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NASA NTRS1983

Atmospheric storm explanation of saturnian electrostatic discharges

Saturn electrostatic discharges (SED) monitored by the Voyager 1 were investigated to determine the source of the phenomena. Consideration has been given to two sources: the atmosphere at equatorial latitudes, where the cloud-top wind velocities correspond to the Saturn 10 hr 10 min rotation period; and the rings at 1.8 Saturn radius. The data were analyzed in terms of time and frequency, revealing a time-varying frequency, few detectable discharges outside of a low threshold, and the appearances and disappearances of the SED with no correlation with frequency. The periodicity of the SED episodes indicated that the source was occulted between revolutions, which ruled out the ring source. The SED signals were only detected on the dayside, suggesting the signals propagated through the dayside ionosphere. Diurnal variations in the ion densities could prohibit the signals from escaping on the nightside, a factor supported by detection of low frequency SED only during close passage of the Voyager. Ray tracing experiments have demonstrated that storm sources have emissions observable with the storm on the limb at the observed 30-40 MHz interval.

Kaiser, M. L.↗

NASA NTRS1983

Light scattering by randomly oriented cubes and parallelepipeds

In this paper, the geometric ray tracing theory for the scattering of light by hexagonal cylinders to cubes and parallelepipeds has been modified. Effects of the real and imaginary parts of the refractive index and aspect ratio of the particle on the scattering phase function and the degree of linear polarization are investigated. Causes of the physical features in the scattering polarization patterns are identified in terms of the scattering contribution due to geometric reflections and refractions. The single-scattering phase function and polarization data presented in this paper should be of some use for the interpretation of observed scattering and polarization data from planetary atmospheres and for the physical understanding of the transfer of radiation in an atmosphere containing nonspherical particles.

Liou, K. N.↗

NASA NTRS1985

A General Optical Systems Evaluation Program

General Optical Systems Evaluation Program, GENOPTICS, aids analysis and evaluation of optical systems that employ lenses, mirrors, diffraction gratings and other geometrical surfaces. GENOPTICS uses geometrical ray tracing based on Snell's law.

Wilson, M.↗

NASA NTRS1983

Deformable subreflector computed by geometric optics

Using a Cassegrainian geometry, the 64-meter antenna with its distorted paraboloidal reflecting surface is forced to produce a uniform phase wavefront by a pathlength-compensating subreflector. First, the computed distortion vectors at the joints or nodes of the main reflector structure supporting the surface panels are best fitted to a paraboloid. Second, the resulting residual distortion errors are used to determine a compensating subreflector surface by ray tracing using geometric optics principles. Third, the totally corrected subreflector surface is defined by the normal directions and distances to the surface of the original symmetric hyperboloid for the purpose of evaluation. Finally, contour maps of distortions of the paraboloid reflector and the compensating subreflector are presented. A field-measured check of the subreflector in focused position as computed by the described methodology is also presented for the antenna position at horizon look with the geometry at 45 degrees elevation.

Katow, M. S.↗

NASA NTRS1984

Luneburg lens and optical matrix algebra research

Planar, as opposed to channelized, integrated optical circuits (IOCs) were stressed as the basis for computational devices. Both fully-parallel and systolic architectures are considered and the tradeoffs between the two device types are discussed. The Kalman filter approach is a most important computational method for many NASA problems. This approach to deriving a best-fit estimate for the state vector describing a large system leads to matrix sizes which are beyond the predicted capacities of planar IOCs. This problem is overcome by matrix partitioning, and several architectures for accomplishing this are described. The Luneburg lens work has involved development of lens design techniques, design of mask arrangements for producing lenses of desired shape, investigation of optical and chemical properties of arsenic trisulfide films, deposition of lenses both by thermal evaporation and by RF sputtering, optical testing of these lenses, modification of lens properties through ultraviolet irradiation, and comparison of measured lens properties with those expected from ray trace analyses.

Wood, V. E.↗

NASA NTRS1984

CYBER 200 Applications Seminar

Applications suited for the CYBER 200 digital computer are discussed. Various areas of application including meteorology, algorithms, fluid dynamics, monte carlo methods, petroleum, electronic circuit simulation, biochemistry, lattice gauge theory, economics and ray tracing are discussed.

Gary, J. P.↗

NASA NTRS1984

Reflectance Modeling

A classical description of the one dimensional radiative transfer treatment of vegetation canopies was completed and the results were tested against measured prairie (blue grama) and agricultural canopies (soybean). Phase functions are calculated in terms of directly measurable biophysical characteristics of the canopy medium. While the phase functions tend to exhibit backscattering anisotropy, their exact behavior is somewhat more complex and wavelength dependent. A Monte Carlo model was developed that treats soil surfaces with large periodic variations in three dimensions. A photon-ray tracing technology is used. Currently, the rough soil surface is described by analytic functions and appropriate geometric calculations performed. A bidirectional reflectance distribution function is calculated and, hence, available for other atmospheric or canopy reflectance models as a lower boundary condition. This technique is used together with an adding model to calculate several cases where Lambertian leaves possessing anisotropic leaf angle distributions yield non-Lambertian reflectance; similar behavior is exhibited for simulated soil surfaces.

Smith, J. A.↗

NASA NTRS1984

Tropospheric Delay Effects in Radio Interferometry

A new tropospheric mapping function is derived which is more accurate than previous mapping functions above elevations of 4 degrees. The error due to the given analytic approximation is estimated to be less than 0.02% for elevation angles larger than 6 degrees, (less than 0.4 cm at 6 degrees, and approximately 0.004% or 0.03 cm at 20 degrees). The mathematical expansion used in the derivation is valid for any laterally homogeneous atmospheric model of refractivity. The new mapping function, computer generated ray tracing tables and other mapping functions are compared. The results can be used in correcting for tropospheric delays of radio signals.

Lanyi, G.↗

NASA NTRS1984

The correction of aberrations computed in the aperture plane of multifrequency microwave radiometer antennas

An analytical/numerical approach to identifying and correcting the aberrations introduced by a general displacement of the feed from the focal point of a single offset paraboloid antenna used in deployable radiometer systems is developed. A 15 meter reflector with 18 meter focal length is assumed for the analysis, which considers far field radiation pattern quality, focal region fields, and aberrations appearing in the aperture plane. The latter are obtained by ray tracing in the transmit mode and are expressed in terms of optical notation. Attention is given to the physical restraints imposed on corrective elements by real microwave systems and to the intermediate near field aspects of the problem in three dimensions. The subject of wave fronts and caustics in the receive mode is introduced for comparative purposes. Several specific examples are given for aberration reduction at eight beamwidths of scan at a frequency of 1.414 GHz.

Schmidt, R. F.↗

NASA NTRS1983

Lightning measurements from the Pioneer Venus Orbiter

The plasma wave instrument on the Pioneer Venus Orbiter frequently detects strong and impulsive low-frequency signals when the spacecraft traverses the nightside ionosphere near periapsis. These particular noise bursts appear only when the local magnetic field is strong and steady and when the field is oriented to point down to the ionosphere thus; the signals have all characteristics of lightning whistlers. We have tried to identify lightning sources between the cloud layers and the planet itself by tracing rays along the B-field from the Orbiter down toward the surface. An extensive data set, consisting of measurements through Orbit 1185, strongly indicates a clustering of lightning sources near the Beta and Phoebe Regios, with an additional significant cluster near the Atla Regio at the eastern edge of Aphrodite Terra. These results suggest that there are localized lightning sources at or near the planetary surface.

Scarf, F. L.↗

NASA NTRS1984

Correlation of auroral hiss and upward electron beams near the polar cusp

Data were obtained from the DE-1 high-altitude plasma instrument (HAPI) and plasma wave instrument (PWI) during outbound passes through the polar cusp near local noon. The observed distribution functions of electron beams are fitted by drifting Maxwellian functions and the observed distribution functions of hot background electrons by isotropic Maxwellian functions. In addition, the cold plasma density is inferred from knowledge of the electron plasma frequency and the measured density of the warm plasma, including the electron beam distribution. The empirically fitted plasma parameters, including density, temperature and drifting energy, are used to solve the linear dispersion equation for the resulting whistler mode emissions. Because the whistler mode becomes quasi-electrostatic for wave-normal angles near the resonance cone, the electrostatic approximation is used for the whistler mode dispersion relation. The results of wave instability analyses are then compared with the wave observations. A ray tracing of cusp hiss emission is conducted to locate the wave source region (at about one earth-radius).

Lin, C. S.↗

NASA NTRS1984

Parametric instabilities during electron cyclotron heating of tandem mirrors

Electron cyclotron resonance heating is one of the most commonly used methods of heating electrons in the plugs and in the thermal barriers of tandem mirrors. The intense coherent electromagnetic waves used for such heating are susceptible to parametric decay into other modes. Significant growth rates are found for the decay of either ordinary or extraordinary waves into two magnetized electron plasma waves. This and related effects may result in electron heating mechanisms rather different than those assumed in linear ray-tracing calculations. These results may help explain the unusual effects observed during heating of the Phaedrus tandem mirror device. In the general case, these instabilities may be strongly inhibited by density gradients.

Nicholson, D. R.↗

NASA NTRS1984

The Io decametric emission cone

The Voyager 1 and 2 Planetary Radio Astronomy observations of Jovian decametric radiation (DAM) were transformed into a fixed Io-Jupiter coordinate system for Io longitudes every 10 deg apart starting from 200 to 260 deg (seven plots per spacecaft). An analysis of the data in this coordinate system has shown that the Io-dependent DAM emissions are generated in hollow cones at all frequencies over the fixed Io longitudes studied. The edges of the emission cones are, in many cases, broad in longitude (30 deg or more), with distinct differences found between the characteristics of DAM emission at the edges of the emission cones. The shapes of the DAM emission cones are explained by using three-dimensional ray-tracing calculations within a model Jovian atmosphere.

Green, J. L.↗

NASA NTRS1983

Optical, thermal, and electrical performance of low-CR solar arrays

This paper describes the analysis and testing of a photovoltaic low-CR concentrator shaped like a truncated pyramid with an aperture of 0.5 m on a side and a geometric concentration ratio of six. The truncated base plane is covered by either silicon (Si) or gallium arsenide (GaAs) solar cells. Ray-trace analysis of the concentrator predicts a peak optical efficiency of 0.77, which falls off only gradually with pointing error. A coupled thermal-electrical analysis of the system shows that the moderately nonuniform illumination produced by the concentrator does not result in significant mismatch losses, provided the solar cells are connected in parallel groups. The results of ground tests involving a full-scale prototype concentrator conform well with theoretical predictions.

French, E. P.↗

NASA NTRS1984

Severe convective storm detection based on satellite infrared imagery analysis

Several cases of GOES digital infrared data and Doppler Sounder array data during the three-hour time period immediately preceding the touchdown of the tornado were analyzed. Tornado-associated clouds are compared with non-tornado-associated clouds using satellite infrared data, ray tracing of gravity waves detected by the Doppler Sounder array and rawinsonde data. The satellite observations are at 15-minute intervals. Our study shows that tornado-associated clouds are always accompanied by overshooting turrets penetrating above the tropopause. The growth rate of the overshooting turret above the tropopause for severe storm-associated clouds is much greater than that for non-severe storm-associated clouds.

Hung, R. J.↗

NASA NTRS1984

Effect of parallel refraction on magnetospheric upper hybrid waves

Large amplitude (not less than 10 mV/m) electrostatic plasma waves near the upper hybrid (UH) frequency have been observed from 0 to 50 deg magnetic latitude (MLAT) during satellite plasma-pause crossings. A three-dimensional numerical ray-tracing calculation, based on an electron distribution measured during a GEOS 1 dayside intense upper-hybrid wave event, suggests how UH waves might achieve such large amplitudes away from the geomagnetic equator. Refractive effects largely control the wave amplification and, in particular, the unavoidable refraction due to parallel geomagnetic field gradients restricts growth to levels below those observed. However, a cold electron density gradient parallel to the field can lead to upper hybrid wave growth that can account for the observed emission levels.

Engel, J.↗

NASA NTRS1984

Optical analysis of the star-tracker telescope for Gravity Probe

A ray tracing modeling of the star tracker telescope for Gravity Probe was used to predict the character of the output signal and its sensitivity to fabrication errors. In particular, the impact of the optical subsystem on the requirement of 1 milliarc second signal linearity over a + or - 50 milliarc second range was examined. Photomultiplier and solid state detector options were considered. Recommendations are made.

Zissa, D. E.↗

NASA NTRS1985

Long range downwind propagation of low-frequency sound

The propagation of low-frequency noise outdoors was studied using as the source a large (80-m diameter) 4-megawatt horizontal axis wind turbine. Acoustic measurements were made with low-frequency microphone systems placed on the ground at five downwind sites ranging from 300 m to 10,000 m (6.3 mile) away from the wind turbine. The wind turbine fundamental was 1 Hz and the wind speed was generally 12 - 15 m/s at the hub height (80 m). The harmonic levels, when plotted versus propagation distance, exhibit a 3 dB per doubling of distance divergence. Two plausible explanations identified for this cylindrical spreading behavior were propagation of the low frequency wind turbine noise via a surface wave and downwind refraction. Surface was amplitude predictions were found to be more than 20 dB smaller than the measured levels. Ray-tracing results were used to qualitatively explain measured trends. A normal mode approach was identified as a candidate method for low-frequency acoustic refraction prediction.

Willshire, W. L., Jr.↗