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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2018

Enhanced Quasiparticle Lifetime in a Superconductor by Selective Blocking of Recombination Phonons with a Phononic Crystal

When quasiparticles in a BCS (Bardeen-Cooper-Schrieffer) superconductor recombine into Cooper pairs, phonons are emitted within a narrow band of energies above the pairing energy at 2 delta. We show that a phonon band gap restricting the escape of recombination phonons from a superconductor can increase the quasiparticle recombination lifetime by more than an order of magnitude. A phonon band gap can be realized and matched to the recombination energy with a phononic crystal, a periodically patterned dielectric membrane. We discuss in detail the nonequilibrium quasiparticle and phonon distributions that arise in a superconductor due to a phonon band gap and a pair-breaking photon signal. Although intrinsically a nonequilibrium effect, the lifetime enhancement in the small-signal regime is remarkably similar to an estimate from an equilibrium formulation. The equilibrium estimate closely follows exp (Omega (sub bg) divided by k (sub B)T (sub b)), where Omega (sub bg) is the phonon band gap energy bandwidth above 2 and T (sub b) is the phonon bath temperature of the coupled electron-phonon system. We discuss the impact of a phononic band gap on the performance of a superconducting circuit element and propose a microwave resonator to measure the enhancement in the quasiparticle lifetime.

Rostem, K.↗
NASA NTRS2018

Changes in Earth’s Energy Budget during and after the “Pause” in Global Warming: An Observational Perspective

This study examines changes in Earth’s energy budget during and after the global warming “pause” (or “hiatus”) using observations from the Clouds and the Earth’s Radiant Energy System. We find a marked 0.83±0.41 Wm−2 reduction in global mean reflected shortwave (SW) top-of-atmosphere (TOA) flux during the three years following the hiatus that results in an increase in net energy into the climate system. A partial radiative perturbation analysis reveals that decreases in low cloud cover are the primary driver of the decrease in SW TOA flux. The regional distribution of the SW TOA flux changes associated with the decreases in low cloud cover closely matches that of sea-surface temperature warming, which shows a pattern typical of the positive phase of the Pacific Decadal Oscillation. Large reductions in clear-sky SW TOA flux are also found over much of the Pacific and Atlantic Oceans in the northern hemisphere. These are associated with a reduction in aerosol optical depth consistent with stricter pollution controls in China and North America. A simple energy budget framework is used to show that TOA radiation (particularly in the SW) likely played a dominant role in driving the marked increase in temperature tendency during the post-hiatus period.

Norman G Loeb↗
NASA NTRS1989

Automatic voice recognition using traditional and artificial neural network approaches

The main objective of this research is to develop an algorithm for isolated-word recognition. This research is focused on digital signal analysis rather than linguistic analysis of speech. Features extraction is carried out by applying a Linear Predictive Coding (LPC) algorithm with order of 10. Continuous-word and speaker independent recognition will be considered in future study after accomplishing this isolated word research. To examine the similarity between the reference and the training sets, two approaches are explored. The first is implementing traditional pattern recognition techniques where a dynamic time warping algorithm is applied to align the two sets and calculate the probability of matching by measuring the Euclidean distance between the two sets. The second is implementing a backpropagation artificial neural net model with three layers as the pattern classifier. The adaptation rule implemented in this network is the generalized least mean square (LMS) rule. The first approach has been accomplished. A vocabulary of 50 words was selected and tested. The accuracy of the algorithm was found to be around 85 percent. The second approach is in progress at the present time.

Botros, Nazeih M.↗
NASA NTRS1982

Radiation from an open-ended waveguide with beam equalizer - A spectral domain analysis

A septum and an impedance matching post are used as a beam equalizer in an open-ended waveguide feed for reflectors used in satellite communications systems. The performance of this design over a frequency band is evaluated using a spectral domain approach. The computed radiation patterns in the E- and H-planes, as well as the results for the impedance match are presented.

Ko, W. L.↗
NASA NTRS1994

Characterizing parallel file-access patterns on a large-scale multiprocessor

Rapid increases in the computational speeds of multiprocessors have not been matched by corresponding performance enhancements in the I/O subsystem. To satisfy the large and growing I/O requirements of some parallel scientific applications, we need parallel file systems that can provide high-bandwidth and high-volume data transfer between the I/O subsystem and thousands of processors. Design of such high-performance parallel file systems depends on a thorough grasp of the expected workload. So far there have been no comprehensive usage studies of multiprocessor file systems. Our CHARISMA project intends to fill this void. The first results from our study involve an iPSC/860 at NASA Ames. This paper presents results from a different platform, the CM-5 at the National Center for Supercomputing Applications. The CHARISMA studies are unique because we collect information about every individual read and write request and about the entire mix of applications running on the machines. The results of our trace analysis lead to recommendations for parallel file system design. First the file system should support efficient concurrent access to many files, and I/O requests from many jobs under varying load conditions. Second, it must efficiently manage large files kept open for long periods. Third, it should expect to see small requests predominantly sequential access patterns, application-wide synchronous access, no concurrent file-sharing between jobs appreciable byte and block sharing between processes within jobs, and strong interprocess locality. Finally, the trace data suggest that node-level write caches and collective I/O request interfaces may be useful in certain environments.

Purakayastha, Apratim↗
NASA NTRS1973

Digital processing of radiographic images

Some techniques are presented and the software documentation for the digital enhancement of radiographs. Both image handling and image processing operations are considered. The image handling operations dealt with are: (1) conversion of format of data from packed to unpacked and vice versa; (2) automatic extraction of image data arrays; (3) transposition and 90 deg rotations of large data arrays; (4) translation of data arrays for registration; and (5) reduction of the dimensions of data arrays by integral factors. Both the frequency and the spatial domain approaches are presented for the design and implementation of the image processing operation. It is shown that spatial domain recursive implementation of filters is much faster than nonrecursive implementations using fast fourier transforms (FFT) for the cases of interest in this work. The recursive implementation of a class of matched filters for enhancing image signal to noise ratio is described. Test patterns are used to illustrate the filtering operations. The application of the techniques to radiographic images of metallic structures is demonstrated through several examples.

Bond, A. D.↗
NASA NTRS1992

Transmission Of Power Via Combined Laser Beams

Laser Diode Array (LDA) appears to be most efficient means of transferring power from Earth to satellites and between satellites, in terms of mass and size, of various laser configurations. To form large-scale-array amplifier (LSAA), element LDA's must generate well-defined diffraction-limited beams. Coherent matching of phases among LDA's enables system to generate good beam pattern in far field over thousands of kilometers. By passing beam from master laser through number of LDA amplifiers simultaneously, one realizes coherence among amplified output beams. LSAA used for transmission of power with efficiency of approximately 80 percent into receiver of moderate size at 5,000 km. Also transmits data at high rates by line-of-sight rather than fiber optics.

Kwon, Jin H.↗
NASA NTRS2012

Intercomparison of MODIS Albedo Retrievals and In Situ Measurements Across the Global FLUXNET Network

Surface albedo is a key parameter in the Earth's energy balance since it affects the amount of solar radiation directly absorbed at the planet surface. Its variability in time and space can be globally retrieved through the use of remote sensing products. To evaluate and improve the quality of satellite retrievals, careful intercomparisons with in situ measurements of surface albedo are crucial. For this purpose we compared MODIS albedo retrievals with surface measurements taken at 53 FLUXNET sites that met strict conditions of land cover homogeneity. A good agreement between mean yearly values of satellite retrievals and in situ measurements was found (R(exp 2)= 0.82). The mismatch is correlated to the spatial heterogeneity of surface albedo, stressing the relevance of land cover homogeneity when comparing point to pixel data. When the seasonal patterns of MODIS albedo is considered for different plant functional types, the match with surface observation is extremely good at all forest sites. On the contrary, in non-forest sites satellite retrievals underestimate in situ measurements across the seasonal cycle. The mismatch observed at grasslands and croplands sites is likely due to the extreme fragmentation of these landscapes, as confirmed by geostatistical attributes derived from high resolution scenes.

Cescatti, Alessandro↗
NASA NTRS1981

The design of a microstrip antenna array for a UHF space telemetry link

An array of microstrip antenna panels was designed for use on a small instrumented satellite as part of a 400-MHz telemetry link between that satellite and NASA's shuttle spacecraft. A roughly omnidirectional phi-plane pattern was desired. The 1.4-wavelength diameter of the satellite and the various ports and structures on its surface presented strong constraints on the antenna array design. Eight antennas, each one a quarter-wavelength panel with one radiating and one shorted edge, were chosen. A phi-plane pattern ripple of 4.4 dB and a gain of at least 0.1 dB relative to a half-wavelength dipole were realized. The design technique used for the individual antenna panels included the study of feed-point location for impedance matching and the effect of losses in the dielectric. A superposition method was used to synthesize the radiation patterns for different numbers and different excitations of antennas on the satellite.

Post, R. E.↗
NASA NTRS1989

A square multiflare horn with 1-megawatt CW power-handling capability

The analysis and low-power testing of a square multiflare horn designed for 1-megawatt CW operation is described. Design considerations for the five-port input section are discussed. The required aperture modes are determined from radiation pattern considerations, and analysis of the multiflare section is carried out using mode matching. Measurements demonstrate that a circularly symmetric beam is produced with relatively low sidelobes.

Hoppe, Dan↗
NASA NTRS1992

A submillimeter tripler using a quasi-waveguide structure

A new type of frequency multiplier structure is being developed which is suitable for application at frequencies above 1 THz. This structure preserves some of the properties of waveguide for mode control, yet is not truly single mode. The device resembles a sectoral horn, with a varactor diode mounted near the throat. Input and output coupling are through the same aperture, requiring a quasi-optical diplexer. Initial tests are directed at building a tripler at 500 GHz, for comparison with waveguide structures. The diplexer is a blazed diffraction grating with appropriate focusing optics. Model studies show that the impedance match to a varactor should be good, and initial tests of the beam patterns of the prototype indicate that optical coupling efficiency should be very high. The structure also has the potential for use as a fundamental mixer, or as a third harmonic mixer.

Erickson, Neal R.↗
NASA NTRS2006

The Supergranule Super-Rotation Illusion

Peculiar aspects of the rotation rate of the supergranules have been noted for over 20 years now. This has culminated in recent reports suggesting that the supergranules have wave-like characteristics and propagate prograde at a rate that exceeds that of the plasma anywhere below the surface. We have simulated supergranules that rotate at a rate that is independent of position or size and find that they appear to rotate at a more rapid rate. This super-rotation of the supergranules is seen in both cross-correlation and Fourier analyses of the Doppler velocity pattern. The amplitude of the rotation excess as a function of-size matches that seen in the Fourier analyses of MDI data. The source of this rotation excess is identified with the effect of projecting velocity signals into the line-of-sight. We conclude that supergranules are merely advected by the flow in the near-surface shear layer and that their apparent super-rotation does not indicate wave-like properties.

Hathaway, David H.↗
NASA NTRS1978

Effect of ambiguities on SAR picture quality

The degradation of picture quality is studied for a high-resolution, large-swath SAR mapping system subjected to speckle, additive white Gaussian noise, and range and azimuthal ambiguities occurring because of the non-finite antenna pattern produced by a square aperture antenna. The effect of the azimuth antenna pattern was accounted for by calculating the aximuth ambiguity function. Range ambiguities were accounted for by adding appropriate pixels at a range separation corresponding to one pulse repetition period, but attenuated by the antenna pattern. A method of estimating the range defocussing effect which arises from the azimuth matched filter being a function of range is shown. The resulting simulated picture was compared with one degraded by speckle and noise but no ambiguities. It is concluded that azimuth ambiguities don't cause any noticeable degradation but range ambiguities might.

Korwar, V. N.↗
NASA NTRS1994

Optical Implementation of Matching Pursuit for Image Representation

We have developed a technique for image analysis, representation, and decomposition. This technique was motivated by Stephane Mallat's matching-pursuit algorithm. We've altered and simplified the mechanics of his algorithm to enable an extremely fast implementation via optical processing. Initial computer simulations show that our algorithm is capable of decomposing and representing a 2-D image as a linear combination of basis images with both high speed and high fidelity.

matching pursuit adaptive wavelet transform image ↗
NASA NTRS2021

Hybrid phenology matching model for robust crop phenological retrieval

Crop phenology regulates seasonal agroecosystem carbon, water, and energy exchanges, and is a key component in empirical and process-based crop models for simulating biogeochemical cycles of farmlands, assessing gross and net primary production, and forecasting the crop yield. The advances in phenology matching models provide a feasible means to monitor crop phenological progress using remote sensing observations, with a priori information of reference shapes and reference phenological transition dates. Yet the underlying geometrical scaling assumption of models, together with the challenge in defining phenological references, hinders the applicability of phenology matching in crop phenological studies. The objective of this study is to develop a novel hybrid phenology matching model to robustly retrieve a diverse spectrum of crop phenological stages using satellite time series. The devised hybrid model leverages the complementary strengths of phenometric extraction methods and phenology matching models. It relaxes the geometrical scaling assumption and can characterize key phenological stages of crop cycles, ranging from farming practice-relevant stages (e.g., planted and harvested) to crop development stages (e.g., emerged and mature). To systematically evaluate the influence of phenological references on phenology matching, four representative phenological reference scenarios under varying levels of phenological calibrations in terms of time and space are further designed with publicly accessible phenological information. The results indicate that the hybrid phenology matching model can achieve high accuracies for estimating corn and soybean phenological growth stages in Illinois, particularly with the year- and region-adjusted phenological reference (R-squared higher than 0.9 and RMSE less than 5 days for most phenological stages). The inter-annual and regional phenological patterns characterized by the hybrid model correspond well with those in the crop progress reports (CPRs) from the USDA National Agricultural Statistics Service (NASS). Compared to the benchmark phenology matching model, the hybrid model is more robust to the decreasing levels of phenological reference calibrations, and is particularly advantageous in retrieving crop early phenological stages (e.g., planted and emerged stages) when the phenological reference information is limited. This innovative hybrid phenology matching model, together with CPR-enabled phenological reference calibrations, holds 3 considerable promise in revealing spatio-temporal patterns of crop phenology over extended geographical regions.

Phenology↗
NASA NTRS1992

A parallel-series-fed microstrip array with high efficiency and low cross-polarization

The requirements of a microstrip array with a vertically polarized fan beam are addressed that correspond to its use in C-band interferometric SAR. A combination of parallel- and series-feed techniques are utilized in an array design with a three-stage parallel-fed configuration to enhance bandwidth performance. The linearly polarized traveling-wave microstrip array antenna is fed by microstrip transmission lines in two rows of 36 elements that resonate at 5.30 GHz. The transmission lines are impedance-matched at every junction for all the waves that travel toward the two ends of the array. The two measured principal-plane patterns are shown, and the measured narrow-beam pattern is found to agree with the calculated values. The VSWR bandwidths and narrow and broad beamwidths of the antenna are found to permit efficient performance. The efficiency is attributed to the parallel and series-feed configuration which allows proper impedance matching, and low cross-polarization is a result of the antiphase feed technique employed in the configuration.

Huang, John↗
NASA NTRS1989

Measuring Thermal Conductivities Of Rough Specimens

Thermal conductivity of irregularly surfaced specimen measured more accurately and reliably by use of heat-transfer adhesive establishing contact between specimen and reference standards. Method useful when specimen made up of fragments of material. Adhesive ensures good thermal contact to pieces not matching others in height. In rough-surfaced or unequally sized specimens, heat-flow pattern becomes distorted, and temperatures at measurement points do not accurately represent those throughout reference standards and specimen. Filling voids with conductive adhesive restores nearly uniform distribution heat flow.

Striepens, Aloys H.↗
NASA NTRS1991

Distribution of silicon between kamacite and taenite

Meteorites formed under highly reducing conditions can incorporate substantial amounts of Si in solid solution in their metallic phases. In enstatite meteorites, reduced Si appears in shreibersite, perryite and, in variable amounts, in the kamacite, taenite, and tetrataenite. Microprobe bulk analysis of Si in Fe,Ni particles of aubrites show a large variation from grain to grain; this large compositional range has been attributed to local equilibrium conditions in the specific T and/or fO2 environments in which they are formed. Si concentrations are homogeneous in the kamacite; in contrast, the Ni-rich phases (taenite and tetrataenite) show a characteristic Si profile. Concentrations of this element in tetrataenite are substantially higher than in the adjacent kamacite, and decrease smoothly towards the center of the taenite grain, matching very closely the M-shaped profile of Ni. Other elements with similar distribution patterns are Ge and Cu. A satisfactory explanation for this behavior cannot be given in the absence of experimental data for the diffusion of Si between alpha and gamma-Fe alloys. However, two end-member possibilities can be envisioned: the distribution of Si between kamacite and taenite may be controlled by (1) the crystallographic structure or (2) the chemical composition of the host phases. These possibilities are explored.

Casanova, Ignacio↗