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At least 667 records · Page 37

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NASA NTRS1982

Development, implementation, and test results on integrated optics switching matrix

A small integrated optics switching matrix, which was developed, implemented, and tested, indicates high performance. The matrix serves as a model for the design of larger switching matrices. The larger integrated optics switching matrix should form the integral part of a switching center with high data rate throughput of up to 300 megabits per second. The switching matrix technique can accomplish the design goals of low crosstalk and low distortion. About 50 illustrations help explain and depict the many phases of the integrated optics switching matrix. Many equations used to explain and calculate the experimental data are also included.

Rutz, E.↗

NASA NTRS1982

Relation of agronomic and multispectral reflectance characteristics of spring wheat canopies

The relationships between crop canopy variables such as leaf area index (LAI) and their multispectral reflectance properties were investigated along with the potential for estimating canopy variables from remotely sensed reflectance measurements. Reflectance spectra over the 0.4 to 2.5 micron wavelength range were acquired during each of the major development stages of spring wheat canopies at Williston, North Dakota, during three seasons. Treatments included planting date, N fertilization, cultivar, and soil moisture. Agronomic measurements included development stage, biomass, LAI, and percent soil cover. High correlations were found between reflectance and percent cover, LAI, and biomass. A near infrared wavelength band, 0.76 to 0.90 microns, was most important in explaining variation in LAI and percent cover, while a middle infrared band, 2.08 to 2.35 microns, explained the most variation in biomass and plant water content. Transformations, including the near infrared/red reflectance ratio and greenness index, were also highly correlated to canopy variables. The relationship of canopy variables to reflectance decreased as the crop began to ripen. the canopy variables could be accurately predicted using measurements from three to five wavelength bands. The wavelength bands proposed for the thematic mapper sensor were more strongly related to the canopy variables than the LANDSAT MSS bands.

Bauer, M. E.↗

NASA NTRS1983

Range safety signal attenuation by the Space Shuttle main engine exhaust plumes

An analysis of attenuation of the range safety signal at 416.5 MHz observed after SRB separation and ending at hand over to Bermuda, during which transmission must pass through the LOX/H2 propelled main engine exhaust plumes, is summarized. Absorption by free electrons in the exhaust plume can account for the nearly constant magnitude of the observed attenuation during this period; it does not explain the short term transient increases that occur at one or more times during this portion of the flight. It is necessary to assume that a trace amount (about 0.5 ppm) of easily ionizable impurity must be present in the exhaust flow. Other mechanisms of attenuation, such as scattering by turbulent fluctuations of both free and bound electrons and absorption by water vapor, were examined but found to be inadequate to explain the observations.

Pearce, B. E.↗

NASA NTRS1982

What has happened in the cores of globular clusters

It is found that some aspects of the observed data for globular cluster cores cannot be easily explained in terms of the existing theory of dynamical evolution. It is noted that globular clusters span only a factor of 40 in core mass, while ranging over a factor of 10 to the 6th in central density, an observation that cannot be explained as a selection effect. A strong correlation is found between the central density and the distance of a cluster from the galactic center. In addition, when cluster cores are evolved backward in time in their initial conditions, using the available theory of dynamical evolution, evidence emerges that many clusters have already undergone core collapse, for which no evolutionary theory exists. It is concluded that these results indicate the incompleteness of the theory of dynamical evolution.

Lightman, A. P.↗

NASA NTRS1981

Theories of dynamical phenomena in sunspots

Attempts that have been made to understand and explain observed dynamical phenomena in sunspots within the framework of magnetohydrodynamic theory are surveyed. The qualitative aspects of the theory and physical arguments are emphasized, with mathematical details generally avoided. The dynamical phenomena in sunspots are divided into two categories: aperiodic (quasi-steady) and oscillatory. For each phenomenon discussed, the salient observational features that any theory should explain are summarized. The two contending theoretical models that can account for the fine structure of the Evershed motion, namely the convective roll model and the siphon flow model, are described. With regard to oscillatory phenomena, attention is given to overstability and oscillatory convection, umbral oscillations and flashes. penumbral waves, five-minute oscillations in sunspots, and the wave cooling of sunspots.

Thomas, J. H.↗

NASA NTRS1983

Interpretation of integrated-disk photometry of Callisto and Ganymede

The Lumme-Bowell (1981) theory has been used to interpret the integrated phase curves of Callisto and Ganymede, and it is noted that while the theory explains the brightness angles of these satellites up to about 80 deg solar phase angle, the observed brightness drops off at larger angles more rapidly than predicted. It is suggested that this discrepancy is due to the fact that single regolith particles must have phase functions which are much more elongated in the forward or backward scattering directions than is allowed for by the Lumme-Bowell theory. The hemispheric asymmetry in Callisto's surface texture can be explained by invoking the formation of an ice film on the trailing side, consistent with Voyager detailed photometry and thermometry of Callisto.

Pang, K.↗

NASA NTRS1983

Charged dust in Saturn's magnetosphere

The overall distribution of fine dust in the Saturnian magnetosphere, its behavior, the cosmogony of the Saturnian ring system, and observations of the magnetosphere and ring system are synthesized and explained using gravito-electrodynamics. Among the phenomena discussed are the formation of waves in the F-ring, the cause of eccentricities of certain isolated ringlets, and the origin and morphology of the broad diffuse E-ring. Magnetogravitational resonance of charged dust with nearby satellites, gyro-orbital resonances, and magnetogravitational capture of exogenic dust by the magnetosphere are used to explain individual observations. The effect of a ring current associated with the charged dust is evaluated. Finally, the cosmogonic implications of the magnetogravitational theory are discussed.

Mendis, D. A.↗

NASA NTRS1983

Evidence for helical kink instability in the Venus magnetic flux ropes

Empirical models of the magnetic field structure of flux ropes found in the Venus ionosphere are seen as suggesting that the ropes are unstable to long-wavelength (more than 100 km) helical-kink perturbations. The onset of such an instability can explain the apparent volume distribution of flux ropes with altitude, as well as their orientation as a function of altitude. In the subsolar region, the fraction of volume occupied by flux ropes increases from approximately 20 percent at high altitudes to more than 50 percent at low altitudes; this is a greater increase than would be expected if ropes convect downward as simple straight horizontal cylinders. The helical kink instability raises the fractional volume occupied by ropes by turning the originally straight, horizontal flux tubes into corkscrew-shaped structures as they convect to lower altitudes. It is noted that this instability also explains why high altitude ropes tend to be horizontal and low altitude ropes appear to have almost any orientation.

Elphic, R. C.↗

NASA NTRS1983

Snow cover and temperature relationships in North America and Eurasia

In this study the snow cover extent during the autumn months in both North America and Eurasia has been related to the ensuing winter temperature as measured at several locations near the center of each continent. The relationship between autumn snow cover and the ensuing winter temperatures was found to be much better for Eurasia than for North America. For Eurasia the average snow cover extent during the autumn explained as much as 52 percent of the variance in the winter (December-February) temperatures compared to only 12 percent for North America. However, when the average winter snow cover was correlated with the average winter temperature it was found that the relationship was better for North America than for Eurasia. As much as 46 percent of the variance in the winter temperature was explained by the winter snow cover in North America compared to only 12 percent in Eurasia.

Foster, J.↗

NASA NTRS1982

Unsymmetrically laminated composites

Classical lamination theory predicts that the room-temperature shapes of all elevated-temperature cure, unsymmetrically laminated composites are saddles. However, experimental observation indicates that the shapes are often cylindrical. In addition, a second cylindrical can sometimes be obtained from the first by a simple snap-through action. A geometrically nonlinear extension to classical lamination theory is used to explain this behavior. Approximate solutions to the nonlinear extension are obtained by using a Rayleigh-Ritz minimization of the laminate's total potential energy. A stability analysis explains the dual cylindrical shapes.

Hyer, M. W.↗

NASA NTRS1983

Collision-induced absorption in the far infrared spectrum of Titan

The effects of collision-induced absorption on the far infrared spectrum of Titan have been investigated. After a review of the procedure for the theoretical calculation of the N2 translation-rotational spectrum, new results for the temperature range o 70 to 120 K are reported. These are used as input data for a simple atmospheric model in order to compute the far infrared radiance, brightness temperature, and specral limb function. This source of opacity alone is not capable of explaining the Voyager results. When the collision-induced methane is included, the results are in closer agreement in the range between 200 and 300/cm, suggesting that a more complete treatment of collision-induced absorption including particularly CH4-N2, N2-H2, and H2-H2 results, may provide sufficient opacity to reduce or obviate the need for opacities due to clouds or aerosols in order to explain the observed spectra.

Hunt, J. L.↗

NASA NTRS1983

The structure of boron in boron fibres

The structure of noncrystalline, chemically vapour-deposited boron fibres was investigated by computer modelling the experimentally obtained X-ray diffraction patterns. The diffraction patterns from the models were computed using the Debye scattering equation. The modelling was done utilizing the minimum nearest-neighbour distance, the density of the model, and the broadening and relative intensity of the various peaks as boundary conditions. The results suggest that the fibres consist of a continuous network of randomly oriented regions of local atomic order, about 2 nm in diameter, containing boron atoms arranged in icosahedra. Approximately half of these regions have a tetragonal structure and the remaining half a distorted rhombohedral structure. The model also indicates the presence of many partial icosahedra and loose atoms not associated with any icosahedra. The partial icosahedra and loose atoms indicated in the present model are in agreement with the relaxing sub-units which have been suggested to explain the anelastic behavior of fibre boron and the loosely bound boron atoms which have been postulated to explain the strengthening mechanism in boron fibres during thermal treatment.

Bhardwaj, J.↗

NASA NTRS1983

Incident reporting: Its role in aviation safety and the acquisition of human error data

The rationale for aviation incident reporting systems is presented and contrasted to some of the shortcomings of accident investigation procedures. The history of the United State's Aviation Safety Reporting System (ASRS) is outlined and the program's character explained. The planning elements that resulted in the ASRS program's voluntary, confidential, and non-punitive design are discussed. Immunity, from enforcement action and misuse of the volunteered data, is explained and evaluated. Report generation techniques and the ASRS data analysis process are described; in addition, examples of the ASRS program's output and accomplishments are detailed. Finally, the value of incident reporting for the acquisition of safety information, particularly human error data, is explored.

Reynard, W. D.↗

NASA NTRS1983

Shock temperatures in anorthite glass

Temperatures of CaAl2Si2O8 (anorthite glass) shocked to pressures between 48 and 117 GPa were measured in the range from 2500 to 5600 K, using optical pyrometry techniques. The pressure dependence of the shock temperatures deviates significantly from predictions based on a single high pressure phase. At least three phase transitions, at pressures of about 55, 85, and 100 GPa and with transition energies of about 0.5 MJ/kg each (approximately 1.5 MJ/kg total) are required to explain the shock temperature data. The phase transition at 100 GPa can possibly be identified with the stishovite melting transition. Theoretical models of the time dependence of the thermal radiation from the shocked anorthite based on the geometry of the experiment and the absorptive properties of the shocked material yields good agreement with observations, indicating that it is not necessary to invoke intrinsic time dependences to explain the data in many cases.

Boslough, M. B.↗

NASA NTRS1984

Energetic electrons in impulsive solar flares

A new analysis was made of a thermal flare model proposed by Brown, Melrose, and Spicer (1979) and Smith and Lilliequist (1979). They assumed the source of impulsive hard X-rays to be a plasma at a temperature of order 10 to the 8th power K, initially located at the apex of a coronal arch, and confined by ion-acoustic turbulence in a collisionless conduction front. Such a source would expand at approximately the ion-sound speed, C sub S = square root of (k T sub e/m sub i), until it filled the arch. Brown, Melrose, and Spicer and Smith and Brown (1980) argued that the source assumed in this model would not explain the simultaneous impulsive microwave emission. In contrast, the new results presented herein suggest that this model leads to the development of a quasi-Maxwellian distribution of electrons that explains both the hard X-ray and microwave emissions. This implies that the source sizes can be determined from observations of the optically-thick portions of microwave spectra and the temperatures obtained from associated hard X-ray observations. In this model, the burst emission would rise to a maximum in a time, t sub r, approximately equal to L/c sub s, where L is the half-length of the arch. New observations of these impulsive flare emissions were analyzed herein to test this prediction of the model. Observations made with the Solar Maximum Mission spacecraft and the Bern Radio Observatory are in good agreement with the model.

Batchelor, D. A.↗

NASA NTRS1983

The PASCAL-HDM Verification System

The PASCAL-HDM verification system is described. This system supports the mechanical generation of verification conditions from PASCAL programs and HDM-SPECIAL specifications using the Floyd-Hoare axiomatic method. Tools are provided to parse programs and specifications, check their static semantics, generate verification conditions from Hoare rules, and translate the verification conditions appropriately for proof using the Shostak Theorem Prover, are explained. The differences between standard PASCAL and the language handled by this system are explained. This consists mostly of restrictions to the standard language definition, the only extensions or modifications being the addition of specifications to the code and the change requiring the references to a function of no arguments to have empty parentheses.

Source record↗

NASA NTRS1984

A critical review of noise production models for turbulent, gas-fueled burners

The combustion noise literature for the period between 1952 and early 1984 is critically reviewed. Primary emphasis is placed on past theoretical and semi-empirical attempts to predict or explain observed direct combustion noise characteristics of turbulent, gas-fueled burners; works involving liquid-fueled burners are reviewed only when ideas equally applicable to gas-fueled burners are pesented. The historical development of the most important contemporary direct combustion noise theories is traced, and the theories themselves are compared and criticized. While most theories explain combustion noise production by turbulent flames in terms of randomly distributed acoustic monopoles produced by turbulent mixing of products and reactants, none is able to predict the sound pressure in the acoustic farfield of a practical burner because of the lack of a proven model which relates the combustion noise source strenght at a given frequency to the design and operating parameters of the burner. Recommendations are given for establishing a benchmark-quality data base needed to support the development of such a model.

Mahan, J. R.↗

NASA NTRS1984

On the detection of underwater bottom topography by imaging radars

A theoretical model which explains basic properties of radar imaging of underwater bottom topography in tidal channels is presented. The surface roughness modulation is described by weak hydrodynamic interaction theory in the relaxation time approximation. In contrast to previous theories on short wave modulation by long ocean waves, a different approximation is used to describe short wave modulation by tidal flow over underwater bottom topography. The modulation depth is proportional to the relaxation time of the Bragg waves. The large modulation of radar reflectivity observed in SEASAT-SAR imagery of sand banks in the Southern Bight of the North Sea are explained by assuming that the relaxation time of 34 cm Bragg waves is of the order of 30-40 seconds.

Alpers, W.↗