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At least 343 records · Page 19
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NASA NTRS2002

A Long-Term Space Astrophysics Research Program: The Evolution of the Quasar Continuum

Four papers have been written. One reports on the major study funded by this grant: a pan-chromatic study of the quasar continuum at redshift 3. Two others make use of the quasar continuum shapes to find the minimum total accretion luminosity of the Universe, and hence the efficiency and spin of supermassive black holes; the second shows that the reemission of absorbed quasar radiation alleviates a major problem with galaxy formation and the FIR background. The last paper recognizes the role quasars may play in the initial formation of dust in the early Universe.

Elvis, M.↗
NASA NTRS1998

Tutorial on the Psychophysics and Technology of Virtual Acoustic Displays

Virtual acoustics, also known as 3-D sound and auralization, is the simulation of the complex acoustic field experienced by a listener within an environment. Going beyond the simple intensity panning of normal stereo techniques, the goal is to process sounds so that they appear to come from particular locations in three-dimensional space. Although loudspeaker systems are being developed, most of the recent work focuses on using headphones for playback and is the outgrowth of earlier analog techniques. For example, in binaural recording, the sound of an orchestra playing classical music is recorded through small mics in the two "ear canals" of an anthropomorphic artificial or "dummy" head placed in the audience of a concert hall. When the recorded piece is played back over headphones, the listener passively experiences the illusion of hearing the violins on the left and the cellos on the right, along with all the associated echoes, resonances, and ambience of the original environment. Current techniques use digital signal processing to synthesize the acoustical properties that people use to localize a sound source in space. Thus, they provide the flexibility of a kind of digital dummy head, allowing a more active experience in which a listener can both design and move around or interact with a simulated acoustic environment in real time. Such simulations are being developed for a variety of application areas including architectural acoustics, advanced human-computer interfaces, telepresence and virtual reality, navigation aids for the visually-impaired, and as a test bed for psychoacoustical investigations of complex spatial cues. The tutorial will review the basic psychoacoustical cues that determine human sound localization and the techniques used to measure these cues as Head-Related Transfer Functions (HRTFs) for the purpose of synthesizing virtual acoustic environments. The only conclusive test of the adequacy of such simulations is an operational one in which the localization of real and synthesized stimuli are directly compared in psychophysical studies. To this end, the results of psychophysical experiments examining the perceptual validity of the synthesis technique will be reviewed and factors that can enhance perceptual accuracy and realism will be discussed. Of particular interest is the relationship between individual differences in HRTFs and in behavior, the role of reverberant cues in reducing the perceptual errors observed with virtual sound sources, and the importance of developing perceptually valid methods of simplifying the synthesis technique. Recent attempts to implement the synthesis technique in real time systems will also be discussed and an attempt made to interpret their quoted system specifications in terms of perceptual performance. Finally, some critical research and technology development issues for the future will be outlined.

Wenzel, Elizabeth M.↗
NASA NTRS2002

Earth Observing-1 Advanced Land Imager Flight Performance Assessment: Absolute Radiometry and Stability During the First Year

The absolute radiometry of the Advanced Land Imager during the first year on orbit (November 21,2000 - November 21, 2001) is presented. Results derived from solar, lunar, ground truth, and internal reference lamp measurements are presented. An 18% drop in the radiometric response of the Band 1p data since preflight calibration at Lincoln Laboratory is observed using all techniques. This decrease cannot be accounted for by preflight calibration errors, stray light, or contamination of the focal plane. A slight drooping of the VNIR response toward the blue and a 5-12% increase in the Band 5 response is also apparent in all the data. Radiometric response correction factors have been calculated and preflight calibration coefficients have been updated in order to provide +/- 5% agreement between the measured solar, lunar, and ground truth data and the expected values. The radiometric stability of the ALI during the first year of operation is also presented for each spectral band. Internal reference lamp data indicate the focal plane has been stable to within 1% for bands 1p, 1, 2, 5p, 5, 7, pan and 3% for Bands 3,4, 4p since launch. Solar, lunar, and ground truth measurements indicate the optical train and solar diffuser of the instrument has been stable to within 1% since initial measurements on orbit in late December 2000.

Mendenhall, J. A.↗
NASA NTRS1998

Recent Progress on the Parallel Implementation of Moving-Body Overset Grid Schemes

Viscous calculations about geometrically complex bodies in which there is relative motion between component parts is one of the most computationally demanding problems facing CFD researchers today. This presentation documents results from the first two years of a CHSSI-funded effort within the U.S. Army AFDD to develop scalable dynamic overset grid methods for unsteady viscous calculations with moving-body problems. The first pan of the presentation will focus on results from OVERFLOW-D1, a parallelized moving-body overset grid scheme that employs traditional Chimera methodology. The two processes that dominate the cost of such problems are the flow solution on each component and the intergrid connectivity solution. Parallel implementations of the OVERFLOW flow solver and DCF3D connectivity software are coupled with a proposed two-part static-dynamic load balancing scheme and tested on the IBM SP and Cray T3E multi-processors. The second part of the presentation will cover some recent results from OVERFLOW-D2, a new flow solver that employs Cartesian grids with various levels of refinement, facilitating solution adaption. A study of the parallel performance of the scheme on large distributed- memory multiprocessor computer architectures will be reported.

Wissink, Andrew↗
NASA NTRS2002

Interannual Variability of the Bimodal Distribution of Summertime Rainfall Over Central America and Tropical Storm Activity in the Far-Eastern Pacific

The summer climate of southern Mexico and Central America is characterized by a mid summer drought (MSD), where rainfall is reduced by 40% in July as compared to June and September. A mid-summer reduction in the climatological number of eastern Pacific tropical cyclones has also been noted. Little is understood about the climatology and interannual variability of these minima. The present study uses a novel approach to quantify the bimodal distribution of summertime rainfall for the globe and finds that this feature of the annual cycle is most extreme over Pan America and adjacent oceans. One dominant interannual signal in this region occurs the summer before a strong winter El Nino/Southern Oscillation ENSO. Before El Nino events the region is dry, the MSD is strong and centered over the ocean, and the mid-summer minimum in tropical cyclone frequency is most pronounced. This is significantly different from Neutral cases (non-El Nino and non-La Nina) when the MSD is weak and positioned over the land bridge. The MSD is highly variable for La Nina years, and there is not an obvious mid-summer minimum in the number of tropical cyclones.

Curtis, Scott↗
NASA NTRS2002

STS 110 Mission Highlights Resource Tape

A continuation of 'STS 110 Mission Highlights Resource Tape'. This video, Part 3 of 4, shows footage from flight days 6 through 9 of STS-110. The spacecrew includes Michael J. Bloomfield, Commander Stephen N. Frick, Pilot; Jerry L. Ross, Mission Specialist; Steven L. Smith, Mission Specialist; Ellen Ochoa, Mission Specialist; Lee M.E. Morin, Mission Specialist; Rex J. Walheim, Mission Specialist. On flight day 6, Ross and Morin conduct an EVA (extravehicular activity) to secure a tripod like strut to the S0 Truss of the International Space Station (ISS). They also move the drag link and keel pins from one face of the Truss to the other to free the rail on the truss for a railcar to move. Smith installs a camera onto the CANADARM robotic arm on flight day 7, and on flight day 8 the restraints are removed from the railcar connected to the S0 Truss in preparation for checkout. The checkout of the railcar is shown, including its inaugural run. Ross and Morin conduct another EVA on flight day 9 to complete the outfitting of the S0 Truss Structure. Notable footage includes views of Ross and Morin at work from the helmet-mounted camera on Ross' EVA suit, including close-ups of the pistol grip tool, CANADARM 2 onboard the ISS lit at sunrise, a 'diamond ring' effect formed by the Sun between the Earth's limb and the ISS, a brief shot of the Yucatan peninsula, and a end-to-end pan down the length of the ISS. The activities from other flights days can be seen on "STS 110 Mission Highlights Resource Tape" Part 1 of 4 (internal ID 2002137575), "STS 110 Mission Highlights Resource Tape" Part 2 of 4 (internal ID 2002137573), and "STS 110 Mission Highlights Resource Tape" Part 4 of 4 (internal ID 2002137517).

Source record↗
NASA NTRS2002

STS-113 Flight Day 3 Highlights

This video shows the activities of the STS-113 crew (Jim Wetherbee, Commander; Paul Lockhart, Pilot; Michael Lopez-Alegria, John Herrington, Mission Specialists) during flight day 3. The major tasks of flight day 3 were rendezvous and docking with the ISS (International Space Station), the transfer of the Expedition 6 crew (Kenneth Bowersox, Commander; Donald Pettit, Nikolai Budarin, Flight Engineers) to the ISS, and preparations for an EVA (extravehicular activity) scheduled for the following day. The approach of Space Shuttle Endeavour to the ISS is shown in detail, including the firing of the Left Orbital Maneuvering System, and the aiming maneuvers the orbiter makes to dock with the ISS. There are centerline views of the ISS before and during the final docking maneuver. The new ISS crew is received by the Expedition 5 crew (Valeri Korzun, Commander; Peggy Whitsun, Sergei Treschev; Flight Engineers), and the transfer of EVA suits is shown. Earth views include a pan along a reddish Earth limb, and the Pacific Ocean with Endeavour's Canadarm robotic arm in the foreground.

Source record↗
NASA NTRS1999

Polycyclic Aromatic Hydrocarbons and the Diffuse Interstellar Bands: a Survey

We discuss the proposal relating the origin of some of the diffuse interstellar bands (DIBs) to neutral and ionized polycyclic aromatic hydrocarbons (PAHs) present in interstellar clouds. Laboratory spectra of several PAHs, isolated at low temperature in inert gas matrices, are compared with an extensive set of astronomical spectra of reddened, early type stars. From this comparison, it is concluded that PAN ions are good candidates to explain some of the DIBs. Unambiguous assignments are difficult, however, due to the shift in wavelengths and the band broadening induced in the laboratory spectra by the solid matrix. Definitive band assignments and, ultimately, the test of the of the proposal that PAH ions carry some of the DIB must await the availability of gas-phase measurements in the laboratory. The present assessment offers a guideline for future laboratory experiments by allowing the preselection of promising PAH molecules to be studied in jet expansions.

Salama, F.↗
NASA NTRS2002

Simulating Snow Over Sea Ice In Climate Models

We have evaluated two methods of simulating the seasonal cycle of snow over sea ice in and around the Arctic: The NCAR global climate model CCM3, with its standard snow hydrology, and the snow pack model SNTHERM, forced with hourly atmospheric output from CCM3. A new dataset providing dates for the onset of snow melt over Arctic sea ice provides a means for assessing basin-wide how well the models simulate melt onset, but contains no information on how long it then takes for all the snow to melt. Use of data from the SHEBA site provides very detailed information on the behavior of the snow before and during the melt season, but only for a very limited area. Russian drift data provide climatological data on the seasonal cycle of snow water equivalent and snow density, over multi-year sea ice in the central Arctic basin. These datasets are used to compare the two modeling methods, and to see if use of the more physically-realistic SNTHERM provides any significant improvements. Conclusions obtained so far include: 1. Both CCM3 and CCM3/SNTHERM do a good job overall of matching the onset of snow melt dataset; although CCM3/SNTHERM consistently trends to underestimate the date and CCM3 to overestimate it. 2. SHEBA and ice drift data for the Arctic show that CCM3/ SNTHERM does a better job than CCM3 at simulating the total melt period. 3. Ice drift snow density and accumulation data suggest that while providing superior results, CCM3/SNTHERM may still suffer from overly vigorous melting. 4. Both the large-scale atmospheric forcing and snow pack physical processes are important in proper simulation of the snow seasonal cycle. Ongoing work includes further diagnosis of CCM3/SNTHERM, use of more observational datasets, especially from marginal seas in the pan-Arctic, and full coupling of SNTHERM into CCM3 (work to date has all been off-line simulations).

Arnold, James E.↗
NASA NTRS2003

Parameters Describing Earth Observing Remote Sensing Systems

The Earth science community needs to generate consistent and standard definitions for spatial, spectral, radiometric, and geometric properties describing passive electro-optical Earth observing sensors and their products. The parameters used to describe sensors and to describe their products are often confused. In some cases, parameters for a sensor and for its products are identical; in other cases, these parameters vary widely. Sensor parameters are bound by the fundamental performance of a system, while product parameters describe what is available to the end user. Products are often resampled, edge sharpened, pan-sharpened, or compressed, and can differ drastically from the intrinsic data acquired by the sensor. Because detailed sensor performance information may not be readily available to an international science community, standardization of product parameters is of primary performance. Spatial product parameters described include Modulation Transfer Function (MTF), point spread function, line spread function, edge response, stray light, edge sharpening, aliasing, ringing, and compression effects. Spectral product parameters discussed include full width half maximum, ripple, slope edge, and out-of-band rejection. Radiometric product properties discussed include relative and absolute radiometry, noise equivalent spectral radiance, noise equivalent temperature diffenence, and signal-to-noise ratio. Geometric product properties discussed include geopositional accuracy expressed as CE90, LE90, and root mean square error. Correlated properties discussed include such parameters as band-to-band registration, which is both a spectral and a spatial property. In addition, the proliferation of staring and pushbroom sensor architectures requires new parameters to describe artifacts that are different from traditional cross-track system artifacts. A better understanding of how various system parameters affect product performance is also needed to better ascertain the utility of existing datasets and products as well as to specify the performance of new sensors and products. Examples of simulations performed for the Landsat Data Continuity Mission illustrate how various parameters affect system and product performance. Specific examples include the effects of ground sample distance, MTF, and band-to-band registration on various products.

Zanoni, Vicki↗
NASA NTRS2004

Mechanical Properties of Heat-treated Carbon Fibers

Carbon fibers are selected for ceramic matrix composites (CMC) are based on their as-fabricated properties or on "that is what we have always done" technical culture while citing cost and availability when there are others with similar cost and availability. However, the information is not available for proper selection of carbon fibers since heat-treated properties are not known for the fibers on the market currently. Heat-treating changes the fiber's properties. Therefore, an effort was undertaken to establish fiber properties on 19 different types of fibers from six different manufactures for both PAN and pitch fibers. Heat-treating has been done at three different temperatures.

Effinger, Michael R.↗
NASA NTRS2004

Microgravity Studies of Liquid-Liquid Phase Transitions in Alumina-Yttria Melts

The scientific objective of this research is to increase the fundamental knowledge base for liquid- phase processing of technologically important oxide materials. The experimental objective is to define conditions and hardware requirements for microgravity flight experiments to test and expand the experimental hypotheses that: 1. Liquid phase transitions can occur in undercooled melts by a diffusionless process. 2. Onset of the liquid phase transition is accompanied by a large change in the temperature dependence of melt viscosity. Experiments on undercooled YAG (Y3A15012)- and rare earth oxide aluminate composition liquids demonstrated a large departure from an Arrhenian temperature dependence of viscosity. Liquid YAG is nearly inviscid at its 2240 K melting point. Glass fibers were pulled from melts undercooled by ca. 600 K indicating that the viscosity is on the order of 100 Pans (1000 Poise) at 1600 K. This value of viscosity is 500 times greater than that obtained by extrapolation of data for temperatures above the melting point of YAG. These results show that the liquids are extremely fragile and that the onset of the highly non-Arrhenian viscosity-temperature relationship occurs at a temperature considerably below the equilibrium melting point of the solid phases. Further results on undercooled alumina-yttria melts containing 23-42 mole % yttrium oxide indicate that a congruent liquid-liquid phase transition occurs in the undercooled liquids. The rates of transition are inconsistent with a diffusion-limited process. This research is directed to investigation of the scientifically interesting phenomena of polyamorphism and fragility in undercooled rare earth oxide aluminum oxide liquids. The results bear on the technologically important problem of producing high value rare earth-based optical materials.

Guynes, Buddy↗
NASA NTRS2004

Stable Isotope Composition of Carbonates Formed in Low-Temperature Terrestrial Environments as Martian Analogs

We examine the carbon and oxygen isotope composition of carbonate minerals that formed in two low-temperature terrestrial environments. Field locations were chosen to represent environments where microcrystalline carbonates (caliches) form. Samples include caliche crusts collected from the tsekel zone of N. Yucatan, Mexico, as well as carbonate mud from the edge of a near-by salt pan, representing both ancient and modern-precipitated carbonates. Additional field samples of surface-coating caliche were collected from two volcanic fields in Arizona. Preliminary results indicate that there is an overall depletion of 18O and 13C as a function of the extent of meteoric diagenesis. These data are used as terrestrial analogs to gauge whether carbonates that have been found within Martian meteorites could possibly have formed under these or similar conditions on Mars.

Socki, Richard A.↗
NASA NTRS1999

Analytical Prediction of Damage Growth in Notched Composite Panels Loaded in Axial Compression

A progressive failure analysis method based on shell elements is developed for the computation of damage initiation and growth in stiffened thick-skin stitched graphite-epoxy panels loaded in axial compression. The analysis method involves a step-by-step simulation of material degradation based on ply-level failure mechanisms. High computational efficiency is derived from the use of superposed layers of shell elements to model each ply orientation in the laminate. Multiple integration points through the thickness are used to obtain the correct bending effects through the thickness without the need for ply-by-ply evaluations of the state of the material. The analysis results are compared with experimental results for three stiffened panels with notches oriented at 0, 15 and 30 degrees to the panel width dimension. A parametric study is performed to investigate the damage growth retardation characteristics of the Kevlar stitch lines in the pan

Ambur, Damodar R.↗
NASA NTRS2003

Caspase inhibitors promote vestibular hair cell survival and function after aminoglycoside treatment in vivo

The sensory hair cells of the inner ear undergo apoptosis after acoustic trauma or aminoglycoside antibiotic treatment, causing permanent auditory and vestibular deficits in humans. Previous studies have demonstrated a role for caspase activation in hair cell death and ototoxic injury that can be reduced by concurrent treatment with caspase inhibitors in vitro. In this study, we examined the protective effects of caspase inhibition on hair cell death in vivo after systemic injections of aminoglycosides. In one series of experiments, chickens were implanted with osmotic pumps that administrated the pan-caspase inhibitor z-Val-Ala-Asp(Ome)-fluoromethylketone (zVAD) into inner ear fluids. One day after the surgery, the animals received a 5 d course of treatment with streptomycin, a vestibulotoxic aminoglycoside. Direct infusion of zVAD into the vestibule significantly increased hair cell survival after streptomycin treatment. A second series of experiments determined whether rescued hair cells could function as sensory receptors. Animals treated with streptomycin displayed vestibular system impairment as measured by a greatly reduced vestibulo-ocular response (VOR). In contrast, animals that received concurrent systemic administration of zVAD with streptomycin had both significantly greater hair cell survival and significantly increased VOR responses, as compared with animals treated with streptomycin alone. These findings suggest that inhibiting the activation of caspases promotes the survival of hair cells and protects against vestibular function deficits after aminoglycoside treatment.

NASA Discipline Neuroscience↗
NASA NTRS1991

Isotopic compositions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the effects of diagenesis and metamorphism

The carbon isotope geochemistry of carbonates and organic carbon in the late Proterozoic Damara Supergroup of Namibia, including the Nama, Witvlei, and Gariep groups on the Kalahari Craton and the Mulden and Otavi groups on the Congo Craton, has been investigated as an extension of previous studies of secular variations in the isotopic composition of late Proterozoic seawater. Subsamples of microspar and dolomicrospar were determined, through petrographic and cathodoluminescence examination, to represent the "least-altered" portions of the rock. Carbon-isotopic abundances in these phases are nearly equal to those in total carbonate, suggesting that 13C abundances of late Proterozoic fine-grained carbonates have not been significantly altered by meteoric diagenesis, although 18O abundances often differ significantly. Reduced and variable carbon-isotopic differences between carbonates and organic carbon in these sediments indicate that isotopic compositions of organic carbon have been altered significantly by thermal and deformational processes, likely associated with the Pan-African Orogeny. Distinctive stratigraphic patterns of secular variation, similar to those noted in other, widely separated late Proterozoic basins, are found in carbon-isotopic compositions of carbonates from the Nama and Otavi groups. For example, in Nama Group carbonates delta 13C values rise dramatically from -4 to +5% within a short stratigraphic interval. This excursion suggests correlation with similar excursions noted in Ediacaran-aged successions of Siberia, India, and China. Enrichment of 13C (delta 13C> +5%) in Otavi Group carbonates reflects those in Upper Riphean successions of the Akademikerbreen Group, Svalbard, its correlatives in East Greenland, and the Shaler Group, northwest Canada. The widespread distribution of successions with comparable isotopic signatures supports hypotheses that variations in delta 13C reflect global changes in the isotopic composition of late Proterozoic seawater. Within the Damara basin, carbon-isotopic compositions of carbonates provide a potentially useful tool for the correlation of units between the Kalahari and Congo cratons. Carbonates depleted in 13C were deposited during and immediately following three separate glacial episodes in Namibia. The correspondence between ice ages and negative delta 13C excursions may reflect the effects of lowered sea levels; enhanced circulation of deep, cold, O2-rich seawater; and/or the upwelling of 13C-depleted deep water. Iron-formation is additionally associated with one of the glacial horizons, the Chuos tillite. Carbon-13 enriched isotopic abundances in immediately pre-glacial carbonates suggest that oceanographic conditions favored high rates of organic burial. It is likely that marine waters were stratified, with deep waters anoxic. A prolonged period of ocean stratification would permit the build-up of ferrous iron, probably from hydrothermal sources. At the onset of glaciation, upwelling would have brought 13C-depleted and iron-rich deep water onto shallow shelves where contact with cold, oxygenated surface waters led to the precipitation of ferric iron.

NASA Program Exobiology↗
NASA NTRS2001

Global Modeling of Tropospheric Chemistry with Assimilated Meteorology: Model Description and Evaluation

We present a first description and evaluation of GEOS-CHEM, a global three-dimensional (3-D) model of tropospheric chemistry driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS) of the NASA Data Assimilation Office (DAO). The model is applied to a 1-year simulation of tropospheric ozone-NOx-hydrocarbon chemistry for 1994, and is evaluated with observations both for 1994 and for other years. It reproduces usually to within 10 ppb the concentrations of ozone observed from the worldwide ozonesonde data network. It simulates correctly the seasonal phases and amplitudes of ozone concentrations for different regions and altitudes, but tends to underestimate the seasonal amplitude at northern midlatitudes. Observed concentrations of NO and peroxyacetylnitrate (PAN) observed in aircraft campaigns are generally reproduced to within a factor of 2 and often much better. Concentrations of HNO3 in the remote troposphere are overestimated typically by a factor of 2-3, a common problem in global models that may reflect a combination of insufficient precipitation scavenging and gas-aerosol partitioning not resolved by the model. The model yields an atmospheric lifetime of methylchloroform (proxy for global OH) of 5.1 years, as compared to a best estimate from observations of 5.5 plus or minus 0.8 years, and simulates H2O2 concentrations observed from aircraft with significant regional disagreements but no global bias. The OH concentrations are approximately 20% higher than in our previous global 3-D model which included an UV-absorbing aerosol. Concentrations of CO tend to be underestimated by the model, often by 10-30 ppb, which could reflect a combination of excessive OH (a 20% decrease in model OH could be accommodated by the methylchloroform constraint) and an underestimate of CO sources (particularly biogenic). The model underestimates observed acetone concentrations over the South Pacific in fall by a factor of 3; a missing source from the ocean may be implicated.

Bey, Isabelle↗