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At least 1,081 records · Page 60

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NASA NTRS1976

Sensitivity limits of an infrared heterodyne spectrometer for astrophysical applications

A discussion and an evaluation of the degradation in sensitivity are given for a heterodyne spectrometer employing a HgCdTe photodiode mixer and tunable diode lasers. The minimum detectable source brightness is considered as a function of the mixer parameters, transmission coefficient of the beam splitter, and local oscillator emission powers. The degradation in the minimum detectable line source brightness that results from the bandwidth being a fraction of the line width is evaluated and plotted as a function of the wavelength and bandwidth for various temperature to mass ratios. It is shown that the minimum achievable degradation in the sensitivity of a practical astronomical heterodyne spectrometer is about 30. Estimates of SNRs with which IR line emission from astronomical sources of interest may be detected are given.

Abbas, M. M.↗

NASA NTRS1976

The effect of suprathermal protons on the physical conditions in Seyfert galaxy nuclei. II

Radiative transfer of Ly-alpha, Ly-beta, and H-alpha in a hydrogen gas containing dust is considered with application to the emitting gas in Seyfert galaxies. The line radiation originates from the kinetic energy of suprathermal particles lost initially to a free-electron gas, which then delivers it to the radiation through excitation of neutral hydrogen atoms. By neglecting the direct escape of line radiation and by averaging over the gas, transfer in space is suppressed, and only transfer in frequency is considered. The dust degrades the line radiation via frequency-independent absorption, converting the energy to infrared luminosity. Solutions are obtained for the source functions in the lines, using appropriate approximations, in order to determine under what conditions the narrow component of the Balmer-line radiation from the gas can be self-absorbed and degraded without similar degradation of the broad component, which originates from the suprathermals themselves. The results of the transfer calculation are employed to find self-consistent values for the temperature and ionization of the gas for various amounts of dust and various concentrations of suprathermal particles. The luminosity in the Balmer continuum is examined which would result under these conditions.

Stoner, R.↗

NASA NTRS1977

The application of ERTS imagery to the FAO/Unesco soil map of the world

The author has identified the following significant results. It was concluded that direct identification and mapping of the various soil degradation forms and intensities from the color composite imager was generally difficult, if not impossible. The imagery, however, provided valuable information on some main environmental criteria which can be used in connection other available field data to assess actual soil degradation and estimate soil degradation hazards.

Dudal, R. J.↗

NASA NTRS1978

Considerations in the design of tip-coupled air-transport systems

It is shown that the lift-drag ratio of tip-coupled systems can be expressed as a simple multiple of the lift-drag ratio of the isolated units comprising the system. When operated for maximum lift-drag ratio, the extent of the coupled system is limited by maximum lift coefficient, high-altitude engine characteristics, and degraded performance of the isolated unit climbing to couple into the system. When operated at constant altitude, the gain from coupling is severely limited. If the cruise altitude is that for best performance of the isolated unit, the system lift-drag ratio can be no better than twice that of the isolated unit even when an infinite number of units are coupled. System performance may be further degraded since span-load distributions which yield good performance for the individual units reduce the efficiency of the coupled system. Coupling a pair of modern transport aircraft results in only about half the expected gain because of a poor span-distribution across the coupled pair. The control deflections required to maintain roll and pitch equilibrium further degrade the possible gain.

Heyson, H. H.↗

NASA NTRS1978

Effect of ambiguities on SAR picture quality

The degradation of picture quality is studied for a high-resolution, large-swath SAR mapping system subjected to speckle, additive white Gaussian noise, and range and azimuthal ambiguities occurring because of the non-finite antenna pattern produced by a square aperture antenna. The effect of the azimuth antenna pattern was accounted for by calculating the aximuth ambiguity function. Range ambiguities were accounted for by adding appropriate pixels at a range separation corresponding to one pulse repetition period, but attenuated by the antenna pattern. A method of estimating the range defocussing effect which arises from the azimuth matched filter being a function of range is shown. The resulting simulated picture was compared with one degraded by speckle and noise but no ambiguities. It is concluded that azimuth ambiguities don't cause any noticeable degradation but range ambiguities might.

Korwar, V. N.↗

NASA NTRS1978

The 1.1 micrometer and visible emission semiconductor diode lasers

In (AlGa)As, the first of three alloy systems studied, Continuous Wave (CW) operation was obtained at room temperature at a wavelength as low as 7260 A. Reliability in this system was studied in the incoherent mode. Zinc doped devices had significant degradation, whereas Ge or Ge plus Zi doped devices had none. The Al2O3 facet coatings were shown to significantly reduce facet deterioration in all types of lasers, longer wavelength units of that type having accumulated (at the time of writing) 22,000 hours with little if any degradation. A CL study of thin (AlGa)As layers revealed micro fluctuation in composition. A macro-scale fluctuation was observed by electroreflectance. An experimental and theoretical study of the effect of stripe width on the threshold current was carried out. Emission below 7000 A was obtained in VPE grown Ga(AsP) (In,Ga)P with CW operation at 10 C. Lasers and LED's were made by LPE in (InGa) (AsP). Laser thresholds of 5 kA/cm2 were obtained, while LED efficiences were on the order of 2%. Incoherent life test over 6000 hours showed no degradation.

Ladany, I.↗

NASA NTRS1977

Interpreting statistics of small lunar craters

Some of the wide variations in the crater-size distributions in lunar photography and in the resulting statistics were interpreted as different degradation rates on different surfaces, different scaling laws in different targets, and a possible population of endogenic craters. These possibilities are reexamined for statistics of 26 different regions. In contrast to most other studies, crater diameters as small as 5 m were measured from enlarged Lunar Orbiter framelets. According to the results of the reported analysis, the different crater distribution types appear to be most consistent with the hypotheses of differential degradation and a superposed crater population. Differential degradation can account for the low level of equilibrium in incompetent materials such as ejecta deposits, mantle deposits, and deep regoliths where scaling law changes and catastrophic processes introduce contradictions with other observations.

Schultz, P. H.↗

NASA NTRS1978

Advantages of thin silicon solar cells for use in space

A system definition study on the Solar Power Satellite System showed that a thin, 50 micrometers, silicon solar cell has significant advantages. The advantages include a significantly lower performance degradation in a radiation environment and high power-to-mass ratios. The advantages of such cells for an employment in space is further investigated. Basic questions concerning the operation of solar cells are considered along with aspects of radiation induced performance degradation. The question arose in this connection how thin a silicon solar cell had to be to achieve resistance to radiation degradation and still have good initial performance. It was found that single-crystal silicon solar cells could be as thin as 50 micrometers and still develop high conversion efficiencies. It is concluded that the use of 50 micrometer silicon solar cells in space-based photovoltaic power systems would be advantageous.

Denman, O. S.↗

NASA NTRS1978

The effect of impact damage and circular holes on the compressive strength of a graphite-epoxy laminate

Specimens were impacted by 1.27-cm-diameter aluminum spheres with speeds ranging from 52 to 101 m/s. Some specimens were impacted without any applied compressive load and then loaded to failure to determine their residual strength. Other specimens were loaded to a prescribed axial compressive strain and impacted while at that applied load. Loaded specimens that did not fail catastrophically on impact were subsequently loaded to failure to determine their residual strength. Low-velocity impact damage was found to degrade seriously the laminate static compressive strength. Low-strain compression-compression cyclic loading was found to degrade further the compressive strength of impact-damaged specimens. Specimens with circular holes having diameters up to a third of the specimen width were loaded to failure in compression. It was found that circular holes can also degrade the static compressive strength of the laminate. The effects of circular holes and impact damage on the compressive strength of the laminate are compared.

Starnes, J. H., Jr.↗

NASA NTRS1979

Improvement and extension of data from ATS-6 Solar Cell Radiation Damage Experiment (SCRDE)

The ATS 6 solar cell radiation damage experiment data through 2 1/3 years of synchronous orbit operation are presented. Comparisons are made of the performances of the 13 different types of solar cell/cover configurations, including solar cell and cover thickness variations, base resistivity variation, new cover processes and materials, and the COMSAT violet cell. These performances are also compared to the performance of the LES 6 solar cell experiment, the ATS 6 main solar arrays, and laboratory spectrum electron irradiations. It is found that the cells of the ATS 6 experiment generally performed as expected through 6 to 9 months in orbit, but that at 2 1/3 years they were severely degraded in current. The short circuit current degradation after 2 1/3 years in orbit appears to exhibit an anomalous additional degradation of 5 to 9 percent over what was experienced in synchronous orbit operation.

Goldhammer, L. J.↗

NASA NTRS1979

Electron impact on atmospheric gases. III - Spatial yield spectra for N2

Spatial yield spectra have been calculated for electron energy degradation into molecular nitrogen gas using a Monte Carlo method for 0.1- to 5.0-keV incident electrons. The spectra contain the spatial yield information about the electron degradation process and can be employed to calculate a 'yield' for any inelastic state at any position in the medium. Because of the spectrum's useful nature and simple characteristics, the three-variable spatial yield spectrum U(E, z, E sub zero) is analytically represented as well. This analytic form can then be easily applied to atmospheric and laboratory problems dealing with energetic electron degradation.

Jackman, C. H.↗

NASA NTRS1979

Thermal exposure effects on the mechanical properties of a polycrystalline alumina fiber/aluminum matrix composite

The effects of thermal exposures and elevated test temperature on the mechanical properties of a unidirectional polycrystalline alumina fiber reinforced aluminum matrix composite were investigated. Test temperatures up to 590 K and 2500 hours exposures at 590 K did not significantly affect fiber dominated properties but did severely degrade matrix dominated properties. Fiber strength, degraded by the fabrication process, was restored by post fabrication thermal exposures. Possible degradation mechanisms are discussed.

Olsen, G. C.↗

NASA NTRS1979

LSA field test

Degradation tests indicate that electrical degradation is not a slow monotonically increasing phenomenon as originally thought but occurs abruptly as the result of some traumatic event. This finding has led to a change in the test philosophy. A discussion of this change is presented along with a summary of degradation and failure data from all the sites and results from a variety of special tests. New instrumentation for in-field measurements are described. Field testing activity was expanded by the addition of twelve remote sites located as far away as Alaska and the Canal Zone. Descriptions of the new sites are included.

Jaffe, P.↗

NASA NTRS1980

Lear jet boundary layer/shear layer laser propagation experiments

Optical degradations of aircraft turbulent boundary layers with shear layers generated by aerodynamic fences are analyzed. A collimated 2.5 cm diameter helium-neon laser (0.63 microns) traversed the approximate 5 cm thick natural aircraft boundary layer in double pass via a reflective airfoil. In addition, several flights examined shear layer-induced optical degradation. Flight altitudes ranged from 1.5 to 12 km, while Mach numbers were varied from 0.3 to 0.8. Average line spread function (LSF) and Modulation Transfer Function (MTF) data were obtained by averaging a large number of tilt-removed curves. Fourier transforming the resulting average MTF yields an LSF, thus affording a direct comparison of the two optical measurements. Agreement was good for the aerodynamic fence arrangement, but only fair in the case of a turbulent boundary layer. Values of phase variance inferred from the LSF instrument for a single pass through the random flow and corrected for a large aperture ranged from 0.08 to 0.11 waves (lambda = .63 microns) for the boundary layer. Corresponding values for the fence vary from 0.08 to 0.16 waves. Extrapolation of these values to 10.6 microns suggests negligible degradation for a CO2 laser transmitted through a 5 cm thick, subsonic turbulent boundary layer.

Gilbert, K.↗

NASA NTRS1979

Tradeoff between picture element dimensions and noncoherent averaging in side-looking airborne radar

An experiment was performed in which three synthetic-aperture images and one real-aperture image were successively degraded in spatial resolution, both retaining the same number of independent samples per pixel and using the spatial degradation to allow averaging of different numbers of independent samples within each pixel. The original and degraded images were provided to three interpreters familiar with both aerial photographs and radar images. The interpreters were asked to grade each image in terms of their ability to interpret various specified features on the image. The numerical interpretability grades were then used as a quantitative measure of the utility of the different kinds of image processing and different resolutions. The experiment demonstrated empirically that the interpretability is related exponentially to the SGL volume which is the product of azimuth, range, and gray-level resolution.

Moore, R. K.↗

NASA NTRS1980

The effects of titanium impurities in N/+//P silicon solar cells

Microscopic and electrical measurements were performed to explain the degradation mechanisms associated with the presence of titanium impurities in silicon. The measurements included X-ray topography, transmission electron microscopy, and deep level transient spectroscopy, before and after processing. The results indicated the presence of TiO2 precipitates, the density of which increased after phosphorus diffusion. A majority carrier trapping level was observed in the wafers before processing. It was concluded that 10% of the Ti in the N(+)/P silicon solar cells formed electrically active centers which caused degradation of the cell junction. 14% of the remaining Ti precipitated out as TiO2, forming electrically active defects, which also caused junction degradation.

Salama, A. M.↗

NASA NTRS1981

Solar energy absorption characteristics and the effects of heat on the optical properties of several coatings

The solar energy absorption characteristics of several high temperature coatings were determined and effects of heat on these coatings were evaluated. Included in the investigation were an electroplated alloy of black chrome and vanadium, electroplated black chrome, and chemically colored 316 stainless steel. Each of the coatings possessed good selective solar energy absorption properties at laboratory ambient temperature. Measured at a temperature of 700 K (800 F), the emittances of black chrome, black chrome vanadium, and colored stainless steel were 0.11, 0.61, and 0.15, respectively. Black chrome and black chrome vanadium did not degrade optically in the presence of high heat (811 K (1000 F)). Chemically colored stainless steel showed slight optical degradation when exposed to moderately high heat (616 K (650 F)0, but showed more severe degradation at exposure temperatures beyond this level. Each of the coatings showed good corrosion resistance to a salt spray environment.

Lowery, J. R.↗

NASA NTRS1981

Effects of lateral resolution on the identification of volcanotectonic provinces on earth and Venus

In an attempt to learn what volcanotectonic features can still be discerned in continental and oceanic areas of the earth when topographic data are degraded to simulate the data sampled by the Pioneer-Venus altimeter, two digital topographic data sets (the 30-second continental U.S. altitude data and the 30 x 30 nautical mile bathymetry data for the North Pacific) were degraded and displayed in the same way as the altimeter data from Venus. The Appalachians were reduced to a gentle swell, with a wavelength of 300 km, and a height of 500 m. The Cordillera was seen as a broad swell, 2500 km wide, and about 2 km high. The east Pacific rise, east Pacific fractures, seamount chains, the Hawaiian swell, and most trenches were discernible in the degraded Pacific data; whereas rises, transforms, seamount chains, and trenches were not seen in the Venus data, even after corrections were made for the higher surface temperature and the absence of oceans on Venus. It was concluded that a plate tectonic regime, similar to earth's does not currently appear to exist on Venus. As shown by the Cordillera data, the Pioneer-Venus information is not of sufficiently high quality to discern whether the highlands of Venus preserve evidence for orogenic events related to plate tectonics.

Arvidson, R. E.↗