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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 883 records · Page 49

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NASA NTRS1985

Atmospheric effects on the remote sensing estimation of forest leaf area index

An analysis is presented of the magnitude and variability of the effect of the atmosphere on high-altitude Daedalus Airborne Thematic Mapper data. By regressing ATM radiances against ground radiances (from measurements by a helicopter-mounted Barnes Modular Multiband Radiometer), it was possible to account for atmospheric conditions and variability across a 250 km transect in Oregon to estimate coniferous forest leaf area index. The technique permitted scene contrast to be increased, providing an improved capability for measurement of ground feature radiance.

Spanner, M. A.↗

NASA NTRS1986

Relationship between atmospheric CO2 variations and a satellite-derived vegetation index

Weekly data on the normalized difference vegetation index (NDVI), obtained from the advanced very-high-resolution radiometer sensors flown on the NOAA's polar-orbiting meteorological satellites, were correlated with monthly changes in atmospheric CO2 concentration measured at the surface stations in Alaska, Hawaii, and the South Pole in the 1982-1984 period. While the South Pole data showed no relationship between CO2 concentration and the NDVI, the CO2 data taken at Alaska and Hawaii showed inverse correlation with the NDVI, demonstrating a measurable link between the CO2 drawdowns and the dynamics of NDVI, which in turn reflect the dynamics of photosynthesis. The data suggest that satellite data can be used for estimations of terrestrial photosynthetic activity.

Tucker, C. J.↗

NASA NTRS1986

Satellite-derived leaf-area-index and vegetation maps as input to global carbon cycle models - A hierarchical approach

A hierarchical procedure for developing a leaf area index (LAI) map of deciduous boreal forests is studied. The collection of spectral reflectance data from the Boundary Waters Canoe area in Minnesota using helicopter-, high-altitude aircraft-, and Landsat-mounted spectral sensors is described. The relationship between LAI and biomass and the reflectance ratio is analyzed. The sensitivity of canopy reflectance in the visible and infrared to the LAI of the canopy for various boreal forest species is evaluated. The data reveal that Landsat data are useful for producing LAI maps of deciduous forest areas and the maps provide data which clarifies the function of vegetation in the global carbon cycle models.

Badhwar, G. D.↗

NASA NTRS1986

Sensitivity of the Lidar ratio to changes in size distribution and index of refraction

In order to invert lidar signals to obtain reliable extinction coefficients, sigma, a relationship between sigma and the backscatter coefficient, beta, must be given. These two coefficients are linearly related if the complex index of refraction, m, particle shape size distribution, N, does not change along the path illuminated by the laser beam. This, however, is generally not the case. An extensive Mie computation of the lidar ratio R = beta/sigma and the sensitivity of R to the changes in a parametric space defined by N and m were examined.

Evans, B. T. N.↗

NASA NTRS1987

Aeronautical engineering: A continuing bibliography with indexes (supplement 211)

A continuing bibliography (NASA SP-7037) lists 519 reports, journal articles and other documents originally announced in February 1987 in Scientific and Technical Aerospace Reports (STAR) or in the International Aerospace Abstracts (IAA). The coverage includes documents on the engineering and theoretical aspect of design, construction, evaluation, testing, operation, and performance of aircraft (including aircraft engines) and associated components, equipment, and systems. It also includes research and development in aerodynamics, aeronautics, and ground support equipment for aeronautical vehicles. Each entry in the bibliography consists of a standard bibliographic citation accompanied in most cases by an abstract. The listing of the entries is arranged by the first nine STAR specific categories and the remaining STAR major categories. The arrangement offers the user the most advantageous breakdown for individual objectives. The citations include the original accession numbers from the respective announcement journals. The IAA items will precede the STAR items within each category. Seven indexes entitled subject, personal author, corporate source, foreign technology, contract number, report number, and accession number are included.

Source record↗

NASA NTRS1986

Bibliographical index of objects observed by IUE 1978-85

A literature search covering the years 1978 to 1985 identified 1134 papers describing studies using data obtained with the International Ultraviolet Explorer (IUE) satellite. The names of greater than 9400 astronomical objects discussed were recorded. The objects were compiled into a list, along with each reference, and sorted by object name or catalog number. Cross-identification tables for the multiple names are provided. The 64-page Table of References includes the titles of the articles. This index enables a user to tell immediately where to find published papers describing IUE observations of the objects of interest.

Mead, Jaylee M.↗

NASA NTRS1986

Estimation of canopy parameters for inhomogeneous vegetation canopies from reflectance data. II - Estimation of leaf area index and percentage of ground cover for row canopies

The canopy reflectance (CR) model for row-planted vegetation proposed earlier has been tested for soybean canopies in three different stages of growth and for corn canopies at early and full growth stages. The model fits the field-measured bidirectional CR data quite well. It is shown that, by inverting this model, one could estimate the leaf area index as well as the percentage of ground cover quite accurately from measured canopy reflectances.

Goel, N. S.↗

NASA NTRS1986

Analysis of the dynamics of African vegetation using the normalized difference vegetation index

Images at a resolution of 8 km are currently being generated for the whole of Africa, displaying the normalized difference vegetation index (NDVI). These images have undergone a process of temporal compositing to reduce the effects of cloud cover and atmospheric variation. When the NDVI is plotted against time, different cover types are shown to have characteristic profiles corresponding closely with their phenology. The resultant pattern of NDVI values displayed on the images is analyzed in terms of the cover types present and local variations in rainfall. Comparison between images for 1983 and 1984 overall showed considerable similarities, but significant differences were observed in the northward extent of the greening wave in the Sahel, the greening up of the Kalahari Desert and East African communities. It is concluded that vegetation monitoring using NDVI images needs to be associated with scene stratification according to cover type.

Townshend, J. R. G.↗

NASA NTRS1987

Radial image transfer by cylindrical, step-index optical waveguides

The theoretical extent of image transfer by perfect, cylindrical, step-index optical waveguides is examined. Azimuthal image structure is essentially removed by such waveguides, while substantial radial image structure is transmitted. The consequences of this behavior for optical-fiber image scramblers are discussed; such devices may be unsuitable for simple photometric applications but appear to be potentially useful in reducing zonal errors in slit spectroscopy.

Heacox, William D.↗

NASA NTRS1986

Prediction filters for the Dst index and the polar cap potential

The technique of linear prediction filtering is used to create filters relating solar wind parameters to the Dst index and to the polar cap potential. The square root of solar wind dynamic pressure and the solar wind electric field together are found to account for nearly 70 percent of the Dst variance. The prediction filter for the polar cap potential requires the square of the solar wind velocity and the solar wind electric field as inputs. The results suggest that both polar cap potential and ring current injection are primarily controlled by the solar wind, and that substorm expansions do not play a major role in ring current injection.

Fay, R. A.↗

NASA NTRS1986

Do we need a geoelectric index?

The need for a geoelectric index (GI) measuring the global level of atmospheric electrical activity for a given time is assessed, and methods for defining a GI are compared. Current problems in atmospheric and space electrodynamics (the global circuit, solar-terrestrial coupling, lightning effects on the ionosphere/magnetosphere, and mesospheric generators), atmospheric chemistry (the stratospheric ozone cycle and atmospheric gravity waves), and meteorology (fog forecasting) are reviewed to illustrate the usefullness of a GI. Derivations of a GI from in situ electrical measurements and from ground or satellite remote sensing of source properties are described, and a system based on ground measurement of the intensity of the Schumann resonance lines (as proposed by Polk, 1982) is found to be the most practical, requiring as few as three (automatically operated) stations for global coverage.

Holzworth, R.↗

NASA NTRS1987

A stress index model for balloon design

A NASA stress index model, SINDEX, is discussed which establishes the relative stress magnitudes along a balloon gore as a function of altitude. Application of the model to a data base of over 550 ballon flights demonstrates the effectiveness of the method. The results show a strong correlation between stress levels, failure rates, and the point of maximum stress coinciding with the observed failure locations. It is suggested that the model may be used during the balloon design process to lower the levels of stress in the balloon.

Smith, I. S.↗

NASA NTRS1987

On the saturation of the refractive index structure function. II - Influence of the correlation length on astronomical 'seeing'

A physical length scale in the wavefront corresponding to the parameter (r sub 0) characterizing the loss in detail in a long exposure image is identified, and the influence of the correlation scale of turbulence as r sub 0 approaches this scale is shown. Allowing for the effect of 2-point correlations in the fluctuations of the refractive index, Venkatakrishnan and Chatterjee (1987) proposed a modified law for the phase structure function. It is suggested that the departure of the phase structure function from the 5/3 power law for length scales in the wavefront approaching the correlation scale of turbulence may lead to better 'seeing' at longer wavelengths.

Venkatakrishnan, P.↗

NASA NTRS1988

Ultralow threshold graded-index separate-confinement heterostructure single quantum well (Al, Ga) As lasers

Broad area graded-index separate-confinement heterostructure single quantum well lasers grown by molecular-beam epitaxy (MBE) with threshold current density as low as 93 A/sq cm (520 microns long) have been fabricated. Buried lasers formed from similarly structured MBE material with liquid phase epitaxy regrowth had threshold currents at submilliampere levels when high reflectivity coatings were applied to the end facets. A CW threshold current of 0.55 mA was obtained for a laser with facet reflectivities of about 80 percent, a cavity length of 120 micron, and an active region stripe width of 1 micron. These devices driven directly with logic level signals have switch-on delays less than 50 ps without any current prebias. Such lasers permit fully on-off switching while at the same time obviating the need for bias monitoring and feedback control.

Derry, P. L.↗

NASA NTRS1988

Index grating lifetime in photorefractive GaAs

The index grating lifetime in liquid encapsulated Czochralski-grown undoped semi-insulating GaAs was measured using a beam coupling technique. The largest lifetime measure was about 8 s under a read beam intensity of 0.7 mW/sq cm with the grating periodicity being 0.63 microns. The measured value decreases to milliseconds as the read beam intensity and the grating periodicity increase to about 10 mW/sq cm and 4 microns, respectively. This range of grating lifetime in this material is adequate for its use in real-time spatial light modulators, reconfigurable beam steering devices, and dynamic memory elements, for optical computing. In addition, the results suggest that the lifetime is sensitive to residual imperfections in the crystal.

Cheng, Li-Jen↗

NASA NTRS1988

Response to soil moisture of spectral indexes derived from bidirectional reflectance in Thematic Mapper wavebands

Laboratory reflectance measurements of 10 soils were used to determine the relationship between soil moisture and three spectral indices: the TM5/7 ratio and the Wetness(R) and Brightness(R) features of the reflectance factor TM Tasseled Cap transformation. Response of the indices to dry mass water percentage was approximately linear for individual soils, except for Wetness(R) and Brightness(R) at high moisture content. Soil differences in the slopes of the Wetness(R)- and Brightness(R)-moisture content relationships were almost entirely eliminated by expressing water content as the percentage of water retained at 0.1 bar (10 kPa) tension (relative water content). The resultant soil lines were offset from one another by the differences in dry soil index value. Slope of the TM5/7 response was not completely normalized by expressing moisture status as relative water content, because slope appeared to vary with dry soil ratio value. Sensitivity to the effects of illumination angle was negligible for the TM5/7 ratio, somewhat greater for Wetness(R) and greatest for Brightness(R).

Musick, H. Brad↗

NASA NTRS1988

Selecting a spatial resolution for estimation of per-field green leaf area index

For any application of multispectral scanner (MSS) data, a user is faced with a number of choices concerning the characteristics of the data; one of these is their spatial resolution. A pilot study was undertaken to determine the spatial resolution that would be optimal for the per-field estimation of green leaf area index (GLAI) in grassland. By reference to empirically-derived data from three areas of grassland, the suitable spatial resolution was hypothesized to lie in the lower portion of a 2-18 m range. To estimate per-field GLAI, airborne MSS data were collected at spatial resolutions of 2 m, 5 m and 10 m. The highest accuracies of per-field GLAI estimation were achieved using MSS data with spatial resolutions of 2 m and 5 m.

Curran, Paul J.↗

NASA NTRS1989

Rapidly-Indexing Incremental-Angle Encoder

Optoelectronic system measures relative angular position of shaft or other device to be turned, also measures absolute angular position after device turned through small angle. Relative angular position measured with fine resolution by optoelectronically counting finely- and uniformly-spaced light and dark areas on encoder disk as disk turns past position-sensing device. Also includes track containing coarsely- and nonuniformly-spaced light and dark areas, angular widths varying in proportion to absolute angular position. This second track provides gating and indexing signal.

Christon, Philip R.↗