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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Remote sensing with spaceborne synthetic aperture imaging radars: A review

A review is given of remote sensing with Spaceborne Synthetic Aperture Radars (SAR's). In 1978, a spaceborne SA was flown on the SEASAT satellite. It acquired high resulution images over many regions in North America and the North Pacific. The acquired data clearly demonstrate the capability of spaceborne SARs to: image and track polar ice floes; image ocean surface patterns including swells, internal waves, current boundaries, weather boundaries and vessels; and image land features which are used to acquire information about the surface geology and land cover. In 1981, another SAR was flown on the second shuttle flight. This Shuttle Imaging Radar (SIR-A) acquired land and ocean images over many areas around the world. The emphasis of the SIR-A experiment was mainly toward geologic mapping. Some of the key results of the SIR-A experiment are given.

Cimino, J. B.↗
NASA NTRS1983

Use of coregistered radar, visible and IR images for geologic remote sensing

The Shuttle Imaging Radar (SIR-A) obtained images over the southern portion of the San Rafael Swell in eastern Utah. SEASAT SAR and LANDSAT MSS images and thermal inertia data from the Heat Capacity Mapping Mission (HCMM) were correlated with the SIR-A data. Radar images obtained with different incidence angles and different illumination directions were compared with images obtained in other portions of the spectrum for geologic remote sensing.

Evans, D. L.↗
NASA NTRS1983

Composite structural materials

Progress and plans are reported for investigations of: (1) the mechanical properties of high performance carbon fibers; (2) fatigue in composite materials; (3) moisture and temperature effects on the mechanical properties of graphite-epoxy laminates; (4) the theory of inhomogeneous swelling in epoxy resin; (5) numerical studies of the micromechanics of composite fracture; (6) free edge failures of composite laminates; (7) analysis of unbalanced laminates; (8) compact lug design; (9) quantification of Saint-Venant's principles for a general prismatic member; (10) variation of resin properties through the thickness of cured samples; and (11) the wing fuselage ensemble of the RP-1 and RP-2 sailplanes.

Ansell, G. S.↗
NASA NTRS1984

Fuel property effects on Navy aircraft fuel systems

Problems of ensuring compatibility of Navy aircraft with fuels that may be different than the fuels for which the equipment was designed and qualified are discussed. To avoid expensive requalification of all the engines and airframe fuel systems, methodologies to qualify future fuels by using bench-scale and component testing are being sought. Fuel blends with increasing JP5-type aromatic concentration were seen to produce less volume swell than an equivalent aromatic concentration in the reference fuel. Futhermore, blends with naphthenes, decalin, tetralin, and naphthalenes do not deviate significantly from the correlation line of aromatic blends, Similar results are found with tensile strenth and elongation. Other elastomers, sealants, and adhesives are also being tested.

Moses, C. A.↗
NASA NTRS1983

VTOL shipboard letdown guidance system analysis

Alternative letdown guidance strategies are examined for landing of a VTOL aircraft onboard a small aviation ship under adverse environmental conditions. Off line computer simulation of shipboard landing task is utilized for assessing the relative merits of the proposed guidance schemes. The touchdown performance of a nominal constant rate of descent (CROD) letdown strategy serves as a benchmark for ranking the performance of the alternative letdown schemes. Analysis of ship motion time histories indicates the existence of an alternating sequence of quiescent and rough motions called lulls and swells. A real time algorithms lull/swell classification based upon ship motion pattern features is developed. The classification algorithm is used to command a go/no go signal to indicate the initiation and termination of an acceptable landing window. Simulation results show that such a go/no go pattern based letdown guidance strategy improves touchdown performance.

Phatak, A. V.↗
NASA NTRS1984

Hygrothermal effects on mechanical behavior of graphite/epoxy laminates beyond initial failure

An investigation was conducted to determine the critical load levels and associated cracking beyond which a multidirectional laminate can be considered as structurally failed. Graphite/epoxy laminates were loaded to different strain levels up to ultimate failure. Transverse matrix cracking was monitored by acoustic and optical methods. Residual stiffness and strength that were parallel and perpendicular to the cracks were determined and related to the environmental/loading history. Results indicate that cracking density in the transverse layers has no major effect on laminate residual properties as long as the angle ply layers retain their structural integrity. Exposure to hot water revealed that cracking had only a small effect on absorption and reduced swelling when these specimens were compared with uncracked specimens. Cracked, moist specimens showed a moderate reduction in strength when compared with their uncracked counterparts. Within the range of environmental/loading conditions of the present study, it is concluded that the transverse cracking process is not crucial in its effect on the structural performance of multidirectional composite laminates.

Ishai, O.↗
NASA NTRS1984

Investigation and Rehabilitation to Extend Service Life of DSS-13 Antenna Concrete Foundation

An investigation to establish the cause and, devise a repair technique to maintain the serviceability of the DSS-13 26 meter antenna is described. Core samples are obtained from the concrete and various laboratory tests conducted. In-place nondestructive type tests are also performed. The tests established that the concrete is deteriorating because of alkali aggregate reactivity. This is a phenomenon wherein certain siliceous constituents present in some aggregates react with alkalies in the portland cement to produce a silica gel which, in turn, imbibes water, swells, and cracks the concrete. The scheme consists of a supplemental steel frame friction pile anchored grade beam encircling the existing foundation. This system provides adequate bracing against base shear and overturning due to seismic loading. Larger cracks are sealed using a pressure injected two-component epoxy.

Riewe, A. A., Jr.↗
NASA NTRS1984

Remote sensing of the marginal ice zone during Marginal Ice Zone Experiment (MIZEX) 83

The remote sensing techniques utilized in the Marginal Ice Zone Experiment (MIZEX) to study the physical characteristics and geophysical processes of the Fram Strait Region of the Greenland Sea are described. The studies, which utilized satellites, aircraft, helicopters, and ship and ground-based remote sensors, focused on the use of microwave remote sensors. Results indicate that remote sensors can provide marginal ice zone characteristics which include ice edge and ice boundary locations, ice types and concentration, ice deformation, ice kinematics, gravity waves and swell (in the water and the ice), location of internal wave fields, location of eddies and current boundaries, surface currents and sea surface winds.

Shuchman, R. A.↗
NASA NTRS1984

Experimental and Theoretical Studies of Moisture and Temperature Effects on the Mechanical Properties of Graphite/Epoxy Laminates and Neat Resins

The properties of high performnce composites which are strongly dependent on the physical properties of the matrix resin were studied. Moisture adversely affect the properties of both neat epoxy resin and epoxy matrix composites. Inhomogeneous swelling as to the moisture degradation of mechanical properties, both in the neat resin and the composite. It is postulated that the postcuring process can change structure/moisture interactions and partially alleviate its adverse effects. The study is directed toward are understanding of the physical/mechanical/thermodynamic aspects of this problem.

Sternstein, S. S.↗
NASA NTRS1984

The spatial evolution of t he directional wave spectrum in the Southern Ocean: Its relation to extreme waves in Agulhas Current

An experiment using the Shuttle Imaging Radar-B (SIR-B) to monitor certain properties of the ocean wave directional spectrum and to track the long swell systems as they propagate northward to encounter the Agulhas near the southeastern coast of Africa is discussed. The experiment is designed around the unique capability of SIR-B to overcome key limitations of the Seasat synthetic aperture radar data set, and to extend the existing Seasat results into new areas. Ocean wave systems will be tracked. The variable-incidence-angle capability to examine wave imaging quality will be utilized. Doppler current measurements will be attempted. An effort will be made to verify that the lower range-to-velocity ratio of SIR-B will lead to the improved response of azimuth-traveling wave systems.

Beal, R. C.↗
NASA NTRS1985

Nickel-cadmium cell life test

Over 6,9000 Low Earth Orbit cycles were accumulated at 30% Depth of Discharge on twelve INTELSAT-design nickel-hydrogen cells. Physical equipment and cells are described. Performance characteristics are seen to be uniform. Further testing is planned to seek a failure mode, and also to investigate the effects of a new additive for nickel-hydrogen cells. Initial results indicate improved performance at higher temperatures and diminished swelling of positive nickel plates.

Wheeler, J. R.↗
NASA NTRS1983

A case study of the energy dissipation of the gravity wave field based on satellite altimeter measurements

Wave breaking is proposed as the primary energy dissipation mechanism for the gravity wave field. The energy dissipation rate is calculated based on the statistical model proposed by Longuet-Higgins (1969) with a modification of the breaking criterion incorporating the surface stress according to Phillips and Banner (1974). From this modified model, an analytic expression is found for the wave attenuation rate and the half-life time of the wave field which depend only on the significant slope of the wave field and the ratio of friction velocity to initial wave phase velocity. These expressions explain why the freshly generated wave field does not last long, but why swells are capable of propagating long distances without substantial change in energy density. It is shown that breaking is many orders of magnitude more effective in dissipating wave energy than the molecular viscosity, if the significant slope is higher than 0.01. Limited observational data from satellite and laboratory are used to compare with the analytic results, and show good agreement.

Huang, N. E.↗
NASA NTRS1984

The reflection of airborne UV laser pulses from the ocean

It is experimentally shown here for the first time that the normalized laser backscatter cross-section of the sea surface is a function of elevation or height position on teh ocean wave. All data were taken off-nadir, resulting in incidence angles of about 6.5 deg measured relative to the normal to mean sea level (MSL). In the limited data sets analyzed to date, the normalized backscatter cross-section was found to be higher in wave crest regions and lower in wave troughs for a swell-dominated sea over which the wind speed was 5 m/s. The reverse was found to be the case for a sea that was driven by a 14 m/s wind. These isolated results show that the MSL, as measured by an off-nadir and/or multibeam type satellite laser altimeter, will be found above, at, or below the true MSL, depending on the local sea conditions existing in the footprint of the altimeter. Airborne nadir-pointed laser altimeter data for a wide variety of sea conditions are needed before a final determination can be made of the effect of sea state on the backscatter cross-section as measured by a down-looking satellite laser system.

Hoge, F. E.↗
NASA NTRS1985

Vortex breakdown simulation

A vortex breakdown was simulated by the vortex filament method, and detailed figures are presented based on the results. Deformations of the vortex filaments showed clear and large swelling at a particular axial station which implied the presence of a recirculation bubble at that station. The tendency for two breakdowns to occur experimentally was confirmed by the simulation, and the jet flow inside the bubble was well simulated. The particle paths spiralled with expansion, and the streamlines took spiral forms at the breakdown with expansion.

Nakamura, Y.↗
NASA NTRS1985

A numerical study of forced lithospheric thinning

Subsolidus lithospheric thinning by mantle plumes may be involved in the creation of swells, hotspots, and rifts. Among the major questions concerning this process are the timescale on which it occurs and the structure of the plumes. The lithosphere is known to have been substantially thinned in 10 Ma or less. Current studies are focused on the lithospheric thinning by time-dependent plumes hypothesized to have large temperature differences across them.

Schubert, G.↗
NASA NTRS1985

Waves across the ocean

The predictions of linear surface wave theory (S = 1/2A sq rt L and S = 1/2A squared P, where S is speed, A is a proportionality constant, L is wavelength, and P is period) are discussed in connection with the remote measurement of swell, i.e., long ocean waves. Particular attention is given to the Scripps experiments and to the APL experiment involving Seasat microwave remote sensing. It is noted that these measurements have yielded an exceptionally clear field verification of the theory, indicating the fidelity of ocean remote sensors.

Irvine, D. E.↗
NASA NTRS1985

Performance of 12Ah aerospace nickel-cadmium cells of design variable groups

The design variable program of NASA is a systematic approach to evaluate the performance of 12Ah aerospace nickel-cadmium cells of 9 important cell designs. These cells were life cycled in a Low-Earth Orbit (LEO) regime for 3 to 4 years. Representative cells taken from the design variable groups after different cycling periods have been examined. The results show that: (1) positive swelling and carbonate content in the electrolyte increases as a function of the number of cycles, (2) electrolyte distribution follows the order NEG greater than POS greater than SEP, 3) control and no PQ groups outperformed the rest of the groups and (4) the polypropylene group shows very heavy cadmium migration and poor performance.

Vasanth, K. L.↗
NASA NTRS1986

Spatial variations of ocean wave directional spectra from the Seasat synthetic aperture radar

Seasat synthetic aperture radar ocean wave spectra for a 900-km pass are analyzed and interpreted in the context of both their probable generation sources and their surface current and bathymetric modifiers. Systematic vector wavenumber variations of several times the standard error of determination (about 1.5 percent in magnitude and 0.9 deg in direction) occur along the entire 900-km pass. The large-scale spatial variation of a 200-m swell system can be accurately accounted for as a result of dispersion from a distant storm. The more local variations are qualitatively well correlated in position with known currents and bathymetry but show systematic biases that appear partly due to an environmentally dependent instrument transfer function in the regions of high current and highest sea state. There is also substantial evidence that a large angular deviation in the center of the pass is the result of a mesoscale eddy just to the east.

Beal, R. C.↗