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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 1,765 records · Page 98
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Probability-matched reflectivity-rainfall relations for a hurricane from aircraft observations

The relation between the distribution of equivalent reflectivity, Z(sub e), measured by an airborne C-band radar and that for rain rate, R, measured concurrently by a disdrometer on the same aircraft in the eyewall and outer bands of Hurricane Anita (1977) was investigated using the probability-matching method (PMM). It was found that the pdf of R is much narrower in the outer rainbands of Hurricane Anita than in the eyewall, and that the mean rain rates are also much smaller. It is shown that the use of PMM provides more realistic relations than the conventional power-law Z-R regression relations based upon scatter-plots of disdrometer measurements, showing higher eyewall reflectivities for the same rain rates at rates in excess of 6.3 mm/h.

Marks, Frank D., Jr.↗
NASA NTRS1992

Extraction of soil and vegetation parameters from high resolution bi-directional reflectance spectra

High spectral resolution reflectance spectra were collected over a semi-desert grassland at both dry and wet season periods. Spectral reflectance measurements were made from several viewing angles at both low and high solar zenith angles. A mixture model was used to separate and extract green vegetation from dry/dead vegetation and soil. The extracted vegetation signal varied greatly with view and sun angle variations such that off-nadir viewing and illuminating angles resulted in the highest vegetation loadings. These variations were normalized with cosine functions for both sun and view angle. These results offer a methodology for standardizing multi-temporal and multi-angular satellite measurements of vegetation activity.

Huete, A. R.↗
NASA NTRS1993

Ultraviolet stability and contamination analysis of Spectralon diffuse reflectance material

A detailed chemical analysis was carried out on Spectralon, a highly Lambertian, diffuse reflectance material. Results of this investigation unambiguously identified the presence of an organic (hydrocarbon) impurity intrinsic to the commercial material. This impurity could be removed by a vacuum bake-out procedure and was identified as the cause of optical changes (degradation) that occur in the material when exposed to UV light. It was found that when this impurity was removed, the Spectralon material was photochemically stable and maintained its reflectance properties even after extensive solar UV exposure.

Stiegman, Albert E.↗
NASA NTRS1993

Correcting for diffraction in the far-infrared reflectance measurement of rough surfaces

The manner in which diffraction can increase the reflectance measured from very rough surfaces is shown by observations of diffracted light within the instrument profile of a far-IR reflectometer system. A correction to the calibration signal based on numerical integration of the diffracted part of the instrument profile is described. Diffraction correction factors as large as 2.94 have been found with small optics at long wavelength (630 microns). The effect of diffraction on diffuse reflectance measurements of a very rough perfect reflector is shown at wavelengths from 56 to 200 microns.

Smith, Sheldon M.↗
NASA NTRS1952

On Reflection of Shock Waves from Boundary Layers

Measurements are presented at Mach numbers from about 1.3 to 1.5 of reflection characteristics and the relative upstream influence of shock waves impinging on a flat surface with both laminar and turbulent boundary layers. The difference between impulse and step waves is discussed and their interaction with the boundary layer is compared. General considerations on the experimental production of shock waves from wedges and cones and examples of reflection of shock waves from supersonic shear layers are also presented.

Liepmann, H W↗
NASA NTRS1943

Jet-boundary corrections for reflection-plane models in rectangular wind tunnel

A detailed method for determining the jet-boundary corrections for reflection-plane models in rectangular wind tunnels is presented. The method includes the determination of the tunnel span local distribution and the derivation of equations for the corrections to the angle of attack, the lift and drag coefficients, and the pitching-, rolling-, yawing-, and hinge-moment coefficients. The principle effects of aerodynamic induction and of the boundary-induced curvature of the streamlines have been considered. An example is included to illustrate the method. Numerical values of the more important corrections for reflection-plane models in 7 by 10-foot closed wind tunnels are presented.

Swanson, Robert S↗
NASA NTRS1994

More About Graded-Reflectance Optics For Coherent Lidar

Report presents theoretical study of energy-extraction and power-transmission efficiency of unstable-resonator laser cavity, output mirror of which has graded reflectance. Super-gaussian reflectance profile may optimize performance. Similar study reported in "Comparative Study of Resonator Optics for Lidar Applications" (NPO-17776).

Tratt, David M.↗
NASA NTRS1993

The state of Fe in the lunar regolith and its relationship to the spectral reflectance properties of the moon

Space weathering may have altered the surfaces of airless bodies such as asteroids and the moon, and effectively shock-darkened their surfaces, resulting in much lower reflectance and shallower absorption bands compared to those of simple, comminuted, and powdered rocks produced in the laboratory. Studies of lunar samples from the Apollo missions have led to the suggestion that these reflection properties are inherent to the fine-grained regolith which has recorded a wide variety of shock-metamorphic effects. In particular, the lunar soil (i.e., that part of the regolith with grain sizes below 1 mm) contains native Fe in quantities above those present in the rocks from which the soil was formed. The origin and nature of the native Fe in the soil is addressed, and speculations concerning its effects upon the soil's optical properties are presented. Portions of this discussion draw heavily from earlier papers by our research group.

Taylor, Lawrence A.↗
NASA NTRS1993

Comparison of reflectance spectra of C asteroids and unique C chondrites Y86720, Y82162, and B7904

Reflectance spectra (0.3-2.6 microns) of 11 carbonaceous (C) chondrite powders (less than 100 or less than 125 microns) including 'unique' ones Y86720, Y82162, and B7904, have been measured and compared with those of 14 C asteroids. Among those C chondrites, only three 'unique' ones had close counterparts among C asteroids (Y86720 and 1 Ceres, Y82162 and 704 Interamnia, and B7904 and 31 Euphrosyne). Mixing calculations of those C chondrites by Hapke's isotropic model were also performed to improve fits of reflectance spectra of all the 14 C asteroids. If the grain-size distributions of those asteroid surfaces are similar to those C chondrite powders, the result suggests that all tested C asteroids contain a high amount of heated C chondrite material, such as Y86720, Y82162, and B7904.

Hiroi, T.↗
NASA NTRS1993

Iron oxide bands in the visible and near-infrared reflectance spectra of primitive asteroids

High resolution reflectance spectra of primitive asteroids (C, P, and D class and associated subclasses) have commonly revealed an absorption feature centered at 0.7 microns attributed to an Fe(2+)-Fe(3+) charge transfer transition in iron oxides and/or oxidized iron in phyllosilicates. A smaller feature identified at 0.43 microns has been attributed to an Fe(3+) spin-forbidden transition in iron oxides. In the spectra of the two main-belt primitive asteroids 368 Haidea (D) and 877 Walkure (F), weak absorption features which were centered near the location of 0.60-0.65 microns and 0.80-0.90 microns prompted a search for features at these wavelengths and an attempt to identify their origin(s). The CCD reflectance spectra obtained between 1982-1992 were reviewed for similar absorption features located near these wavelengths. The spectra of asteroids in which these absorption features have been identified are shown. These spectra are plotted in order of increasing heliocentric distance. No division of the asteroids by class has been attempted here (although the absence of these features in the anhydrous S-class asteroids, many of which have presumably undergone full heating and differentiation should be noted). For this study, each spectrum was treated as a continuum with discrete absorption features superimposed on it. For each object, a linear least squares fit to the data points defined a simple linear continuum. The linear continuum was then divided into each spectrum, thus removing the sloped continuum and permitting the intercomparison of residual spectral features.

Jarvis, Kandy S.↗
NASA NTRS1993

High-resolution seismic reflection survey at the Manson crater, Iowa

Approximately 17.4 km of high-resolution reflection data were acquired along an east-west radius of the Manson Impact Structure (MIS) to delineate the shallow (upper 300 m) subsurface structural configuration. The geometry of the shallow structure is poorly known due to a 30-90 m thick Pleistocene till cover. The resolution of the new seismic data is roughly 5-10 times that of existing Vibroseis data. Data quality varies rapidly along the line from exceptional to poor, due primarily to velocity variations associated with the geological complexity of the area. Preliminary results indicate subsurface structural blocks previously envisioned to be several hundreds of meters in size are actually an order of magnitude smaller and more complex. A seismogram-by-seismogram analysis is necessary to confidently identify intricate stratigraphic and structural relationships seen on preliminary CDP sections, as numerous faults, diffractions, and complicated reflection patterns create potential pitfalls.

Keiswetter, D. A.↗
NASA NTRS1993

Bi-directional reflectance and other radiation parameters of cirrus from ER-2 observations

Passive and active remote sensing of cirrus were acquired from the ER-2 high altitude aircraft in the 1991 Cirrus Experiment. The observations include direct measurements of cirrus bi-directional reflectance from a new translinear scanning radiometer and the previously employed measurements by lidar and visible-infrared imaging radiometers. For any cirrus radiative transfer application, it is necessary to know the appropriate model for visible reflectance in relation to angle and also the optical thickness and infrared emissivity of the clouds. At a more complicated level, for remote sensing and overall cloud effects it is ultimately required to understand effects from multiple cloud layers, broken clouds, and variable microphysics. Our overall data set from the scanning radiometers and lidar is intended to provide the necessary observations to investigate these problems.

Spinhime, James↗
NASA NTRS1994

Possible spinel absorption bands in S-asteroid visible reflectance spectra

Minor absorption bands in the 0.55 to 0.7 micron wavelength range of reflectance spectra of 10 S asteroids have been found and compared with those of spinel-group minerals using the modified Gaussian model. Most of these S asteroids are consistently shown to have two absorption bands around 0.6 and 0.67 micron. Of the spinel-group minerals examined in this study, the 0.6 and 0.67 micron bands are most consistent with those seen in chromite. Recently, the existence of spinels has also been detected from the absorption-band features around 1 and 2 micron of two S-asteroid reflectance spectra, and chromite has been found in a primitive achondrite as its major phase. These new findings suggest a possible common existence of spinel-group minerals in the solar system.

Hiroi, T.↗
NASA NTRS1994

Case study of a low-reflectivity pulsating microburst: Numerical simulation of the Denver, 8 July 1989, storm

On 8 July 1989, a very strong microburst was detected by the Low-Level Windshear Alert system (LLWAS), within the approach corridor just north of Denver Stapleton Airport. The microburst was encountered by a Boeing 737-200 in a 'go-around' configuration which was reported to have lost considerable air speed and altitude during penetration. Data from LLWAS revealed a pulsating microburst with an estimated peak velocity change of 48 m/s. Wilson et al. reported that the microburst was accompanied by no apparent visible clues such as rain or virga, although blowing dust was present. Weather service hourly reports indicated virga in all quadrants near the time of the event. A National Center for Atmospheric Research (NCAR) research Doppler radar was operating; but according to Wilson et al., meaningful velocity could not be measured within the microburst due to low radar-reflectivity factor and poor siting for windshear detection at Stapleton. This paper presents results from the three-dimensional numerical simulation of this event, using the Terminal Area Simulation System (TASS) model. The TASS model is a three-dimensional nonhydrostatic cloud model that includes parameterizations for both liquid and ice phase microphysics, and has been used in investigations of both wet and dry microburst case studies. The focus of this paper is the pulsating characteristic and the very-low radar reflectivity of this event. Most of the surface outflow contained no precipitation. Such an event may be difficult to detect by radar.

Proctor, Fred H.↗
NASA NTRS1995

Trend analysis of in-situ spectral reflectance data from the Thermal Control Surfaces Experiment (TCSE)

The Thermal Control Surfaces Experiment (TCSE) on the LDEF was a comprehensive experiment that combined in-space measurements with extensive pre- and post-flight analyses of thermal control surfaces to determine the effects of exposure to the low earth orbit (LEO) space environment. The TCSE is the first space experiment to directly measure in-situ total hemispherical reflectance of thermal control surfaces in the same way they are routinely measured in the laboratory. In-space optical measurements performed by the TCSE provide the unique opportunity for trend analysis of the performance of materials in the space environment. Such trend analysis of flight data offers the potential to develop an empirical life time prediction model for several thermal control surfaces. For material research, trend analysis of the TCSE flight data, particularly the spectral data, can provide insight into the damage mechanisms of space exposure. Trend analysis for the TCSE samples has been limited to those materials that were not significantly eroded by the atomic oxygen (AO) environment. The performance of several materials on the LDEF mission was dominated by AO effects. Trend analysis was performed on both the detailed spectral reflectance measurements (in-space, pre-flight, and post-flight) and on the integrated solar absorptance. Results of this analysis for the five selected TCSE materials are presented along with the spectral flight data. Possible degradation and effects mechanisms will be discussed to better understand and predict the behavior of these materials in the LEO space environment.

Wilkes, Donald R.↗
NASA NTRS1994

New composite reflectance spectra of Mars from 0.4 to 3.14 micron

To verify the quality and accuracy of Phobos-2 Imaging Spectrometer for Mars (ISM) spectra of Mars, we have performed comparisons with telescopic data obtained contemporaneously and which have spatial and spectral overlap with the ISM measurements. We find general agreement between the independently-calibrated data sets in terms of spectral shape and absorption band strength and position in the 0.77 to 0.93 micron overlap region. The telescopic data were scaled to the ISM reflectance values in this overlap region to yield new bright and dark region composite reflectance spectra from 0.40 to 3.14 microns. We ascribe the increase in band depth and overall spectral heterogeneity of the ISM data relative to previous telescopic observations to the more than order of magnitude increase in spatial resolution achieved by ISM from Martian orbit. Further increases in spatial resolution from future orbital imaging spectroscopic instruments can be expected to reveal even greater degrees of compositional and mineralogic diversity on Mars.

Mustard, John F.↗
NASA NTRS1994

Comparison of daily averaged reflection, transmission, and absorption for selected radiative flux transfer approximations

This paper compares accuracy for the daily averaged reflection, transmission, and absorption of solar flux derived from the delta-four-stream approximation and a few selected two-stream approximations. In the chosen variety of two-stream approximations, a two-stream approximation suggested by Zhu produces the most accurate reflection and transmission for conservative scattering with optical thickness tau(sub 1) less than 3. Otherwise, the delta-Eddington approximation is superior to other two-stream approximations. The delta-four stream approximation produces errors that are smaller than the delta-Eddington approximation by at least a factor of 2.

Zhu, Xun↗
NASA NTRS1995

Classification of rain regimes by the three-dimensional properties of reflectivity fields

An automated scheme to characterize precipitation echoes within small windows in the radar field is presented and applied to previously subjectively classified tropical rain cloud systems near Darwin, Australia. The classification parameters are (a) E(sub e), effective efficiency, as determined by cloud-top and cloud-base water vapor saturation mixing ratios; (b) BBF, brightband fraction, as determined by the fraction of the radar echo area in which the maximal reflectivity occurs within +/- 1.5 km of the 0 C isotherm level; and (c) del(sub r) Z, radial reflectivity gradients (dB/km). These classification criteria were applied to tropical rain cloud systems near Darwin, Australia, and to winter convective rain cloud systems in Israel. Both sets of measurements were made with nearly identical networks of C-band radars and rain gauge networks. The results of the application of these objective classification criteria to several independently predetermined rain regimes in Darwin have shown that better organized rain systems have smaller del(sub r) Z and larger BBF. Similarly, smaller del(sub r)Z and larger BBF were also observed from maritime rain cloud systems, as compared to continental rain cloud systems with the same degree of organization. Continental rain cloud system, regardless of their degree of organization, have larger depths, as expressed by E(sub e). The rainfall analyses presented in this study are based exclusively on rain gauge measurements, while radar information was used only to classify the individual gauge measurements.

Rosenfeld, Daniel↗