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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

TM report

Key spectral, radiometric, and geometric characteristics of the TM sensor are examined. Spectrally, band 5 is slightly wider than specified and its upper edge at 50% of miximum of 1784 nm includes some unintended overlap with water absorption bands. Thermal band 6 is half the expected width. Radiometrically, the TM has extreme linearity and stability in comparison with previous MSS photomultiplier systems. Forward and reverse scans on TM show some unexplained radiometric differences of up to four digital levels. Primary focal plane bands show a monotomic decrease in gain of about 5% in the five months since launch. The relative internal gains of bands 5 and 7 varied with a period of about 60 days and an amplitude of about 6%. Radiometric calibration must be applied before histogram equalization algorithms to preserve radiometric accuracy. Geometrically, channel 4 in band 2 has a smaller modulation transfer function than desired. There is apparent misregistration of .75 pixels along scan and 0.2 pixels across scan.

Barker, J. L.↗
NASA NTRS1983

GOES satellite time code dissemination

The GOES time code system, the performance achieved to date, and some potential improvements in the future are discussed. The disseminated time code is originated from a triply redundant set of atomic standards, time code generators and related equipment maintained by NBS at NOAA's Wallops Island, VA satellite control facility. It is relayed by two GOES satellites located at 75 W and 135 W longitude on a continuous basis to users within North and South America (with overlapping coverage) and well out into the Atlantic and Pacific ocean areas. Downlink frequencies are near 468 MHz. The signals from both satellites are monitored and controlled from the NBS labs at Boulder, CO with additional monitoring input from geographically separated receivers in Washington, D.C. and Hawaii. Performance experience with the received time codes for periods ranging from several years to one day is discussed. Results are also presented for simultaneous, common-view reception by co-located receivers and by receivers separated by several thousand kilometers.

Beehler, R. E.↗
NASA NTRS1981

Imaging radar observations of volcanic features in Medicine Lake Highland, California

Synthetic aperture L-band radar images of Medicine Lake Highland, California, as obtained from the JPL-NASA aircraft and Seasat orbital systems, are presented. Image interpretation is based on two types of information: slope and topographic effects for geomorphic information, and reflectivity or backscatter in flat terrain which is related to surface roughness and to the surface dielectric constant. Cinder cones and lava tubes are visible (geomorphic features), and three types of lava surfaces are: aa, pahoehoe, and block. In addition, to infer lava flow relative ages, overlapping flow fronts and roughness attenuation observations may be used. It is hoped that this research will improve the understanding of the radar signature of volcanic areas, and will be helpful in interpreting data obtained from spaceborne sensors over the earth and, in particular, Venus.

Farr, T. G.↗
NASA NTRS1982

Active region magnetic fields inferred from simultaneous VLA microwave maps, X-ray spectroheliograms, and magnetograms

The VLA maps at 6-cm wavelength discussed here are generated from observations of a solar active region (NOAA 2363) on March 29 and 30, 1980. The X-ray spectroheliograms were acquired for this region in the lines of O VIII, Ne IX, Mg XI, Si XIII, S XV, and Fe XXV. It is found that the peaks of X-ray and 6-cm emission do not coincide but that the sources in the two wavelength domains tend to overlap. These facts are seen as evidence for the existence of opacity mechanisms other than thermal bremsstrahlung. To quantify this assertion, differential emission measures are computed to derive densities and temperatures. Using these and calculated force-free magnetic fields from Kitt Peak magnetograms, an assessment is made of the mechanism of gyroresonance absorption at low harmonics of the electron gyrofrequency as the source of opacity responsible for the microwave features. It is concluded that large-scale currents must be present in the active region loops to account for the bright 6-cm sources far from sunspots.

Schmahl, E. J.↗
NASA NTRS1982

Heat transport in porous cometary nuclei

Heat transport in cometary nuclei is important because it leads to the formation of larger tails and comae after perihelia than before, to early outbursts caused by phase transitions, and to fragmentation. The role of gases in pores in icy (H2O and H2O + CO2) nuclei in the heat transport has been investigated in the viscous and Knudsen diffusion regions. It appears that in pure H2O-ices the heat flow is affected by the presence of pores but is affected only very close to the perihelion by water vapor contained in them. On the other hand, the presence of CO2 vapor in the pores significantly increases the heat flow in mixed nuclei at temperatures above 140 - 150 K. Thus for heliocentric distances between 3 and 5 AU, the above mentioned phenomena should be considerably enhanced in proportion to the admixture of CO2 ices. The overlap and channel formation by pores in nuclei with high porosities is discussed.

Smoluchowski, R.↗
NASA NTRS1983

The origins of Birkeland currents

Fundamental observations and zero-order theories for Birkeland currents are employed, together with recent observational data, to formulate a first-order model for the currents. Five categories of Birkeland currents are detailed, including region 1 current sheets which follow the auroral oval, region 2 currents with opposite polarity forming on the equatorward side, cusp currents poleward of region 1, currents appearing after extended periods of the interplanetary magnetic field, and a three-layer overlap of current sheets in the Harang discontinuity. The region 1 currents near noon are found to be driven by the interplanetary electric field on open field lines, while away from noon the inner edge of the plasma sheet is the source. Charge separation drives the region 2 currents, and neutralization is effected in the ionosphere. The magnetopause is concluded to be an insignificant factor for the Birkeland currents. Convection reversal is predicted to occur in the region 1 interior. Finally, four lobed polar electric fields are considered to be generated by the boundary layer dynamo, and indicate a closed magnetospheric configuration.

Stern, D. P.↗
NASA NTRS1982

Geologic observations of the northern boundary of the Caribbean plate across central America as seen by Seasat and SIR-A

The radar data analyzed here extend from the Amatique Bay (Golfo de Honduras) in the northeast to the Pacific Ocean (Puenta Remedios) in the southwest. Space Shuttle Imaging Radar (SIR-A) data-take 18 overlaps the principal part of the Seasat mosaic. SIR-A data make possible more observations over the Central American Cordillera, where strong layover limited the amount of information obtained by Seasat. The radar coverage delineates the principal strike-slip faults of the region (Cuilco-Chixoy-Polochic, Motagua and Jocotan), which have acted as the Caribbean-Americas plate boundary. It also demarcates volcanic terranes related to subduction of the Cocos Plate under the Caribbean Plate. Within pumice fields of the Tertiary volcanic belt, the use of two Seasat look directions (rev 759 and rev 1211), in conjunction with SIR-A data, makes possible some rock discrimination.

Rebillard, P.↗
NASA NTRS1983

Errors in scatterometer-radiometer wind measurement due to rain

The behavior of radiometer corrections for the scatterometer is investigated by simulating simple situations using footprint sizes comparable with those used in the SEASAT-1 experiment and also actual footprints and rain rates from a hurricane observed by the SEASAT-1 system. The effects on correction due to attenuation and wind speed gradients are examined independently and jointly. It is shown that the error in the wind-speed estimate can be as large as 200% at higher wind speeds. The worst error occurs when the scatterometer footprint overlaps two or more radiometer footprints and the attenuation in the scatterometer footprint differs greatly from those in parts of the radiometer footprints. This problem could be overcome by using a true radiometer-scatterometer system having identical coincident footprints comparable in size with typical rain cells.

Moore, R. K.↗
NASA NTRS1983

A new AM Her-like X-ray source

The discovery of a new X ray source associated with a binary and classifiable as a member of the AM Her class of objects is reported. The object was located with a detector on the HEAO A-2 spacecraft and by the imaging proportional counter on the Einstein Observatory. Detection occurred in the 0.15-0.4 keV band, and intensity variations of a factor of 2 were observed to happen on a time scale of around 2 hr. A location error box 0.9 deg on a side was constructed from the 2.95 deg overlapping FWHM field of view of the HEAO detector. The object, designated HO139-68, was reexamined using photometric and spectroscopic scans of it and the companion, which has been observed to be an ordinary late type star. Evidence for the presence of a magnetic white dwarf in the binary is discussed, noting indications of cyclotron emission. The X ray source flux is estimated to be 2.3/10 billion erg per sq cm per sec.

Agrawal, P. C.↗
NASA NTRS1983

Communications and tracking - Light and IR will help carry high traffic

The space station currently under consideration by NASA must simultaneously track and communicate with the many vehicles and objects surrounding it in orbit. While the Space Shuttle has 23 antennas, more than 50 will be required by the space station. In addition to Shuttle-compatible equipment at P, L, C, S, and Ku bands, the station system will probably incorporate Ka, W, IR and optical frequency equipment for tracking and communications. A major design challenge is foreseen in the placement of separate antennas, lenses and reflectors over the station's external geometry in order to give both the overlapping fields of view required for spherical coverage and the radiation of unambiguous navigation guide beams and markers. Adaptive distributed element arrays are under consideration. Another approach to spherical coverage involves the use of omnidirectional antennas which both transmit and receive RF energy over a wide range of angles.

Dickinson, R. M.↗
NASA NTRS1983

The evolution of Tollmien-Schlichting waves near a leading edge

The method of matched asymptotic expansions is used to study the generation of Tollmien-Schlichting waves by free-stream disturbances incident on a flat-plate boundary layer. Near the leading edge, the motion is governed by the unsteady boundary-layer equation, while farther downstream it is governed (to lowest order) by the Orr-Sommerfeld equation with slowly varying coefficients. It is shown that there is an overlap domain where the Tollmien-Schlichting wave solutions to the Orr-Sommerfeld equation and appropriate asymptotic solutions of the unsteady boundary-layer equation match, in the matched-asymptotic-expansion sense. The analysis explains how long-wavelength free-stream disturbances can generate Tollmien-Schlichting waves of much shorter wavelength. It also leads to a set of scaling laws for the asymptotic structure of the unsteady boundary layer.

Goldstein, M. E.↗
NASA NTRS1982

Electron impact excitation of SO2 - Differential, integral, and momentum transfer cross sections

Electron impact excitation of the electronic states of SO2 was investigated. Differential, integral, and inelastic momentum transfer cross sections were obtained by normalizing the relative measurements to the elastic cross sections. The cross sections are given for seven spectral ranges of the energy-loss spectra extending from the lowest electronic state to near the first ionization limit. Most of the regions represent the overlap of several electronic transitions. No measurements for these cross sections have been reported previously.

Vuskovic, L.↗
NASA NTRS1983

Analysis of optical imagery for Seyfert's Sextet and VV 172

Seyfert's Sextet and VV 172, 5-m photographs have been subjected to image processing to yield field-galaxy density analysis, redshift-scaled imagery, interaction morphology display and enhancement, color difference imagery,modeling of the VV 172 halo, and image texture analysis of the spiral galaxy components of Seyfert's Sextet. An effort is made to evaluate the evidence for physical association of the discordant redshift components of these groups. An especially noteworthy characteristic of the groups is their extended luminous halos. The halo of VV 172 cannot be explained by the overlapping envelopes of galaxies with normal luminosity profiles, and the high redshift spiral galaxy in the Sextet is found to have an asymmetric internal structure and associated filament which suggest gravitational perturbation by the other members of the group.

Sulentic, J. W.↗
NASA NTRS1983

Combining techniques to enhance protection against high sustained accelerative forces

Five volunteer subjects were tested for acceleration tolerance under eight different experimental conditions representing relaxed and unprotected tolerance and tolerance with all possible combinations of the anti-G suit, and M-1 maneuver, and supination in a PALE seat. The individual and combined effects of the various acceleration protective techniques were examined as they related to various models for acceleration protection, and the data revealed no statistically significant deviations from a simple additive model. The apparent net additivity was interpreted as resulting from a combination of additive, synergistic, and overlapping mechanisms.

Cohen, M. M.↗
NASA NTRS1983

On the nature of the bonding in Cu2 - An ab initio viewpoint

The bonding in Cu2 has been calculatively investigated. A CASSCF calculation is performed where the two 4s electrons are distributed in all possible ways among the 4s and 4p orbitals. The CAS results have a 3d population of 9.99. The corresponding orbitals between the CAS core orbitals at r(e) and the atomic SCF core orbitals show an overlap of 13.9992 out of a possible 14, with all of the change in sigma space. Thus, the CAS results show virtually no change in the d orbitals relative to the atom and can be considered as showing only a single bond. The reduction in 3d population, the decrease in r(e), and the increase in D(e) upon correlating the 3d shell indicates that the 3d electrons are slightly involved in the bonding, but there is no indication of the formation of a triple bond.

Bauschlicher, C. W., Jr.↗
NASA NTRS1983

Three-dimensional algebraic grid generation

One of the major approaches to numerical grid generation is the explicit algebraic expression of a physical grid as a function of a uniform grid in a rectangular computational coordinate system. The algebraic methods are based on mathematical interpolation, and the primary advantages are speed and directness. The relation between interpolation and grid generation is described. For three-dimensional grid generation, transfinite interpolation using the coordinate control processes developed in the multisurface method and two-boundary technique are advocated. Grid singularities encountered in three dimensions are discussed, and the exploration of multiple overlapping grids is proposed. Some aspects of interactive algebraic grid computation in three dimensions are discussed.

Smith, R. E.↗
NASA NTRS1983

X-ray imaging of extended objects using nonoverlapping redundant array

A common problem associated with X-ray imaging using coded apertures is the reconstruction of low-intensity extended objects. In the decoding of such objects, the overlapping images from the multiple pinholes give rise to noise cross talk and, in many cases, also to signal cross talk. In this paper, an alternate approach is proposed based on the principle used in an earlier (Yin et al., 1980) laboratory device for the real-time viewing of X-ray objects. It is shown that with this approach, the nonoverlapping redundant array, the sidelobes in the point spread function are not eliminated but merely displaced through a suitable choice of geometry. In this manner, the sidelobes no longer contribute to the background in the vicinity of a reconstructed image, and both signal and noise cross talks are completely eliminated. It may now be possible to reconstruct extended X-ray objects in 3-D by simple optical correlation and tomographically by a computer.

Yin, L.-I.↗
NASA NTRS1982

Advanced 30/20 GHz multiple beam antenna for future communications satellites

A multiple offset reflector system is being used to provide multiple fixed and scanning beams at 30 GHz by means of a polarization grid diplexer to separate two orthogonally polarized feed assemblies. Each feed assembly consists of multiple fixed and scanning beam feeds. The technology developments required for successful operation of this proposed system are presented. These technologies include: (1) an offset Cassegrain system for low sidelobe wide angle scan beams; (2) a diplexed beam forming network for overlapping low sidelobe beams; (3) a fast switching network for scanning beams; and (4) fabrication of precision feed components and large offset reflectors.

Chen, C. C.↗