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At least 469 records · Page 26

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NASA NTRS1982

Effects of body shape on disk-integrated spectral reflectance

The effects of scattering geometry on the spectral reflectance curves of various powdered substances representing possible planetary surfaces are examined. Reflectance measurements were made as a function of incidence angle and emission angle for ground samples of a slightly oxidized basalt, the C3 carbonaceous chondrite Allende, and the ordinary chondrite Bruderheim, and compared with various photometric functions. The best fit to the observations is obtained with the photometric function of Goguen (1981) and Hapke (1981), and with a simpler function representing a linear combination of a Hapke-Irvine law and Lambert's law. The wavelength dependence of the adopted functions implies that the spherical reflectance of a flat sample will differ from that of a spherical planet of the same material, and that there will be a difference between the spectral reflectance curves of spherical and elliptical planets.

Gradie, J.↗

NASA NTRS1982

Radar reflectivities and satellite imagery of severe storms 20 May 1977

Storms on 20 May 1977 generated a vast cirrus deck. Disturbed areas at storm top had equivalent black-body temperatures (T sub BB) much lower than the tropopause temperature, indicative of overshooting tops. The area of T sub BB not greater than -71 C represents the area of convective activity penetrating 2 km above the tropopause. This area was relatively large after cloud tops and radar reflectivities reached their maximum heights. It became much smaller during tornadoes when reflectivities were decreasing. T sub BB was at a minimum at the time of mesocyclone formation. The Del City storm had two periods of growth, as indicated both by reflectivities and the T sub BB areas. The mesocyclone was first detected during the second less intense period of growth; the tornado occurred during decreasing reflectivities. The maintenance of large areas of relatively low T sub BB after tornado dissipation is ascribed to continued convection on the flanks of the storm and to residual updrafts in a thick anvil cloud.

Wexler, R.↗

NASA NTRS1981

On the uncertainty in the determination of ground reflectance and temperature from TM measurements

Uncertainties in the derivation of the ground reflectance and temperature relative to the uncertainty in the estimation of aerosol loading and water vapor amount are presented. To compute the reflectance for TM 1 (thematic mapper) through TM6, a doubling model is used, and to compute satellite-received radiance for TM7, an emission-transmission model is employed. It is shown that at a water vapor concentration of higher than 50% above normal, the relative sensitivity remains as low as 8% for ground reflectance determinations. The relative sensitivity for determining ground temperature for midlatitude summer climatological profile is less than 5%, but as the ground temperature is approximately 300 K, low sensitivity still means a variation of a few degrees. An increase in water vapor amount causes an uncertainty in ground temperature and reflectance calculations comparable to the uncertainty in aerosol loading and water vapor amount estimations.

Kiang, R. K.↗

NASA NTRS1982

Variation of directional reflectance factors with structural changes of a developing alfalfa canopy

Directional reflectance factors of an alfalfa canopy were determined and related to canopy structure, agronomic variables, and irradiance conditions at four periods during a cutting cycle. Nadir and off-nadir reflectance factors decreased with increasing biomass in Thematic Mapper band 3(0.63-0.69 micrometer) and increased with increasing biomass in band 4(0.76-0.90 micrometer). The sensor view angle had less impact on perceived reflectance as the alfalfa progressed from an erectophile canopy of stems after harvest to a near planophile canopy of leaves at maturity. Studies of directional reflectance are needed for testing and upgrading vegetation canopy models and to aid in the complex interpretation problems presented by aircraft scanners and pointable satellites where illumination and viewing geometries may vary widely. Distinct changes in the patterns of radiance observed by a sensor as structural and biomass changes occur are keys to monitoring the growth and condition of crops.

Kirchner, J. A.↗

NASA NTRS1983

Dynamics of directional reflectance factor distributions for vegetation canopies

Directional reflectance factors that span the entire exitance hemisphere are collected on the ground for a variety of homogeneous vegetation canopies and bare soils. NOAA 6/7 AVHRR bands 1 (0.58-0.68 micron) and 2 (0.73-1.1 microns) are used. When possible, geometric measurements of leaf orientation distributions are taken simultaneously with each spectral measurement. Other supporting structural and optical measurements are made. These data sets are taken at various times of the day for each cover type. These unique sets, together with pertinent data in the literature, are used to investigate the dynamics of the directional reflectance factor distribution as a function of the geometric structure of the scene, solar zenith angle, and optical properties of the scene components (leaves and soil). For complete homogeneous vegetation canopies, the principal trend observed at all sun angles and spectral bands is a minimum reflectance near nadir and increasing reflectance with increasing off-nadir view angle for all azimuth directions.

Kimes, D. S.↗

NASA NTRS1983

Analysis of an axial compressor blade vibration based on wave reflection theory

The paper describes application of the theory of wave reflection in turbomachines to rotor blade vibrations measured in an axial compressor stage. The blade vibrations analyzed could not be predicted using various flutter prediction techniques. The wave reflection theory, first advanced in 1966, is expanded, and more general equations for the rotor blade excitation frequencies are derived. The results of the analysis indicate that all examined rotor blade vibrations can be explained by forced excitations caused by reflecting waves (pressure pulses). Wave reflections between the rotor blades and both the upstream and downstream stator vanes had to be considered.

Owczarek, J. A.↗

NASA NTRS1983

Seasonal soybean crop reflectance

Data are presented from field measurements of 1980 including 5 acquisitions of handheld radiometer reflectance measurements, 7 complete sets of parameters for implementing the Suits mode, and other biophysical parameters to characterize the soybean canopy. LANDSAT calculations on the simulated Brazilian soybean reflectance are included along with data collected during the summer and fall on 1981 on soybean single leaf optical parameters for three irrigation treatments. Tests of the Suits vegetative canopy reflectance model for the full hemisphere of observer directions as well as the nadir direction show moderate agreement for the visible channels of the MSS and poor agreement in the near infrared channel. Temporal changes in the spectral characteristics of the single leaves were seen to occur as a function of maturity which demonstrates that the absorptance of a soybean single leaf is more a function of thetransmittancee characteristics than the seasonally consistent single leaf reflectance.

Lemaster, E. W.↗

NASA NTRS1984

Characterization of Surface Reflectance Variation Effects on Remote Sensing

The use of Monte Carlo radiative transfer codes to simulate the effects on remote sensing in visible and infrared wavelengths of variables which affect classification is examined. These variables include detector viewing angle, atmospheric aerosol size distribution, aerosol vertical and horizontal distribution (e.g., finite clouds), the form of the bidirectional ground reflectance function, and horizontal variability of reflectance type and reflectivity (albedo). These simulations are used to characterize the sensitivity of observables (intensity and polarization) to variations in the underlying physical parameters both to improve algorithms for the removal of atmospheric effects and to identify techniques which can improve classification accuracy. It was necessary to revise and validate the simulation codes (CTRANS, ARTRAN, and the Mie scattering code) to improve efficiency and accommodate a new operational environment, and to build the basic software tools for acquisition and off-line manipulation of simulation results. Initial calculations compare cases in which increasing amounts of aerosol are shifted into the stratosphere, maintaining a constant optical depth. In the case of moderate aerosol optical depth, the effect on the spread function is to scale it linearly as would be expected from a single scattering model. Varying the viewing angle appears to provide the same qualitative effect as modifying the vertical optical depth (for Lambertian ground reflectance).

Pearce, W. A.↗

NASA NTRS1984

A Study of the Relation Between Crop Growth and Spectra Reflectance Parameters

The relationship between crop growth and spectra reflectance parameters was studied. The dynamic behavior of canopy reflectance versus time and the relationship of trajectories to canopy dynamics, ontogeny and morphology was investigated. The determination of which transformations of spectral data best suited for observing key canopy characteristics, such as leaf area index and vegetation identification are understood. A physical basis to understand how various spectral transform trajectories depend on vegetation type and models that describe the time delayed interaction of reflectance in the chlorophyll and water absorption bands to monitor stress were formulated. The general process of physical understanding of canopy reflectance is evaluated and existing canopy models are improved.

Badhwar, G.↗

NASA NTRS1983

Fresnel zone considerations for reflection and scatter from refractive index irregularities

Several different echoing mechanisms are proposed to explain VHF/UHF scatter from clear air; (1) anisotropic scatter; (2) Fresnel reflection, and (3) Fresnel scatter, in order to account for the spatial (angle and range) and temporal dependence of the echoes. The term diffuse reflection describes the echoing mechanism when both scatter and reflection coexist. A unifying formulation is presented incorporating a statistical approach that embraces all mechanisms the above mechanisms and gives conditions under which reflection or scatter dominates. A distinction between Fraunhofer and Fresnel scatter and a criterion is presented under which Fresnel scatter is important.

Doviak, R. J.↗

NASA NTRS1984

Atmospheric transfer of radiation above an inhomogeneous non-Lambertian reflective ground. I - Theory

A method for solving the three-dimensional equation of transfer for a vertically inhomogeneous atmosphere bounded by a reflecting surface of non-uniform bidirectional reflectance is presented. The technique incorporates a two-dimensional spatial Fourier transform of the transfer equation and solution of the resulting expressions for each Fourier component of the radiation field using the method of Gauss-Seidel iteration. The intensity field in the spatial domain, which is calculated for a variety of altitudes, zenith angles, and azimuths, is reconstructed using the inverse Fourier transform. An empirical surface bidirectional reflectance function is employed, permitting the consideration of non-Lambertian reflective properties.

Diner, D. J.↗

NASA NTRS1983

Simulation studies and the need for field reflectance studies

There is a need to calibrate satellite data obtained at large polar (i.e., off-nadir) view angles for effects caused by scanner geometry and by the atmosphere. If such effects are not corrected for, then recorded radiance data will contain systematic and random errors which will make accurate target identification and quantification difficult. Adequate calibration of digital radiance data requires both an inductive analysis of digital satellite images and a deductive analysis based upon a priori simulation studies. The inputs to a simulation model (ground reflectance, atmospheric transmission and bandscatter) and their variability are dependent upon (for example) sun-target-sensor geometry. There is a major need to make accurate ground reflectance measurements to provide calibration information on the anisotropy of ground reflectance. There is also a need to assess atmospheric transmission and backscatter from the image itself. A potential simple, inexpensive method for making ground reflectance measurements in the above context is discussed.

Duggin, M. J.↗

NASA NTRS1984

The effect of direct and reflected radiation on the temperatures of space structures

As an orbiting structure passes from the bright side to the dark side of the earth, substantial differences in surface heating may take place due to direct sunlight and albedo. Most structures are built of convex curved elements to spread out the reflected energy. A change in the shortwave absorptivity and emissivity maintains the temperature of electronic components within a narrow range. Temperature extremes may also be reduced by the use of thermal capacity of the system or by shielding the surface from direct sunlight. For an instrument deployed from a Shuttle, a temperature must be established for it in the cargo bay before deployment. The effect of obstructed views by surfaces such as the Shuttle arm complicates solar flux computations because of the object's mobility. For typical orbits, the earth's albedo and the direct long wavelength radiation may be as much as 30 percent of the direct solar flux. Irradiation due to direct and reflected solar, long wave radiation and reflected radiation from earth, and from Shuttle surfaces is largely dependent on the specular reflection from the radiators as the Shuttle rolls and its surfaces block the sun. Through a judicious choice of surface properties, thermal capacitance and orbit attitude programming, the temperature response of a system can be controlled.

Emery, A. F.↗

NASA NTRS1984

Reflectance spectroscopy - Quantitative analysis techniques for remote sensing applications

The empirical methods and scattering theories that are important for solving remote sensing problems are among the methods for remotely sensed reflectance data analysis presently compared. In the case of the photon mean optical path length concept's implications for reflectance spectra modeling, it is shown that the mean optical path length in a particulate surface is in roughly inverse proportion to the square root of the absorption coefficient. Absorption bands, which are Gaussian in shape when plotted as true absorptance vs photon energy, are also Gaussians in apparent absorptance, although they have a smaller intensity. An apparent continuum in a reflectance spectrum is modeled as a mathematical function that is used to isolate a particular absorption feature for analysis, and it is noted that this continuum should be removed by dividing it into the reflectance spectrum.

Clark, R. N.↗

NASA NTRS1985

Telescope With Reflecting Baffle

Telescope baffle made from combination of reflecting surfaces. In contrast with previous ellipsoidal reflecting baffles, new baffle reflects skew rays more effectively and easier to construct. For infrared telescopes, reflecting baffles better than absorbing baffles because heat load reduced, and not necessary to contend with insufficiency of infrared absorption exhibited by black coatings.

Linlor, W. I.↗

NASA NTRS1986

Reflecting Slit for Imaging Spectrometer

Optical slit that reflects rather than transmits performs multiple functions for imaging infrared spectrometer. Serves as field reflector, field flattener, and entrance aperture for instrument. Reflecting slit narrow strip of spherical reflecting surface with intersecting spherical reflecting side surfaces that reject radiation originating outside slit region. Precision of slit geometry crucial in optical system.

Page, N. A.↗

NASA NTRS1984

Growth and reflectance characteristics of winter wheat canopies

A valuable input to crop growth and yield models would be estimates of current crop condition. If multispectral reflectance indicates crop condition, then remote sensing may provide an additional tool for crop assessment. The effects of nitrogen fertilization on the spectral reflectance and agronomic characteristics of winter wheat (Triticum aestivum L.) were determined through field experiments. Spectral reflectance was measured during the 1979 and 1980 growing seasons with a spectroradiometer. Agronomic data included total leaf N concentration, leaf chlorophyll concentration, stage of development, leaf area index (LAI), plant moisture, and fresh and dry phytomass. Nitrogen deficiency caused increased visible, reduced near infrared, and increased middle infrared reflectance. These changes were related to lower levels of chlorophyll and reduced leaf area in the N-deficient plots. Green LAI, an important descriptor of wheat canopies, could be reliably estimated with multispectral data. The potential of remote sensing in distinguishing stressed from healthy crops is demonstrated. Evidence suggests multispectral imagery may be useful for monitoring crop condition.

Hinzman, L. D.↗

NASA NTRS1983

Environmental responses of solar reflective surfaces

An assessment is undertaken of the environmental responses of solar reflective surfaces, with emphasis on dish-type concentrator surfaces exposed to the conditions of Southern California. A generalized mathematical model for specific solar reflective surfaces can be formulated on the basis of either experimental or assumed site degradation/corrosion data. In addition, the fabrication parameters of a parabolic reflecting surface and its substrate can be used to model combined reflective characteristics for the postulated environmental conditions.

Bouquet, F. L.↗