Proceedings Volume 1688

Atmospheric Propagation and Remote Sensing

Anton Kohnle, Walter B. Miller
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Proceedings Volume 1688

Atmospheric Propagation and Remote Sensing

Anton Kohnle, Walter B. Miller
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 1 August 1992
Contents: 4 Sessions, 72 Papers, 0 Presentations
Conference: Aerospace Sensing 1992
Volume Number: 1688

Table of Contents

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Table of Contents

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  • Propagation and Imaging Through Aerosols
  • Remote Sensing and Measurements
  • Optical Turbulence: Propagation and Adaptive Techniques
  • Transmission and System Performance
  • Optical Turbulence: Propagation and Adaptive Techniques
  • Propagation and Imaging Through Aerosols
  • Remote Sensing and Measurements
Propagation and Imaging Through Aerosols
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Ocean aerosol measurements and models in the straits of Florida (the Key-90 experiment)
Stuart G. Gathman
The Navy Oceanic Vertical Aerosol Model (NOVAM) has been under development for some time. The model showed considerable promise in its first verification test during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) in the eastern Pacific. Because much of the development work on NOVAM was done in this oceanic region, the model needed to be tested in very different environments to see just how universally it could be applied to other regions. KEY-90 was an experiment carried out in the tropical waters between the Florida Keys and Cuba in July 1990 to test the model. It included investigators from the U.S., U.K., and the Netherlands. The experiment included two lidars, two aircraft, a small boat, buoys, and several shore installations. The experiment provided an excellent database not only to test the model for the tropical water scenario, but also to further investigate the convective marine boundary layer and to enhance further modeling schemes in this region. This paper describes the optical, IR, and meteorological measurements made during KEY-90 and shows the comparison between the NOVAM model predictions and measurement.
NOVAM evaluation from aerosol and lidar measurements in a tropical marine environment
Gerrit de Leeuw, Gerard J. Kunz
NOVAM, the Naval Oceanic Vertical Aerosol Model, has been developed to predict the nonuniform and nonlogarithmic extinction profiles that are often observed in the marine atmospheric boundary layer. The kernel of NOVAM is the Navy Aerosol Model (NAM) that calculates the aerosol size distribution at 10 m ASL from meteorological parameters. The aerosol profile is calculated from the surface layer size distribution with a physical model. Extinction profiles are calculated from the aerosol profiles using a Mie code. NOVAM requires validation in different meteorological scenarios. During the KEY-90 experiment, July 1990 near Marathon, Florida, NOVAM was validated in a tropical marine environment. We measured the surface layer particle size distribution profile at levels from 0.5 to 4 m ASL to evaluate the large-particle end of NAM. The NOVAM prediction of the aerosol profile in the mixed layer was evaluated by lidar measurements of the 1.06 micrometers backscatter profile. The time-serial lidar measurements show the convective plumes and the variability in both the aerosol content at higher levels in the boundary layer and in the boundary layer height itself. Consequences for application of NOVAM for slant path transmission are discussed.
Modeling aerosol extinction in a coastal environment
Alexander M. J. van Eijk, Gerrit de Leeuw
Aerosol particle size distributions were measured during the HEXMAX experiment in the fall of 1986 from Meetpost Noordwijk (MPN), a tower in the North Sea at 9 km from the Dutch coast. Extinction coefficients for wavelengths from 0.55 to 10.6 micrometers were calculated using a Mie routine. The initial analysis showed that straightforward empirical modeling in terms of meteorological parameters did not yield meaningful results. Therefore, the aerosol particle size distributions were modeled in terms of parameters that determine the production, dispersal, and deposition of the aerosol in the coastal marine boundary layer. For an adequate description, the database had to be partitioned into wind-direction sectors based on industrial, rural continental, and predominant marine influences. Based on this empirical aerosol model, the extinction coefficients can be predicted with an accuracy of better than a factor of 1.8. The reliability of the model decreases during frontal passages and periods of rain. The model is validated versus the experimental data and by comparison with the Navy Aerosol Model.
Aerosol-induced loss of image resolution
An approach aimed at completely simulating the operation of camera or telescope, including direct calculation of images, in a spatially complex, turbid environment is presented. A Monte Carlo radiative transfer program coupled with an optics design program is used to compute aureoles at the image plane of a camera. The procedure uses the camera MTF so that the operation of the camera in a scattering environment can be completely simulated. The code is validated through a comparison of the analytical solutions for cameras with plane and Gaussian apertures.
Imaging through the atmosphere: practical instrumentation-based theory and verification of aerosol MTF
Dan Sadot, Norman S. Kopeika
A practical instrumentation-based atmospheric aerosol MTF which is a modification of the classical aerosol MTF theory is proposed. The approach based on radiative transfer theory takes into account the effect of finite field-of-view, finite dynamic sensitivity, and finite spatial bandwidth of every existing imaging system. The asymptote value of the measured aerosol MTF approaches at high spatial frequencies is found to be significantly higher than the theoretical prediction of turbid medium transmittance. It is concluded that the aerosol MTF cannot be referred to as constant attenuation, and in many cases it is the dominant part in the actual overall atmospheric MTF. An image resolution-image irradiance relationship can be determined by the system designer through considering field-of-view, instrumentation dynamic range, and spatial frequency bandwidth. It is necessary to choose between imaging of faint and bright objects at the expense of image quality, or imaging of either faint or bright objects with improved image quality.
Investigation of multiple scattering contribution to transmission of radiation in scale model clouds
Wladyslaw W. Szymanski
A new dual beam laser system was developed for systematic study of the influence of various aerosol parameters such as size, concentration and refractive index on the transmission of light through an aerosol under well-controlled laboratory conditions. The measuring technique is based on simultaneous observation of transmitted and scattered light fluxes in growing aerosol droplets. Multiple spattering effects were determined by concurrent use of two optical systems for the same model droplet cloud, of which one (the reference system) was negligibly affected by multiple scattering. Modeling of transmittance was performed using a solution of the radiative transfer equation in small angle approximation, as well as Monte Carlo simulations of beam propagation in turbid media. It was found that Monte Carlo results were in good agreement with the experimental data, whereas the small angle approximation considerably underestimated the multiple scattering contributions to the transmitted light for our experimental conditions.
Numerical method for modeling nonspherical aerosol modulation transfer functions
Aerosol scattering produces image degradation effects which can be characterized by the MTF. This paper describes a technique for calculating MTFs for atmospheres containing nonspherical particles. The approach uses the extended boundary condition method and a Monte Carlo simulation to model a scattering atmosphere. Model MTF results for 8.0 and 9.2 microns are presented for media containing spheroidal particles.
Measurement of the aerosol component of the modulation transfer function in desert atmospheres
Carlos McDonald, Gonzalo Romero, Carmen Ortiz, et al.
A unique experimental technique has made it possible to make measurements in desert atmospheres of the horizontal modulation transfer function (MTFA) and its components which are attributed to contrast, aerosols, and turbulence. 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 that include two, optically identical, castellated targets which are located at different distances and are contrasted against the horizon sky. Ratios of contrast and FFT calculations may be used to determine the absolute values and spatial cutoff frequencies of the MTFA and its three components, independent of the imaging system and actual properties of the targets. The experimental technique is described along with preliminary MTFA measurements of the aerosol component.
Specific marine boundary layer aerosol model
Tom B. Low, Norman G. Loeb
A model is described which accepts meteorological input data from marine areas and calculates vertical profiles of size distributions and infrared extinction coefficients in the marine boundary layer under clear sky, fog, and precipitation conditions. For the clear case, aerosols consist of a mixture of dust, water soluble and sea salt particle components represented by the sum of four log-normal size distributions having meteorological and airmass dependent amplitude and modal radius parameters. These were derived by empirical fits to actual ship measurements recorded over the Atlantic Ocean. The calculations account for particle growth using empirical formulations which take particle chemistry into account, while extinction coefficients are determined using Mie theory. Continental component contributions are inferred using a meteorological interpreter model which also corrects for errors or uncertainties in the input meteorological data by ensuring consistency between calculated visibility and observed visibility. Vertical aerosol profiles are modelled considering the production, mixing, and deposition of aerosols in the marine boundary layer, with simpler parameterizations for fog and precipitation cases.
New sun-photometer for remote sensing of aerosols
Kun Tan, Hong Hu, Jun Zhou, et al.
A new sun-photometer intended to perform accurate measurements of the spectral characteristics of both direct solar and aureole radiation is presented. Controlled by a microcomputer, the photometer carries out the following functions: active tracking of the sun's center, transforming optical filters, adjusting the gain of an amplifier, sampling and storing the data on time.
Planewave scattering by a simply moving, electrically small, bianisotropic sphere: dyadic analysis
A dyadic procedure is given for the planewave scattering response of an electrically small, bianisotropic sphere moving with a constant velocity in free space. Lorentz transformations and the polarizability dyadics of a stationary bianisotropic sphere are used in this procedure. The canonical problem solved here is applicable to moving aerosol particles.
Prediction of aerosol distributions parameters according to weather forecast: status report
Prediction of wave propagation parameters through an aerosol medium such as scattering and absorption coefficients and scattering phase function is possible if the particulate size distribution is known. In this paper an effort is made to relate the desert particulate size distribution parameters to simple meteorological parameters. All of the size distribution curves showed clear `knees' in their characteristic indicating that the source of the particles is more than one. The particulate size distributions were well fitted to a Trimodal log normal distribution. This good agreement suggests that the particulate size distribution is composed from three separate sources. The first and second sources are due to the local particles which exist in the location of the measurement, contributing to the smaller radii of the size distribution. The third source is large particles which are transferred from large deserts like the Sahara desert by dust carrying winds. These particles contribute to the larger radii end of the distribution. The size distribution parameters were related to meteorological parameters. The most dominant parameter was relative humidity. Using this model it should be possible to predict the particulate size distribution from meteorological data. Prediction of particulate size distribution allows the prediction of aerosol Modulation Transfer Function which is crucial in the prediction of image quality propagating through aerosols.
Analytic functions for modeling vertical profiles of extinction in and beneath very low stratus clouds
Neal Harold Kilmer, Henry Rachele
A theoretically based microphysics model developed by the authors was used to simulate a large number of vertical drop size distribution profiles in very low stratus clouds and subcloud regions. These drop size distributions were used with Mie calculations to simulate vertical profiles of extinction coefficients at wavelengths of 0.55, 1.06, 4.0, and 10.6 micrometers . These profiles were then fit with smooth analytic functions in order to develop a model that can be used quickly and easily to simulate gross behavior of vertical profiles of extinction coefficients. Values of parameters that appear in two selected analytic functions were determined from fitting simulated data and are presented for a few cases in this work. It is proposed that one of these analytic functions with associated parameter values be considered for possible worldwide application in modeling very low stratus clouds.
Light scattering of fractal aerosol aggregates using T-matrix method
Kung-Hau Ding
The aerosols in atmosphere are often random cluster of small primary particles with disordered appearance. They are considered to be fractal aggregates with a noninteger dimensions and have the important property of invariance under scale transformation. The essential fractal morphology are included in this study of scattering properties for aggregated aerosol particles. The fractal aggregates are simulated by using the growth model of hierarchical cluster-cluster aggregation. The T-matrix approach together with the translation addition theorem for vector spherical waves is employed to solve the aggregate scattering problem. Numerical results for scattering cross sections are illustrated for fractal clusters.
Remote Sensing and Measurements
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Retrieving atmospheric temperature profiles: a constrained eigenvalue approach coupled to high-spectral-resolution simulations
Joseph M. Coughlin
We present the results of a study to determine the effect atmospheric properties have on the retrieval of temperature profiles. A high spectral resolution line-by-line atmospheric radiative transfer code, Emission Spectra, is used to compute radiances for different atmospheric conditions. Given the radiance due to variations in aerosols, observation angles, initial conditions, and spectral noise, we retrieve atmospheric temperature profiles using a Backus- Gilbert constrained eigenvalue approach. Limitations of using a constrained eigenvalue approach are discussed. Analysis of the eigenvalues provides valuable insight into the operation of this algorithm and may yield information to refine this methodology.
Lidar inversion technique using total attenuation and the lidar system constant
George H. Goedecke, Radon B. Loveland, Raymond Todd Lines, et al.
It is desirable to determine the internal structure of clouds during smoke tests. One method for doing this involves using lidar returns from a cloud and inversion of the lidar equation. Conventionally, the latter requires a knowledge of the attenuation at the end of the path of interest. This is seldom known or measurable with sufficient accuracy. In this presentation, a new iterative inversion algorithm is described. The new algorithm uses the total attenuation through the desired path length as input, rather than the attenuation of the last range cell. The total attenuation is easy to determine accurately. For example it may he measured using the lidar and a backstop of known reflectivity placed at the end of the desired path.
Timing and sequencing of operational cycles of an ATmospheric LIDar (ATLID) system
Robert Lange, Martin J. Endemann
ATLID is a spaceborne scanning lidar system planned for implementation on the European Polar Platform, an earth observation satellite. It measures cloud top heights and atmospheric layers above ground. The operational cycles of ATLID are discussed.
Water vapor, temperature, and wind velocity measurements from space using 2-um Tm:Ho;YAG
Jean-Bernard Ghibaudo, Rodolphe Krawczyk
A differential absorption lidar (DIAL) and a Doppler wind lidar (DWL) for future ESA mission dedicated to water, vapor, temperature, and wind velocity measurements are described. A feasibility study of all solid state technologies for spaceborne DIAL and DWL systems in the spectral range of 1.5 to 10 microns is presented. Topics discussed include the spectral line selection in the above mentioned spectral range, the performance parametric analyses of all lidar candidates, and a subsystem study that took into account the payload architecture and optical and thermal constraints.
Low-altitude cross-wind velocity measurements using pulsed visible fringe-type laser anemometry
Eckhart W. Sommer, Hans J. Pfeifer
A pulsed fringe type laser Doppler anemometer has been developed during a feasibility study for remote wind velocity measurements. In contrast to conventional Doppler systems, this technique directly measures the cross-components of the wind vector. Field measurements over a range of l00 m have demonstrated successfully the performance of this method and have also shown eventual restricting atmospheric conditions.
Comparison of two retrieval methods for ground reflectance
Carmen Tornow, Rudolf Richter
A comparison of two retrieval methods is presented to calculate ground reflectance from Landsat Thematic Mapper satellite data. The first method is based on a modified two-stream approximation to simulate the radiative transfer above an inhomogeneous surface. The atmosphere is parametrized by the optical depth and the single scattering albedo. The theory of fractal geometry is employed to compute the structure measures of a scene, from which the ground variability is estimated. By a linear regression, the ground variability can be related to the atmospheric optical depth. The independent second method is based on model ATCOR (including LOWTRAN-7). Here, a priori knowledge is used (shape of spectral reflectance curve for vegetation, water, bare soil) to determine the unknown atmospheric parameters like optical depth and type of aerosol (single scattering albedo). The adjacency effect, which describes the influence of atmospheric crosstalk in modifying the radiances of adjacent fields of different reflectance, is taken into account by both procedures. Typically, deviations between both methods are up to 2% in reflectance for low to medium reflection (< 30%) targets and up to 4% for high reflectance (> 40%) targets of Landsat imagery. In view of the independent approaches, this level of agreement in retrieved ground reflectance is fairly good. The new method is particularly valuable if no a-priori knowledge is available and if the scene has a large dynamic range of spatial frequencies.
U.S. Army atmospheric profiler research facility: a preliminary view
John R. Hines, Frank D. Eaton, William H. Hatch, et al.
The U.S. Army Atmospheric Sciences Laboratory has established a unique atmospheric research facility at White Sands Missile Range (WSMR), New Mexico. The Atmospheric Profiler Research Facility (APRF) monitors three primary atmospheric parameters: wind (speed and direction), refractive index structure parameter C-squared(n), and ambient temperature. The APRF includes a suite of high-performance (temporal and spatial resolution) phased-array, multiple reflector, and multiple aperture type atmospheric profilers. Specialized and standard surface and balloon-borne instrumentation are deployed to provide local ground truth, comparison, and calibration data. The Facility achieves a high-resolution height coverage to about 20 km. The products of the APRF support research applications, including research and development testbed analyses, imaging and propagation system design and testing, meteorological and earth-observation satellite ground comparison, and environmental forecasting improvement at WSMR. The design and capabilities of the APRF are described. Preliminary measured and derived atmospheric parameters as well as initial comparisons are presented.
Wind and refractive turbulence profiling using imaging-filtering technique
Valery U. Zavorotny
The theoretical description of incoherent source imaging through an extended turbulent medium is considered. It is demonstrated that the temporal-spatial power spectra of the defocused images contain information about the transverse wind velocity and the refractive turbulence strength along an optical path. The information about wind and turbulence profiles can be recovered from these spectra using a class of quasi one-dimensional natural sources, and a spatial filter in the aperture plane.
Holographic scintillometry technique for measuring atmospheric parameters
John F. Dove
A holographic spatial filtering technique for measuring atmospheric turbulence (C sup 2 (n)), wind speed, and wind direction developed for the NOAA wind profiler II is presented. The technique is based on a device that incorporates a laser array and a holographic filter (the transmitter), a holographic filter, and a detector array (the receiver). The 20cm x 20cm four-channel holographic zero mean filter is being tested in the HOLODAR system which provides simplicity of design, reduced signal/data processing requirements, and improved atmospheric measurement capability. The device can be used for improvements in radars, optical communications, imaging, coherent laser radar, and windshear/vortex detection.
Sensing refractive-turbulence profiles (Cn2) using wavefront slope measurements from two reference sources
Michael D. Rejack, Byron M. Welsh, Steven K. Rogers
This paper examines a remote sensing technique for measuring the atmospheric structure constant, C-squared(n) as a function of altitude by performing spatial correlation of wave front sensor measurements. Two point sources are used to irradiate separate wave front sensors in the aperture plane of an optical system. The geometric relationship between the sources and the sensors gives rise to crossed optical paths. At the point where the paths cross, the turbulence contributions in both paths are highly correlated. The correlation of the slope sensor measurements is shown to be mathematically related to the structure constant in terms of an integral of C-squared(n) multiplied by a path weighting function. The value of the structure constant can be inferred from the correlation. The resolution of the technique depends on the angle between the optical paths and the size of the wave front sensor apertures. For sensing the vertical profile of C-squared(n), resolution on the order of 100 m can be obtained by using sources separated by 0.29 deg.
Intercomparison of C2n data derived from VHF and UHF radars
Frank D. Eaton, William H. Hatch, John R. Hines, et al.
Results of backscatter, or radar reflectivity, from VHF (50 MHz) and UHF (404 MHz) profiling radars are inter-compared. Both systems are operated as clear-air Doppler radars providing vertical profiles of horizontal and vertical wind, but the 50 MHz system is also calibrated for the refractive index structure parameter (Cn2) that is directly proportional to the backscatter. The method of calibration is outlined. Both radars employ phased array antennas with collinear-coaxial (CO-CO) elements. The VHF radar system transmits a peak power of 250 kW and has an antenna area of 15,600 m2 while the UHF system transmits 7 kW peak power and has an antenna size of 170 m2. Performance characteristics and operating modes are presented for both systems. Results presented include simultaneous comparisons of `shapes' of Cn2 profiles as well as comparisons between different beams applying appropriate statistical tests. A method to empirically calibrate the UHF radar is discussed with recommendations for long-term use. Measurements from the radar systems are also compared to the Aeronomy Laboratory's turbulence model. Utility of the radar results with various optical and acoustic derived turbulence values is discussed.
Two-wavelength lidar inversion technique
Raul Gonzalez, Young P. Yee, Jill C. Thompson, et al.
A simple technique for extracting the extinction and backscatter coefficients from lidar returns signals is presented. The technique uses lidar signals from two different wavelengths taken with the same lidar system, and is based on the assumptions that the extinction coefficient at one wavelength is related to the extinction at the other wavelength, likewise for the backscatter coefficients. The technique is tested on data generated by the Nd:YAG laser 2nd (532 nm) and 3rd (355 nm) harmonics. The relationship between extinction coefficients was obtained by a power law fitting to data generated by the UVTRAN model. For the backscatter coefficient a simple proportionality law is assumed.
Statistical properties of SHF radar sea return: fine structure of temporal and spatial spectra
Leonid A. Slavutsky, A. V. Volkov
The paper presents a technique to observe experiment ally backscatter o SHF rrom the sea . Analysis o spatial stru cture o microwave sea echo shows that spectral characteristics or the radar reflected pulse (wavelengths 3 ,1 0 cm ) are determined in general by angle or incidence and the azimuthal divergence or the illuminated area. A non—linear errect is considered to attempt to explain rine structure in the spectrum or the backscat tered signal. Temporal characteristics or radar backscattering were s tudied by Doppler measurements (wavelength 1 .5 cm ) . We inves tigated. a deviation or radar signal statistical distribution rrom the Raleigh one and how this deviation connects with sea surrace state. Variations or Doppler spectrum gives us the possi bility to obtain some inrormation about capillary- gravitational waves which are modulated by large-scale waves.
Development of an integrated wind profile from several sources using a continuity model
William H. Hatch
An approach based on the Wuertz-Weber continuity model (1989) coupled with a Gaussian smoothing technique developed by the U.S. Army Atmospheric Sciences Laboratory is presented. The approach is aimed at providing an integrated profile of wind from the surface to 20 km mean sea level. The continuity model demonstrated excellent results on the test data.
Ultrahigh-resolution Cn2 profiles derived from an FM-CW radar
Jim R. Jordan, Scott McLaughlin
The U.S. Army Atmospheric Science Laboratory operates a frequency modulated-continuous wave (FM-CW) radar at White Sands Missile Range, New Mexico. This 10 cm wavelength radar has the unique capability of measuring 2 m resolution Cn2 profiles to 2 km above ground level. At this short wavelength, scattering from point targets, presumably insects, seriously contaminates the turbulence measurements. The ability of the FM-CW radar to resolve individual insects even at two km allows the insect signature to be removed from the turbulent backscatter. Radar calibration, data, and a technique for removing insect contamination are presented.
U.S. Army ultrahigh-resolution turbulence profiling FM-CW radar: description and data
Scott McLaughlin
The U.S. Army Atmospheric Sciences Laboratory, White Sands Missile Range, New Mexico, recently received a new frequency-modulated continuous-wave (FM-CW) turbulence profiling radar. The radar is capable of high temporal (10 s) and spatial (1 to 2 m) resolution profiles of Cn2 up through 2 km above ground level. Data from this system can detect electromagnetic/atmospheric effects such as possible laser blooming/scattering regions, microwave tunneling and multipathing layers, and other clear-air and hydrometeor meteorological phenomena. The radar is superior in performance to previous high-resolution clear-air FM-CWs incorporating newer technologies and real-time processing. A system description and sample data are shown.
Wind measurements from Doppler radar profilers and rawinsondes: a comparison of 50-MHz and 404-MHz profilers and rawinsonde data at White Sands Missile Range
Wayne Flowers, Elaine Santantonio, Glenn Hoidale, et al.
Wind measurements derived from collocated VHF and UHF Doppler radar profilers were analyzed for statistical trends in vertical and horizontal variability. Nearly continuous-6-min data from 20 March through 30 April 1991 are analyzed and also compared with rawinsonde data. Consistency of wind data from these three sources is discussed.
Wind profile comparisons between rawinsonde flights and a 50-MHz atmospheric profiling radar
Jeffrey O. Johnson
Comparisons were made and statistical analyses performed incorporating wind data from a 50-MHz atmospheric profiling clear-air radar at White Sands Missile Range, New Mexico, and rawinsonde flights made at selected locations near the profiler site. Four separate periods were examined: December 1990, February 1991, April 1991, and June 1991. The data were concentrated on 3 to 5 nearly contiguous days of each month. Rawinsonde flights were tracked and plotted relative to the radar site. The radar data were averaged to better intercompare the two very different profiling systems. Effects of seasonal variation on the comparison are discussed.
Lidar wind shear measurements in the planetary boundary layer
Ivan N. Kolev, Orlin Parvanov, Boiko K. Kaprielov
During the last years various remote and conventional techniques are widely used for th investigation of the processes in the planetary boundary layer (PBL) . The knowledge about regional meteorology at a corn— plex terrain is a significant problem which is currently justifying thus motivates a number of studies. The lidars allow profiles of the meteorological parameters to be remotely obtained with high time and spatial resolutions. The absence of contact sensors makes them convenient instruments particularly in two cases: 1. When investigating transient processes such as formation and destruction of a given atmospheric stratification; 2. For the observation of interaction areas at the boundary of different layers where significant changes in the meteorological parameters behaviour occur, e.g. in case of front invasion, etc. The investigation of fronts advections is an interesting problem for the mesoscale meteorology since their occurrence modify the evolution of the planetary boinIary layer parameters. Various techniques (radars, sodars and lidars) ' are used to monitor these modifications in order to reveal the characteristics of frontal dynamics. The obtained results are generally compared with data from traditional means. Lidar measurements of wind velocity profiles by slant sounding in the PBL accompanied by ground based measurements and a comparison with radio and kytoon data have been carried out in the country during 32 hours . We also carried out similar investigations studying the influence of various miro- and mesoscale phenomena on the wind velocity stratification ' In the present work parts of the preliminary results from two lidar campaigns (1988, 1990) in the region of Sofia city are described: A). Some results of lidar measurements of wind velocity profiles in case of a stable PBL formation after the sunset are presented. The results are obtained during the BLEX'90 (Boundary Layer EXperiment). This campaign aimed to perform an investigation of various phenomena and processes in the PBL over an urban area. B). Some observations of a wind shear appearance during a cold front invasion over the region of Sofia are also presented. These results are derived in the course of the international boundary layer experiment ZOND '88 (teams from Institute of Atmospheric Optics of Siberian Branch of USSR Academy of Sciences and from Institute of Electronics of Bulgarian Academy of Sciences).
Calibration errors in cloud-base temperature remote sensing with ground-based infrared radiometers
Cloud-base temperature is a useful parameter in atmospheric and climate research, meteorology, and hydrology. This parameter can be sensed remotely by inferring the cloud- base temperature from the brightness temperature observed with a ground-based infrared radiometer. Infrared radiometric measurements in the 10 - 12 micrometers `atmospheric window' spectral region routinely can have accuracies of +/- 1.0 degree(s)C if the radiometer is calibrated carefully. However, subtle calibration errors easily can increase this uncertainty many times. These errors arise largely because the calibration temperatures are less than the ambient temperature. In this paper I describe errors due to inadequate blackbody simulation, fluctuations in radiometer temperature, and imprecise voltage measurements during calibration. I also discuss simple ways of avoiding or minimizing each of these errors.
Neural network for retrieval of indirectly measured information
Edward M. Measure, Young P. Yee, Teddy L. Barber, et al.
Microwave radiation emitted by the atmosphere contains information about the composition and temperature structure of the atmosphere. Given the vertical temperate profile and humidity structure of the atmosphere, it is possible to compute the microwave radiance received by an upward-looking surface-based radiometer operating in the 20 - 60 GHz range. The inverse problem is ill-posed and requires additional information for its solution. We have found a method for solving this problem using the techniques of neural networks. In our calculation, the network has as inputs the set of microwave brightness temperature measurements made at the surface, surface temperature, and surface pressure, or surface altitude, or both. The output is the atmospheric temperature as a function of height. The neural network computes the outputs from the inputs using its internal weights which have been established by a training process. The training consists of presenting the system with input brightness temperatures calculated from radiosonde observations, and comparison of the resulting outputs with the radiosonde temperature profiles using a backpropagation training rule. The trained system was then tested on a separate set of brightness temperatures. Similar techniques should work with many types of inverse problems.
Lidar sounding of the smoke plumes: water coating of sol particles in power-station plumes
The water coating of the smoke soles, which takes place in the humid atmosphere, is one of the main factors reducing the precision of the lidar measurements of smoke plumes. The current paper presents the results of theoretical investigations of the process of water shell growth on the smoke particles. These investigations were conducted on the basis of a Gaussian model of smoke plumes (that takes into account the sedimentation of the coated particles during the plume diffusion) at various meteorological conditions and plume parameters. The numerical calculations show that when the plume is initially very humid, a zone with supersaturated water vapor and extremely high moistening of the particles appears near the smoke source. The process of water coating is essentially influenced also by the seasonal and diurnal variations of the temperature and humidity in the surface layer of the atmosphere. The degree of water coating of the particles is established to be maximal in winter at night and slow wind. The paper also presents some calculation of the optical properties of smoke particles.
Optical Turbulence: Propagation and Adaptive Techniques
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Beam and pulse waves, imaging, and enhanced backscattering in turbulence
Akira Ishimaru
Optical and beam propagation characteristics in the turbulent atmosphere and short-time and long-time beam spreads are reviewed. Particular attention is given to pulse wave propagation in terms of coherence bandwidths, imaging through turbulence for an incoherent source, backscattering enhancement through turbulence, and speckle imaging.
Energy balance model for imagery and electromagnetic propagation
Henry Rachele, Arnold Tunick
The optical turbulence structure parameter Cn2 typically appears in formulations used to estimate the effects of temperature and moisture (gradients) on imagery and electro- magnetic propagation. Temperature and moisture gradients can be approximated from sensible and latent heat flux estimates, and these fluxes can be obtained from radiation/energy balance equations. Numerous energy balance models exist requiring different kinds and numbers of inputs. The semiempirical model developed and presented in this paper was constrained to require a minimum number of conventional measurements at a reference level (2 m). These measurements include temperature, pressure, relative humidity, and windspeed. The model also requires a judgment of soil type and moisture (dry, moist, or saturated), cloud characteristics (tenths of cloud cover and density and an estimation of cloud height), day of the year, time of day, and longitude and latitude of the site of interest. Model estimates of net radiation, sensible and latent heat fluxes, and Cn2 are compared with measured values.
Nondimensional beam parameters and the wave structure function
Larry C. Andrews, Walter B. Miller, Jennifer C. Ricklin
Two pairs of nondimensional beam parameters are identified, either of which combined with wavelength and path length completely characterizes the diffractive propagation environment of a Gaussian beam. One pair is associated with the beam radius and radius of curvature at the transmitter while the other pair is associated with these same quantities at the receiver. The fundamental nature of the receiver parameters is revealed in the simple analytic forms for the wave structure function derived under the assumption of a modified Kolmogorov spectral model for refractive index fluctuations.
Short-exposure imaging of nonisoplanatic objects through turbulence
An approach to short-exposure imaging that takes into account anisoplanatic perturbations of the image is proposed. The short exposure modulation transfer function is formulated using the Marcovian stochastic process approximation. This formula which is free from the drawbacks of Fried's approximations describes the diffraction effects and medium fluctuation distribution along the propagation path.
Effect of turbulence scale and spectral function on optical image degradation
Ven H. Shui, Barry L. Reeves
Propagation of optical signals through a turbulent medium such as the atmosphere or the boundary layer around a missile or an aircraft, causes image degradation. This paper examines the characteristics of such degradation, including log-amplitude variance, phase variance, attenuation, and blur. In particular, the effect of using different turbulence spectral/correlation functions and the dependence on various characteristic length scales, are analyzed.
High-resolution imaging at the Multiple Mirror Telescope using adaptive optics
The next generation of 6 to 10 m class telescopes is being planned to include the capability for adaptive wavefront correction. The MMT with its 7-m baseline, provides an ideal testbed for novel techniques of adaptive optics. Using a new instrument based on a six-segment adaptive mirror, a number of wavefront sensing algorithms including an artificial neural network have been implemented to demonstrate the high resolution imaging capability of the telescope. These algorithms rely on a freely available property of starlight, namely, its coherence over large scales, to sense directly atmospheric and instrumental phase errors across large distances. In this paper, we report results obtained so far with resolutions between 0.08 and 0.3 arcsec at 2.2-micron wavelength. We also show data indicating that at the level of 0.1-arcsec imaging in good seeing, the isoplanatic patch at this wavelength is at least 20 arcsec across.
Spectral and dynamic characteristics of the adaptive imaging system
Adaptive imaging systems characterized by the time delay and spectral dependence on the quality of phase correcting are analyzed using the most reliable model profiles of spectral density fluctuations of atmospheric refractivity. The optical adaptive systems are classified as dynamic feedback systems. Emphasis is placed on high-speed and predicting adaptive systems.
Irradiance variance for convergent Gaussian beams using nondimensional beam parameters
Jennifer C. Ricklin, Walter B. Miller, Larry C. Andrews
The longitudinal and radial irradiance variance for a convergent Gaussian beam is examined in terms of two pairs of nondimensional beam parameters, the first pair (Omega) equals (lambda) L/(pi) Wo2 and (Omega) o equals 1 - L/Ro associated with the beam when transmitted, and the second pair given by (Lambda) equals (lambda) L/(pi) W2 and (Theta) equals 1 + L/R associated with the received beam. Here, (lambda) is wavelength, L is path length, Ro and R are radii of curvature of the phase front at the transmitter and receiver, respectively, and Wo and W are the beam radii at the transmitter and receiver. With the addition of (lambda) and L, either of these beam parameter pairs completely characterizes the diffractive propagation environment for a lowest- order paraxial Gaussian beam, and is fundamental in the analytic expression of the irradiance variance. Special attention is paid to differences between the perfectly focused beam and the nearly focused beam. We also show that every beam has a convergent counterpart with identical diffractive irradiance behavior at the receiver, but decreased irradiance variance.
Estimating effects of temperature and moisture on Cn2 in the damp unstable boundary layer for visible, infrared, radio, and millimeter wavelengths
Arnold Tunick, Henry Rachele
Data collected over barren and vegetated ground surfaces were used to obtain estimates of Cn2 for the damp unstable boundary layer. These data consisted of latent and sensible heat fluxes. Results from this study show that moisture effects on Cn2 can be larger than generally reported in the literature. Results presented emphasize the relative contributions of temperature and moisture to Cn2 for visible, infrared, radio, and millimeter wavelengths.
Modeling of the image observed through a turbulent atmosphere
Boris V. Fortes, Vladimir P. Lukin
Computer simulations of a wavefront distorted by atmospheric turbulence are considered focusing on large scale and dynamic simulations. Attention is also given to wavefront sensor and wavefront corrector simulations. Computer modeling of adaptive optical systems takes into account an optical wave field in the plane of receiving aperture, a wavefront distortion sensor, a wavefront distortion corrector, and quantum fluctuation of the optical wave intensity.
Effects of predetection atmospheric compensation and postdetection image processing on imagery collected at a ground-based telescope
Marsha F. Bilmont, Michael C. Roggemann, David W. Tyler, et al.
Short exposure imagery of single and binary stars was collected at the 1.6-m Air Force Maui Optical Station (AMOS) telescope, using a low-noise CCD camera. Atmospheric turbulence effects were partially mitigated using the Compensated Imaging System (CIS), a predetection wavefront sensor and deformable mirror adaptive optical system. We present images and power spectra from both partially compensated and uncompensated short exposure simulations and field data. Our results illustrate that the use of lower-cost partially compensating adaptive optical systems combined with post-detection processing provides a viable alternative to expensive, fully compensated adaptive imaging systems for achieving high-resolution imagery through the atmosphere.
Estimating contributions of turbulent layers to total wavefront phase aberration
Dustin C. Johnston, Byron M. Welsh
Multiple deformable mirrors (DMs) and an array of artificial guide stars have been used to analyze a method for widening the compensated field of view of an adaptive optical telescope. The contribution of an atmospheric region to the cumulative phase distortion is estimated using wavefront sensor (WFS) measurements. The analysis takes into account the effects of measurement noise, WFS spatial bandwidth, and reconstruction of the wavefront from slope measurements for a 2-layer atmosphere.
Development and implementation of first-order adaptive-optics algorithms
John A. Qualtrough, James J. Drexler, David B. Soules, et al.
First order adaptive optics algorithms have been developed for the ATMOS high speed imaging system. Design considerations and implementation techniques are discussed along with performance characteristics measured in the laboratory. Preliminary image motion and power spectral density (PSD) results of tip-tilt image correction on the KPNO 2.1 meter telescope equipped with a 'fast guider' secondary are presented. Implications for infrared applications are also discussed.
Atmospheric modeling with the intent of training a neural net wavefront sensor
D'nardo Colucci, Michael Lloyd-Hart, Peter L. Wizinowich, et al.
Modeling atmospheric turbulence which plays a critical role in the training of neural network wavefront sensors is discussed in the framework of an adaptive optics program for the multiple mirror telescope. It is concluded that the accuracy of the wavefront correction possible with a neural network directly depends on the similarity of the training images to those seen in the telescope. The image simulations used in the training of neural network wavefront sensors are based on a random mid-point displacement (RMD) algorithm and sine wave summation algorithms. The RMD algorithm is considered to be an extremely fast method of wavefront generation used for very large arrays and image sequences without time evolution. Multiple turbulent layer atmospheric models based on the sine wave summation algorithm create image sequences with temporal structure functions that closely match real structure function data.
Effect of an adaptive optical system's temporal response on imaging performance
Byron M. Welsh
The performance degradation due to the finite time response of an adaptive optical system is studied. It is concluded that the degradation is a strong function of the time delay between wavefront sensing and wavefront correction. The degradation can be also expressed as a strong function of the aperture size and overall wave front tilt removal.
Time-domain analysis simulation for advanced tracking approaches to compensated imaging
James F. Riker, Robert Russell Butts
The TASAT code is a complete end-to-end system simulation of tracking and pointing systems. In its ground-based laser (GBL) system configuration, the code will treat tracking and imaging systems that look through atmospheric turbulence at high fidelity. In particular, the effects of atmospheric turbulence, adaptive optics servo lag, anisoplanatism, and deformable mirror (DM) fitting error are treated with a novel time average point-spread function (PSF). Tracking and imaging sensors are treated as a sequence of physical effects, including allocation of the image energy to pixels with finite dead bands, sensor noise effects, and integrating sensor dwell with analog-to-digital conversion. We treat the physics of atmospheric turbulence and adaptive optics in some detail in this paper, and discuss results for variations in system bandwidth, actuator spacing, atmospheric coherence length, and anisoplanatism effects. The DM is treated as a spatial and temporal transfer function acting on Kolmogorov atmospheric turbulence. We develop the time average PSF using the residual atmospheric structure function. Finally, we use the convolution of the PSF with realistic satellite imagery to assess tracking and imaging performance.
Effect of wavefront undersampling on signal-to-noise ratio in adaptive-optics systems
Adaptive-optics (AO) systems for imaging of exoatmospheric objects employ a wavefront sensor designed to sample the phase imposed by the atmosphere. Whether a Hartmann array or shearing interferometer, such a sensor is typically designed to sample at spatial frequencies corresponding to the expected value of the Fried coherence diameter, r(o), at the site. However, undersampling of the wavefront occurs in practice during periods of bad seeing, and the cost of adaptive-optics systems designed to sample at high frequency and control a large number of actuators make deliberately 'underdesigned' systems attractive. In this paper, we use a detailed computer simulation in a preliminary investigation of the effect of wavefront undersampling on the SNR of the power-spectrum estimate of resulting point-source images. We have found doubling subaperture size in an Hartmann-sensor-driven AO system can nearly approximate the performance given by a fully-sampled system for a 3.67-m telescope.
Performance analysis of self-referenced speckle holography
Byron M. Welsh, Rudolf N. VonNiederhausern
Self-referenced speckle holography (SRSH) is a post-detection turbulence-compensation technique for obtaining diffraction limited imagery from ground based telescopes degraded by atmospheric turbulence. In SRSH image plane information is used together with wave front distortion information to reconstruct an estimate of the object spectrum. The wave front distortion information is obtained from a wave front sensor (WFS) in the pupil plane of the telescope. The data from the WFS is used in a post-processing environment to estimate the point spread function (PSF) of the combined telescope and atmosphere. The PSF is then used to obtain an estimate of the object via deconvolution. We present the results of a detailed performance analysis of SRSH. Performance is quantified in terms of a 'system transfer function' and a 'system point spread function'. The results show how the performance of the technique is dependent on the WFS sampling intervals and shot noise. The results also indicate how the technique, for a given set of design parameters, responds to changing seeing conditions. For subaperture dimensions on the order of a Fried coherence cell size ro and adequate light levels, SRSH boosts the high spatial frequencies (those near the diffraction limit of the telescope) to nearly 0.6. The 'system strehl ratio' approaches 0.6.
Analysis of phase correction efficiency when laser radiation is propagated through nonlinear layer
Valerii V. Kolosov, Sergey I. Sysoev
A partially coherent laser beam propagated in the far region of diffraction through a nonlinear layer adjacent to the radiation plane is considered. The propagation is described using a small-angle approximation of a transfer equation. It is demonstrated that 'slow' phase correction is effective only in the limited region of the beam energy parameters. Beyond the limits of the region even the optimum phase correction does not decrease angular expansion, and the use of nonoptimum algorithms causes deterioration of radiation characteristics. 'Fast' phase correction makes it possible to derive the angular expansion value that is less than the diffractive value, when a 'focusing lens' formed in a nonlinear layer.
Transmission and System Performance
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Impact of atmospheric layering on LOWTRAN radiance calculations
Fritz G. Wollenweber
The LOWTRAN-7 computer code for calculating the propagation of IR radiation through the atmosphere is evaluated in the 8-12 microns wavelength region. Good agreement was found between calculated and measured radiances, except for the LOWTRAN 7 single scattering mode that is based on a conservative scattering approach which results in too high sky radiances.
High-altitude atmospheric attenuation on near- and submillimeter wave propagation
Robert H. Au
The Spreadsheet Attenuation Model (SAM) has been developed for assessing the atmospheric attenuation of millimeterwave (MMW) radars operating at frequencies up to 1000 GHz and at altitudes from sea level to 25 km. The operational frequency windows can be determined as well as the sensitivity of the MMW radar range to the attenuation factor contained in the radar equation. Predictions are presented for a MMW radar operating with a line of sight looking upwards and horizontally at frequencies 210, 400, 680, 880 and 935 GHz.
Adjacency-blurring effect of scenes modeled by the radiosity method
Christoph C. Borel, Siegfried A. W. Gerstl
A method to compute the point spread function for any view direction and any layered atmosphere is proposed which is based on an extended radiosity method (Borel and Gerstl, 1991). The adjacency blurring effect is simulated for a scene containing vegetated, bare soil, and water surfaces taking into account aerosol scattering phase functions and ground bidirectional reflectance distributions.
Estimation of atmospheric phase stability from sky-brightness temperature fluctuations at millimeter wavelengths
Mark A. Holdaway
Atmospheric phase stability was studied on two high elevation sites in the southwestern U.S. during the winter months. Results indicate that the phase stability is significantly better on the South Baldy site than on the Springerville site on long baselines at high frequency. On the South Baldy site high quality imaging is possible in a dynamic range greater than or equal to 100 at 230 GHz over half the time in the compact 70 m configuration. Phase stable observations at 230 GHz in the 3000 m array are possible only about 15 percent of the time, but self-calibration or fast external calibration make it possible to perform high frequency, long baseline observations for about 40 percent of the time.
Intermountain measurement campaigns on atmospheric propagation in the near infrared
Adolfo Comeron, Javier Bara, Aniceto Mounta Belmonte, et al.
Propagation effects at around 830 nm were evaluated in an intermountain scenario in the Canary Islands (Spain) in the framework of an ESA Free Space Optical Communication program. It is concluded that the extinction effects in clear weather conditions do not limit possible ground experiments. The different turbulence-induced effects are found to be reasonably consistent with simplified models. Power scintillation and point-spread function widening can be disturbing effects on the horizontal path.
Atmospheric effects on laser radar performance at 2 microns
A series of tests designed to quantify the effects of various atmosphere conditions on 2 micron laser radar performance is described. The coherent laser radar system was setup at the Table Mountain Test Facility outside of Boulder. The 2 micron system is operated for the Air Force Wright Laboratory Avionics Directorate by Coherent Technologies, Inc. The system is based on a flashlamp pumped laser that produces 25 mj pulses at a 3 Hz rate. Data on coherent laser radar reveal that aerosol backscatter is directly related to extinction at the laser wavelength considered. Atmospheric conditions have significantly different effects on the ladar performance depending on the application.
Atmospheric transmittance measurements of CO2 and near-IR laser radiation over 8.6 km
Karin Weiss-Wrana, Anton Kohnle, Dieter H. Hoehn
A long-range laser transmissometer to measure atmospheric extinction at different laser wavelengths simultaneously from near to thermal infrared, (1.06 and 10.6 micron) was designed, constructed, and operated under different atmospheric conditions over a distance of 8.6 km in hilly terrain near Tuebingen, Germany. Beam extinction was obtained by measuring the ratio of the total transmitted laser radiation to the total received radiation as collected by a mirror and focused onto a pyroelectrical detector array. Measured values of Nd:YAG and CO2 laser extinction are compared with model predictions (FASCODE 3P) based on simultaneously measured meteorological data as model input parameters. The agreement between measurement and calculation is better than expected. The corresponding extinction coefficients cover the range 0.05/km to 0.17/km for Nd:YAG laser radiation and 0.07/km to 0.25/km for CO2 laser radiation.
Electro-optical climatology microcomputer version 2.0
Robert L. Miller, Bradley T. Regan
Electro-optical Climatology (EOCLIMO) Ver 2.0 was developed to enhance employment of precision guided munitions and target acquisition systems. This paper explains how EOCLIMO was produced, and interpretation of each output type. Guidance is provided on EOCLIMO 2.0 program manipulation, station/data comparisons, geographical map/narrative display, and transmittance versus ceiling output.
WKB: approximation for inverse problem of radiowaves refraction
Leonid A. Slavutsky
A technique for estimating conditions of radiowave propagation in a case of superrefraction has been offered. A possibility has been shown to use WKB-approximation in profile reconstruction of tropospheric waveguides from a spatial oscillations of the path loss. When two or more modes are "captured" in a waveguide channel the relative error does not exceed 1 0%.
Application of invariant imbedding method to simulate numerically beyond-the-horizon propagation of SHF over the sea
Konstantin V. Koshel
The present paper studies beyond-the-horizon propagation ot SEF in evaporation d.uct in presence or strong anisotropic fluctuations or the refractivity index. Errective technique to obtain numerically complex eigen values is suggested. On the base or this approach the modeling method. or SHF propagation in media with layer fluctuations o rerracti vity index is formed. Some examples or the influence oI'! fluctuations on beyond-the-horizon propagation are considers. A generalization or adia batic approximation ror opening systems with complex spectrum o eigen values and indefinite eigen runctions is suggested. Mechanism o exiting the evaporation duct by the high located SElF source is also discussed.
Spatial structure of radar sea return: influence of refraction
Konstantin V. Koshel, Leonid A. Slavutsky, Boris M. Shevtsov, et al.
The paper presents a technique to investigate experimentally a structure or radar pulses scattered by the sea sarrace ror racli— ation wavelengths o 3 and. 10 cm. It is shown that a signal level in beyond-the-horizon region is clerined. in general by rerractivity and. its spatial structure is directly connected. with path loss.
Comparison of turbulence MTF and aerosol MTF
An imaging system with a narrow field of view was used to measure the overall atmospheric MTF of an horizontal path and the turbulence MTF at the same time and over the same optical path over the visible and near IR. Results suggest that the atmospheric MTF is composed not only of turbulence MTF alone. Image quality is found to be degraded by aerosols, although the spatial frequencies of degradation were up to 5 tp 15 cycles per mrad rather than radian.
Effects of aerosol forward scatter on atmospheric coherence diameter: theory and validation
Dan Sadot, Norman S. Kopeika
A correction to the definition of the atmospheric coherence diameter is suggested here. The basis of this new approach is the existence of an aerosol MTF which is often the dominant ingredient of the atmospheric MTF. As defined by Fried about 25 years ago, atmospheric MTF was related to turbulence MTF only. In the case of a Gaussian approximation of the aerosol MTF, an analytical expression is derived for the aerosol-caused coherence diameter. This parameter is related to the aerosol MTF's cutoff frequency, and to its asymptote at high spatial frequencies which was recently shown to be higher than the atmospheric transmittance. Qualitative validation of the theory is presented, based on measured MTFs in the open atmosphere. Overall atmospheric coherence diameter is generally the smaller between the turbulence and aerosol coherence diameters.
Optical Turbulence: Propagation and Adaptive Techniques
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Effect of random layers on atmospheric propagation
Valentin Freilikher
Theoretical study of microwaves propagation through the atmosphere may be reduced to the `one-dimensional' problem of waves propagation in a randomly layered media with effective attenuation. The randomly stratified layer that is homogeneous on the average represents a fluctuation waveguide arising as a result of interference of multiply scattered waves.
Propagation and Imaging Through Aerosols
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Monte Carlo calculations of point spread function and MTF of an optical system operating in a turbid medium
Piero Bruscaglioni, P. Donelli, Andrea Ismaelli, et al.
A numerical procedure is applied to studying the effect of some geometrical parameters on point spread function and modulation transfer function of an optical system operating in a turbid medium. The possibilities offered by an inverse scheme of calculation are also examined.
Remote Sensing and Measurements
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Error analysis of restoring stratospheric aerosol parameters from single-frequency lidar sounding
Bernd Mielke, Valerij N. Cherbakov, Bernhard Stein, et al.
An algorithm for restoring stratospheric backscatter coefficients from ground-based lidar measurement is proposed. Error sources of the algorithm are analyzed including the boundary condition, the molecular backscatter coefficients, and the influence of the aerosol light extinction.
Conceptual design of a spaceborne radar for rain and cloud sensing
A conceptual design for a mid-latitude orbiting precipitation and cloud mapping radar is discussed. In this conceptual design the radar utilizes a narrow, dual-frequency beam, electronically scanned antenna to achieve 4-km spatial resolution and 300-km cross-track swath. Vertical resolution of 500 m is achieved by short-pulse transmission. It is expected that such system can measure rain rates up to 100 mm/hr for precipitation at the cloud base, surface precipitation up to 20 mm/hr, and cloud reflectivities as low as -39 dBz. By averaging over 100 independent samples, signal reflectivities can be estimated to better than 20 percent. Other rain and cloud characteristics, such as height, thickness, and cell size, can also be extracted from the data.