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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Coding for spread spectrum packet radios

Packet radios are often expected to operate in a radio communication network environment where there tends to be man made interference signals. To combat such interference, spread spectrum waveforms are being considered for some applications. The use of convolutional coding with Viterbi decoding to further improve the performance of spread spectrum packet radios is examined. At 0.00001 bit error rates, improvements in performance of 4 db to 5 db can easily be achieved with such coding without any change in data rate nor spread spectrum bandwidth. This coding gain is more dramatic in an interference environment.

Omura, J. K.↗
NASA NTRS1979

Spread spectrum techniques for the Space Shuttle

The Space Shuttle will employ spread spectrum techniques for both communication and navigation. During on-orbit phases of flight, two-way S-band and Ku-band communication links between the Shuttle and the ground will be available via the tracking and data relay satellite system (TDRSS). A pseudonoise (PN) code will spread the forward-link data spectrum from 32 kbps to 216 kbps to reduce the signal spectral density impinging on the earth's surface, with the S-band code search and acquisition being performed at C/NO-values of around 5 dB-Hz, and with a code length of 2047 chips at a rate of 11.232 Mchips/sec. C/NO-values in the region of 60 dB-Hz to 63 dB-Hz will characterize PN code acquisition in the Ku-band, whose forward-link PN clock is 3.03 Mchips/sec on a 1023-chip length sequence. Primary operational navigation will be provided by the Global Positioning System (GPS), which, after measuring transit time of the PN spread spectrum signal between a number of GPS satellites and itself, scales it by the velocity of light. GPS will enable position estimation accuracy to 30 feet and velocity estimation accuracy to 0.02 ft/sec.

Batson, B. H.↗
NASA NTRS1984

The Effect of Point-spread Function Interaction with Radiance from Heterogeneous Scenes on Multitemporal Signature Analysis

The point-spread function is an important factor in determining the nature of feature types on the basis of multispectral recorded radiance, particularly from heterogeneous scenes and particularly from scenes which are imaged repetitively, in order to provide thematic characterization by means of multitemporal signature. To demonstrate the effect of the interaction of scene heterogeneity with the point spread function (PSF)1, a template was constructed from the line spread function (LSF) data for the thematic mapper photoflight model. The template was in 0.25 (nominal) pixel increments in the scan line direction across three scenes of different heterogeneity. The sensor output was calculated by considering the calculated scene radiance from each scene element occurring between the contours of the PSF template, plotted on a movable mylar sheet while it was located at a given position.

Duggin, M. J.↗
NASA NTRS1984

Gravitational effects on flames spreading over thick solid surfaces

A theoretical model for the downward spread of a flame in the gas over a semi-infinitely thick fuel bed is presented. Gas-phase chemistry is assumed to proceed at an infinite rate while the fuel bed is taken to pyrolyze at the surface in an Arrhenius fashion. Spread rates and flame shapes are computed for two different ambient oxygen concentrations for gravitational accelerations both below and above that of the earth. Predicted spread rates decrease and actual flame size increases with decreasing gravity. Results agree favorably with available, experimental results and expected results from reduced gravity experimentation.

Vedha-Nayagam, M.↗
NASA NTRS1983

Point spread function for a segmented mirror system

A procedure was developed to generate point spread functions for a segmented mirror system for a deployable reflector for submillimeter astronomy. These point spread functions were generated using ACCOS V and special purpose software. This procedure allows tilt and piston sensitivities to be evaluated. Point spread functions with tilt and piston errors are discussed.

Smith, S. T.↗
NASA NTRS1985

Seafloor hydrothermal activity and spreading rates - The Eocene carbon dioxide greenhouse revisited

A suggestion has been made that enhanced rates of hydrothermal activity during the Eocene could have caused a global warming by adding calcium to the ocean and pumping CO2 into the atmosphere (Owen and Rea, 1984). This phenomenon was purported to be consistent with the predictions of the CO2 geochemical cycle model of Berner, Lasaga and Garrels (1983) (henceforth BLAG). In fact, however, the BLAG model predicts only a weak connection between hydrothermal activity and atmospheric CO2 levels. By contrast, it predicts a strong correlation between seafloor spreading rates and pCO2, since the release rate of CO2 from carbonate metamorphism is assumed to be proportional to the mean spreading rate. The Eocene warming can be conveniently explained if the BLAG model is extended by assuming that the rate of carbonate metamorphism is also proportional to the total length of the midocean ridges from which the spreading originates.

Kasting, J. F.↗
NASA NTRS1989

Dynamical consequences of compositional and thermal density stratification beneath spreading centers

Dynamical consequences of compositional buoyancy and the combined effects of compositional and thermal buoyancy on mantle flow and crustal production are explored. The results show that for a low enough mantle viscosity, buoyant upwelling can significantly enhance the crustal thickness relative to that which would be produced by plate spreading alone, while for a mantle viscosity of 10 to the 22nd Pa s, upwelling due to plate spreading is dominant and crustal thickness is predicted to be a function of spreading rate. The results indicate that thermal and compositional density variations result in opposing buoyancy forces that can cause time-dependent upwelling.

Sotin, C.↗
NASA NTRS1989

Near-Limit Flame Spread over a Thin Solid Fuel in Microgravity

Diffusion flame spread over a thin solid fuel in quiescent and slowly moving atmospheres is studied in microgravity. The flame behavior is observed to depend strongly on the magnitude of the relative velocity between the flame and the atmosphere. In particular, a low velocity quenching limit is found to exist in low oxgen environments. Using both the microgravity results and previously published data at high opposed-flow velocities, the flame spread behavior is examined over a wide velocity range. A flammability map using molar oxygen percentages and characteristic relative velocities as coordinates is constructed. Trends of flame spread rate are determined and mechanisms for flame extinction are discussed.

Olson, Sandra L.↗
NASA NTRS1989

Opposed-flow flame spread and extinction in mixed-convection boundary layers

Experimental data for flame spread down thin fuel samples in an opposing, mixed-convection, boundary-layer flow are analyzed to determine the gas-phase velocity that characterizes how the flame reacts as it spreads toward the leading edge of the fuel sample into a thinning boundary layer. In the forced-flow limit where the cube of the Reynolds number divided by the Grashof number, Re exp 3/Gr, is large, L(q)/L(e), where L(q) is a theoretical flame standoff distance at extinction and L(e) is the measured distance from the leading edge of the sample where extinction occurs, is found to be proportional to Re exp n with n = -0.874 and Re based on L(e). The value of n is established by the character of the flow field near the leading edge of the flame. The Re dependence is used, along with a correction for the mixed-convection situation where Re exp 3/Gr is not large, to construct a Damkohler number with which the measured spread rates correlate for all values of Re exp 3/Gr.

Altenkirch, R. A.↗
NASA NTRS1991

Opposed Flow Flame Spread in Normal, Enhanced and Reduced Gravity

Experimental and theoretical aspects of opposed-flow flame spread over solid fuels are presented with emphasis on the microgravity environments of spacecraft. For high opposing flow velocities, spread rate decreases with increasing velocity eventually leading to flame blowoff due to kinetic effects. At low opposing flow velocities, where diffusional effects are slowed and radiation becomes important, flame spread rate increases with increasing flow velocity. Extinction at low velocities is due to radiative effects. Modeling efforts that include radiation, both solid surface radiation and gas-phase radiation, predict qualitatively the experimental trends observed. Computationally, gas-phase radiation is conveniently included in solution of the conservation equations by employing a Plank mean absorption coefficient, a fraction of radiation that is fed back to the surface, and a shape function that describes the radiative flux distribution along the surface.

Altenkirch, Robert A.↗
NASA NTRS1990

Opposed-flow flame spread with implications for combustion at microgravity

Various regimes of the flame spread phenomenon in an opposing flow environment are classified here depending on the mechanism of flame spread. Simple analyses of energy balances at the tip of the flame are used to complement more sophisticated numerical analysis of the field problem associated with flame spreading in microgravity. The microgravity flames are shown to constitute a separate regime or class by themselves and are discussed in some detail.

Altenkirch, Robert A.↗
NASA NTRS1992

Bispectral analysis of equatorial spread F density irregularities

Bispectral analysis has been applied to density irregularities at frequencies 5-30 Hz observed with a sounding rocket launched from Peru in March 1983. Unlike the power spectrum, the bispectrum contains statistical information about the phase relations between the Fourier components which make up the waveform. In the case of spread F data from 475 km the 5-30 Hz portion of the spectrum displays overall enhanced bicoherence relative to that of the background instrumental noise and to that expected due to statistical considerations, implying that the observed f exp -2.5 power law spectrum has a significant non-Gaussian component. This is consistent with previous qualitative analyses. The bicoherence has also been calculated for simulated equatorial spread F density irregularities in approximately the same wavelength regime, and the resulting bispectrum has some features in common with that of the rocket data. The implications of this analysis for equatorial spread F are discussed, and some future investigations are suggested.

Labelle, J.↗
NASA NTRS1993

Computational predictions of flame spread over alcohol pools

The effects of buoyancy and thermocapillarity on pulsating and uniform flame spread above n-propanol fuel pools have been studied using a numerical model. Data obtained indicate that the existence of pulsating flame spread is dependent upon the formation of a gas-phase recirculation cell which entrains evaporating fuel vapor in front of the leading edge of the flame. The size of the recirculation cell which is affected by the extent of liquid motion ahead of the flame, is shown to dictate whether flame spread is uniform or pulsating. The amplitude and period of the flame pulsations are found to be proportional to the maximum extent of the flow head. Under conditions considered, liquid motion was not affected appreciably by buoyancy. Horizontal convection in the liquid is the dominant mechanism for transporting heat ahead of the flame for both the pulsating and uniform regimes.

Schiller, D. N.↗
NASA NTRS1993

Numerical Computation of Low-Speed Concurrent Flow Flame Spread in Mixed Buoyant and Forced Flow

The effect of low-speed mixed convection (forced plus buoyant) on concurrent flow flame spread over a thin solid is examined. Computations are carried out using an existing model. Results indicate that seemingly small levels of gravity can significantly alter flame spread rates. Starting with a purely forced flow condition, as gravity is added, the entrainment due to buoyancy lengthens the flame and increases the spread rate significantly. Buoyancy has an influence on the extinction limits. At low speed, the presence of a small gravity level widens the flammability limit.

Ferkul, Paul V.↗
NASA NTRS1993

Downward Diffusion Flame Spread and Extinction in Variable Gravitational Fields - Lunar and Martian Simulations

This paper describes experimental observations of downward, opposed-flow flame spreading made under partial-gravity conditions aboard NASA research aircraft. Flame spreading and flammability limit behavior of a thin cellulosic fuel tested at normal pressure in oxygen/nitrogen mixtures of 21 percent oxygen, by volume, and below are described over effective acceleration levels ranging from 0.05 to 0.6 times normal earth gravity. Downward burning flammability and flame spread rates are shown to be enhanced by reductions in gravitational acceleration. These data have fire safety implications for the planning of lunar and Martian outposts.

Sacksteder, Kurt R.↗
NASA NTRS1994

VHF radar and rocket observations of equatorial spread F on Kwajalein

VHF radar data from the Summer 1990 Equatorial Spread F campaign on Kwajalein are presented. The Cornell 50 MHz portable radar interferometer (CUPRI) operated concurrently with the Altair UHF incoherent scatter radar throughout July and August and supported two sounding rocket flights on July 30 and August 2. This experiment provided the first opportunity to simultaneously diagnose equatorial spread F using the three prime experimental techniques: VHF/UHF coherent scatter, incoherent scatter, and in situ probe measurements of electric field and density fluctuations. The intensity of the coherent echoes observed was consistent with typical Jicamarca spread F observations, but chains of periodic, large-scale plasma upwellings were observed more often and for much longer durations on Kwajalein than have been seen over Peru. CUPRI also measured Doppler frequencies in one upwelling corresponding to 1200-m/s plasma drift velocities. This measurement agrees with recent observations of supersonic drift rates at the magnetic equator by spacecraft. Near the most active localized plasma upwellings, interferometer data reveal that the zonal drift rate of plasma irregularities can vary sharply in space, as one would expect for two-dimensional incompressible flow. We introduce a semiempirical model of the three-dimensional spectrum of F region irregularities that is consistent with the one-dimensional spectra of density fluctuations observed by sounding rockets and with the axial ratio of irregularities determined recently. Normalized to data from one of the rocket flights on Kwajalein, the model predicts the 3-m scattering cross-section measured by CUPRI to within a few decibels.

Hysell, D. L.↗
NASA NTRS1995

Low velocity opposed-flow frame spread in a transport-controlled environment DARTFire

The overall objectives of the DARTFire project are to uncover the underlying physics and increase understanding of the mechanisms that cause flames to propagate over solid fuels against a low velocity of oxidizer flow in a low-gravity environment. Specific objectives are (1) to analyze experimentally observed flame shapes, measured gas-phase field variables, spread rates, radiative characteristics, and solid-phase regression rates for comparison with previously developed model prediction capability that will be continually extended, and (2) to investigate the transition from ignition to either flame propagation or extinction in order to determine the characteristics of those environments that lead to flame evolution. To meet the objectives, a series of sounding rocket experiments has been designed to exercise several of the dimensional, controllable variables that affect the flame spread process over PMMA in microgravity, i.e., the opposing flow velocity (1-20 cm/s), the external radiant flux directed to the fuel surface (0-2 W/cm(exp 2)), and the oxygen concentration of the environment (35-70%). Because radiative heat transfer is critical to these microgravity flame spread experiments, radiant heating is imposed, and radiant heat loss will be measured. These are the first attempts at such an experimental control and measurement in microgravity. Other firsts associated with the experiment are (1) the control of the low velocity, opposed flow, which is of the same order as diffusive velocities and Stefan flows; (2) state-of-the-art quantitative flame imaging for species-specific emissions (both infrared and ultraviolet) in addition to novel intensified array imaging to obtain a color image of the very dim, low-gravity flames.

West, Jeff↗
NASA NTRS1998

The Geometric Spreading of Coronal Plumes and Coronal Holes

The geometric spreading in plumes and in the interplume region in coronal holes is calculated, using analytic and numerical theoretical models, between 1.0 and 5.0 solar radius. We apply a two-scale approximation that permits the rapid local spreading at the base of plumes (f(sub t)) to be evaluated separately from the global spreading (f(sub g)) imposed by coronal hole geometry. We show that f(sub t) can be computed from a potential-field model and f(sub g) can be computed from global magnetohydrodynamic simulations of coronal structure. The approximations are valid when the plasma beta is mail with respect to unity and for a plume separation small with respect to a solar radius.

Suess, S. T.↗