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The angular reflectance signature of the canopy hot spot in the optical regime

Description: When any three-dimensional surface, e.g. a plant canopy is illuminated by a directional light source such as the sun, an angular reflectance distribution results that shows a narrow intensity peak in the direction of retro-reflection. This is called the Heiligenschein or hot spot (HS) of vegetation canopies and is caused by the absence of mutual shading of leaves when the observation direction coincides with the illumination direction. The angular intensity distribution of this hot spot, its brightness and contrast against the background, are therefore indicators of the plant's geometry. We show from experimental data and by modeling that the hot spot angular reflectance signature carries information about plant stand architecture that is often more distinctive for different plant species than their spectral signatures. 8 refs., 11 figs.
Date: January 1, 1988
Creator: Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Off-nadir optical remote sensing from satellites for vegetation identification

Description: Today's satellite remote sensing systems rely heavily on spectral signatures for scene identification from nadir observations. We propose to use angular signatures as complementary scene identifiers when off-nadir sensing is possible. Specifically, the hot spot (Heiligenschein) of plant canopies is recognized as an atmosphere-invariant angular reflectance signature that carries information about the plant stand architecture which may be useful for instant crop identification from off-nadir satellite measurements.
Date: May 30, 1986
Creator: Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Application of the adjoint method in atmospheric radiative transfer calculations

Description: The transfer of solar radiation through a standard mid-latitude summer atmosphere including different amounts of aerosols (from clear to hazy) has been computed. The discrete-ordinates (S/sub N/) method, which has been developed to a high degree of computational efficiency and accuracy primarily for nuclear radiation shielding applications, is employed in a forward as well as adjoint mode. In the adjoint mode the result of a transfer calculation is an importance function (adjoint intensity) which allows the calculation of transmitted fluxes, or other radiative responses, for any arbitrary source distribution. The theory of the adjoint method is outlined in detail and physical interpretations are developed for the adjoint intensity. If, for example, the downward directed solar flux at ground level, F/sub lambda/ (z = 0), is desired for N different solar zenith angles, a regular (forward) radiative transfer calculation must be repeated for each solar zenith angle. In contrast, only 1 adjoint transfer calculation gives F/sub lambda/ (z = 0) for all solar zenith angles in a hazy aerosol atmosphere, for 1 wavelength interval, in 2.3 seconds on a CDC-7600 computer. A total of 155 altitude zones were employed between 0 and 70 km, and the convergence criterion for the ratio of fluxes from successive iterations was set at 2 x 10/sup -3/. Our results demonstrate not only the applicability of the highly efficient modern S/sub N/ codes, but indicate also conceptual and computational advantages when the adjoint formulation of the radiative transfer equation is used.
Date: January 1, 1979
Creator: Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Uncertainty analysis for secondary energy distributions

Description: In many transport calculations the integral design parameter of interest (response) is determined mainly by secondary particles such as gamma rays from (n,..gamma..) reactions or secondary neutrons from inelastic scattering events or (n,2n) reactions. Standard sensitivity analysis usually allows to calculate the sensitivities to the production cross sections of such secondaries, but an extended formalism is needed to also obtain the sensitivities to the energy distribution of the generated secondary particles. For a 30-group standard cross-section set 84% of all non-zero table positions pertain to the description of secondary energy distributions (SED's) and only 16% to the actual reaction cross sections. Therefore, any sensitivity/uncertainty analysis which does not consider the effects of SED's is incomplete and neglects most of the input data. This paper describes the methods of how sensitivity profiles for SED's are obtained and used to estimate the uncertainty of an integral response due to uncertainties in these SED's. The detailed theory is documented elsewhere and implemented in the LASL sensitivity code SENSIT. SED sensitivity profiles have proven particularly valuable in cross-section uncertainty analyses for fusion reactors. Even when the production cross sections for secondary neutrons were assumed to be without error, the uncertainties in the energy distribution of these secondaries produced appreciable uncertainties in the calculated tritium breeding rate. However, complete error files for SED's are presently nonexistent. Therefore, methods will be described that allow rough error estimates due to estimated SED uncertainties based on integral SED sensitivities.
Date: January 1, 1978
Creator: Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Minimum-thickness blanket/shield with optimum tritium breeding and shielding effectiveness

Description: A blanket/shield assembly for a fusion reactor has been designed through extensive optimization studies. The design was optimized under the following constraints: (a) minimum overall thickness, (b) tritium breeding ratio of 1.10, (c) thermal energy recovery of 90%, (d) acceptably flat temperature distribution, and (e) excluding all ''exotic'' or problematic materials. The optimized blanket/shield has an overall thickness of 36 cm and conforms with all the above requirements. All tritium breeding is accomplished in a 24-cm-thick breeding zone using stagnant enriched lithium-6, and lead as a neutron multiplier. The energy recovery in this breeding zone is 71% which, together with an additional energy extraction of 19% in a 12-cm-thick laminated stainless steel/boron carbide shield zone, results in the desired overall thermal efficiency of 0.90 which is considered adequate if normal-conducting magnets are used for plasma confinement.
Date: January 1, 1978
Creator: Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Extension of the method of the small angle approximation: open detector

Description: We use the radiative transfer equation to study the multiple scattering undergone by a laser beam propagating through a turbid medium. During the propagation, we view the beam as first scattering into a narrow forward cone, and then into a diffuse pattern. To describe this process, we propose a systematic and practical method to combine the small angle approximation with the diffusion approximation. The method works when the scattering cross-section describing scattering from aerosols can be written as the sum of a gaussian sigma/sub s/ to describe scattering into small angles, and a term sigma/sub d/, that can be represented by the first two terms of a Legendre expansion to describe scattering into large/diffuse angles. We use a Green's function formalism to perform partial resummations and set up a hierarchy of approximations in the form of coupled radiative transfer equations to describe the scattering of radiation from small angles into large angles. The adjoint operator formalism then provides a simple way to obtain the net flux received by an open detector at any given point. Our approximations may be described rigorously as a power series expansion in sigma/sup 0//sub d//sigma/sup 0//sub s/, the ratio of the diffusion scattering cross-section to the forward scattering cross-section. Thus our technique works well when small angle scattering dominates.
Date: August 18, 1986
Creator: Chitanvis, S.M. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Off-axis scattering of laser beams using single and multi-Gaussian phase function approximation

Description: Within the small-angle approximation, a multi-Gaussian phase function model provides a new description of the multiple scattering process and leads to improved agreement with experiment data. Future work will be concerned with the combination of small-angle and diffusion approximations in the radiative transfer. This should lead to an improved predictive model of the performance of electroopitcal systems in aerosol-loaded atmospheres. 10 refs., 3 figs.
Date: September 1, 1986
Creator: Zardecki, A. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Vaporization and recondensation in propagation and imaging of laser beams

Description: To study the response of a medium to a localized disturbance, the coupled aerosol-beam equations---in which the dominant interactions are diffusive mass transport and conductive energy transport---are solved numerically, thus giving the spatio-temporal behavior of the propagating beam and the irradiated aerosols. In the context of the cloud clearing problem, a deleterious effect of recondensation is assessed. We examine the effect of turbulence on the distribution of droplet sizes during the recondensation. Results relevant to propagation and imaging are given. 12 refs., 10 figs.
Date: February 1, 1989
Creator: Zardecki, A. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Influence of the atmosphere on remotely sensed reflection from vegetation surfaces

Description: Multiple scattering of solar radiation in a vegetation canopy is modelled equivalent to absorbing and scattering in a turbid medium with direction-dependent cross sections. Perturbations of plant reflection patterns due to atmospheric effects are computed at different altitudes and compared to the angular reflection characteristics caused by Lambertian surfaces of varying albedoes.
Date: January 1, 1985
Creator: Simmer, C. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Calculations of increased solar UV fluxes and DUV doses due to stratospheric-ozone depletions

Description: Accurate radiative transfer calculations are performed in the middle ultraviolet spectral region for aerosol-loaded atmospheres with the goal of determining the solar irradiance at the ground and quantifying the irradiance perturbations due to the presence of aerosols and various ozone depletions. The extent of the increase of UV-B radiation as a function of wave-length and solar zenith angle is calculated for five model atmospheres. In addition, the damaging ultraviolet dose rates and radiation amplification factors are evaluated at different latitudes and seasons for erythemal and DNA action spectra.
Date: February 1, 1982
Creator: Zardecki, A. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Contributon Monte Carlo. [Application in MCNP code]

Description: The contributon Monte Carlo method is based on a new recipe to calculate target responses by means of volume integral of the contributon current in a region between the source and the detector. A comprehensive description of the method, its implementation in the general-purpose MCNP code, and results of the method for realistic nonhomogeneous, energy-dependent problems are presented. 23 figures, 10 tables.
Date: May 1, 1979
Creator: Dubi, A. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Application of biasing techniques to the contributon Monte Carlo method

Description: Recently, a new Monte Carlo Method called the Contribution Monte Carlo Method was developed. The method is based on the theory of contributions, and uses a new receipe for estimating target responses by a volume integral over the contribution current. The analog features of the new method were discussed in previous publications. The application of some biasing methods to the new contribution scheme is examined here. A theoretical model is developed that enables an analytic prediction of the benefit to be expected when these biasing schemes are applied to both the contribution method and regular Monte Carlo. This model is verified by a variety of numerical experiments and is shown to yield satisfying results, especially for deep-penetration problems. Other considerations regarding the efficient use of the new method are also discussed, and remarks are made as to the application of other biasing methods. 14 figures, 1 tables.
Date: January 1, 1980
Creator: Dubi, A. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Reduction of photosynthetically active radiation under extreme stratospheric aerosol loads

Description: The recently published hypothesis that the Cretaceous-Tertiary extinctions might be caused by an obstruction of sunlight is tested by model calculations. First we compute the total mass of stratospheric aerosols under normal atmospheric conditions for four different (measured) aerosol size distributions and vertical profiles. For comparison, the stratospheric dust masses after four volcanic eruptions are also evaluated. Detailed solar radiative transfer calculations are then performed for artificially increased aerosol amounts until the postulated darkness scenario is obtained. Thus we find that a total stratospheric aerosol mass between 1 and 4 times 10/sup 1/ g is sufficient to reduce photosynthesis to 10/sup -3/ of normal. We also infer from this result tha the impact of a 0.4- to 3-km-diameter asteroid or a close encounter with a Halley-size comet may deposit that amount of particulates into the stratosphere. The darkness scenario of Alvarez et al. is thus shown to be a possible extinction mechanism, even with smaller size asteroids of comets than previously estimated.
Date: August 1, 1981
Creator: Gerstl, S.A.W. & Zardecki, A.
Partner: UNT Libraries Government Documents Department

Reduction of photosynthetically active radiation under extreme stratospheric-aerosol loads

Description: The recently published hypothesis that the Cretaceous-Tertiary extinctions might be caused by an obstruction of sunlight is tested by model calculations. First we compute the total mass of stratospheric aerosols under normal atmospheric conditions for four different (measured) aerosol size distributions and vertical profiles. For comparison, the stratospheric dust masses after four volcanic eruptions are also evaluated. Detailed solar radiative transfer calculations are then performed for artificially increased aerosol amounts until the postulated darkness scenario is obtained. Thus we find that a total stratospheric aerosol mass between 1 and 4 times 10/sup 16/ g is sufficient to reduce photosynthesis to 10/sup 3/ of normal. We also infer from this result that the impact of a 0.4- to 3-km-diameter asteroid or a close encounter with a Halley-size comet may deposit that amount of particulates into the stratosphere. The darkness scenario of Alvarez et al., is thus shown to be a possible extinction mechanism, even with smaller size asteroids or comets than previously estimated.
Date: January 1, 1981
Creator: Gerstl, S.A.W. & Zardecki, A.
Partner: UNT Libraries Government Documents Department

High-energy laser-assisted imaging through vaporizing aerosols

Description: The degradation of image quality due to multiple scattering in a turbid medium is analyzed various conditions of illumination. The emphasis is on the forward-peaked multiple scattering effects, which can adequately be described by the small-angle approximation. In the case of incoherent illumination, the modulation transfer function (MTF) can be given explicity both in the low- and high-frequency limits. For scattering with smaller degree of anisotropy, the MTF should be imputed numerically by considering numerical by considering solutions to the equation of radiative transfer with a line or point source. As the beam power increases, the turbid medium becomes modified by its interactions with the beam, thus affecting the image resolution. In this nonlinear transport regime (flux levels of the order of 10/sup 6/ Wcm/sup 2/ and higher) the propagation leads actually to beam narrowing. In the context of the imaging problem, an apparent paradosical situation in which the image of a point source narrows down as the high-energy laser (HEL) beam propagates is discussed. 14 refs., 12 figs
Date: February 1, 1988
Creator: Zardecki, A. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

An atmospheric correction algorithm for remote identification of non-Lambertian surfaces and its range of validity

Description: The usefulness of remotely sensed surface data depends on the ability to correct for atmospheric pertubations on the image. An atmospheric correction algorithm has been proposed which removes atmospheric pertubations from off-nadir measured radiances at the top of the atmosphere in the visible and near-infrared wavelength region. The ability of the model to reproduce radiance distributions at the surface from radiances at the top of the atmosphere is tested and found to be better than 15%. The correction formalism requires as minimum information the total optical depth of the atmosphere and the surface albedo. In this study the accuracy of the model to assumptions about the aerosol phase function, the single-scattering albedo and the vertical profile of the optical depth is also tested.
Date: February 20, 1987
Creator: Gratzki, A. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Application of sensitivity analysis to a quantitative assessment of neutron cross-section requirements for the TFTR: an interim report

Description: A computational method to determine cross-section requirements quantitatively is described and applied to the Tokamak Fusion Test Reactor (TFTR). In order to provide a rational basis for the priorities assigned to new cross- section measurements or evaluations, this method includes quantitative estimates of the uncertainty of currently available data, the sensitivity of important nuclear design parameters to selected cross sections, and the accuracy desired in predicting nuclear design parameters. Perturbation theory is used to combine estimated cross-section uncertainties with calculated sensitivities to determine the variance of any nuclear design parameter of interest. (auth)
Date: September 1, 1975
Creator: Gerstl, S.A.W.; Dudziak, D.J. & Muir, D.W.
Partner: UNT Libraries Government Documents Department

Canopy hot-spot as crop identifier

Description: Illuminating any reflective rough or structured surface by a directional light source results in an angular reflectance distribution that shows a narrow peak in the direction of retro-reflection. This is called the Heiligenschein or hot-spot of vegetation canopies and is caused by mutual shading of leaves. The angular intensity distribution of the hot-spot, its brightness and slope, are therefore indicators of the plant's geometry. We propose the use of hot-spot characteristics as crop identifiers in satellite remote sensing because the canopy hot-spot carries information about plant stand architecture that is more distinctive for different plant species than, for instance, their spectral reflectance characteristics. A simple three-dimensional Monte Carlo/ray tracing model and an analytic two-dimensional model are developed to estimate the angular distribution of the hot-spot as a function of the size of the plant leaves. The results show that the brightness-distribution and slope of the hot-spot change distinctively for different leaf sizes indicating a much more peaked maximum for the smaller leaves.
Date: May 1, 1986
Creator: Gerstl, S.A.W.; Simmer, C. & Powers, B.J.
Partner: UNT Libraries Government Documents Department

Hydrodynamics of evaporating aerosols irradiated by intense laser beams

Description: An analysis is presented describing the interactions of atmospheric aerosols with a high-intensity laser beam propagating along an atmospheric path. For the case of moderate beam irradiances, diffusive mass transport and conductive energy transport dominate the aerosol-beam interactions. In this regime, the coupled aerosol-beam equations are solved numerically to obtain the spatic-temporal behavior of the propagating beam, and of the irradiated aerosols. For higher beam irradiances, convective transport of mass, energy and momentum away from the irradiated aerosols must be considered. The hydrodynamic equations are solved in the surrounding medium for this regime subject to appropriate ''jump conditions'' at the surface of the irradiated aerosol. Numerical examples illustrative of both regimes are given for the case of irradiated water aerosol droplets. 11 refs., 6 figs.
Date: January 1, 1985
Creator: Armstrong, R.L.; Zardecki, A. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Angular signatures, and a space-borne measurement concept

Description: The nature and value of angular signatures in remote sensing are reviewed with emphasis on the canopy hot-spot as a directionally localized angular signature and an important special case of a BRDF (bidirectional reflectance distribution function). A new concept is presented that allows hot spot measurements from space by using active (laser) illumination and bistatic detection. The detectors are proposed as imaging array sensors that are circulating the illumination source (or vice versa) and are connected with it through tethers in space which also provide the directional controls needed so that the entire system becomes pointable like a search light. Near infrared or IR operation in an atmospheric transmission winodw is envisioned with night-time data acquistion. Detailed feasibility and systems analyses have yet to be performed.
Date: May 1, 1996
Creator: Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

A measurement concept for hot-spot BRDFs from space

Description: Several concepts for canopy hot-spot measurements from space have been investigated. The most promising involves active illumination and bistatic detection that would allow hot-spot angular distribution (BRDF) measurements from space in a search-light mode. The concept includes a pointable illumination source, such as a laser operating at an atmospheric window wavelength, coupled with a number of high spatial-resolution detectors that are clustered around the illumination source in space, receiving photons nearly coaxial with the reto-reflection direction. Microwave control and command among the satellite cluster would allow orienting the direction of the laser beam as well as the focusing detectors simultaneously so that the coupled system can function like a search light with almost unlimited pointing capabilities. The concept is called the Hot-Spot Search-Light (HSSL) satellite. A nominal satellite altitude of 600 km will allow hot-spot BRDF measurements out to about 18 degrees phase angle. The distributed are taking radiometric measurements of the intensity wings of the hot-spot angular distribution without the need for complex imaging detectors. The system can be operated at night for increased signal-to-noise ratio. This way the hot-spot angular signatures can be quantified and parameterized in sufficient detail to extract the biophysical information content of plant architectures.
Date: September 1, 1996
Creator: Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Smoke clearing by high energy laser beams

Description: We describe the clearing phenomenon that occurs when a continuous wave (CW) high energy laser beam, incident upon a cloud of hygroscopic droplets, vaporizes these droplets. We consider the case when the incident wavelength is greater than the average droplet radius. Williams' model is used to describe the vaporization of a single droplet. The propagation of the laser beam is described by the radiative transfer equation in a slab geometry. The radiative transfer equation is solved using the method of successive orders of scattering.
Date: April 10, 1987
Creator: Chitanvis, S.M.; Gerstl, S.A.W. & Zardecki, A.
Partner: UNT Libraries Government Documents Department

Modelling of atmospheric effects on the angular distribution of a backscattering peak

Description: If off-nadir satellite sensing of vegetative surfaces is considered, understanding the angular distribution of the radiance exiting the atmosphere in all upward directions is of interest. Of particular interest is the discovery of those reflectance features which are invariant to atmospheric perturbations. When mono-directional radiation is incident on a vegetative scene a characteristic angular signature called the hot-spot is produced in the solar retro-direction. The remotely sensed hot-spot is modified by atmospheric extinction of the direct and reflected solar radiation, atmospheric backscattering, and the diffuse sky irradiance incident on the surface. It is demonstrated, however, by radiative transfer calculations through model atmospheres that at least one parameter which characterizes the canopy hot-spot, namely its angular half width, is invariant to atmospheric perturbations. 7 refs., 4 figs., 1 tab.
Date: February 20, 1987
Creator: Powers, B.J. & Gerstl, S.A.W.
Partner: UNT Libraries Government Documents Department

Imaging through a multiple scattering medium

Description: The existing theory of imaging through an aerosol medium, based on the small-angle approximation to radiative transfer, is extended to the general case of multiple scattering with an arbitrary degree of anisotropy. By applying the discrete-ordinates, finite-element radiation transport code TWOTRAN, we compute the modulation transfer function for a medium characterized by optical depth, single scattering albedo, and a symmetry parameter. An extended version of this investigation will appear in a 1984 issue of Applied Optics.
Date: January 1, 1984
Creator: Zardecki, A.; Gerstl, S.A.W. & Embury, J.F.
Partner: UNT Libraries Government Documents Department