar backscatter intensity are computed from the lidar data and compared with the magnitude of the surface fluxes. The results show that the surface latent heat flux is most strongly correlated with entrainment zone top, bottom and the integrated MABL lidar backscatter, with corresponding correlation coefficients of 0.62, 0.67 and 0.61, respectively.

In early tests of an airborne lidar platform, we confirmed the utility of an elevated, ground-based infrared camera as a chemical plume diagnostic. For a series of lidar tests during the summer of 1998, we carried this concept a step further, by adding a digital data link to pass infrared camera video and other data streams to and from the aircraft in real time. In addition, the entire system had to be operated from a distance, for safety considerations. To achieve this goal under a restricted budget and significant time and effort constraints, we assembled a system using primarily off-the-shelf components and software requiring little customization. Remote system control was achieved by a set of radio modems, while the aircraft data link was effected via wireless ethernet connection. The system performed reliably throughout the test series.