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At least 415 records · Page 23

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NASA NTRS1978

Axisymmetric disturbances in a disk galaxy

The Lin-Shu dispersion relation, with corrections to match experimental conditions and in combination with a quantization rule, permits growth rates to be calculated for axisymmetric disturbances in an axisymmetric disk of stars. Growth rates calculated this way are in reasonable quantitative agreement with recently published results from a series of numerical experiments. The agreement confirms the physical basis of the theory and demonstrates that the theory remains valid even when strained somewhat in the application to galactic models.

Miller, R. H.↗

NASA NTRS1979

A high order accurate numerical solution algorithm for turbulent boundary layer flow

A fourth-order accurate numerical solution algorithm is derived using finite element interpolation theory for the non-linear parabolic equations governing turbulent boundary layer flow including a two-equation turbulence closure model. The results of carefully controlled numerical experiments firmly quantize for the first time performance differences between finite element and finite difference solution methodology for this type of equation. The developed algorithm takes advantage of the apparent semi-analytical formulational procedure, in establishment of a single, retarded-evaluation Jacobian matrix iterative solution algorithm. Numerical results document performance of solution economy features in terms of computer requirements and solution accuracy. The developed algorithm should find wide application in aerodynamics analysis.

Soliman, M. O.↗

NASA NTRS1978

Observable phase factors and symmetry of electric and magnetic charges

The observable phase factor is taken as a basic concept for the description of electromagnetism. Generalization of this concept to SU(2) and SU(2) x U(1) groups is carried out in such a way that the monopoles with quantized charges appear naturally and that the symmetry between the electric and magnetic phenomena is preserved. Some physical implications are discussed.

Hsu, J. P.↗

NASA NTRS1979

A theory of stationary long waves. III - Quasi-normal modes in a singular waveguide

The present paper deals with the fundamental issue of whether one can treat waves as normal modes when critical surface, where the phase speed of the wave matches the zonal wind speed, are present. In particular the question of whether a Rossby critical level (such as the zero-wind line for stationary waves) is absorbing or reflecting is raised and subsequently addressed. It is found that the critical level is never totally absorbing; Rossby waves are partially reflected even if the critical layer is dominated by dissipative processes. The relevance of nonlinearity in planetary-scale Rossby wave critical layers is also discussed and it is found to be the dominant mechanism. With the relative magnitudes of nonlinearity versus viscosity relevant to the earth's atmosphere it is found that the steady-state critical level should be almost perfectly reflecting to incident Rossby waves. Consequently, normal-mode solutions can be found; the quantization condition for these waves is also derived.

Tung, K. K.↗

NASA NTRS1979

Utility of a finite element solution algorithm for initial-value problems

The Galerkin criterion within a finite element Weighted Residuals formulation is employed to establish an implicit solution algorithm for an initial-value partial differential equation. Numerical solutions of a transient parabolic and a hyperbolic equation, obtained using linear, quadratic and two cubic finite element basis functions, are employed to quantize accuracy and confirm and refine theoretical convergence rate estimates. The linear basis algorithm for the hyperbolic equation displays excellent accuracy on a coarse computational grid and a high-order convergence rate with discretization refinement. Good accuracy and a strong convergence rate in surface flux are determined for a nonhomogeneous Neumann boundary constraint applied to a parabolic equation. The results amply demonstrate the impact of the nondiagonal finite element initial-value matrix structure on solution accuracy and/or convergence rate.

Baker, A. J.↗

NASA NTRS1979

Implications of cyclotron features in the X-ray spectrum of Her X-1

The X-ray spectra from binary pulsars is investigated through the modification of physical processes due to an intense magnetic field. An effective scattering cross section for line photons and the Coulomb cross section for electron-ion collisions, treating the electrons relativistically, are calculated. The quantization of electron orbits in strong fields are discussed. The absorption cross sections and emission rates for cyclotron photons are calculated. An application of the results to the data from Hercules X-1 is examined.

Bussard, R.↗

NASA NTRS1979

Weighting in digital synthetic aperture radar processing

Weighting is employed in synthetic aperture radar (SAR) processing to reduce the sidelobe response at the expense of peak center response height and mainlobe resolution. The weighting effectiveness in digital processing depends not only on the choice of weighting function, but on the fineness of sampling and quantization, on the time bandwidth product, on the quadratic phase error, and on the azimuth antenna pattern. The results of simulations conducted to uncover the effect of these parameters on azimuth weighting effectiveness are presented. In particular, it is shown that multilook capabilities of future SAR systems may obviate the need for consideration of the antenna pattern, and that azimuth time-bandwidth products of over 200 are probably required before the digital results begin to approach the ideal results.

Dicenzo, A.↗

NASA NTRS1979

Program documentation for the data noising program

Problems in the Space Shuttle aerodynamic data that are expected to occur in the actual Orbital Flight Test (OFT) flight data are introduced. The user selects from 1 to 100 channels to be operated on by this program. For each selected channel number, the user may introduce up to three problems in the channel data: time skew, data noise in the frequency domain, and data-quantization. The output data is the input data plus the data for each selected channel number with NOISE selected by the program user.

Nguyen, M. T.↗

NASA NTRS1980

Considerations in the placement of phase calibration tones

In the use of tones to calibrate unwanted instrumental phases for very long baseline interferometry experiments, certain problems exist which are related to the placement of these phase calibrator tones. (1) A bias exists in an analytically generated stopping function used during correlation if its frequency satisfies the following condition; f = (f sub s)x(m)/n, where f sub s = sampling frequency, m = any integer, and n = any odd integer. (2) Due to the quantized representation of sine waves in the stopping function, odd harmonics of the fundamental frequencies are generated. Several mechanisms are available through which these harmonics can cause errors in the residual phase extracted from the recorded tones. (3) When multiple tones are injected into a pass band, intermodulation products can occur. The magnitude of these effects is discussed along with strategies designed to avoid them.

Young, L. E.↗

NASA NTRS1980

The sensitivity of a very long baseline interferometer

The theoretical sensitivity of various methods of acquiring and processing interferometer data are compared. It is shown that for a fixed digital recording capacity one bit quantization of single sideband data filtered with a rectangular bandpass and sampled at the Nyquist rate yields the optimum signal to noise ratio. The losses which result from imperfect bandpass, poor image rejection, approximate methods of fringe rotation, fractional bit correction, and loss of quadrature are discussed. Also discussed is the use of the complex delay function as a maximum likelihood fringe estimator.

Rogers, A. E. E.↗

NASA NTRS1980

Interpretation of laser/multi-sensor data for short range terrain modeling and hazard detection

A terrain modeling algorithm that would reconstruct the sensed ground images formed by the triangulation scheme, and classify as unsafe any terrain feature that would pose a hazard to a roving vehicle is described. This modeler greatly reduces quantization errors inherent in a laser/sensing system through the use of a thinning algorithm. Dual filters are employed to separate terrain steps from the general landscape, simplifying the analysis of terrain features. A crosspath analysis is utilized to detect and avoid obstacles that would adversely affect the roll of the vehicle. Computer simulations of the rover on various terrains examine the performance of the modeler.

Messing, B. S.↗

NASA NTRS1980

On extracting brightness temperature maps from scanning radiometer data

The extraction of brightness temperature maps from scanning radiometer data is described as a typical linear inverse problem. Spatial quantization and parameter estimation is described and is suggested as an advantageous approach to a solution. Since this approach takes into explicit account the multivariate nature of the problem, it permits an accurate determination of the most detailed resolution extractable from the data as well as explicitly defining the possible compromises between accuracy and resolution. To illustrate the usefulness of the method described for algorithm design and accuracy prediction, it was applied to the problem of providing brightness temperature maps during the NOSS flight segment. The most detained possible resolution was determined and a curve which displays the possible compromises between accuracy and resolution was provided.

Argentiero, P.↗

NASA NTRS1979

Nebular condensation of Ga, Ge and Sb and the chemical classification of iron meteorites

The quantization of the Ga and Ge contents in iron meteorites, which is used as a key parameter in the chemical classification of iron meteorites, is discussed in terms of nebular condensation. The calculation of nebular equilibrium condensation is examined, taking into account the dependence of the activity coefficient on temperature and composition, and recent calculations of the condensation temperatures of Ga, Ge, Sb, Au, As and Cu are presented, noting that Ge is the most volatile siderophile, followed by Ga and Sb. The narrow intragroup ranges of Ga and Ge are interpreted in terms of minimal fractionation during core crystallization, while the larger ranges of Sb are attributed to its significantly smaller solid/liquid distribution coefficient in IIIAB meteorites.

Wai, C. M.↗

NASA NTRS1979

On-board radiometric preprocessing for multispectral linear arrays /MLA/

A program that was undertaken to design, fabricate, and test a real-time hardwired data preprocessor is described, which applies a calibration normalization to each detector in a 576-element linear photodiode array. Various calibration problems were uncovered, such as those (1) due to system noise in recording the calibration tables, or (2) due to thermal drift and (3) due to the original quantization process. It was determined that in this experiment, noise and thermal drift led to fixed errors in the normalization of responses on the order of + or - 10 counts, out of 255 counts for many of the detectors.

Thompson, L. L.↗

NASA NTRS1979

Spatial quantification of maps or images - Cell size or pixel size implications

The paper discusses spatial quantification of maps or images. Spatial accuracies evaluated by mapping and inventory measures were observed with a range of cell sizes for a 10.36 sq km soil survey segment. The distribution of interboundary distances in a map was evaluated as unique, characterizing information; experimentally observed relationships between accuracy and cell size and interboundary distance distribution parameters and cell size led to a model of the process of spatial quantization. Grid positioning effects were evaluated and were significant only at the level of individual mapping units; two generations of 'universal' process models were derived, implemented, and evaluated.

Wehde, M. E.↗

NASA NTRS1979

Gravitational radiation detection with spacecraft Doppler tracking - Limiting sensitivities and prospective missions

The prospects of using spacecraft Doppler tracking, in NASA missions, for the detection of gravitational waves are examined. The sensitivity limits of such detection are characterized in terms of plasma scintillation, troposphere scintillation, receiver noise, MDA and ODA quantization error, and clock jitter. Current and possible future NASA missions that will involve gravitational wave experiments are briefly reviewed, including the Galileo, solar polar, Halley/Tempel-2, and solar probe missions.

Estabrook, F. B.↗

NASA NTRS1980

Saturn's E ring revisited

Images of the E ring of Saturn obtained by the image processing of photographs of the 1966 edge-on presentation of the planet's ring plane are presented. Two methods of image enhancement were used: scanning with an image quantizer operated in the derivative mode to enhance contrast and computerized subtraction of a circularly symmetric image of the overexposed Saturn disk. Further photographic and CCD observation confirming the existence of the ring extending to twice the diameter of the A ring, which was not detected by the Pioneer 11 imaging photopolarimeter, is indicated.

Feibelman, W. A.↗

NASA NTRS1980

Application of digital terrain data to quantify and reduce the topographic effect on LANDSAT data

Integration of LANDSAT multispectral scanner (MSS) data with 30 m U.S. Geological Survey (USGS) digital terrain data was undertaken to quantify and reduce the topographic effect on imagery of a forested mountain ridge test site in central Pennsylvania. High Sun angle imagery revealed variation of as much as 21 pixel values in data for slopes of different angles and aspects with uniform surface cover. Large topographic effects were apparent in MSS 4 and 5 was due to a combination of high absorption by the forest cover and the MSS quantization. Four methods for reducing the topographic effect were compared. Band ratioing of MSS 6/5 and MSS 7/5 did not eliminate the topographic effect because of the lack of variation in MSS 4 and 5 radiances. The three radiance models examined to reduce the topographic effect required integration of the digital terrain data. Two Lambertian models increased the variation in the LANDSAT radiances. The nonLambertian model considerably reduced (86 per cent) the topographic effect in the LANDSAT data. The study demonstrates that high quality digital terrain data, as provided by the USGS digital elevation model data, can be used to enhance the utility of multispectral satellite data.

Justice, C. O.↗