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At least 703 records · Page 39

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NASA NTRS1990

Measurement of the complex refractive index and complex dielectric permittivity of T.P.S. Space Shuttle tile materials at millimeter wavelengths

Complex refractive index and dielectric permittivity studies of presently used Space Shuttle tile materials at millimeter wavelengths reveal these tiles to exhibit similar absorption characteristics to those of fused silica materials. This absorption is mainly related to the water content in the specimen. A strong birefringence is observed at least in one of these fibrous refractory composite materials.

Afsar, Mohammed Nurul↗

NASA NTRS1990

Calibration correction of an active scattering spectrometer probe to account for refractive index of stratospheric aerosols

The use of the active scattering spectrometer probe (ASAS-X) to measure sulfuric acid aerosols on U-2 and ER-2 research aircraft has yielded results that are at times ambiguous due to the dependence of particles' optical signatures on refractive index as well as physical dimensions. The calibration correction of the ASAS-X optical spectrometer probe for stratospheric aerosol studies is validated through an independent and simultaneous sampling of the particles with impactors; sizing and counting of particles on SEM images yields total particle areas and volumes. Upon correction of calibration in light of these data, spectrometer results averaged over four size distributions are found to agree with similarly averaged impactor results to within a few percent: indicating that the optical properties or chemical composition of the sample aerosol must be known in order to achieve accurate optical aerosol spectrometer size analysis.

Pueschel, R. F.↗

NASA NTRS1991

A new activity index for comets

An activity index (AI) is derived from observational data to measure the increase of activity in magnitudes for comets when brightest near perihelion as compared to their inactive reflective brightnesses at great solar distances. In all, AI determinations are made for 32 long-period (L-P) comets and for 14 short-period (S-P). The range of values of AI is the order of 3 to 10 magnitudes with a median about 6. An expected strong correlation with perihelion distance, q, was found to vary as approx. q(-2.3). Residuals from a least-square solution, delta AI, were used for comparing comets of different orbital classes, the standard deviation of a single value of delta AI was only plus or minus 1(m).1 for L-P comets and plus or minus 1(m)2 for S-P comets. Among the L-P comets, 19 of Period, P greater than 10(exp 4) yr yielded delta AI greater than or equal to -0(m).27 compared to +0(m).39 for 13 of 10(exp 4) greater than P greater than 10(exp 2) yr. This denies any effect of aging among the L-P comets. The 14 S-P comets yielded (delta AI) less 0(m).3 plus or minus 0(m).3 than the 32 L-P comets. The results suggest a common actively level, nature and probably origin for comets of all orbits, sizes and ages.

Whipple, Fred L.↗

NASA NTRS1990

Ultra-low-frequency wave power in the magnetotail lobes. I - Relation to substorm onsets and the auroral electrojet index

Time-series observations of the magnetotail-lobe magnetic field have been Fourier analyzed to compute the frequency-weighted energy density Pfz in the range 1-30 mHz. Pfz is generally observed in the range 0.0001-0.01 gamma-squared Hz with a mean value of 0.0012 during substorm growth phases and 0.001 in the comparison intervals. No strong correlation of Pfz is found with the auroral electrojet index in either set of intervals, but during substorm growth phases Pfz may vary by an order of magnitude over time scales of 30 min, with a tendency for higher power levels to occur later in the growth phase. Increases in Pfz precede by about 10 min localized expansive phase activity observed in individual magnetograms.

Smith, R. A.↗

NASA NTRS1990

Sensitivity of the normalized difference vegetation index to subpixel canopy cover, soil albedo, and pixel scale

An analytical framework is provided for examining the physically based behavior of the normalized difference vegetation index (NDVI) in terms of the variability in bulk subpixel landscape components and with respect to variations in pixel scales, within the context of the stochastic-geometric canopy reflectance model. Analysis focuses on regional scale variability in horizontal plant density and soil background reflectance distribution. Modeling is generalized to different plant geometries and solar angles through the use of the nondimensional solar-geometric similarity parameter. Results demonstrate that, for Poisson-distributed plants and for one deterministic distribution, NDVI increases with increasing subpixel fractional canopy amount, decreasing soil background reflectance, and increasing shadows, at least within the limitations of the geometric reflectance model. The NDVI of a pecan orchard and a juniper landscape is presented and discussed.

Jasinski, Michael F.↗

NASA NTRS1991

Chaotic appearance of the AE index

Results are reported from a stochastic analysis of a 5-day time series of the geomagnetic AE index during an active period. The original data, the power spectrum, and the autocorrelation function are shown, and the steps in the analysis are described in detail. It is found that the autocorrelation time scale is about 50 min, giving a correlation dimension (for the construction of a time series of m-dimensional vectors) of 2.4. This result is consistent with either colored-noise or deterministic-chaos magnetosphere models, indicating the need for further investigation.

Shan, L.-H.↗

NASA NTRS1991

Mean and inter-year variation of growing-season normalized difference vegetation index for the Sahel 1981-1989

Images are presented that show the mean and coefficient of variation of nine years (1981-1989) of NOAA AVHRR normalized difference vegetation index (NDVI) data for the growing season (July-October) in Africa, north of the equator. The variation in the growing season NDVI is represented by the coefficient of variation image that shows the large variation in the Sahelian growing season between years. It is concluded that these images illustrate some aspects of the perspective being brought to regional and continental scale processes by coarse resolution satellite sensors and the potential of these sensors to provide consistent, long-term datasets.

Tucker, C. J.↗

NASA NTRS1991

The effect of water vapour on the normalized difference vegetation index derived for the Sahelian region from NOAA AVHRR data

The near-infrared channel of the NOAA advanced very high resolution radiometer (AVHRR) contains a water vapor absorption band that affects the determination of the normalized difference vegetation index (NDVI). Daily and seasonal variations in atmospheric water vapor within the Sahel are shown to affect the use of the NDVI for the estimation of primary production. This water vapor effect is quantified for the Sahel by radiative transfer modeling and empirically using observations made in Mali in 1986.

Justice, Christopher O.↗

NASA NTRS1991

Elliptical-core two mode fiber sensors and devices incorporating photoinduced refractive index gratings

Results of experiments performed using germanium-doped, elliptical core, two-mode optical fibers whose sensitivity to strain was spatially varied through the use of chirped, refractive-index gratings permanently induced into the core using Argon-ion laser light are presented. This type of distributed sensor falls into the class of eighted-fiber sensors which, through a variety of means, weight the strain sensitivity of a fiber according to a specified spatial profile. We describe results of a weighted-fiber vibration mode filter which successfully enhances the particular vibration mode whose spatial profile corresponds to the profile of the grating chirp. We report on the high temperature survivability of such grating-based sensors and discuss the possibility of multiplexing more than one sensor within a single fiber.

Greene, Jonathan A.↗

NASA NTRS1990

The use of remotely sensed surface temperature and a spectral vegetation index for evaluating heat fluxes over the Konza Prairie

The relationship between surface temperature T(s) and the normalized difference vegetation index (NDVI) is examined to determine whether it can be used to characterize latent heat fluxes (LEs). The regression of T(s) on NDVI is studied in the context of surface heat fluxes over the prairie with a limited data set for four flux-measurement sites. The 17 flux-measurement stations at the sites provided data that are consistently significant at the 99-percent confidence level for the regression of T(s) on NDVI. Variations in the regression slopes are found to correlate with variations in the ground heat flux but not with LE. Important components of the regression of T(s) on NDVI include fractional vegetation cover and surface-soil moisture conditions. The relationship is important for determining the relative contributions of the soil and vegetation components of an area's total evaporative flux.

Hope, A. S.↗

NASA NTRS1990

Indexing system for optical beam steering

This paper describes the design and testing of an indexing system for optical-beam steering. The cryogenic beam-steering mechanism is a 360-degree rotation device capable of discrete, high-precision alignment positions. It uses low-precision components for its rough alignment and kinematic design to meet its stringent repeatability and stability requirements (of about 5 arcsec). The principal advantages of this design include a decoupling of the low-precision, large angular motion from the high-precision alignment, and a power-off alignment position that potentially extends the life or hold time of cryogenic systems. An alternate design, which takes advantage of these attributes while reducing overall motion, is also presented. Preliminary test results show the kinematic mount capable of sub-arc second repeatability.

Sullivan, Mark T.↗

NASA NTRS1992

Airborne in situ computation of the wind shear hazard index

An algorithm for airborne in situ computation of the wind shear hazard index (F-factor) was developed and evaluated in simulation and verified in flight. The algorithm was implemented on NASA's B-737-100 airplane, and tested under severe maneuvering, nonhazardous wind conditions, and normal takeoffs and landings. The airplane was flown through actual microburst conditions in Orlando, FL, where the algorithm produced wind shear measurements which were confirmed by an independent, ground-based radar measurement. Flight test results indicated that the in situ F-factor algorithm correctly measured the effect of the wind environment on the airplane's performance, and produced no nuisance alerts.

Oseguera, Rosa M.↗

NASA NTRS1992

Vegetation canopy PAR absorptance and the normalized difference vegetation index - An assessment using the SAIL model

Relationships are studied between the normalized-difference vegetation index (NDVI) and absorbed photosynthetically active radiation (APAR) in a vegetation canopy. The SAIL model of bidirectional canopy radiative transfer is employed to compare NDVI measurements that are instantaneous with diurnally integrated canopy APAR capacity. The NDVI measurements - taken at solar-zenith angles of more than 60 deg and sensor views of less than 40 deg from nadir - give stable near-linear estimates of diurnal APAR capacity. Discrepancies in the relations between APAR and NDVI are associated with variations in the optical properties of the canopy and with background spectral reflectance. The results are significant for the practical use of these remote sensing techniques but suggest that instantaneous observations can be used to characterize the diurnally integrated APAR in vegetation canopies.

Goward, Samuel N.↗

NASA NTRS1992

Atmospherically Resistant Vegetation Index (ARVI) for EOS-MODIS

ARVI, which is here proposed as a tool for the remote sensing of vegetation by the EOS MODIS sensor, takes advantage of the presence of the blue channel in MODIS, together with the red and near-IR channels composing the normalized-difference vegetation index (NDVI). Unlike the NDVI, ARVI resists atmospheric effects via a self-correction process for the effects of the atmosphere on the red channel. ARVI is presently shown to have a dynamic range similar to NDVI's, but is on average less sensitive to atmospheric effects by a factor of 4. A single combination of the blue and red channels in the ARVI may be used in all or most remote-sensing applications.

Kaufman, Yoram J.↗

NASA NTRS1992

Toxic substances registry system: Index of material safety data sheets

The Material Safety Data Sheets (MSDSs) listed reflect product inventories and associated MSDSs which were submitted to the Toxic Substance Registry data base maintained by the Base Operations Contractors of the Biomedical Operations and Research Office of NASA Kennedy. The purpose of the index is to provide a means of accessing information on the hazards associated with the toxic and otherwise hazardous chemicals stored and used at NASA Kennedy. Indices are provided for manufacturers, trademarks, and stock numbers.

Source record↗

NASA NTRS1993

Simulation Of Fluctuating Geomagnetic Index

Mathematical model produces synthetic geomagnetic-index (ap) data including short-term fluctuations like those of real ap data. Measures geomagnetic activity computed from measurements of fluctuations in geomagnetic field taken at 12 high-latitude stations every 3 hours. Used in studies of interactions between solar wind and Earth, especially in studies of effect of geomagnetic field upon heating of thermosphere by impacts of energetic charged solar-wind particles.

Vedder, John↗

NASA NTRS1993

Indexing Mount For Rotation Of Optical Component

Indexing mount for polarizer, wave plate, birefringent plate, or other optical component facilitates rotation of component to one or more preset angles. Includes hexagonal nut holding polarizer or other optical component. Ball bearing loaded by screw engages notch on cylindrical extension of nut engaging bracket. Time-consuming and tedious angular adjustment unnecessary: component turned quickly and easily, by hand or by use of wrench, to preset angular positions maintained by simple ball-detent mechanism.

Reichle, Donald J., Jr.↗

NASA NTRS1992

Recursively indexed differential pulse code modulation

The performance of a differential pulse code modulation (DPCM) system with a recursively indexed quantizer (RIQ) under various conditions, with first order Gauss-Markov and Laplace-Markov sources as inputs, is studied. When the predictor is matched to the input, the proposed system performs at or close to the optimum entropy constrained DPCM system. If one is willing to accept a 5 percent increase in the rate, the system is very forgiving of predictor mismatch.

Sayood, Khalid↗