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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Leaf bidirectional reflectance and transmittance in corn and soybean

Bidirectional optical properties of leaves must be adequately characterized to develop comprehensive and reliably predictive canopy radiative-transfer models. Directional reflectance and transmittance factors of individual corn and soybean leaves were measured at source incidence angles (SIAs) 20, 45, and 70 deg and numerous view angles in the visible and NIR. Bidirectional reflectance distributions changed with increasing SIA, with forward scattering most pronounced at 70 deg. Directional-hemispherical reflectance generally increased and transmittance decreased with increased SIA. Directional-hemispherical reflectance factors were higher and transmittances were lower than the nadir-viewed reflectance component.

Walter-Shea, E. A.↗
NASA NTRS1990

Removing spurious reflections from CFD solutions by using the Complex Cepstrum

The Complex Cepstrum is shown to remove spurious reflections from artificial boundaries in computational fluid dynamic (CFD) solutions. First, the Complex Cepstrum theory is presented. A model time sequence consisting of a direct signal and reflections is analyzed theoretically with the Complex Cepstrum, and it is shown that the direct signal uncontaminated by reflections may be recovered in the time domain. Next, the Complex Cepstrum is applied to one- and three-dimensional CFD solutions, and spurious reflections from the boundary conditions are removed. By eliminating spurious reflections introduced by artificial boundary conditions, the applicability of CFD methods to aeroacoustic problems is greatly enhanced.

Meadows, Kristine R.↗
NASA NTRS1989

Bidirectional reflectances of three soil surfaces and their characterization through model inversion

Spectral bidirectional reflectances were measured over three natural soil sites using a specially designed radiometer called the Parabola. Two of the sites were bare soils, and the third had a sparse cover of desert scrub. The reflectances were strongly non-Lambertian for all three surfaces, but with markedly different patterns. The measured data were fitted with a quasi-physical reflectance model in which the surface backscattering and forwardscattering are separately formulated. A soil reflectance characterization was obtained by assessing the contributions of the forward, backward, and Lambertian components. This three-parameter characterization produced a satisfactory fit to the measured reflectances and appears promising as a basis for soils categorization.

Deering, D. W.↗
NASA NTRS1991

Alteration of the UV-visible reflectance spectra of H2O ice by ion bombardment

Satellite in the Jovian and Saturnian system exhibit differences in reflectivity between their 'leading' and 'trailing' surfaces which can affect the local vapor pressure. Since these differences are thought to be due to differences in the flux of bombarding magnetospheric ions, the influence of ion impact on the UV-visible reflectance of water ice surfaces (20-90 K) by keV ion bombardment was studied. An observed decrease in reflectance in the UV is attributed to rearrangement processes that affect the physical microstructure and surface 'roughness'. The ratio in reflectance of bombarded to freshly deposited films is compared to the ratio of the reflectance of the leading and trailing hemispheres for Europa and Ganymede.

Sack, N. J.↗
NASA NTRS1991

Bi-directional reflectance of finite and infinite clouds

The transfer of solar irradiance in plane parallel and broken cloud fields is modeled using a Monte Carlo method. The angular distribution pattern of radiances exiting the cloud layer is studied with varying cloud geometries, optical thicknesses, cloudiness and solar zenith angles. A rather large anisotropy of the reflected flux is found, usually increasing with solar zenith angle and with patterns that strongly depend on cloud geometry. The main features are: a local maximum of reflected intensity in the forward direction for all cases; a limb darkening for the plane parallel case; and a limb brightening and a local maximum of reflected intensity in the backward direction for broken clouds. The method is used to study the cloud reflectance sensitivity to various parameters. A more precise description of cloud field internal and external structure is needed in order to obtain accurate bi directional reflectance diagrams.

Breon, Francois-Marie↗
NASA NTRS1992

Characteristics of hypervelocity impact craters on LDEF experiment S1003 and implications of small particle impacts on reflective surfaces

The Ion Beam Textured and Coated Surfaces Experiment (IBEX) was designated S1003 on the Long Duration Exposure Facility (LDEF) at a location of 98 degrees relative to the ram direction. Thirty-six diverse materials were exposed to the micrometeoroid (and some debris) environment for 5.8 years. Optical property measurements indicated no changes for almost all of the materials except S-13G, Kapton, and Kapton-coated surfaces, and these changes can be explained by other environmental effects. From the predicted micrometeoroid flux of NASA SP-8013, no changes in optical properties of the surfaces due to micrometeoroids were expected. However, there were hypervelocity impacts on the various diverse materials flown on IBEX. The characteristics of these craters were documented using scanning electron microscopy (SEM) and are presented. Interest in placing large solar concentrator/solar dynamic systems in space for power generation has again brought up a concern for maintaining the integrity of the optical properties of highly specular reflecting surfaces in the near-Earth space environment. It has been shown that highly reflective polished metals and thin film coatings degrade when exposed to simulated micrometeoroids in the laboratory. At LeRC, a shock tube was used to simulate the phenomenon of micrometeoroid optical properties of surfaces exposed to this impact were then evaluated. A calibrated sensor, 2000 A Al/stainless steel, was developed to not only detect the small size micrometeoroid environment, but also to evaluate the degradation of the optical properties of thin aluminum films in space. This sensor was flown on LDEF experiment S1003 and also on the OSO 3 and SERT 2 satellites that were launched in 1967 and 1970, respectively. No changes in the optical properties of the highly reflective surface sensor on SERT 2 were measured during 20 years in space. The results, as determined by the accuracy of the sensor, indicate that a highly reflective surface should lose less than 1 percent of its specular reflectance in near-Earth orbit during 20 years.

Mirtich, Michael J.↗
NASA NTRS1991

Reflectivities of uniform and broken layered clouds

Differences between the reflectivities of uniform layered clouds and of broken clouds taken from the same layers of the Californian coast were investigated using 1 km data obtained by the AVHRRs on the NOAA-9 and -10 satellites during the FIRE Marine Stratocumulus Intensive Field Observations (June 29 - July 18, 1987). The reflectivities are compared at 0.63 micron, where cloud droplets are nonabsorbing, and at 3.7 micron, where the cloud droplets absorb and the single-scattering albedo is about 0.9. It was found that, at visible wavelengths, the reflectivities of broken clouds were only 80-85 percent of those of the same clouds where they formed regions of extensive uniform layers. The radiation not reflected was transmitted to the surface. The results suggest that the presatellite estimates of the planetary albedo may be too large because they failed to allow for the effects of cloud boundaries on cloud reflectivities.

Coakley, James A., Jr.↗
NASA NTRS1991

The effect of ionospheric reflected noise on the performance of an orbital-debris radar system

An orbital-debris radar system was designed to detect the presence of small objects in low earth orbit by reflecting radio waves off the objects. The author provides a rigorous derivation of the ionospheric reflected noise power and provides an integration over the complete pulse period. Free electrons and ions contained in the ionosphere cause incoherent scatter of the radar signal. This ionospheric reflection tends to increase the noise at the terrestrial radar receiver. A parameter called the ionospheric scattering cross section per unit volume, which is a function of altitude, is useful for computing the power of the ionospheric reflection signal. The Doppler frequency speed of the ionospheric reflected signal is a function of altitude also. The ionospheric noise of a 9-GHz orbital-debris radar receiver is computed using these concepts. Annual and diurnal variations of the noise are included.

Bishop, Dennis F.↗
NASA NTRS1992

Reflectance of broken cloud fields - Simulation and parameterization

The transfer of solar irradiance in plane parallel and broken cloud fields is simulated using a Monte Carlo method. The angular distribution pattern of radiances exiting the cloud layer is studied with varying cloud geometries, optical thicknesses, cloudiness, and solar zenith angles. A rather large anisotropy of the reflected flux is found, usually increasing with solar zenith angle and with patterns that strongly depend on cloud geometry. The main features are (1) a local maximum of reflected intensity in the forward direction for all cases, (2) a limb darkening for the plane parallel case, and (3) a limb brightening and a local maximum of reflected intensity in the backward direction for broken clouds. A parameterization for the azimuth-average reflectance function is developed. It reproduces the Monte Carlo simulation with a reasonable accuracy and illustrates that, when azimuthally averaged, the reflectance function is dominated by side viewing and intercloud shadowing effects.

Breon, Francois-Marie↗
NASA NTRS1993

Study of atmospheric and bidirectional effects on surface reflectance and vegetation index time series: Application to NOAA AVHRR and preparation for future space missions

The objectives of the investigation, namely 'to characterize the atmospheric and directional effects on surface reflectance and vegetation index using the First International Satellite Cloud Climatology Project (ISLCSP) Field Experiment (FIFE) data set, develop new algorithms to obtain better Advanced Very High Resolution Radiometer (AVHRR) indices, and define possible improvements for future satellite missions', were addressed in three separate, yet complementary studies. First, it was shown, from theoretical calculations, that visible and near infrared reflectances combined linearly at optimum (one or two) viewing angles relate linearly to the fraction of photosynthetically available radiation absorbed by plants, f(sub par), can be used independently of the type of foliage and substrate, eliminate the effects of sub-pixel spatial heterogeneity, and improve the accuracy of the f(sub par) estimates when compared to the Normalized Difference Vegetation Index, NDVI. Second, it was demonstrated that NDVI, even though it is not a linear combination of radiances or reflectances, can be spatially integrated without significant loss of information from scales of 300 to 1000 m. Third, AVHRR visible and near-infrared reflectances over the FIFE site, separating temporal and bidirectional components and determining the model parameters through an original iterative scheme was successfully modeled. It appears that NDVI generated from the top-of-atmosphere reflectances normalized by the bidirectional effects (as determined in the scheme) is a better vegetation index than maximum NDVI. Details about the three studies are presented.

Frouin, Robert↗
NASA NTRS1992

Influence of atmospheric aerosols and desert reflectance properties on satellite radiance measurements

The influence of surface bidirectional reflectance factors, including shadowing, and of atmospheric aerosol variability are modeled for their effects on the remote sensing of desert targets from space in the 0.7-micron region at high spatial resolution. The white sand reflectance data of Salomonson (1968) are used as the basis for the simulation. The effects of the surface bi-directional reflectance and atmospheric aerosol on the nadir-normalized reflectance measured at the satellite are discussed individually and jointly. The net influence of these two factors is shown to depend on the magnitude of other parameters, such as the surface reflectance and solar zenith angle.

Bowker, D. E.↗
NASA NTRS1993

Automated measurement of polarized bidirectional reflectance

An Automated Bidirectional Reflection Acquisition Measurement System (ABRAMS) has been constructed to facilitate measurement of bidirectional reflectance from soil and vegetative samples in the laboratory. The system illuminates a sample with linearly-polarized laser light, lambda = 632.8 nm, and measures the like- and cross-polarized scattered intensities over half a hemisphere. System design and polarized bidirectional reflectance measurements from a soil sample and SiO2 spherical particles are discussed in this work. It is shown that polarization information in the plane of incidence is useful for identifying certain scattering mechanisms associated with soil reflectance. This is because the like-polarized intensity, Ivv, is influenced by single-scattered light and the cross-polarized intensity, IHv, is strongly influenced by multiple-scattered light. For example, comparable levels of Ivv and IHv indicated that the reflectance of soils, is dominated by significant multiple scattering because single scattering causes minimal depolarization in the plane of incidence.

Gibbs, Daniel P.↗
NASA NTRS1992

High dimensional reflectance analysis of soil organic matter

Recent breakthroughs in remote-sensing technology have led to the development of high spectral resolution imaging sensors for observation of earth surface features. This research was conducted to evaluate the effects of organic matter content and composition on narrowband soil reflectance across the visible and reflective infrared spectral ranges. Organic matter from four Indiana agricultural soils, ranging in organic C content from 0.99 to 1.72 percent, was extracted, fractionated, and purified. Six components of each soil were isolated and prepared for spectral analysis. Reflectance was measured in 210 narrow bands in the 400- to 2500-nm wavelength range. Statistical analysis of reflectance values indicated the potential of high dimensional reflectance data in specific visible, near-infrared, and middle-infrared bands to provide information about soil organic C content, but not organic matter composition. These bands also responded significantly to Fe- and Mn-oxide content.

Henderson, T. L.↗
NASA NTRS1992

Developments of new force reflecting control schemes and an application to a teleoperation training simulator

Two schemes of force reflecting control, position-error based force reflection and low-pass-filtered force reflection, both combined with shared compliance control, were developed for dissimilar master-slave arms. These schemes enabled high force reflection gains, which were not possible with a conventional scheme when the slave arm was much stiffer than the master arm. The experimental results with a peg-in-hole task indicated that the newly force reflecting control schemes combined with compliance control resulted in best task performances. As a related application, a simulated force reflection/shared compliance control teleoperation trainer was developed that provided the operator with the feel of kinesthetic force virtual reality.

Kim, Won S.↗
NASA NTRS1993

The X-ray reflectivity of the AXAF VETA-I optics

The study measures the X-ray reflectivity of the AXAF VETA-I optic and compares it with theoretical predictions. Measurements made at energies of 0.28, 0.9, 1.5, 2.1, and 2.3 keV are compared with predictions based on ray trace calculations. Results on the variation of the reflectivity with energy as well as the absolute value of the reflectivity are presented. A synchrotron reflectivity measurement with a high-energy resolution over the range 0.26 to 1.8 keV on a flat Zerodur sample is also reported. Evidence is found for contamination of the flat by a thin layer of carbon on the surface, and the possibility of alteration of the surface composition of the VETA-I mirror, perhaps by the polishing technique. The overall agreement between the measured and calculated effective area of VETA-I is between 2.6 and 10 percent. Measurements at individual energies deviate from the best-fitting calculation to 0.3 to 0.8 percent, averaging 0.6 percent at energies below the high energy cutoff of the mirror reflectivity, and are as high as 20.7 percent at the cutoff.

Kellogg, E.↗
NASA NTRS1993

Reflectance calibrations of AXAF mirror samples at absorption edges using synchrotron radiation

We are developing a system to calibrate reflectances of witness coupons to the AXAF flight mirrors at the National Synchrotron Light Source over the 0.05-12 keV energy range. These witness coupons will be coated in the same process as the AXAF mirror elements. One of the key issues is the accurate determination of mirror efficiencies across the absorption edges of the mirror coating elements. We present a series of reflectance measurements with 2 eV resolution of a nickel-coated flat mirror in the region of the Ni L-II (870 eV) and L-III (853 eV) absorption edges. Scans of reflectance versus grazing angle at fixed energies in this region show distinct interference fringes at grazing angles larger than the critical angle which are extinguished as the photon energy is increased beyond the low point of the L-III edge, indicating total absorption of the evanescent wave within the Ni film. At 51 arc minutes grazing angle, measured reflectance decreases smoothly by 35 percent and then recovers in an 8 eV band at the L-III edge. We have also measured reflectances in the M absorption edge region for gold, platinum, and iridium coated mirrors. We derive optical parameters n and k specific to the film for comparison to the existing data tables.

Graessle, D. E.↗
NASA NTRS1993

Exploratory study of shock reflection near an expansion corner

Experiments were performed at Mach 8 in which a shock was reflected off a low Reynolds number, turbulent boundary layer past an expansion corner. The shock was generated by 2- and 4-deg sharp wedges, and the corner was either 2.5 or 4.25 deg. The inviscid shock reflection was one boundary layer thickness ahead or behind the corner or at the corner itself. All interactions were unseparated. The dynamic surface pressure distributions were examined together with the case of shock reflection on a flat plate. With shock reflection ahead of the corner, the mean surface pressure downstream was attenuated due to the proximity of the corner. With shock reflection downstream of the corner, the surface pressure distribution showed a reduced upstream influence. The highly swept expansion fan produced a surface pressure which rose gently downstream with no minima, unlike in supersonic flows with the same shock-corner separation distance. In many of the interactions, an anomalous pressure peak was found downstream. The surface pressure through the interaction exhibited unsteadiness.

Lu, Frank K.↗
NASA NTRS1993

A simple bidirectional-reflectance model applied to a tallgrass canopy

The present bidirectional reflectance model is used to simulate spectral reflectance direction-changes in a tallgrass canopy; comparisons with field measurements show generally good agreement. An examination of the effects of 'hot spot' and soil-reflection inhomogeneous canopy characteristics indicates that both modeled and observed reflectances are strongly backscattered in the red band, but there is a more symmetric back/forward-scattering distribution in the near-IR. Modeled simulations for three leaf-angle distributions show that hemispherical reflectance errors could be reduced at an optimal solar zenith angle having the same magnitude for the three leaf-angle distributions, but decreasing with increasing canopy leaf-area index.

Gao, W.↗