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NASA NTRS1987

Effect of ship-stack effluents on cloud reflectivity

Under stable meteorological conditions the effect of ship-stack exhaust on overlying clouds was detected in daytime satellite images as an enhancement in cloud reflectivity at 3.7 micrometers. The exhaust is a source of cloud-condensation nuclei that increases the number of cloud droplets while reducing droplet size. This reduction in droplet size causes the reflectivity at 3.7 micrometers to be greater than the levels for nearby noncontaminated clouds of similar physical characteristics. The increase in droplet number causes the reflectivity at 0.63 micrometer to be significantly higher for the contaminated clouds despite the likelihood that the exhaust is a source of particles that absorb at visible wavelengths. The effect of aerosols on cloud reflectivity is expected to have a larger influence on the earth's albedo than that due to the direct scattering and absorption of sunlight by the aerosols alone.

Coakley, James A., Jr.↗

NASA NTRS1988

Improved canopy reflectance modeling and scene inference through improved understanding of scene pattern

The Li-Strahler reflectance model, driven by LANDSAT Thematic Mapper (TM) data, provided regional estimates of tree size and density within 20 percent of sampled values in two bioclimatic zones in West Africa. This model exploits tree geometry in an inversion technique to predict average tree size and density from reflectance data using a few simple parameters measured in the field (spatial pattern, shape, and size distribution of trees) and in the imagery (spectral signatures of scene components). Trees are treated as simply shaped objects, and multispectral reflectance of a pixel is assumed to be related only to the proportions of tree crown, shadow, and understory in the pixel. These, in turn, are a direct function of the number and size of trees, the solar illumination angle, and the spectral signatures of crown, shadow and understory. Given the variance in reflectance from pixel to pixel within a homogeneous area of woodland, caused by the variation in the number and size of trees, the model can be inverted to give estimates of average tree size and density. Because the inversion is sensitive to correct determination of component signatures, predictions are not accurate for small areas.

Franklin, Janet↗

NASA NTRS1987

Polarized and non-polarized leaf reflectances of Coleus blumei

A polarization photometer has been used to measure the reflectance of three variegated portions of Coleus blumei, Benth. in five wavelength bands of the visible and near-infrared spectrum. The polarized component of the reflectance factor was found to be independent of wavelength, indicating that the polarized reflectance arises from the leaf surface. It is suggested that differences in the polarized component result from variations in surface features. The nonpolarized component of the reflectance factor is shown to be related to the internal leaf structure. The variation of the degree of polarization with wavelength was found to be greatest in the regions of the spectrum where absorption occurs.

Grant, Lois↗

NASA NTRS1988

Evidence for highly reflecting materials on the surface and subsurface of Venus

Radar images at a 12.5-centimeter wavelength made with the Goldstone radar interferometer in 1980 and 1986, together with lunar radar images and recent Venera 15 and 16 data, indicate that material on the surface and subsurface of Venus has a Fresnel reflectivity in excess of 50 percent. Such high reflectivities have been reported on the surface in mountainous regions. Material of high reflectivity may also underlie lower reflectivity surficial materials of the plains regions, where it has been excavated by impact cratering in some areas.

Jurgens, R. F.↗

NASA NTRS1988

Measurement of temperature and emissivity of specularly reflecting glowing bodies

A new method of measuring the thermodynamic temperature of an object as well as the surface emissivity based on laser reflectivity has been developed. By using rotator analyzer ellipsometry, the light reflected from the sample at a specific angle of incidence can be analyzed for its ellipticity. The normal incidence reflectivity and emissivity are then extracted using standard relations. The thermodynamic temperature of the body is obtained simultaneously by measuring the intensity of emitted light at the same angle of incidence. Room temperature measurements are carried out on selected metals to test the system. Elevated temperature measurements on platinum foils show that this technique is reliable and accurate for monitoring and measuring the temperature and emissivity of specularly reflecting, glowing bodies.

Hansen, G. P.↗

NASA NTRS1988

Airborne Visible/Infrared Imaging Spectrometer (AVIRIS): Inflight radiometric calibration and the determination of surface reflectance

The inflight radiometric performance of AVIRIS is presented together with a comparison of methods of recovering surface spectral reflectance from the data. Performance is evaluated by comparing radiance predicted from AVIRIS with radiance generated from the LOWIRAN 6 atmospheric model and measured surface reflectance. Comparisons show apparent agreement to within a few percent between 1800 and 2450 nm. Between 600 and 1800 nm the response of AVIRIS is systematically low by as much as 70 percent, and between 400 and 600 nm it is higher than expected. These problems are traced to thermal distortions of the instrument, and to detachment during flight of optical fibers connecting foreoptics to two of four spectrometers in the instrument. Of three methods studied, an empirical one involving calibration curves constructed from field reflectance measurements returns accurate predictions of the surface reflectance independent of the actual radiometric significance of the flight data.

Conel, J. E.↗

NASA NTRS1988

Bidirectional reflectance properties of iron-nickel meteorites

The effects of viewing geometry and small-scale (less than 1 mm) surface roughness on the bidirectional reflectance of iron-nickel meteorites were investigated by measuring the reflectance properties of Gibeon and Canyon Diablo iron-nickel meteorites. Several cut surfaces of each of these meteorites were polished with abrasive grits to controlled surface roughness, and a series of bidirectional reflectance spectra were taken at a variety of viewing geometries using NASA's RELAB facility. The measurements show that, for optically smooth surfaces, only the specular component of reflectance exhibited red-sloped continuum characteristic of iron. Spectra of surfaces that were much rougher than the wavelength of incident light exhibited iron's characteristic red sloped continuum and could be modeled by linear combinations of facets that are oriented in specular and nonspecular geometries with respect to the observer. Spectra of this latter type exhibit good agreement with the published spectra of M-type asteroids.

Britt, D. T.↗

NASA NTRS1987

Atmospheric correction of NS-001 data and extraction of multiple angle reflectance data sets

The percentage of incident solar flux reflected by a surface is a quantity of considerable interest in remote sensing studies. To calculate reflectance from remotely sensed radiance data some estimate of incident flux is needed. Since simultaneous ground-based radiometric measurements are often not available for observations by aircraft or satellite sensors, a procedure based on modeling atmospheric transmittance and scattering was developed. The primary application is to an aircraft data set collected with the NASA C-130 over the Superior National Forest, Minnesota. Atmospherically corrected multiple angle reflectance data sets and reflectance images are generated for areas of natural forest vegetation. These data and the technique may be useful for studies of the interactions of light with forested canopies.

Strebel, D. E.↗

NASA NTRS1989

Frequency-selective reflection and transmission by a periodic dielectric layer

The feasibility of using a periodic dielectric layer, composed of alternating bars having dielectric constants epsilon sub 1 and epsilon sub 2, as a frequency selective subreflector in order to permit feed separation in large aperture reflecting antenna systems was examined. For oblique incidence, it is found that total transmission and total reflection can be obtained at different frequencies for proper choices of epsilon sub 1, epsilon sub 2, and the geometric parameters. The frequencies of total reflection and transmission can be estimated from wave phenomena occurring in a layer of uniform dielectric constant equal to the average value for the periodic layers. About some of the frequencies of total transmission, the bandwidth for 90 percent transmission is found to be 40 percent. However, the bandwidth for 90 percent reflection is always found to be much narrower; the greatest value found being 2.5 percent.

Bertoni, Henry L.↗

NASA NTRS1989

Transmission and reflection of compressive waves at a nonmagnetic-magnetic interface

The transmission and reflection properties of compressive waves at a plane interface between uniform nonmagnetic and magnetic regions in the absence of gravity are examined. Using stereographic polar projection, these properties are presented as functions of the two angles determining the direction of incidence. It is shown that the reflection coefficient and the direction of propagation of the transmitted wave are dependent on the direction of propagation of the incident wave for several representative parametric values. It is found that the incident, reflected, and transmitted wavenumber vectors always lie in the same plane, although the group velocity of the transmitted wave does not always lie in this plane. When the transmitted wave is a fast mode, there is generally weak reflection.

Abdelatif, Toufik E.↗

NASA NTRS1989

Reflections, diffractions, and surface waves for an interior impedance wedge of arbitrary angle

The asymptotic-impedance wedge solution for plane-wave illumination at normal incidence is examined for interior wedge diffraction. An efficient method for calculating the diffraction coefficient for arbitrary wedge angle is presented. The asymptotic solution isolates the incident, singly reflected, multiply reflected, diffracted, surface-wave, and associated-surface-wave transition fields. Multiply reflected fields (of any order) from the exact solution arise as ratios of auxiliary Maliuzhinets functions; however, by using properties of these functions, the representation can be reduced to products of reflection coefficients, much more efficient for calculation. A surface-wave transition field is added to the surface wave to retain continuity of the total field at the surface wave boundaries. This formulation is equally valid for both exterior and interior wedges with uniform but different impedances on each face, for both soft and hard polarizations.

Griesser, Timothy↗

NASA NTRS1989

From primal sketches to the recovery of intensity and reflectance representations

A local change in intensity (edge) is a characteristic that is preserved when an image is filtered through a bandpass filter. Primal sketch representations of images, using the bandpass-filtered data, have become a common process since Marr proposed his model for early human vision. Here, researchers move beyond the primal sketch extraction to the recovery of intensity and reflectance representations using only the bandpass-filtered data. Assessing the response of an ideal step edge to the Laplacian of Gaussian (NAb/A squared G) filter, they found that the resulting filtered data preserves the original change of intensity that created the edge in addition to the edge location. Using the filtered data, they can construct the primal sketches and recover the original (relative) intensity levels between the boundaries. It was found that the result of filtering an ideal step edge with the Intensity-Dependent Spatial Summation (IDS) filter preserves the actual intensity on both sides of the edge, in addition to the edge location. The IDS filter also preserves the reflectance ratio at the edge location. Therefore, one can recover the intensity levels between the edge boundaries as well as the (relative) reflectance representation. The recovery of the reflectance representation is of special interest as it erases shadowing degradations and other dependencies on temporal illumination. This method offers a new approach to low-level vision processing as well as to high data-compression coding. High compression can be gained by transmitting only the information associated with the edge location (edge primitives) that is necessary for the recovery

Alter-Gartenberg, Rachel↗

NASA NTRS1989

Force-reflective teleoperated system with shared and compliant control capabilities

The force-reflecting teleoperator breadboard is described. It is the first system among available Research and Development systems with the following combined capabilities: (1) The master input device is not a replica of the slave arm. It is a general purpose device which can be applied to the control of different robot arms through proper mathematical transformations. (2) Force reflection generated in the master hand controller is referenced to forces and moments measured by a six DOF force-moment sensor at the base of the robot hand. (3) The system permits a smooth spectrum of operations between full manual, shared manual and automatic, and full automatic (called traded) control. (4) The system can be operated with variable compliance or stiffness in force-reflecting control. Some of the key points of the system are the data handling and computing architecture, the communication method, and the handling of mathematical transformations. The architecture is a fully synchronized pipeline. The communication method achieves optimal use of a parallel communication channel between the local and remote computing nodes. A time delay box is also implemented in this communication channel permitting experiments with up to 8 sec time delay. The mathematical transformations are computed faster than 1 msec so that control at each node can be operated at 1 kHz servo rate without interpolation. This results in an overall force-reflecting loop rate of 200 Hz.

Szakaly, Z.↗

NASA NTRS1989

Reflection and trapping of transient Alfven waves propagating in an isothermal atmosphere with constant gravity and uniform magnetic field

A time-dependent linear magnetohydrodynamic numerical model was used to investigate the propagation of Alfven waves in an isothermal and stratified atmosphere with constant gravity and uniform vertical magnetic field. Results show that the Alfven wave transit time for the wave source to infinity is finite and that the wave exhibits continuous partial reflection which becomes total reflection as the front approaches infinity. The total reflection causes the waves to be trapped in the cavity that extends from the wave source to infinity and in which the wave energy is stored. The results suggest that the reflection of Alfven waves (of sufficiently long period) from the outer corona is an intrinsic phenomenon for any stellar atmosphere stratified by gravity and an open magnetic field, and that, therefore, such waves may be trapped in the stellar atmosphere.

An, C.-H.↗

NASA NTRS1989

Photometric phase functions of common geologic minerals and applications to quantitative analysis of mineral mixture reflectance spectra

The mass fractional abundance of components in intimately mixed, particulate surfaces is calculated from laboratory reflectance spectra using Hapke's (1981) model for bidirectional reflectance. It is found that a simplified version of the model is accurate to within 7 percent for mixtures not containing low albedo components. The model is not appropriate for mixtures with very low and high albedo components. Consideration is given to the possible improvement of the method's accuracy by using an empirical single-particle phase function to describe the scattering characteristics of all minerals, or by solving for the single-particle phase function of the minerals involved exactly using extensive bidirectional reflectance measurements and Hapke's equations for bidirectional reflectance. Also, the results are used to examine the general scattering behavior of particulate mineral surfaces.

Mustard, John F.↗

NASA NTRS1990

Studies of dust grain properties in infrared reflection nebulae

A model has been developed for reflection nebulae around luminous IR sources embedded in dense dust clouds. The shape of the IR spectrum is shown to be the result of a combination of the scattering properties of the dust, the spectrum of the illuminating source, and foreground extinction, while geometry plays a minor role. Comparison of the model results with IR observations of the reflection nebula surrounding OMC-2/IRS 1 shows that either a grain size distribution like that found in the diffuse ISM, or consisting of larger grains, can explain the observed shape of the spectrum. However, the absolute intensity level of the scattered light, as well as the observed polarization, requires large grains. By adding water-ice mantles to the silicate and graphite cores, the 3.08 micron ice-band feature observed in the spectra of several IR reflection nebulae has been modeled. It is shown that this ice band arises naturally in optically thick reflection nebulae containing ice-coated grains.

Pendleton, Y. J.↗

NASA NTRS1990

Survey of coherent ion reflection at the quasi-parallel bow shock

This paper presents the results of a survey of ion beam observations near the earth's quasi-parallel bow shock. Particular attention is given to three issues: (1) the spatial locations of the cold coherent ion beams relative to the shock, (2) the velocity space locations of the ion beams relative to that expected for specular reflection, and (3) the parameter regimes over which the beams are observed. Evidence is found that coherent ion reflection occurs commonly in the quasi-parallel regime, over essentially the full range of upstream plasma parameters normally found at the earth's bow shock. It was also observed that the cold coherent reflected beams spread rapidly in the velocity space upstream from the shock. The observations presented serve as an extension to those presented by Gosling et al. (1989) and further strengthen the conclusion that specularly reflected ions are contributing directly to the downstream thermalization at the earth's quasi-parallel bow shock.

Onsager, T. G.↗

NASA NTRS1989

The relationship between leaf water status, gas exchange, and spectral reflectance in cotton leaves

Measurements of leaf spectral reflectance, the components of water potential, and leaf gas exchanges as a function of leaf water content were made to evaluate the use of NIR reflectance as an indicator of plant water status. Significant correlations were determined between spectral reflectance at 810 nm, 1665 nm, and 2210 nm and leaf relative water content, total water potential, and turgor pressure. However, the slopes of these relationships were relatively shallow and, when evaluated over the range of leaf water contents in which physiological activity occurs (e.g., photosynthesis), had lower r-squared values, and some relationships were not statistically significant. NIR reflectance varied primarily as a function of leaf water content, and not independently as a function of turgor pressure, which is a sensitive indicator of leaf water status. The limitations of this approach to measuring plant water stress are discussed.

Bowman, William D.↗