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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1971

Investigation of line-of-sight propagation in dense atmosphere, phase 3, part 1

The investigation of microwave absorption in the 1 to 10 GHz frequency band by the Jovian atmosphere has continued, and an estimate of the strength of signal fading at these frequencies due to layers of turbulence in Jupiter's atmosphere is given. The microwave absorption due to gaseous ammonia is estimated both in terms of a power loss in dB/km, and in total power loss in dB for slant-path communication with a probe at altitudes down to pressures of several tens of atmospheres. The graphs indicate a frequency-squared scaling of the absorption, and appreciable losses at altitudes where the pressure is several atmospheres. An estimate of turbulence strength is given. This may turn out to be quite crude considering the absence of any relevant data. A planetary scaling law which appears to hold reasonably well for Earth to Venus, is extrapolated to Jupiter. No reasonable modifications of the estimate can alter the conclusion that direct-path fading is negligible for pressure regimes up to 20 atm.

Dewolf, D. A.↗
NASA NTRS1975

Thermo/structural design considerations to achieve the Large Space Telescope line-of-sight requirements

The Large Space Telescope (LST) which is scheduled for launch in 1982, is a long-life, precision-pointing, earth-orbiting satellite requiring a structural system that provides high dimensional stability, minimum thermal distortion, and minimum response to onboard dynamic environments (e.g., reaction wheels). The results of a detailed thermostructural finite element computer analysis show that the telescope structure, even though fabricated from a material with a zero coefficient of thermal expansion, must be isolated from the external structure by a three-point support (flex joints or spherical bearings will accomplish this). Other thermo/structural analysis of the metering structure showed that second-order deformations have a significant effect on the alignment of the primary and secondary mirrors.

Tenerelli, D. J.↗
NASA NTRS1977

The ratio of deuterium to hydrogen in interstellar space. III - The lines of sight to Zeta Puppis and Gamma Cassiopeiae

An extensive set of measurements of the ratio of deuterium to hydrogen in the interstellar medium has been undertaken. The general observing program is described along with the data reduction techniques used to separate the complex deuterium and hydrogen profiles, and the results of the measurements for two stars, Zeta Pup and Gamma Cas. For Gamma Cas, log N(D)/N(H) is found to be approximately -4.8, in agreement with previous measurements. For Zeta Pup, a complicated structure of absorbing clouds in the interstellar medium is inferred. The best value of log N(D)/N(H) is about -4.6 for each of two components. One component is probably more complex than can be demonstrated directly at the resolution employed, and log N(D)N(H) could be -4.0 or larger in this component.

Vidal-Madjar, A.↗
NASA NTRS1982

IUE observations of lines of sight with peculiar ultraviolet extinction

Low resolution IUE data were used to derive UV extinction curves for a group of stars known to have peculiar extinction parameters from ANS data. The resulting curves have a wide range of appearances. Although the ratio E(BUMP)/E(B-V) differs by a factor of three in the extreme cases, the wavelength of maximum absorption does not appear to change. No evidence for new fine structure in UV extinction was found. The structure near 62 micrometers in the existing mean extinction curves appears to be the result of luminosity mismatch errors. The new extinction curves have shapes that separate into two distinct classes; those associated with clear field extinction and those associated with extinction in dense nebular environments. The range of variation in the curves is so large, the common practice of ironing out the bump can produce enormous errors in the resultant UV energy distributions when E(B-V) 0.3.

Massa, D.↗
NASA NTRS1983

Description and overview of an instrument designed to measure line-of-sight delay due to water vapor

Eight dual channel microwave radiometers were constructed as a research and development effort for the Crustal Dynamics Project and the Deep Space Network. These instruments, known as water vapor radiometers, are primarily intended to demonstrate that the variable path delay imposed by atmospheric water vapor can be calibrated in microwave tracking and distance measuring systems but could also be used in other applications involving moist air meteorology and propagation studies. They are being deployed to various stations and observatories that participate in Very Long Baseline Interferometry experiments. The development history of these instruments are reviewed, the theory of operation and overall design considerations are outlined, and the instrumental parameters and performance characteristics are described.

Resch, G. M.↗
NASA NTRS1984

Effects of line-of-sight velocity on spaced-antenna measurements, part 3.5A

Horizontal wind velocities in the upper atmosphere, particularly the mesosphere, have been measured using a multitude of different techniques. Most techniques are based on stated or unstated assumptions about the wind field that may or may not be true. Some problems with the spaced antenna drifts (SAD) technique that usually appear to be overlooked are investigated. These problems are not unique to the SAD technique; very similar considerations apply to measurement of horizontal wind using multiple-beam Doppler radars as well. Simply stated, the SAD technique relies on scattering from multiple scatterers within an antenna beam of fairly large beam width. The combination of signals with random phase gives rise to an interference pattern on the ground. This pattern will drift across the ground with a velocity twice that of the ionospheric irregularities from which the radar signals are scattered. By using spaced receivers and measuring time delays of the signal fading in different antennas, it is possible to estimate the horizontal drift velocities.

Royrvik, O.↗
NASA NTRS1987

Vibration isolation for line of sight performance improvement

Diagrams of the Reaction Wheel Assembly (RWA) are presented along with charts and graphs illustrating jitter error model, induced vibration tests, radial displacement transfer function, and axial displacement power spectra density. The RWA isolator specification requirements are listed.

Rodden, J. J.↗
NASA NTRS1986

High-velocity interstellar gas in the line of sight to the Wolf-Rayet star HD 50896

The large shell of interstellar gas (IG) discovered toward HD 50896 by Heckathorn and Fesen (1984) is characterized on the basis of high-dispersion IUE SWP and LWR spectra of 19 objects located within 4 deg of HD 50896 (but outside the optical ring nebula S308) at distances 0.6-2.9 kpc (compared to 1.5 kpc for HD 50896). The IG is found to have two components (at velocities -80 and -125 km/s), diameter 90 pc or greater, and distance 1.0 + or - 0.2 kpc, demonstrating that it is not related to HD 50896 and suggesting that it is a highly evolved supernova remnant associated with cluster Cr 121.

Nichols-Bohlin, J.↗
NASA NTRS1986

Line-of-sight performance improvement with reaction-wheel isolation

Design techniques used to isolate and damp vibrations from the reaction control wheels assembly (RCA) from the structural dynamics of the Hubble Space Telescope are described. The four RCAs can generate sufficient torque for large maneuvers and precision control during fine pointing. The RCAs are to operate continuously during observations and thereby pose the potential of inducing jitter. The image stability must be constrained to less than 0.007 arcsec rms per 24 hr and pointing accuracy must be 0.01 arcsec for 100 hr. Vibration-producing characteristics of the RCAs are identified, and test results are provided from measurements of the magnitudes of the associated disturbances. Design features and test results for the vibration isolators employed with the RCAs to effect viscous damping of RCA axial perturbations are presented. The dampers are expected to reduce jitter effects of RCA operation by a half.

Rodden, J. J.↗
NASA NTRS1987

Design of robust line-of-sight pointing control system for the SCOLE configuration

Robust compensator design for attitude control of the Spacecraft Control Laboratory Experiment (SCOLE) configuration is considered. A loop-shaping procedure similar to that used in the LQG/LTR method is used to iteratively design the compensator. A satisfactory compensator is obtained by including the rigid modes and three elastic modes in the design model.

Joshi, S. M.↗