Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Spreading”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Evidence for variations in magma production along oceanic spreading centers - A critical appraisal

Recent geologic, geophysical, and geochemical studies of the oceanic lithosphere near fracture zones have resulted in the proposal that the 'magma budget,' defined as the amount of magma delivered to magma chambers or conduits beneath a spreading center for a given amount of spreading, decreases as fracture zones are approached. Seismic reflection and refraction studies indicate that the crust becomes anomalously thin near fracture zones, but reference must be made to the boundary between residual upper mantle peridotites and overlying cumulate rocks in order to assess potential variations in the magma budget. The position of this interface, however, generally is not constrained by geophysical studies. Geochemical variations in basaltic glasses collected near fracture zones are consistent with a decrease in partial melting as fracture zones are approached, but they could also be produced by variations in open-system magmatic processes with no change in the extent of partial melting. Although a decrease in the magma budget as fracture zones are approached is consistent with these data, so are alternative models that incorporate a constant magma budget.

Karson, J. A.↗
NASA NTRS1986

Sample band spreading phenomena in ground and space-based electrophoretic separators

Sample stream spreading in the transverse direction (direction perpendicular to the chamber center plane) is calculated for two ground-based and one space-based continuous flow electrophoresis chambers. The magnitude of transverse sample stream spreading is shown to be a function of the difference in electrical conductivity between the sample and curtain buffer. Sample/buffer conductivity mismatch distorts the normally uniform electrical field in the vicinity of the sample to produce transverse field components which, in turn, deflect sample material toward the chamber walls. These results indicate that nongravity effects are important and must be resolved in order for space-based continuous flow electrophoresis to achieve its maximum potential.

Rhodes, Percy H.↗
NASA NTRS1987

Evidence for variations in magma production along oceanic spreading centers - A critical appraisal

Recent studies of the oceanic lithosphere near fracture zones have resulted in the proposal that the 'magma budget', defined as the amount of magma delivered to magma chambers or conduits beneath a spreading center for a given amount of spreading, decreases as fracture zones are approached. Geochemical variations in basaltic glasses collected near fracture zones are consistent with a decrease in partial melting as fracture zones are approached, but they could also be produced by variations in open-system magmatic processes with no change in the extent of partial melting. Although a decrease in the magma budget as fracture zones are approached is consistent with these data, so are alternative models that incorporate a constant magma budget.

Karson, J. A.↗
NASA NTRS1988

Propagation effects on spread-spectrum mobile satellite systems

In contrast to the situation at L-band, wide bandwidths of 500 MHz or more have been allocated for mobile satellite service at frequencies between 20 and 50 GHz. These broad bandwidths are well suited for the use of spread-spectrum. Certain system considerations about the use of such high frequencies for mobile satellite service are mentioned first, and attention is then given to propagation effects on high-frequency broad-band systems. Attenuation due to rain is a constant at 20 to 50 MHz, but would not be a serious problem if outages occurring for one to three percent of the time, depending on location, are considered to be acceptable. Clear air absorption becomes a significant factor above 40 GHz, but should not exceed 2 dB at a 10 degree elevation angle and frequencies below 40 GHz. Spread-spectrum provides a form of frequency diversity that helps to minimize the effects of multipath.

Flock, Warren L.↗
NASA NTRS1987

Evidence for divergent plate-boundary characteristics and crustal spreading on Venus

Detailed examination of the topography and morphology of western Aphrodite Terra reveals numerous features that are similar to terrestrial divergent plate-boundary characteristics. Individual domains between fracture-zone-like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests, rifted and separated, and moved laterally to their present symmetrical positions. The topographic and morphologic similarities, together with strikingly similar mirror-image map patterns on both sides of the rise axis, suggest that western Aphrodite Terra shares the characteristics of oceanic divergent plate boundaries, and is the site of crustal spreading on Venus. Topographic profiles are consistent with spreading rates of the order of several centimeters per year.

Head, James W., III↗
NASA NTRS1988

Comparative point-spread function calculations for the MOMS-1, Thematic Mapper and SPOT-HRV instruments

Point-spread functions (PSF) comparisons were made between the Modular Optoelectronic Multispectral Scanner (MOMS-01), the LANDSAT Thematic Mapper (TM) and the SPOT-HRV instruments, principally near Lake Nakuru, Kenya. The results, expressed in terms of the width of the point spread functions at the 50 percent power points as determined from the in-scene analysis show that the TM has a PSF equal to or narrower than the MOMS-01 instrument (50 to 55 for the TM versus 50 to 68 for the MOMS). The SPOT estimates of the PSF range from 36 to 40. When the MOMS results are adjusted for differences in edge scanning as compared to the TM and SPOT, they are nearer 40 in the 575 to 625 nm band.

Salomonson, V. V.↗
NASA NTRS1988

Formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate - Application to the 95.5 deg W Galapagos propagator

Possible rigid plate models and shear models for the formation of curved seafloor lineaments by rift propagation are investigated and are applied to the Galapagos propagation rift system at 95 deg W. It is shown that the geometry noted at the Galapagos can result from either rift propagation acceleration or a spreading rate decrease during the last few hundred thousand years. It is postulated that the reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate. The data interpreted as requiring a shear zone are found to be equally consistent with two distinct models based on rigid plate tectonics.

Acton, Gary↗
NASA NTRS1988

Coded multiple chirp spread spectrum system and overlay service

An asynchronous spread-spectrum system called coded multiple chirp is proposed, and the possible spread-spectrum overlay over an analog FM-TV signal is investigated by computer simulation. Multiple single-sloped up and down chirps are encoded by a pseudonoise code and decoded by dechirpers (pulse-compression filters) followed by a digital code correlator. The performance of the proposed system, expressed in terms of in probability of bit error and code miss probability, is similar to that of FSK (frequency shift keying) using codewords if sufficient compression gain is used. When chirp is used to overlay an FM-TV channel, two chirp signals with data rate up to 25 kb/s could be overlaid in a 36-MHz satellite transponder without significant mutual interference. Performance estimates for a VSAT (very small aperture terminal) earth station operating at C-band show that a 2.4-m antenna and 300-mW transmitter could send a 2.4-kb/s signal to a large central earth station over an occupied channel.

Kim, Junghwan↗
NASA NTRS1990

Acquisition Of Spread-Spectrum Code

Effects of Doppler shift and data modulation taken into account. Two advanced schemes for acquisition of direct-sequence spread-spectrum codes proposed. M1-Lag correlator in each strip of spread-spectrum-code detector operates at different offset code-chip time. Each offset represents assumed (tentative) Doppler shift. Schemes have highly parallel architecture implemented with currently available technology. Possible to use hybrid parallel/serial architecture in which acquisition time varies in inverse proportion to number of correlators and fast-Fourier-transform processors.

Cheng, Unjeng↗
NASA NTRS1989

Divergent plate boundaries and crustal spreading on Venus: Evidence from Aphrodite Terra

The modes of lithospheric heat transfer and the tectonic styles may differ between Earth and Venus, depending on how the high surface temperature (700 K = 430 C), dense and opaque atmosphere (approx. 10 MPa = 100 bars), lack of water oceans, and the other known ways in which Venus differs from Earth, influence basic lithospheric processes, thermal gradient, upper mantle temperature, thermal and chemical evolution, and convection. A fundamental question is whether the lithosphere of Venus is horizontally stable, like the other terrestrial planets, or is mobile like that on Earth. The variety of characteristics, their integrated relationships, and their predictable behavior throughout Western Aphrodite Terra are similar to those features known to occur in association with the terrestrial seafloor at spreading centers and divergent plate boundaries. It is concluded that Western Aphrodite Terra represents the site of crustal spreading centers and divergent plate boundaries. The extent of similar characteristics and processes elsewhere on Venus outside of the 13,000 km long Western and Eastern Aphrodite Terra rise is unknown at the present, but their presence in other areas of the equatorial highlands, suggested from recent analysis, may be tested with forthcoming Magellan data.

Crumpler, L. S.↗
NASA NTRS1989

Image processing by Intensity-Dependent Spread (IDS)

As retinal illuminance is lowered, the human visual system integrates the effects of photon absorptions over larger areas and longer times. A theory of the process that might underlie these changes is called Intensity-Dependent Spread (IDS). Each input point gives rise to a pattern of excitation that spreads to a region of the total excitation it sees. The unique aspect of the theory is the assumption that, although the amplitude of the excitation pattern at its center increases with input illuminance, its width decreases in such a way that its volume remains constant. Application of this theory to image processing reveals that it displays a number of unexpected and potentially useful properties. Among them are edge enhancement and independence from scene illumination.

Cornsweet, Tom N.↗
NASA NTRS1990

Lateral spreading of Au contacts on InP

The contact spreading phenomenon observed when small area Au contacts on InP are annealed at temperatures above about 400 C was investigated. It was found that the rapid lateral expansion of the contact metallization which consumes large quantities of InP during growth is closely related to the third stage in the series of solid state reactions that occur between InP and Au, i.e., to the Au3In-to-Au9In4 transition. Detailed descriptions are presented of both the spreading process and the Au3In-to-Au9In4 transition along with arguments that the two processes are manifestations of the same basic phenomenon.

Fatemi, Navid S.↗
NASA NTRS1989

Explosive spread F caused by lightning-induced electromagnetic effects

Lightning-produced electromagnetic effects may produce significant modifications in the ionospheric plasmas. An outstanding phenomenon investigated in this paper is the so-called explosive spread F, whose close link with lightning has been identified (Woodman and Kudeki, 1984). Parametric instability excited by the lightning-induced whistler waves is proposed as a potential source mechanism causing the explosive spread F.

Liao, C. P.↗
NASA NTRS1990

Venus trough-and-ridge tessera - Analog to earth oceanic crust formed at spreading centers?

The similarity between the morphologies of Venus trough-and-ridge tessera and the earth's ocean floor is discussed. The hypothesis that tessera texture might be related to a crustal fabric produced at spreading centers is examined. It is suggested that the proccesses that produce the ocean floor fabric on earth are good candidates for the origin and production of the trough-and-ridge tessera. To support this hypothesis, the characteristics of the trough-and-ridge terrain in Laima Tessera are described and compared to the seafloor at spreading centers.

Head, James W.↗
NASA NTRS1990

Pulse spreading and range correction analysis for satellite laser ranging

The pulse spreading resulting from light detection and ranging measurements of the range to earth-orbiting satellites is described. An analysis quantifying this pulse spreading and the calculation of corrections to be applied to the lidar range determination of satellites is detailed.

Schwartz, Jon A.↗
NASA NTRS1990

Numerical investigation of the spreading of self-excited stratified jets

The structure and evolution of self-excited subsonic periodic arrays of jets of constant and variable density are studied using spectral-element direct numerical simulations. The governing equation of motion is presented, and a method based on spectral element discretizations appropriate for simulating arbitrarily complex geometry jets and large density variations for subsonic flows is developed. Variable density fields are found to be more unstable than the corresponding uniform density fields with much higher rms values; as a result, their spreading is also considerably larger. There is a dramatic increase in spreading after a few pairings occur. Findings presented for low and high side-momentum flux reveal a shifting of the origin of instability from the near-field to the far-field, respectively, and suggest possible routes of stabilization.

Batcho, P. F.↗
NASA NTRS1991

Heat Transfer to a Thin Solid Combustible in Flame Spreading at Microgravity

The heat transfer rate to a thin solid combustible from an attached diffusion flame, spreading across the surface of the combustible in a quiescent, microgravity environment, was determined from measurements made in the drop tower facility at NASA-Lewis Research Center. With first-order Arrhenius pyrolysis kinetics, the solid-phase mass and energy equations along with the measured spread rate and surface temperature profiles were used to calculate the net heat flux to the surface. Results of the measurements are compared to the numerical solution of the complete set of coupled differential equations that describes the temperature, species, and velocity fields in the gas and solid phases. The theory and experiment agree on the major qualitative features of the heat transfer. Some fundamental differences are attributed to the neglect of radiation in the theoretical model.

Bhattacharjee, S.↗
NASA NTRS1990

Spread spectrum mobile communication experiment using ETS-V satellite

The spread spectrum technique is attractive for application to mobile satellite communications, because of its random access capability, immunity to inter-system interference, and robustness to overloading. A novel direct sequence spread spectrum communication equipment is developed for land mobile satellite applications. The equipment is developed based on a matched filter technique to improve the initial acquisition performance. The data rate is 2.4 kilobits per sec. and the PN clock rate is 2.4552 mega-Hz. This equipment also has a function of measuring the multipath delay profile of land mobile satellite channel, making use of a correlation property of a PN code. This paper gives an outline of the equipment and the field test results with ETS-V satellite.

Ikegami, Tetsushi↗