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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Enhancement of Far Field Sound Levels by Refractive Focusing

The enhancement of sound pressure levels resulting from refractive focusing was calculated for meteorological conditions representative of those observed at the MOD-1 site near Boone, N.C. The results show that 10 to 20dB enhancements can occur over ranges of several hundred meters. Localized enhancements in excess of 20dB can occur but will probably be of limited duration as a consequence of normal temporally varying meteorological conditions.

Thomson, D. W.↗
NASA NTRS1981

Radio jet refraction in galactic atmospheres with static pressure gradients

A theory based on the refraction of radio jets in the extended atmosphere of an elliptical galaxy, is proposed for double radio sources with a Z or S morphology. The model describes a collimated jet of supersonic material that bends self-consistently under the influence of external static pressure gradients, and may alternatively be seen as a continuous-jet version of the buoyancy model proposed by Gull (1973). Emphasis is placed on (1) S-shaped radio sources identified with isolated galaxies, such as 3C 293, whose radio structures should be free of distortions resulting from motion relative to a cluster medium, and (2) small-scale, galaxy-dominated rather than environment-dominated S-shaped sources such as the inner jet structure of Fornax A.

Henriksen, R. N.↗
NASA NTRS1982

Inversion of the anomalous diffraction approximation for variable complex index of refraction near unity

The Fymat analytic inversion method for retrieving a particle-area distribution function from anomalous diffraction multispectral extinction data and total area is generalized to the case of a variable complex refractive index m(lambda) near unity depending on spectral wavelength lambda. Inversion tests are presented for a water-haze aerosol model. An upper-phase shift limit of 5 pi/2 retrieved an accurate peak area distribution profile. Analytical corrections using both the total number and area improved the inversion.

Smith, C. B.↗
NASA NTRS1983

Autonomous satellite navigation by stellar refraction

This paper describes an error analysis of an autonomous navigator using refraction measurements of starlight passing through the upper atmosphere. The analysis is based on a discrete linear Kalman filter. The filter generated steady-state values of navigator performance for a variety of test cases. Results of these simulations show that in low-earth orbit position-error standard deviations of less than 0.100 km may be obtained using only 40 star sightings per orbit.

Gounley, R.↗
NASA NTRS1984

Refractive turbulence effects on truncated Gaussian beam heterodyne lidar

A monostatic heterodyne lidar performance model is formulated to study the combined effects of beam truncation and refractive turbulence in the weak scintillation regime. The results show that there is a loss of signal power due to beam truncation and coherence loss, but there is also an enhancement of signal power due to log-amplitude covariance in suitable conditions of long paths with weak turbulence.

Murty, R.↗
NASA NTRS1984

Advanced wide-field broad-passband refracting field correctors for large telescopes

Design objectives, constraints, and optical data are presented for specific corrector designs under consideration for several large telescope projects. These include a preliminary 30-arcmin prime focus (f/2.0) refracting field corrector system for the University of California Ten-Meter Telescope (UC TMT); a compact 40-arcmin internal Cassegrain (f/1.75 hyperbola to f/5.0) broad-passband (3300 A to 1.0 micron) corrector suitable for imaging and multi-object spectroscopy at the UC TMT; three 60-arcmin Cassegrain correctors for 300-inch f/1.8 and f/2.0 parabolic primary mirrors suitable for a Fifteen-Meter NNTT/MMT; and a 300-inch 40-arcmin external Cassegrain (f/1.0 parabola to f/4.0) broad-passband (3300 A to 1.0 micron) corrector with ADC.

Epps, H. W.↗
NASA NTRS1986

Wave propagation in pulsar magnetospheres - Refraction of rays in the open flux zone

The propagation of waves through a relativistically outflowing electron-positron plasma in a very strong dipolar magnetic field, conditions expected in pulsar magnetospheres, is investigated. Halmilton's equations is derived for the propagation of rays through a plasma which is inhomogeneous in density, magnetic field directions, and Lorentz factor. These equations are solved for rays propagating through the plasmas outflowing along the 'open' dipolar field lines in which the density decreases inversely as the radius cubed and in the case where gradients transverse to the radial direction exist. In the radial case, the effects of refraction on pulse profiles, spectrum, and polarization are examined, and the effects of a transverse gradient are indicated. Attention is given to models in which the observed broad bandwidth in the radio emission has its origin in a radius to frequency map. Models with broad-band emission at a single radius are also studied. These are compared to observations of pulse width and pulse component separation as a function of frequency. The origin of 'orthogonal modes' is discussed.

Barnard, J. J.↗
NASA NTRS1986

Crustal structure of the Southern Rio Grande rift determined from seismic refraction profiling

As part of a major cooperative seismic experiment, a series of seismic refraction profiles have been recorded in south-central New Mexico with the goal of determining the crustal structure in the southern Rio Grande rift. The data gathered greatly expand the seismic data base in the area, and consist of three interlocking regional profiles: a reversed E-W line across the rift, an unreversed N-S axial line, and an unreversed SW-SE line. The reversed E-W line shows no significant dip along the Moho (32 km thick crust) and a 7.7 km/s Pn velocity. Results from the N-S axial line and the NW-SE line indicate an apparent Pn velocity of 7.95 km/s and significant dip along the Moho with crustal thinning toward the south and southeast. When interpreted together, these data indicate a crustal thinning in the southern rift of 4-6 km with respect to the northern rift and the adjacent Basin and Range province, and establish the regional Pn velocity to be approximately 7.7 km/s. These results suggest that the Rio Grande rift can be identified as a crustal feature separate and distinct from the Basin and Range province.

Sinno, Y. A.↗
NASA NTRS1987

Diffractively Coupled, Refractively Guided Lasers

Semiconductor-laser arrays more reliable, more powerful, and easier to make. Improved design intended to eliminate undesired electromagnetic modes and mode shifts sometimes occuring in gain-guided variety. Reflected from mirror/window at end of common resonator section of laser, energy refracted from each laser enters adjacent laser. Mutual coupling establishes phase relationships among lasers. Monolithic laser array made by standard epitaxial techniques. Made in part with polymeric materials to mitigate some deleterious effects of all-expitaxial processing. Potential applications include optical communications, ranging, printing, and recording.

Katz, Joseph↗
NASA NTRS1986

Sensitivity of the Lidar ratio to changes in size distribution and index of refraction

In order to invert lidar signals to obtain reliable extinction coefficients, sigma, a relationship between sigma and the backscatter coefficient, beta, must be given. These two coefficients are linearly related if the complex index of refraction, m, particle shape size distribution, N, does not change along the path illuminated by the laser beam. This, however, is generally not the case. An extensive Mie computation of the lidar ratio R = beta/sigma and the sensitivity of R to the changes in a parametric space defined by N and m were examined.

Evans, B. T. N.↗
NASA NTRS1986

Crystalline sulfur dioxide: Crystal field splittings, absolute band intensities and complex refractive indices derived from infrared spectra

The infrared absorption spectra of thin crystalline films of sulfur dioxide at 90 K are reported in the 2700 to 450/cm region. The observed multiplicity of the spectral features in the regions of fundamentals is attributed to factor group splittings of the modes in a biaxial crystal lattice and the naturally present minor S-34, S-36, and O-18 isotopic species. Complex refractive indices determined by an iterative Kramers-Kronig analysis of the extinction data, and absolute band strengths derived from them, are also reported in this region.

Khanna, R. K.↗
NASA NTRS1987

On the saturation of the refractive index structure function. II - Influence of the correlation length on astronomical 'seeing'

A physical length scale in the wavefront corresponding to the parameter (r sub 0) characterizing the loss in detail in a long exposure image is identified, and the influence of the correlation scale of turbulence as r sub 0 approaches this scale is shown. Allowing for the effect of 2-point correlations in the fluctuations of the refractive index, Venkatakrishnan and Chatterjee (1987) proposed a modified law for the phase structure function. It is suggested that the departure of the phase structure function from the 5/3 power law for length scales in the wavefront approaching the correlation scale of turbulence may lead to better 'seeing' at longer wavelengths.

Venkatakrishnan, P.↗
NASA NTRS1988

Magnetically induced neutrino oscillations and neutrino refractive effects in the early universe

The interrelation between hypothetical neutrino magnetic dipole moments (MDMs) and primeval magnetic fields is discussed. The parameter range over which these possibilities are mutually exclusive is determined, taking into account for the first time neutrino refractive effects in the early universe. It is shown that an independent determination of either the neutrino MDMs or the primeval magnetic field strength would set powerful bounds on the other quantity.

Fukugita, Masataka↗
NASA NTRS1988

Infrared band extinctions and complex refractive indices of crystalline C2H2 and C4H2

Thermal IR absorption intensities are obtained for thin films of crystalline C2H2 and C4H2 at 70 K, and their n and k complex refractive indices are ascertained by separating true film absorption from interface reflection on the basis of an analysis of the transmission spectrum ratio for two sample thicknesses. This method significantly simplifies the n and k iteration process. The n and k values determined in the laboratory will in most cases reproduce a given sample thickness' observed transmission to within + or - 5 percent.

Khanna, R. K.↗
NASA NTRS1989

The role of refractive interstellar scintillation in the low frequency variability of extragalactic radio sources

A statistical analysis is presented of the 408 MHz flux density time series of 39 extragalactic radio sources, to study the role of refractive interstellar scintillation in the decimeter wavelength variability of compact extragalactic radio sources. It is found that the product of the scintillation index and the variability timescale is systematically larger for sources at lower galactic latitudes. This variation is obscured by much larger random variations. Possible explanations of these results are discussed.

Spangler, S.↗
NASA NTRS1989

Evidence for self-refraction in a convergence zone: NPE (Nonlinear progressive wave equation) model results

The nonlinear progressive wave equation (NPE) model was developed by the Naval Ocean Research and Development Activity during 1982 to 1987 to study nonlinear effects in long range oceanic propagation of finite amplitude acoustic waves, including weak shocks. The NPE model was applied to propagation of a generic shock wave (initial condition provided by Sandia Division 1533) in a few illustrative environments. The following consequences of nonlinearity are seen by comparing linear and nonlinear NPE results: (1) a decrease in shock strength versus range (a well-known result of entropy increases at the shock front); (2) an increase in the convergence zone range; and (3) a vertical meandering of the energy path about the corresponding linear ray path. Items (2) and (3) are manifestations of self-refraction.

Mcdonald, B. Edward↗
NASA NTRS1990

Absorption intensities and complex refractive indices of crystalline HCN, HC3N, and C4N2 in the infrared region

IR absorption intensities are presented for thin crystalline films of HCN, HC3N, and C4N2, together with n and k complex refractive indices determined on the basis of an iterative program for the Kramers-Konig integral via a least-squares, point-by-point fitting of the experimental transmission data. It is established that the transmission spectra generated by means of these n and k values can reproduce the experimental transmission observation values to within + or - 2 percent.

Masterson, C. M.↗