Multiple scattering effects in droplet aerosols were investigated theoretically using three different numerical approaches and experimentally by means of a specially developed measuring system. The system allows quantitative determination of laser light extinction and scattering. Measurements were performed in scale model clouds formed by growing water droplets. This was accomplished in an expansion cloud chamber under well-controlled laboratory conditions. In many cases however, the investigated models and the experimental evidence showed substantial discrepancies depending on parameters and ranges of variables within which a model was used. The work is in progress and preliminary experimental results are discussed and compared with computations. Based on the experimental conditions chosen in this study, an attempt was made to explain the accuracy of the applied models to describe multiple scattering effects in aerosols.

The propagation of pulsed laser radiation along the atmospheric ground paths containing droplet aerosols (fog, drizzle, rain) belongs to a class of problems in nonlinear optics in which the multiple-factor nature and nonadditivity of the processes affecting the transmission of the radiation channel are strongly pronounced. The interaction of the radiation with the medium along the propagation path strongly depends on the type and microphysical properties of the specific meteorological formation as well as on the energy parameters of the beam, the structure of the beam, the temporal regime, and the conditions of focusing. The purpose of this work is to analyze the affect of these factors on the optical characteristics of the channel (the integral transmission) under conditions of explosive vaporization of droplet media.

Linear and nonlinear time-dependent backscattering calculations for dielectric spheres are presented. The time-dependent backscattered linear fields depend upon: (1) the linewidth of the pulse and the linewidth of the resonance of the sphere, and (2) the frequency separation between the center frequency of the pulse and the resonance frequency of the sphere. When the refractive index or absorption of the sphere is intensity-dependent, then the backscattering also depends upon the intensity of the laser pulse. When the droplets have a large intensity- dependent index of refraction or a large saturable absorption, and are illuminated near a resonance with a high-intensity laser pulse, hysteresis may be seen in the back-scattered intensity.

The design of a new semi-analytical Monte Carlo simulation is discussed in detail in this paper. This model uses two stages. In the first stage, the simulation itself, the contribution of each scattering event to the total reflectance and transmittance is evaluated. Thus the photon energy decreases more rapidly during its random walk and fewer steps are required to obtain a given accuracy. The reduced number of necessary steps makes it possible to store all events positions and energies. In the second stage, the results of the first stage can be used to calculate analytically any desired result. Examples are given for scattering slabs of isotropic or anisotropic scatterers when collimated beam incidence is used. Reflections at the boundaries are taken into account. The results obtained with this new method and classical Monte Carlo methods are identical. However, the convergence of our new model is much better and, because of the separation in two stages, any quantity related to the problem can be easily calculated afterwards without recomputing the simulation.

A Monte Carlo procedure to deal with propagation of radiation in a turbid medium presenting some simple kinds of inhomogeneity is briefly outlined. Cases of lidar returns from inhomogeneous clouds are shown.

Both active and passive atmospheric modulation transfer function (MTF) measurements carried out simultaneously in both thermal imaging atmospheric windows are presented. Results indicate rather significant angular spatial frequency dependence of the MTF, in contradiction to the conventional approach which assumes contrast transfer is atmospheric transmission only. A theoretical explanation is discussed based upon aerosol forward scattering and absorption effects which are shown to be angular spatial frequency dependent and yield MTF results similar to those measured. This means that small targets are blurred much more than large targets by the atmosphere, thus also affecting target acquisition probabilities.

An analysis is presented of aerosol particle size distributions measured over the North Atlantic and extinction coefficients derived from these data. Two empirical models, an aerosol model and an extinction model, are formulated in terms of simple meteorological parameters (wind speed, relative humidity, air temperature and sea temperature). The choice of these parameters is based on considerations of their effects on the aerosol physics in the marine atmosphere. The performance of the models for predictions of the extinction coefficients at (laser) wavelengths in the visible and IR atmospheric windows is assessed. The results are compared with predictions of the Navy Aerosol Model (NAM).

This paper discusses the phenomena associated with signal attenuation through dust and foliage and quantifies the impact of these environments on signal propagation. Specifically, models used to characterize dust and foliage attenuation are developed and experimental data presented. Based on the model's results, conclusions are drawn concerning the relative impact of dust versus foliage on current satellite communications (SATCOM) links. Geopolitical events of recent years have given rise to an increased likelihood of operations requiring communications in these environments. Thus, the importance of these findings in today's world cannot be overstated.

A simple and convenient tool to investigate the radiative characteristics of the atmosphere- underlying surface system is described here. It is the ATMOTOOLS package implemented for IBM PC and synthesizing up-to-date optical atmosphere models, new procedures for refining these models, and special radiation calculation procedures. Considerable attention is paid to aerosols models and arrangement of data banks on optical aerosol properties. Calculation procedures for radiative transfer are developed with regard to atmosphere stratification, light polarization, multiple scattering, and so on. Some examples of investigating different problems connected with remote sensing of ocean, ozone, and clouds using ATMOTOOLS are given.

This paper deals with the following problems of the theory of vision: t-effect, influence of a scattering medium and geometric conditions of observation on the image resolution, and hardware and software for elimination of a distorting effect of scattering media on the image quality of extended objects. Additionally, the information on the software developed at the Institute of Atmospheric Optics for modeling the images distorted with specified scattering media is presented.

In this paper, we study theoretically light scattering by polydisperse, randomly oriented, rotationally symmetric particles of size comparable to the wavelength of radiation. In our computations, we use the T-matrix approach, as extended recently to randomly oriented particles [M. I. Mishchenko, J. Opt. Soc. Am. A 8, 871 (1991)]. Results of extensive numerical calculations for particles of different shape are presented. The influence of particle size distribution, shape, and refractive index on the scattering patterns is examined and implications for remote sensing of nonspherical aerosols are discussed.

This paper evaluates equivalent sphere approaches for modeling nonspherical aerosols in MTFs calculations. A Monte Carlo simulation is used to calculate the image of point source in a scattering medium. The MTF is then found using a 2-D FFT on the resulting image (power distribution). The aerosol phase functions are computed using Mie theory for spherical particles and the Method of Extended Boundary Conditions for nonspherical particles. Spheres with radii determined by the nonspherical aerosol volume, surface area, orientation averaged radius, semi-minor axis length, and semi-major axis length are considered. Sample MTF results are presented for a 2:1 axial ratio prolate spheroid at a wavelength of 9.2 micrometers .

The purpose of this work is to develop and to test the approach combining path integral technique and complex-valued Monte Carlo method for calculating the moments of the Green function of stochastic wave equation for random media with the large-scale inhomogeneities. The numerical results for the second and fourth moment have been obtained in the framework of the three models: Markov approximation, parabolic, and Hankel's function approximation with cluster integrals. The finiteness of the correlation radius has been shown to be the reason for significant difference between Markov approximation and the two others. Comparison has been made in good agreement with reliable results for the 1-D case.

We discuss the validity and the precision of scaling principles which can be used to transform problems of radiative transfer in media that scatter anisotropically (g does not equal 0, g being the mean cosine of the scattering angle) to equivalent isotropic problems (g equals 0). A Monte Carlo model is used to calculate the reflection (R) and the transmission (T) factors for a collimated incident beam on an anisotropically scattering media, for a wide range of absorption ((kappa) ) and scattering ((sigma) ) coefficients per unit length. The results (R,T) obtained for anisotropic scatterers (g does not equal 0, (kappa) , (sigma) ) are systematically compared to those (R', T') obtained in the case of the equivalent isotropic configuration (g' equals 0, (kappa) ', (sigma) ') as deduced from the similarity relations.

The transmission of a stationary beam of optical radiation through a thick layer of a turbid medium with strongly anisotropic scattering (sea water, turbid atmosphere, clouds, etc.) is considered. To describe the process of light propagation the procedure of the self-consistent renormalization of the photon diffusion coefficient in the radiative transfer equation is proposed. This approach enables one to obtain simple analytical equations for the parameters of the radiance distribution (the angular width and the polar angle corresponding to the apparent peak of the radiance) and the total flux of radiation. In the case of power-law phase functions the self-consistent method results in the novel scaling relations between the radiance parameters and optical coefficients of turbid media. The results obtained are convenient for practical applications in turbid atmosphere and sea water.

A study, performed outside the framework of the Fokker-Planck approximation, is presented of the depth mode of light propagation in turbid media with slowly decreasing scattering phase function X((gamma) ), when the scattering probability decreases more slowly, than (gamma) ^-4, as the single scattering angle (gamma) is enhanced. Postulating the approximate shape of the angular spectrum in the depth mode, we propose and realize a regular procedure for optimum determination of the parameters of this spectrum. The dispersion in the depth mode and the depth damping coefficient are found analytically. The obtained results are in good agreement with the results of numerical calculations and transform, in the limit, into known results of the diffusion approximation.

A study is presented for the 2-D case of light propagation in a turbid medium involving sharply anisotropic one-center scattering, in accordance with the Heney-Greenstein law, with strong absorption (when the photon absorption length is shorter than the transport length). An exact analytic solution, that takes into account fluctuations of the photon paths, has been obtained, within the framework of the small-angle approximation. Angular spectrum and attenuation coefficient for the deep-propagating mode are analyzed in detail.

Analytical methods are considered for calculating the parameters of light in media with sharply anisotropic phase functions. Compact expressions are obtained that are suitable for performing numerical calculation of the principal parameters of a broad non-stationary light beam normally incident upon the surface of a medium.

It is well known that the presence of aerosols in the atmospheric boundary layer can have a significant impact on electromagnetic propagation, and the underlying physical processes involving extinction, multiple scattering, and thermal emission are reasonably well understood. In this paper we examine a related, but less well understood, aspect which we term aerosol-induced `radiative damping' that can alter the local atmospheric stability and the vertical profiles of temperature and humidity which, in turn, can alter the vertical profiles of optical turbulence and hence image propagation.

Recent studies of beam wave propagation have led to a new and useful method of analysis for describing the diffractive geometry of a Gaussian beam and to a number of new results involving optical turbulence effects. This paper is primarily a review of these studies.

It has been shown in previous work that double passage imaging combined with spatial filtering leads to an improvement in the quality of the time-averaged image obtained from coherently reflecting objects (mirrors) which are placed in turbulence. In this paper, the results of experimental investigation of the imaging of partially coherent objects are presented. We have demonstrated that double passage imaging leads to an improvement in image quality only for objects whose correlation scale is larger than the coherence radius of the illuminating radiation.

In the theory of atmospheric turbulence, the strength of the spatial variations of the index of refraction n is proportional to a parameter known as the atmospheric structure constant, denoted C_n², which is a function of position along the optical path z. The strength of the temporal variations of the index of refraction is directly related to the transverse velocity ^YLDV of the turbulence along z. Current optical techniques for remotely sensing C_n² and ^YLDV rely primarily upon the spatial or temporal cross-correlation properties of the intensity of the optical field. In the technique proposed here, we exploit the correlation properties of the wave front slope measured from two different point sources in order to obtain vertical profiles of C_n²(z) and ^YLDV(z). Resolution on the order of 100 meters is possible with reference sources separated by 0.2 degrees. Additionally, signal-to-noise ratio (SNR) calculations for a single measurement of C_n² and ^YLDV are presented for a Hufnagel-Valley C_n² profile and a Bufton wind profile. The SNR results indicate the need for multiple measurements to obtain useful estimates of the desired quantities.

Several papers have treated the implementation of adaptive optics systems in astronomical imaging from the ground, for the correction of wavefronts degraded by the atmosphere. The case of imaging at horizontal paths, parallel to the ground, has not been as thoroughly discussed. It is shown here that when the statistics of the turbulence are homogeneous along the path, the distortions of the wavefronts, originating from different points in the field of view, are uncorrelated. Thus, the adaptive optics compensate the wavefront degradation only for a very small area in the field of view, while increasing the degradation outside of this area. It is concluded, that for terrestrial reconnaissance systems, adaptive optics can not give a solution for the image degradation caused by atmospheric turbulence, in a practical field of view.

When a high-frequency electromagnetic wave propagating in a random medium is scattered by the embedded obstacles, the reflected backwards radiation can traverse through the same random inhomogeneities as the incident one, leading to the anomalous intensity distribution of the scattered radiation. Here, we construct the intensity response at the image plane of an optical system, resulting from a backward reflection from a target having discontinuities. The object plane - image plane relations are obtained by using the multiscale propagators for the coherence measures and the resolution effects are considered for point source - two-point scatterer configuration.

The question of interdependence of statistical characteristics of wave field upon its behavior in particular realizations is analyzed with the aid of a number of the simplest examples.

The possibility of improving the image quality of a reflecting object, viewed through a randomly inhomogeneous medium, is theoretically investigated. Three cases of coherent illumination of the object are considered: illumination by unperturbed wave, illumination and viewing through different inhomogeneities of a medium, illumination and viewing through the same inhomogeneities (double passage imaging). It is shown that, when the fluctuations of the intensity in the plane of the receiving lens aperture are strong, the use of a suitable spatial filter always gives an improvement of the average image for the object, correlation scale of which is larger than coherence radius of a spherical wave propagating in the medium.

Statistical values of intensity of laser beams propagating in the turbulent atmosphere with precipitation are discussed. Regimes of weak and strong scintillation are determined. The phenomena of saturation and of decrease of the scintillation index in the strong scintillation regime are experimentally observed and theoretically explained. Dependence of the statistical values on the geometry of experiments is investigated too. Some methods of retrieval of the atmospheric parameters are proposed.

Based on the technique of effective beam parameters are found the conditions of existence of the exact aberration solutions for the effective width, radius of phase front curvature, and limiting divergence. The effect of similarity of processes under the strong nonlinear distortions for different classes of a collimated beam self-action and mechanisms of nonlinear interaction are revealed. Regimes for formation of the limiting divergence in the primarily homogeneous and inhomogeneous nonlinear refractive media are studied. Relationships between the parameters of inhomogeneous path and the initial parameters of laser beams are determined for the case of weak nonlinear distortions.

The paper is dedicated to theoretical treatment of laser beam propagation along vertical and elevated paths from the ground surface through all thickness of the atmosphere. The length of the propagation paths is proposed therewith to be far more than thickness of the distortion layer since the random distortions of the wave beam due to turbulent fluctuations of the air refractive index arise mainly in lower layers of the atmosphere. The mean intensity of laser beam as well as relative variance, and spatial and temporal correlation function of its intensity are considered.

It is shown that in a regime of weak turbulence the edge diffraction on a reflector has a profound effect on intensity distribution of the reflected wave and on the manifestation of backscatter amplification. Therewith the dependence of the amplification factor on the Fresnel number of the reflector radius has an oscillating nature both for the corner reflector and for the plane mirror one. In going from a weak turbulence regime to a strong one the effect of diffraction on reflector edges on backscatter enhancement becomes negligible.

In recent years there were published many papers detailing an important problem on the use of adaptive means and adaptive optics systems to improve the image of objects observed through the atmosphere. These papers concerned the theoretical and technical aspects of the problem on constructing an adaptive optical telescope.

The atmospheric turbulence spectrum even in the surface layer differ by the larger dynamical range and, in accordance with it, in view of the finite correctness of the optical measurements by themselves, cannot be reconstructed from the measuring of any one of the optical wave parameters. Measurements of the fluctuations of the optical wave phase can be used for investigation of the energy range of the turbulence spectrum.

The propagation of partially coherent beam in refractive media (linear or nonlinear) obeys the equation for the second order coherence function. Numerical solution of this equation is encountered with difficulties, because this equation is in five independent variables. A ray method for solving this task is constructed. The advantage of this approach is that partial differential equation reduces to the set of ordinary differential equations similar to the geometric optics equations. But there are no divergences in the present method due to the presence of diffractive term in the ray equation. The accuracy of this method is discussed. It is shown that propagation of the coherent beam is exactly described by this technique. Based on the solutions obtained with this technique a comparison of self-action of a coherent and partially coherent beams with the same Fresnel number is made. Relations of this technique to the ray methods of solving small angle radiation transfer equation are discussed.

We define the diffractive limit of phase-only compensation and compute this limit in the form of the strehl ratio of a perfectly phase-only compensated scoring laser beam on target. A comparison of uplink, downlink including horizontal compensated propagation, is made.

Efficiency of phase-conjugate correction for random tilts of wavefront in formation of laser beams propagating through the turbulent atmosphere is studied. Increase of the mean intensity of radiation and the damping degree of the intensity fluctuations on the beam axis in the recording plane are investigated.

A study of optical turbulence measurements at 1 m above ground level over an arid desert terrain was conducted at White Sands Missile Range, New Mexico, in the spring of 1992. The optical turbulence was characterized by the index of refraction structure function, C_n², measured directly with scintillometers. Following a side-by-side comparison of the scintillometers along essentially identical 1-km paths, the calibrated sensors were installed along a 3750-m path for this study. The path was segmented into four nearly equal subdivisions, each equipped with a scintillometer transmitter-receiver pair. This paper describes the terrain variation by segment and the local and synoptic weather conditions during the study, and summarizes the observations and correlations drawn from intercomparing the four simultaneously sampled scintillometer measurements acquired along the 3750-m path.

A theoretical and experimental investigation of image quality through an inhomogeneous turbulent medium under controlled laboratory conditions is presented. In addition, a theoretical analysis based upon simple physical principles leads to the same theoretical results obtained 25 years ago by Fried on the basis of phase covariance and phase structure function. The present analysis, however, can be expanded to indicate how effects of turbulence depend upon field of view. This work is applicable toward determining optimum elevation for imaging through inhomogeneous turbulence near the surface of the earth.

The problem of phase distortion correction caused by atmospheric turbulence is considered. It is shown that the accuracy of angular measurements and the resolution of a linear aperture may be improved by making use of a reference source with known position. The limits of such a correction are investigated in the case of Kolmogorov turbulence spectrum.

The short-term statistical characteristics of the point spread function of a linear aperture in the presence of turbulent atmosphere are introduced. It is shown that these characteristics exhibit adequately the behavior of the random diffraction pattern in the case of atmospheric turbulence having Kolmogorov spectrum.

A short-term beam spread theory developed starting from the path integral representation of the field in random media. The new approximate formula is obtained for average short term intensity distribution. This formula takes into account all geometrical and diffraction beam parameters. Results of average short-term on-axis intensity and beam size calculations are presented. The beam parameters optimization is performed for horizontal and slant paths. The difference between short-exposure and tilt-corrected beam spread and applications of developed theory to adaptive optics problems are discussed.

Expressions are derived for the mutual coherence function (MCF) for a Gaussian beam wave using a bump spectral model for refractive-index fluctuations with inner scale parameter but assuming infinite outer scale. The analysis is based on both weak and strong fluctuation theories, and comparisons are made between theories where possible. The presence of a bump in the spectrum is shown to slightly increase the beam spread and reduce implied spatial coherence length.

Although analyses general enough to account for both the temporal and spatial characteristics of the adaptive optical system exist, they are complex and require detailed information regarding the wave front sensor, the deformable mirror, and the control algorithm. This investigation develops a frequency domain analysis that predicts the performance of an adaptive optical system having an arbitrary temporal response. The analysis takes into account aperture piston and tilt removal and spatial bandwidth limitations due to the wave front sensor's finite subaperture size and the deformable mirror's finite actuator spacing. The unique aspect of this model is the relative ease with which performance characteristics of different spatial and temporal system response functions can be investigated.

Laser propagating through atmosphere is distorted by thermal blooming and turbulence. Thermal blooming in atmospheric laser propagation is studied using Fast Fourier Transform (FFT). Variation of steady state thermal blooming with laser power and propagation distance are shown and power threshold is also calculated. Time-dependent thermal blooming effects with and without wind are investigated. Comparisons between simulated and experimental results reach good agreements.

Methods for atmospheric calibration of reflected and thermal images using ground truth are briefly reviewed. These methods set the stage for consideration of `in-scene' and atmospheric propagation modeling methods. The emphasis is on placing the `in-scene' and radiation propagation methods in a common radiometric framework and discussing the strengths and weaknesses of the various methods. The methods treated include multiple altitude, multiple look angle, multiple bandpass, and LOWTRAN-style radiation propagation models for calibration of thermal infrared airborne and/or satellite systems. A limited treatment of selected methods for calibration of atmospheric effects in the reflected region is also included.

The representation problem of the LDA wind measurements in the boundary layer under different atmospheric conditions has been investigated theoretically and experimentally. The calculations of the mean wind velocity measurement errors for the surface layer under different types of thermal stratification and for the boundary layer under neutral conditions have been carried out. The theory conclusions are confirmed by the experimental results.

Space-based lidar systems are planned for a number of applications. One application being considered is to use a space-based lidar to infer information about the visibility near the surface from remote or inaccessible areas. This can be accomplished if one can obtain information about the optical properties near the surface. The concept would involve using a lidar on a space platform probing the atmosphere and underlying surface along its orbital path. The purpose of this research has two goals. The first is to determine if a unique relationship can be found at a suitable laser wavelength to relate the extinction coefficients near the surface to the visibility at the surface. The second goal is to determine if lidar back scatter measurements can be inverted in a reasonable fashion to obtain the extinction coefficient near the surface. If these goals can be met, then visibility can be obtained on a routine basis from space-based lidars. This paper presents the results from the study. In the study, a number of different lidar wavelengths have been studied to see if one is more suitable than the others. Also, an examination of the assumptions required to perform the inversion of the lidar equation has been made.

The structure constant of humidity fluctuations is an important statistical characteristic of the turbulent atmosphere. We propose a new method for the remote sensing of this structure constant using bistatic acoustic sounding. First of all, the new approximate equation for a sound wave in a turbulent medium with random inhomogeneities of the adiabatic sound speed, density and medium velocity is presented. This equation has a wider range of applicability than equations used before in the theory of sound propagation in turbulent media. Starting from this equation, the sound scattering cross-section (sigma) in the turbulent atmosphere is calculated. From the obtained equation it follows that at the scattering angle (theta) equals 90 degree(s) the sound wave is scattered only by humidity fluctuations. Therefore, measuring the scattering cross-section (sigma) (90 degree(s)) by bistatic acoustic sounding we can obtain the structure constant of humidity fluctuations in the atmosphere. Some problems of practical realization of the proposed method are considered.

MODTRAN2 (1992) is the most recent version of MODTRAN, the Moderate Resolution Atmospheric Radiance and Transmittance Model, first released by the Geophysics Directorate, Phillips Laboratory, in 1990. It encompasses all the capabilities of LOWTRAN 7, the historic 20 cm^-1 resolution radiance code, but incorporates a much more sensitive molecular band model with 2 cm^-1 resolution. For inversion algorithm applications, MODTRAN2 must prove to be sufficiently accurate when calculating layer- specific perturbations. First steps in establishing this capability have recently been accomplished. DREV (Defence Research Establishment Valcartier, Canada), in conjunction with the Geophysics Directorate, has taken measurements with a surface-based Bomem interferometer (approximately 1 cm^-1 resolution), with full supporting sonde profiles (z, T, p, and relative humidity). This suggests that the derivative matrices, typically required for inversion algorithms, may be readily (and rapidly) calculated using MODTRAN whenever its spectral resolution is adequate.

The design of a three-channel solar radiometer for obtaining total columnar water vapor using solar transmittance and differential absorption is presented. Water vapor transmittance is determined using a modified Langley approach and converted to columnar water vapor using a band model developed at the University of Arizona. Several cases are presented in which columnar water vapor amounts determined using the current instrument and method are compared to sounding balloon results. Tests using simulated data indicate that columnar water vapor may be retrieved with an uncertainty less than 10%.

The information content of radiance measurements in the O₂A- and O₂B-bands over the oceans for aerosol monitoring from space is discussed. A simple model of radiation transfer in the system atmosphere/ocean shall be used for estimation of optical thicknesses of different aerosol layers and ocean reflection. There are 3 nonredundant spectrometer channels in the O₂A-band around 762 nm and 2 nonredundant channels in the O₂B-band around 687 nm. High measurement accuracy of about 1% is more important for the measurements than a halfwidth (Delta) (lambda) _FWHM of spectrometer channels smaller than 1 nm. A sensitivity analysis shows that the optical thickness of stratospheric aerosols can be estimated from measurements in the O₂A- and O₂B-bands. From these results the chance to use combined measurements in both bands for distinction between volcanic and background- stratospheric aerosols exists. There also is a chance to use measurements in the O₂A-band for estimation of the optical thickness of aerosols in the free troposphere. The estimation of the optical thickness of aerosols within the maritime boundary layer can be expected from measurements in the O₂A-band, if ocean reflection is known. To estimate optical thicknesses of different aerosol layers is not only useful for aerosol studies, but also for better atmospheric correction of satellite images of the ocean surface.

Determining the precise location of low altitude ground based lidar returns is the topic of this paper. This involves consideration of the curvature of the earth and atmospheric refractivity. A closed form geometrical optics solution is obtained for a spherical earth and a modified index of refraction which has a quadratic dependence on altitude as determined by a linear temperature profile. In this way the ray path of a laser can be plotted as a function of range and azimuth in a variety of realistic atmospheres. Resulting ray paths and corresponding altitude errors are presented for near horizon measurements.

The in-flight calibration of satellite radiometers using ground truth measurements relies on the use of an atmospheric radiative transfer code. The accuracy of the calibration depends largely on the aerosol model used in the radiative transfer codes. In order to improve the calibrations, a camera system has been developed for the determination of the aerosol size distribution, index of refraction, and scattering phase function. In addition, the camera can be used to measure ozone and water vapor content. The camera uses a two dimensional silicon CCD array to image the sun and the solar aureole. A filter wheel provides eight spectral bands from 310 nm to 1045 nm. The camera is mounted on an altitude-azimuth mount for tracking the sun. An external computer allows automatic or manual data acquisition. This paper presents the design and calibration of the camera system. A companion paper presents the data collection and inversion techniques used to retrieve the parameters of interest.

A camera has been developed to image the sun and the solar aureole for use in aerosol studies. The design and calibration of the system are presented in a companion paper. The camera has eight spectral channels in the wavelength range from 310 nm to 1045 nm. The spectral bands and data collection procedure are chosen to allow retrieval of the atmospheric aerosol size distribution, index of refraction, and scattering phase function. In addition, the camera provides data for the retrieval of the atmospheric water vapor and ozone. The aerosol size distribution retrieval is based on the combined inversion of solar extinction and solar aureole data. The real part of the aerosol refractive index is determined using scattering measurements in the near-backward direction, while diffuse-to-global measurements provide the imaginary part. The performance of the inversion schemes is illustrated for in-flight satellite signal predictions over both bright and dark targets.

It is known that the use of lidar systems for investigation of the atmospheric aerosol makes it possible to obtain the optical characteristics and, with some definite assumptions, the scattering particles concentration. To have information about the aerosol microstructure, and especially about its space-time variations, it is necessary to enlarge the measuring information volume. With the monostatic pulse scheme of sounding, such an enlargement can be realized in the lidar using several lasers with different wavelengths. These optical systems are complicated technically and need automatization of the measuring complex. In order for the multifrequency laser sounding to be an effective method for remote measurements of the aerosol characteristics, it is necessary to develop mathematical methods of the algorithms for solution of the corresponding inverse problems taking into account their mathematical incorrectness. The results of these investigations, and their practical application, are described in detail in the papers.

The paper analyzes the results of nighttime lidar measurements of vertical distribution of aerosol and ozone at altitudes ranging from 10 to 35 km obtained from 1986 (aerosol) and 1988 (ozone) until the present time above Tomsk, Western Siberia. For investigations the lidar was used with a transmitter at (lambda) equals 308, 353, and 532 nm and a receiver with a 1 m diameter mirror operating in the photon counting mode. Space and time resolutions of aerosol and ozone were 375 meters and 15 - 20 minutes as well as 500 meters and 20 - 30 minutes, respectively. The basic parameters employed for the analysis of the aerosol and ozone stratification were the scattering ratio R(H) and ozone concentration n(H).

In recent years aerosol optical depths have been estimated from AVHRR satellite images on a regular basis. One of the primary uncertainties of these methods is in the choice of the aerosol phase function. In this paper the possibility of using the ratio of AVHRR channel 1 and channel 2 to infer the value of the aerosol phase function is studied. For this purpose a large number of marine aerosol measurements are employed to study the response of NOAA AVHRR satellites for various aerosol cases.

The technique and instrumentation for remote (ground) measurements of the optical refractive index structure parameter C_n² using an acoustic sounder (sodar) is described. The procedure of the sodar absolute calibration is discussed. The comparison between the sodar and in situ measurements suggests that the errors which are inherent to the method proposed are essentially less than the space-time variability of C_n². The results of the study of optically active turbulence in the atmospheric boundary layer above mountainous and urban areas are presented.

A study of two methods for computing temperature and velocity structure parameters C_T² and C_v² is presented. Fluctuating temperature and wind data obtained from sonic anemometers were processed utilizing time and frequency analyses to derive C_T² and C_v². The time-domain method is less rigorous and simplifies processing of the sonic anemometer data for deriving the turbulence parameters. The frequency-domain method has the advantage of providing the spectral characteristics of turbulence. This feature greatly facilitates in identifying data that contain spike noise. Thus, corrections may be made to adjust the derived values accordingly. The comparison shows a very good correlation between the two methods. However, the frequency-domain method yield results that are 2 2/3 times greater than the corresponding values from the time-domain method. The difference was determined to be a bias in the frequency-domain method, resulting from the coefficient used in the inertial subrange form of the one-dimensional spectrum, (Phi) _theta((kappa) ) equals (alpha) C_theta²(kappa) ^-5/3. In order to remove the bias, the coefficient (alpha) must be changed from the original value of 0.25 to 0.50.

We develop a dynamic stochastic model for refractive turbulence based on a linearization of the Navier-Stokes equations. The model is needed to estimate wind profiles from measurements of point to point propagation of a laser beam. We outline the model, which is meant to be an improvement on Taylor's hypothesis, and explore its implications in terms of the second order steady state statistics.

The problem concerned with the photocurrent frequency fluctuations of the cw-Doppler lidar and the pulsed Doppler lidar caused by the wind velocity fluctuations is considered. This problem is formulated based on the theoretical results of the study of the optical radiation scattered by suspended particles in a turbulent flow. It is shown that the measurement error of the average Doppler wind velocity depends on the photocurrent statistical characteristics. In the case of the non-Gaussian statistical characteristics the accuracy of the average wind velocity measurements increases with the increase of the scattering volume length. In the case of the Gaussian statistical characteristics the error of the average wind velocity measurements increases and saturates at the constant level with the increase of the scattering volume length. The theoretical results obtained are in agreement with the experimental data.

Use of airborne DIAL return signals from the Earth's surface in combination with a tomographic data processing technique enables one to essentially lower the sounding radiation power sufficient for effective sensing of the atmosphere. An optical arrangement of sounding and an algorithm of lidar data inversion aimed at reconstruction of two-dimensional fields of atmospheric parameters are proposed. Some results of numerical simulations of the technique are presented.

We present a modification to the laser radar equation (LRE) which includes the combined first-order effects of rough surface (RPS) and double passage (DP) RPS. A thin random phase screen atmospheric turbulence model is first included in the LRE. Rough surface RPS and DP- RPS are included individually, then we present the reciprocal path modified LRE (RPMLRE) for the combination of the two processes. The RPMLRE is formulated such that the predicted diffuse and coherent received power components are easily extracted from the total received power. Calculations are presented for a 1.064 micrometers ladar operating against a 1 m² space target. A substantially increased received power is predicted by the RPMLRE over that predicted by the LRE including turbulence only.

An experiment was performed to analyze the effects of random phase screen correlation upon reciprocal path scattering (RPS) for a mirror target. It is well known that when a propagation wave is passed through a random phase screen, then back reflected through the same phase screen, signal enhancement and increased signal fluctuations occur. As the phase screen encountered by the back reflected beam evolves from that which the transmitted beam encountered, the RPS phenomena degrades. The results due to Taylor phase screen slewing are presented.

A method for determining the transfer function of a continuous system from sampled responses to single and multiple pulse excitation is presented. The method is an extension of the sampled edge response method pioneered by the theoretical work of A. Papoulis in 1962 and the application of the theory to optical systems by B. Tatian in 1965. Occasions arise when pulse rather than step function stimuli are available for system excitations. In such cases, the method presented is practical for determining the system transfer function. The use of anti- aliasing filters and estimation of non-bandwidth limited transfer functions are also discussed.

An interactive system for predicting and taking into account the effect of the totality of linear optical phenomena in the real atmosphere on the accuracy and energy characteristics of opto- electronic systems and devices is presented. The interactive system consists of two packages of programs for IBM PC/AT.

A new edition of the spectroscopic molecular database, HITRAN, became available in March 1992. This current edition contains over 70 Megabytes of high-resolution data for transitions of 31 species and their atmospherically-significant isotopic variants. In addition, there is a file of cross-sections for molecular species with densely packed spectra, such as the cholorofluorocarbons, with bands at several representative temperatures that can be used for atmospheric retrievals with some transmission and radiation codes such as FASCODE3P. HITRAN is now available on alternative media: floppy diskettes and optical CD-ROM. Unlike the mainframe-standby (nine-track magnetic tape), these media permit rapid access and new user features. This presentation summarizes some of the major updates, improvements, modifications, and future directions to HITRAN as well as the new workstation-oriented media options.

Forward scattering by particles along a transmissometer path can increase the measured transmittance. The contribution of forward scattering is determined by the scattering phase function of the scatterers, their concentration and the optical characteristics of the transmissometer. We present simultaneous measurements of transmittance through fog, rain, and snow made with transmissometers of different wavelength and geometry. Converting the measured transmittances to extinction using the Beer-Lambert law, we find a linear relation between the simultaneously measured extinction values. The linear relationship suggests a simple model for the forward scattering effect.

In order to investigate the smoke cloud visibility of small wildfires a series of controlled biomass burning experiments has been carried out to investigate the characteristics of smoke clouds using various remote sensing techniques. These techniques include simultaneous scattering and transmission measurements in four wavelength bands, near-, mid-, and far- infrared video imagery, high resolution Fourier spectrometry, and particle size distribution measurements. The characterization and, in particular, knowledge on the contrast of smoke from small, beginning wildfires against a vegetation background is required in order to predict the performance of autonomous surveillance systems. This paper describes the preliminary analysis of experiments which have been carried out in Ypenburg (the Netherlands) in 1992. The results of these experiments are used to estimate the wildfire detection efficiency of a demonstration sensor which is being developed in a project financed by the Commission of the European Communities and by Bosschap. The autonomous wildfire detection sensor is described.

Calculations of the atmospheric attenuation at wave numbers between 33 and 333 cm^-1 have been made for the water vapor continuum and air. This region covers the submillimeter to the infrared. Results are given for wave numbers 25, 300, 350, and 236 cm^-1. The 236 cm^-1 wave number was selected because it is a potential propagation window. The local lines are not included in the calculations.

Ground level atmospheric extinction from 0.5 to 12 (mu) was determined as a function of date and time of day by a combination of direct visibility measurements and Mie calculations made using simultaneously measured particle size distributions. The measurements were made over a three month period in the Indian Wells Valley of the Mojave Desert during quiescent weather conditions. From previous work and a review of the literature an estimate was made of the composition of the dust and combustion produced aerosols. Results of Mie computations agree very well with direct visibility measurements. However, extinction in the 8 - 12 (mu) range was frequently greater than in the 3 - 5 (mu) range consistent with unquantified field observations made using HgCdTe detectors. This unexpected result is due to ammonium sulfate absorption bands centered at 7 and 8.8 (mu) indicating that a detector designed to have a narrow spectral response centered at 7 (mu) could experience difficulties. This work surveys detector data available in the open literature and compares the effect of detector spectral response on calculated transmission in the Indian Wells Valley as a function of time of day and frequency of occurrence of aerosol conditions.

Based on a unique experimental technique, measurement results are presented on the passive, remote sensing of the optical modulation transfer function of desert atmospheres (MTF_A), including the dc, low, and high spatial cutoff frequency components which are attributed to contrast, aerosol, and turbulence, respectively. In particular, use of this technique has made it possible, for the first time, to directly measure the low spatial frequency cutoff of the aerosol component. This technique is based on utilizing digital image processing of remote video scenes which include two, optically identical, castellated targets which are located at different distances and are contrasted against the horizontal sky. Ratios of apparent contrast and FFT calculations are used to determine the MTF_A components, including the spatial cutoff frequencies of the aerosol and turbulence components, independent of the imaging system and actual properties of the targets. The experimental technique is described along with current MTF_A component measurements.

The goal of this research was to map the regimes of validity of the weak-line and strong-line limits as a function of temperature, pressure, and path length for the 2.7 micrometers and 6.3 micrometers H₂O absorption bands. These calculations were done using an updated version of the NASA band model. A parametric study was performed where the error in assuming the validity of the two limits was calculated as a function of the physical parameters temperature, pressure, and path length. Results were generated in the form of spectral plots of the error and as band-integrated error presented in contour plots as a function of temperature and path length. Results indicate that for both bands, the weak-line error is localized in regions of intermediate temperatures and pressures at all path lengths. The strong-line limit error shows a linear increasing trend with pressure at short path lengths, while varying as a saddle-shaped function with respect to temperature and pressure at longer path lengths.

This paper discusses an aspect of spatially varying dissipative (extinction) and reflective (backscattering) losses that ultimately may allow a relatively quick appraisal of measured data quality, using a new figure of merit q(x), after some limited preprocessing. In order to do this, the paper presents the exact analytical, closed-form, finite solution for the inversion of the classical first-order nonlinear Riccati differential equation (RDE) which relates the reflection coefficient (R) and its derivative (dR/dx) to the otherwise arbitrary profiles of the media parameters [(epsilon) ₁(X) equals (epsilon) ₁(X) - j(epsilon) ₂(X)] in the coefficients of the RDE. This synthesis assumes a deterministic medium which can be accurately represented by the usual media parameters.

On the basis of invariant imbedding method the disturbance theory for complex constants of the propagation is proposed. This approach allowed us to consider influence of layer and anisotropic fluctuations of refractive index on the beyond-the-horizon propagation in the adiabatic approximation at the existence of the evaporation duct. Finally, the paper considers some examples of the calculations and investigates stochastic effects.

Measurements of wide band signal propagation peculiarities have been carried out in 37 GHz range under different season and meteorologic conditions on a 13 km long direct visibility path over rugged country near Kharkov (Ukraine). Dispersion effects conditioned by direct and indirect meteorologic and season factors have been discovered and measured. A method which allows measurements of difference-phase shifts in received signals has been developed. Quantitative evaluation of atmospheric communication channel coherence band variation has been carried out.

The short wave radiation in atmosphere will go through a multiple scattering process. In this paper, a special function, radiative function (RF), is studied to deal with the integration on optical depth involved in the calculation of every order of scattering lights, providing that the atmosphere is a plane parallel layer and the product of atmospheric scattering phase function and atmospheric single scattering albedo is a polynomial of optical depth. The properties of RF will be very useful to improve the calculation efficiency of multiple scattering.

The Conical Scanning Horizon Sensors on the Indian Remote Sensing satellite IRS-1B which detect the Earth's radiation in the 14 - 16 micrometers Carbon dioxide band have provided radiance data versus latitude and longitude. The present paper describes the method of generating the radiance data and the results obtained for the period January to July 1992.

Extinction of laser rangefinder (LRF) pulses by the atmosphere depends on the LRF wavelength, weather conditions, and the aerosol concentration along the optical path. The total atmospheric extinction (alpha) ((lambda) ) is the sum of the molecular and aerosol contributions, (alpha) _m((lambda) ) and (alpha) _a((lambda) ). We present simple expressions for (alpha) _m((lambda) ) and (alpha) _a((lambda) ) for the LRF sources: Er:glass, Ho:YAG, and CO₂ which operate near 1.54, 2.1, and 10.6 micrometers respectively. Also included are results for Nd:YAG which may be made to lase at the eyesafe wavelength of 1.444 micrometers . The expressions give an estimate of (alpha) ((lambda) ) as a function of standard meteorological parameters, assuming horizontal beam propagation. The effect of forward scattering on the received LRF signal is also discussed.

Propagation of electro-optical signals through a turbulent medium such as the atmosphere or the boundary/shear layer around an aircraft or a missile, causes image degradation. This paper examines the characteristics of such aero-optical degradation, including blur and strehl distribution. In particular, the effect of using different turbulence correlation approximations is analyzed.

Characterization of small scale structures within high speed turbulent flow fields requires instrumentation that is capable of acquiring high speed data at rates exceeding one megahertz. From experimental studies performed by the Teledyne Brown Engineering (TBE) Experimental Aero-Optics Group in conjunction with SY Technology, it has been observed that structures within a high speed turbulent flow have a limited lifetime. With the development of the Ultranac computer controlled high speed camera, the collection of high speed images was possible. The camera was capable of 8 to 24 short sub-microsecond exposure times and fast MHz frequency frame rates, all of which was variable and could be set independently for each frame recorded by the camera. An application for this system was demonstrated using a collimated beam of HeNe laser light to record shadowgraphs of turbulent flow structures generated by TBE's Aero-Optic Simulator (AOS). Argon gas was exhausted at a low speed from one nozzle and neon gas was exhausted at a higher speed from the other nozzle to give a calculated shear layer flow velocity of approximately 450 m/s. Frame-by- frame comparisons were made and flow structures were observed to persist for periods on the order of a microsecond. Based on experience from this preliminary demonstration, improvements for future experiments have been suggested. These tests clearly demonstrate the potential of the Ultranac camera to aid in the characterization of high speed turbulent flows.

A simple technique has been developed to record both the high and low frequency fluctuations contained within an aerooptically distorted 2-D point spread function. A collimated beam of light from an Nd:YAG laser, operated at 1.064 jim, was passed through a high velocity wrbulent flow field and imaged on the focal plane of a 128 x 128 array CCD camera (60 jun square pixels). Extremely short duration (50 nsec) pulses from the laser, synched with a high speed (92 frames per second), video data acquisition system captured one pulse per frame and effectively froze all motion in the flow. Post test averaging of single pulsed frames made it possible to visualize the full range of frequencies contained in the Fourier plane. Investigations are currently being made into the mathematical relationship between these high resolution images and the 2-D power spectrum for the refractive index fluctuations. The technique has the advantages of being non-intrusive and capable of acquiring multiple samples in a short period of time to insure statistical validity. Experimental measurements were performed on the center of a turbulent mixing shear layer (—8 mm thick), generated by back-to-back supersonic nitrogen/argon gas nozzles, with a mean flow velocity of ''380 rn/sec. Calculations were made using horizontal scans through several frames from a single test run. Turbulent scale lengths down to 0.6 mm were resolved.

Two problems are examined: the potential accuracy estimation of phase distortions and limiting resolution of two equal power point sources of noise signals. Having solved the first problem one can determine the limiting resolution of the adaptive telescope. From solution of the second problem it follows that the limiting resolution of antenna doesn't depend on the presence of arbitrary phase distortions in aperture.

A new remote sensing technique is proposed for determining the turbulent parameters of the atmosphere using a single-ended lidar system. This technique is based on the enhanced backscattering effect and is insensitive to the scattering volume averaging effect on the intensity fluctuations of the reflected wave and the sounding beam. The corresponding measurements are independent of the turbulent scintillation spectrum and that permits the use of high power pulsed lasers with a relatively low repetition rate for determining the refractive index structure characteristic C_n², its vertical profile C_n²(h) and inner scale of turbulence l_o in the atmosphere. A theory of the method is developed, and the conditions are obtained for observing the backscattering amplification effect in the atmosphere with a laser beam scattered by aerosol. The signal-to-noise ratio and the sensitivity of the measured quantities to the inner scale of turbulence l_o variations are estimated. A planned demonstration of this technique in the boundary layer of the atmosphere with an eyesafe lidar which has been developed at Georgia Tech is discussed.

We present the results of a Horizontal Propagation Experiment (HoPE) that was performed at the Phillips Laboratory Starfire Optical Range. In this experiment a laser beam was phase corrected using an adaptive optics system located at the transmitting site and focused toward a target located two miles away. Irradiance patterns of the corrected and uncorrected beam were recorded at the target site. Weather and atmospheric turbulence characteristics along the optical path were recorded at the same time. Strehl ratios calculated from the recorded images show that phase-only correction of a horizontally propagated laser beam can significantly improve the energy collected on-axis even under strongly scintillated conditions. Time- averaged strehl ratios were improved by as much as a factor of 5. Improvements in strehl for varying turbulence conditions and the effect of hardware limitations on the results are discussed.