Pulsed Lidar Research Articles

Variational Analysis for Airborne Conically Scanned Doppler Lidar to Retrieve Mesoscale Wind Fields

An airborne pulsed Doppler lidar implementing a downlooking conical scan rotating around the vertical axis is under development. The information contained in the measured radial velocities is studied to assess the capacity to retrieve the 3D wind field at mesoscale. First, in the frame of a variational analysis, it is shown that the observations cannot resolve horizontal scales shorter than approximately 5 km. Then, constraints of mass conservation and regularity as well as various boundary conditions are added to improve the resolution. The various constraints are tested on synthetic data. Further, a full analysis scheme is proposed that combines a preanalysis at low resolution followed by a finer resolution analysis. The achievable performances are discussed as well as the main limitations.

Comparison of the pseudorandom noise code and pulsed direct-detection lidars for atmospheric probing

The advantages and limitations of using the pseudorandom-noise (PN)-code technique in comparison with pulsed direct detection are discussed. Because the signal-to-noise ratio (SNR) of the PN codes is dependent on the average atmospheric return, strong returns from short ranges and clouds determine the SNR for all ranges. The SNR of a profile obtained by pulsed direct detection exceeds that of the PN-code technique for laser beams with the same average power.

Effects of Wind Turbulence on Coherent Doppler Lidar Performance

Abstract The effects of wind turbulence on pulsed coherent Doppler lidar performance are investigated theoretically and with computer simulations. The performance of velocity estimators is determined for the case of a single realization of a wind field described by a Kolmogorov spatial spectrum and for an ensemble average over many realizations. The results are compared with previously published data. The important normalized physical parameters are identified to reduce the parameter space. For a given realization of a random wind field, the mean Doppler lidar velocity (an ensemble average over the random aerosol particles) is a function of the lidar parameters and the instantaneous radial velocity over the sensing volume of the lidar pulse. Various approximations for the mean Doppler lidar velocity are compared using computer simulations. The best approximation for the mean Doppler lidar velocity is used to calculate the effects of the spatial averaging of the radial velocity by the lidar pulse on Dopple...

Remote sensing of atmospheric wind velocities using solid-state and CO/sub 2/ coherent laser systems

Coherent lidar/laser radar systems have been used for the remote measurement of atmospheric wind velocity since 1966. Both CW and pulsed coherent lidars have been developed and applied to a variety of ground-based and airborne applications. In recent years, most efforts have concentrated on pulsed CO/sub 2/ and solid-state Doppler lidars for the remote measurement of atmospheric wind velocities. Issues associated with the design and application of coherent lidars to atmospheric wind measurement are discussed. Coherent lidar/laser radar systems have been applied to detection and tracking of aircraft wake vortices, measurement of atmospheric wind fields during the space shuttle takeoff and landing, airborne windshear detection, ground-based airport microburst windshear monitoring, meteorological research, and environmental monitoring. Numerous examples of measurement results are presented. An assessment of the status of coherent lidar technology as practical devices for operational and research uses is addressed.

Pulsed LIDAR for obstacle detection in the automotive field: the measurement of reflectance range data in scene analysis

The performance of an active laser radar obstacle detection system (LIDAR) of the third generation applied to an automobile is presented. Dynamic measurements of the detector, mounted on a car and tested on the road, exhibit the improvements in this system in comparison with the preceding prototypes.

Compact Eye Safe Lidar Systems.

New technology for compact, eye safe lidar is surveyed. Advances with solid state lasers anddetectors permit efficient lidar designs that meet requirements for full time atmospheric monitoring. Micro pulse lidar is an approach that employs kHz pulse rate Nd : YLF lasers and microJoule pulse energies at visible wavelengths. Eye safety is an essential factor and is obtained bybeam expansion. High performance is realized by solid state Geiger mode avalanche photodiodephoton counting signal acquisition. The capability to monitor all significant cloud andaerosol scattering by a compact instrument has been demonstrated. Pseudorandom modulation CW lidar is under development. CW diode lasers are used directly as transmitters with theadvantage of wavelength tunability for water vapor and other differential absorption measurements. Background photon noise and dynamic range are problems for CW lidar. Advancedconventional pulsed diode laser lidar and near infrared lidar at fundamentally eye safewavelengths are in development for some applications.

Characterization of pulsed coherent Doppler LIDAR with the speckle effect

The relationships among heterodyne efficiency γ, number of speckle cells M, and the ratio of receiver area to coherence area S(R)/S(C) for a pulsed coherent laser radar (CLR) are written through the use of mutual coherence functions. It is shown that numerical values for S(R)/S(C) that follow Goodman's definition [J. W. Goodman, in Laser Speckles and Related Phenomena, J. C. Dainty, ed. (Springer-Verlag, Berlin, 1975), pp. 9-75] or that are obtained through the use of a transverse-field coherence length agree. In the frame of the Gaussian model proposed by Frehlich and Kavaya [Appl. Opt. 30, 5325 (1991)] a new equation is derived: M = (1 + S(R)/S(C)). This equation agrees with our experimental results. Our theoretical analysis shows that the number of speckle cells for an optimal monostatic CLR system is M ~ 4. An experiment has been conducted with a ground-based pulsed CO(2) LIDAR and remote hard targets to study the probability density function of LIDAR returns as a function of M and to study the dependence of M on S(R)/S(C). An assessment of CLR performance through the use of M or the collecting aperture S(R) is discussed.

32.O.02 Technique to simulate the time-dependent signal of the pulsed lidar with the separately located illuminating and receiving platforms for the aerosol space sounding

32.O.02 Technique to simulate the time-dependent signal of the pulsed lidar with the separately located illuminating and receiving platforms for the aerosol space sounding

A lidar technique to quantify surface deposition from atmospheric releases of bulk liquids

We show that a scanning, pulsed lidar can be used to quantify the time history and areal concentration of mass deposited on the ground from an elevated release of bulk liquid. Aircraft measurements, witness car depositions and evaporative modelling crudely support results from analysed lidar data.

Evolution of the Monterey Bay Sea-Breeze Layer As Observed by Pulsed Doppler Lidar

Abstract As part of the Land/Sea Breeze Experiment (LASBEX) to study the sea breeze at Monterey Bay, the pulsed Doppler lidar of the NOAA/ERL Wave Propagation Laboratory performed vertical and nearly horizontal scans of the developing sea breeze on 12 days. Analyses of Doppler velocity data from these scans revealed details on the growth of the sea-breeze layer and on the horizontal variability of the sea breeze resulting from inland topography. Two days were selected for study when the ambient flow was offshore, because the onshore flow of the sea breeze was easy to discern from the background flow. Sequences of vertical cross sections taken perpendicular to the coast showed the beginnings of the sea breeze beneath the land breeze at the coast and the subsequent growth of the sea-breeze layer horizontally and vertically. On one of the days a transient precursor—a “minor sea breeze”—appeared and disappeared before the main sea breeze began in midmorning. Other issues that the lidar was well suited to stud...

A high-pulse-repetition-frequency CO2 Doppler lidar for atmospheric monitoring

A monostatic pulsed CO2 Doppler lidar operating in a short-pulse (<1 μs), high-pulse-repetition-frequency (≳1 kHz), low-pulse energy (1–10 mJ) mode has been developed for Doppler and return-power measurements within the planetary boundary layer. The system has achieved near-quantum-limited performance, demonstrating a Doppler measurement capability to approximately 5 km with pulse energies of 1–2 mJ.

Micro pulse lidar

An eye safe, compact, solid-state lidar for profiling atmospheric cloud and aerosol scattering is described. The transmitter of the micro pulse lidar is a diode pumped microjoule pulse energy, high-repetition-rate Nd:YLF laser. Eye safety is obtained through beam expansion. The receiver uses a photon counting solid-state Geiger mode avalanche photodiode detector. Data acquisition is by a single card multichannel scaler. Daytime background induced quantum noise is controlled by a narrow receiver field-of-view (FOV) and a narrow bandwidth temperature controlled interference filter. Dynamic range of the signal is limited by optical geometric signal compression. Signal simulations and initial atmospheric measurements indicate that systems built on the micro pulse lidar concept are capable of detecting and profiling all significant cloud and aerosol scattering through the troposphere and into the stratosphere. The intended applications are scientific studies and environmental monitoring. which require full-time unattended measurements of the cloud and aerosol height structure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Frequency monitoring a LIDAR transmitter for pulse-by-pulse analysis and control

An electronic technique is described whereby the heterodyne mixing frequency of a pulsed LIDAR transmitter is displayed on a color-coded linear bar graph. The technique is of great utility for manually fine tuning the laser output frequency when the use of conventional frequency counters and spectrum analyzers is impractical in a rapidly firing system.

Aerosol backscatter measurements at 10.6 micrometers with airborne and ground‐based CO2 Doppler lidars over the Colorado High Plains: 1. Lidar intercomparison

An airborne continuous wave (CW) focused CO2 Doppler lidar and a ground‐based pulsed CO2 Doppler lidar were used to obtain seven pairs of comparative measurements of tropospheric aerosol backscatter profiles at 10.6 μm wavelength, near Denver, Colorado, during a 20‐day period in July 1982. In regions of uniform backscatter the two lidars show good agreement, with differences usually less than ∼50% near 8‐km altitude and less than a factor of 2 or 3 elsewhere but with the pulsed lidar often lower than the CW lidar. Near sharp backscatter gradients the two lidars show poorer agreement, with the pulsed lidar usually higher than the CW lidar. Most discrepancies arise from a combination of atmospheric factors and instrument factors, particularly small‐scale areal and temporal backscatter heterogeneity above the planetary boundary layer, unusual large‐scale vertical backscatter structure in the upper troposphere and lower stratosphere, and differences in the spatial resolution, detection threshold, and noise estimation for the two lidars.

Aerosol backscatter measurements at 10.6 micrometers with airborne and ground‐based CO2 Doppler lidars over the Colorado High Plains: 2. Backscatter structure

Measurements of tropospheric aerosol volume backscatter coefficients at 10.6‐μm wavelength were obtained with airborne continuous wave and ground‐based pulsed CO2 Doppler lidars over the Colorado High Plains during a 20‐day period in summer 1982. A persistent “background” layer was found between 6‐ and 10‐km altitude, with a generally uniform backscatter mixing ratio of ∼10−10 m2 kg−1 sr−1. The upper boundary of this background layer varied with the tropopause height; the lower boundary varied with the strength and diurnal cycle of convective mixing in the planetary boundary layer (PBL). For quiescent meteorological conditions the transition from the PBL to the background layer was usually very sharp, with backscatter decreases sometimes as large as 3 decades in ∼70 m. Sharp gradients were also found at the boundaries of shallow (tens of meters) subvisible cirrus clouds. For less stable conditions, associated with vertical aerosol transport by deep cumuliform clouds, backscatter tended to decrease exponentially with altitude.

RamBackscattering, extinction, and liquid water content in fog: a detailed study of their relations for use in lidar systems

A solution to the single-scattering lidar equation to infer optical extinction, liquid water content W, and visibility in a fog through a monostatic pulsed lidar, requires the use of the relations, and combinations of them, between backscattering coefficient beta, extinction coefficient sigma, and W. To this end, beta and sigma have been computed for 239 droplet size spectra and forty wavelengths from 0.25 to 12 microm, together with W, and the three relations beta vs sigma, W vs sigma and sigma(0.55 microm) vs sigma have been determined. The analysis of their behavior with wavelength shows that (1) the relation beta vs sigma is mostly reliable in the lambda = 0. 25-2-microm region, where the dispersion of the beta values around the best-fit curve is within 20%; (2) the relation W vs sigma is generally not well verified (the dispersion is centered around 30%), and when used in connection with the beta vs sigma relation to infer liquid water content, their joint dispersion P(D) is always greater than 20%; (3) the relation sigma(0.55 microm) vs sigma is well verified in the region lambda = 0.25-2 microm (dispersion within 15%), and when used in connection with the beta vs sigma relation to infer visibility P(D) appears to be minimum at lambda congruent with 0.35 microm (P(D) = 4.7%).

Receiving efficiency of monostatic pulsed coherent lidars 1: Theory

The receiving efficiency eta as a function of range z is investigated for pulsed coherent lidars using a theory that relates eta(z) to the transmitted laser intensity and the point-source receiving efficiency eta(s)(r,z). The latter can be calculated either by a forward method, or by a backward method that employs the back-propagated local oscillator (BPLO) approach. The BPLO method is efficient and accurate provided that cascaded diffraction effects inside the lidar system are properly taken into account. The theory is applied to the ideal case to examine the optimization of the system when both transmitted and BPLO fields at the antenna are Gaussian, including optimum telescope aperture.

Receiving efficiency of monostatic pulsed coherent lidars 2: Applications

Using the theory developed in Part 1, the receiving efficiency as a function of range, eta(z), is calculated under different conditions for the NOAA/ERL/Wave Propagation Laboratory CO(2) Doppler lidar. Theoretical analyses, numerical calculations, and experimental measurements are carried out to quantify the sensitivity of eta(z) to transmitted laser beam quality, telescope focal setting, telescope power, scanner astigmatism, LO beam divergence, and system misalignment. These results bring insight to the design of practical coherent lidar systems.

Atmospheric Velocity Spectral Width Measurements Using the Statistical Distribution of Pulsed CO2Lidar Return Signal Intensities

A pulsed CO2 lidar with coherent detection has been used to measure the correlation time of backscatter from an ensemble of atmospheric aerosol particles which are illuminated by the pulsed radiation. The correlation time of the backscatter of the return signal, which is directly related to the velocity spectral width, can be used to study the velocity structure constant of atmospheric turbulence and wind shear. Various techniques for correlation time measurement are discussed, and several measurement results are presented for the technique using the information contained in the statistical distribution of a set of lidar return signal intensities.

Doppler Lidar Observations of a Downslope Windstorm

Abstract During January and February 1987, the NOAA/WPL pulsed Doppler lidar was deployed in the foothills west of Boulder, Colorado, to study orographically induced flows over the Continental Divide. On 29 January 1987, the lidar, with its unique spatial and temporal data-gathering capabilities, documented a downslope windstorm affecting the Boulder area and the rest of the Front Range. The lidar recorded in detail 1) a low-level leeside wind maximum, 2) propagating wind gusts exhibiting two distinct periodicities, and 3) the eastern edge of a mountain wave feature where a jumplike flow reversal occurred. Such structures have not previously been observed with comparable detail by conventional in situ and remote sensing instruments. The observed phenomena were similar to results obtained from mountain-wave numerical models. The most notable of the structural similarities was between the observed and modeled wind gusts.