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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Active localization of virtual sounds

We describe a virtual sound display built around a 12 MHz 80286 microcomputer and special purpose analog hardware. The display implements most of the primary cues for sound localization in the ear-level plane. Static information about direction is conveyed by interaural time differences and, for frequencies above 1800 Hz, by head sound shadow (interaural intensity differences) and pinna sound shadow. Static information about distance is conveyed by variation in sound pressure (first power law) for all frequencies, by additional attenuation in the higher frequencies (simulating atmospheric absorption), and by the proportion of direct to reverberant sound. When the user actively locomotes, the changing angular position of the source occasioned by head rotations provides further information about direction and the changing angular velocity produced by head translations (motion parallax) provides further information about distance. Judging both from informal observations by users and from objective data obtained in an experiment on homing to virtual and real sounds, we conclude that simple displays such as this are effective in creating the perception of external sounds to which subjects can home with accuracy and ease.

Loomis, Jack M.↗
NASA NTRS1994

RF optics study for DSS-43 ultracone implementation

The Ultracone feed system will be implemented on DSS 43 to support the S-band (2.3 GHz) Galileo contingency mission. The feed system will be installed in the host country's cone, which is normally used for radio astronomy, VLBI, and holography. The design must retain existing radio-astronomy capabilities, which could be impaired by shadowing from the large S-band feed horn. Computer calculations were completed to estimate system performance and shadowing effects for various configurations of the host country's cone feed systems. Also, the DSS-43 system performance using higher gain S-band horns was analyzed. A new S-band horn design with improved return loss and cross-polarization characteristics is presented.

Lee, P.↗
NASA NTRS1993

Mineralogic variability of the Kelso Dunes, Mojave Desert, California derived from Thermal Infrared Multispectral Scanner (TIMS) data

Mineral identification and mapping of alluvial material using thermal infrared (TIR) remote sensing is extremely useful for tracking sediment transport, assessing the degree of weathering and locating sediment sources. As a result of the linear relation between a mineral's percentage in a given area (image pixel) and the depth of its diagnostic spectral features, TIR spectra can be deconvolved in order to ascertain mineralogic percentages. Typical complications such as vegetation, particle size and thermal shadowing are minimized upon examination of dunes. Actively saltating dunes contain little to no vegetation, are very well sorted and lack the thermal shadows that arise from rocky terrain. The primary focus of this work was to use the Kelso Dunes as a test location for an accuracy analysis of temperature/emissivity separation and linear unmixing algorithms. Accurate determination of ground temperature and component discrimination will become key products of future ASTER data. A decorrelation stretch of the TIMS image showed clear color variations within the active dunes. Samples collected from these color units were analyzed for mineralogy, grain size, and separated into endmembers. This analysis not only revealed that the dunes contained significant mineralogic variation, but were more immature (low quartz percentage) than previously reported. Unmixing of the TIMS data using the primary mineral endmembers produced unique variations within the dunes and may indicate near, rather than far, source locales for the dunes. The Kelso Dunes lie in the eastern Mojave Desert, California, approximately 95 km west of the Colorado River. The primary dune field is contained within a topographic basin bounded by the Providence, Granite Mountains, with the active region marked by three northeast trending linear ridges. Although active, the dunes appear to lie at an opposing regional wind boundary which produces little net movement of the crests. Previous studies have estimated the dunes range from 70% to 90% quartz mainly derived from a source 40 km to the west. The dune field is assumed to have formed in a much more arid climate than present, with the age of the deposit estimated at greater than 100,000 years.

Ramsey, Michael S.↗
NASA NTRS1995

Analysis of solar receiver flux distributions for US/Russian solar dynamic system demonstration on the MIR Space Station

The United States and Russia have agreed to jointly develop a solar dynamic (SD) system for flight demonstration on the Russian MIR space station starting in late 1997. Two important components of this SD system are the solar concentrator and heat receiver provided by Russia and the U.S., respectively. This paper describes optical analysis of the concentrator and solar flux predictions on target receiver surfaces. The optical analysis is performed using the code CIRCE2. These analyses account for finite sun size with limb darkening, concentrator surface slope and position errors, concentrator petal thermal deformation, gaps between petals, and the shading effect of the receiver support struts. The receiver spatial flux distributions are then combined with concentrator shadowing predictions. Geometric shadowing patterns are traced from the concentrator to the target receiver surfaces. These patterns vary with time depending on the chosen MIR flight attitude and orbital mechanics of the MIR spacecraft. The resulting predictions provide spatial and temporal receiver flux distributions for any specified mission profile. The impact these flux distributions have on receiver design and control of the Brayton engine are discussed.

Kerslake, Thomas W.↗
NASA NTRS1995

Total Solar Eclipse of 1997 March 9

A total eclipse of the Sun will be visible from Asia and the Pacific Ocean on 1997 March 9. The path of the Moon's umbral shadow begins in eastern Kazakhstan and travels through Mongolia and eastern Siberia, where it swings northward to end at sunset in the Arctic Ocean. A partial eclipse will be seen within the much broader path of the Moon's penumbral shadow, which includes eastern Asia, the northern Pacific, and the northwest corner of North America. Detailed predictions for this event are presented and include besselian elements, geographic coordinates of the path of totality, physical ephemeris of the umbra, topocentric limb profile corrections, local circumstances for 280 cities, maps of the eclipse path, weather prospects, the lunar limb profile, and the sky during totality. Tips and suggestions are also given on how to safely view and photograph the eclipse.

Espenak, Fred↗
NASA NTRS1994

Spatial distribution of plasma wave activity in the nightside ionosphere of Venus

In this study we use 14 years of Pioneer Venus Orbiter Electric Field Detector (OEFD) data to define the characteristics of (VLF) burst activity in the nightside ionosphere of Venus. Our statistical results show that there are essentially four types of VLF signals. The first type of signal is only observed in the 100 Hz channel and not in any of the higher frequency channels (730 Hz, 5.4 kHz or 30 kHz). Occurrence of these waves is controlled by the magnetic field with a weaker dependence on electron density. The occurrence of these waves is controlled by the magnetic field with a weaker dependence on electron density. The occurrence rate decreases with increasing altitude to a height of 600 km. For higher altitudes beyond 600 km the occurrence rate remains roughly constant. The statistics of these signals are what one would expect for whistler mode waves from a subionospheric source. The second type of signal is broadline wave activity appearing below 300 km in the low altitude ionosphere. These signals often occur in all four channels of the OEFD. These signals are also thought to come from a subionospheric source. The third type of signal appearing near the edge of the planetary optical shadow. They are probably ion acoustic waves generated by a current driven instability associated with plasma clouds in the wake. The fourth type of signal is a narrow band wave. It occurs in either of the two high frequency channels in the high altitude tail region, and is attributed to locally generated Langmuir waves. In addition, we also observe spacecraft interference noise in both the 100 and 730 Hz channels. These signals mainly occur near the edge of the planetary optical shadow and have an inbound and outbound asymmetry in activity.

Ho, C.-M.↗
NASA NTRS1995

Measurement of satellite PCS fading using GPS

A six-channel commercial GPS receiver with a custom-made 40 deg tilted, rotating antenna has been assembled to make fade measurements for personal satellite communications. The system can measure up to two times per minute fades of up to 15 dB in the direction of each tracked satellite from 10 to 90 deg elevation. Photographic fisheye lens images were used to categorize the fade data obtained in several test locations according to fade states of clear, shadowed, or blocked. Multipath effects in the form of annular rings can be observed when most of the sky is clear. Tree fading by a Pecan exceeding 3.5 dB and 12 dB at 50 to 10 percent probability, respectively, compared with median fades of 7.5 dB measured earlier and the discrepancy is attributed to the change in ratio when measuring over an area as opposed to along a line. Data acquired inside buildings revealed 'rf-leaky' ceilings. Satellite diversity gain in a shadowed environment exceeded 6 dB at the 10 percent probability.

Vogel, Wolfhard J.↗
NASA NTRS1994

Robotic vision techniques for space operations

Automation and robotics for space applications are being pursued for increased productivity, enhanced reliability, increased flexibility, higher safety, and for the automation of time-consuming tasks and those activities which are beyond the capacity of the crew. One of the key functional elements of an automated robotic system is sensing and perception. As the robotics era dawns in space, vision systems will be required to provide the key sensory data needed for multifaceted intelligent operations. In general, the three-dimensional scene/object description, along with location, orientation, and motion parameters will be needed. In space, the absence of diffused lighting due to a lack of atmosphere gives rise to: (a) high dynamic range (10(exp 8)) of scattered sunlight intensities, resulting in very high contrast between shadowed and specular portions of the scene; (b) intense specular reflections causing target/scene bloom; and (c) loss of portions of the image due to shadowing and presence of stars, Earth, Moon, and other space objects in the scene. In this work, developments for combating the adverse effects described earlier and for enhancing scene definition are discussed. Both active and passive sensors are used. The algorithm for selecting appropriate wavelength, polarization, look angle of vision sensors is based on environmental factors as well as the properties of the target/scene which are to be perceived. The environment is characterized on the basis of sunlight and other illumination incident on the target/scene and the temperature profiles estimated on the basis of the incident illumination. The unknown geometrical and physical parameters are then derived from the fusion of the active and passive microwave, infrared, laser, and optical data.

Krishen, Kumar↗
NASA NTRS1996

The effects of cloud inhomogeneities upon radiative fluxes, and the supply of a cloud truth validation dataset

The ASTER polar cloud mask algorithm is currently under development. Several classification techniques have been developed and implemented. The merits and accuracy of each are being examined. The classification techniques under investigation include fuzzy logic, hierarchical neural network, and a pairwise histogram comparison scheme based on sample histograms called the Paired Histogram Method. Scene adaptive methods also are being investigated as a means to improve classifier performance. The feature, arctan of Band 4 and Band 5, and the Band 2 vs. Band 4 feature space are key to separating frozen water (e.g., ice/snow, slush/wet ice, etc.) from cloud over frozen water, and land from cloud over land, respectively. A total of 82 Landsat TM circumpolar scenes are being used as a basis for algorithm development and testing. Numerous spectral features are being tested and include the 7 basic Landsat TM bands, in addition to ratios, differences, arctans, and normalized differences of each combination of bands. A technique for deriving cloud base and top height is developed. It uses 2-D cross correlation between a cloud edge and its corresponding shadow to determine the displacement of the cloud from its shadow. The height is then determined from this displacement, the solar zenith angle, and the sensor viewing angle.

Welch, Ronald M.↗
NASA NTRS1995

Laser Light Scattering by Shock Waves

Scattering of coherent light as it propagates parallel to a shock wave, formed in front of a bluff cylindrical body placed in a supersonic stream, is studied experimentally and numerically. Two incident optical fields are considered. First, a large diameter collimated beam is allowed to pass through the shock containing flow. The light intensity distribution in the resultant shadowgraph image, measured by a low light CCD camera, shows well-defined fringes upstream and downstream of the shadow cast by the shock. In the second situation, a narrow laser beam is brought to a grazing incidence on the shock and the scattered light, which appears as a diverging sheet from the point of interaction, is visualized and measured on a screen placed normal to the laser path. Experiments are conducted on shocks formed at various free-stream Mach numbers, M, and total pressures, P(sub 0). It is found that the widths of the shock shadows in a shadowgraph image become independent of M and P(sub 0) when plotted against the jump in the refractive index, (Delta)n, created across the shock. The total scattered light measured from the narrow laser beam and shock interaction also follows the same trend. In the numerical part of the study, the shock is assumed to be a 'phase object', which introduces phase difference between the upstream and downstream propagating parts of the light disturbances. For a given shape and (Delta)n of the bow shock the phase and amplitude modulations are first calculated by ray tracing. The wave front is then propagated to the screen using the Fresnet diffraction equation. The calculated intensity distribution, for both of the incident optical fields, shows good agreement with the experimental data.

Panda, J.↗
NASA NTRS1996

A Proposed Change to ITU-R Recommendation 681

Recommendation 681 of the International Telecommunications Union (ITU) provides five models for the prediction of propagation effects on land mobile satellite links: empirical roadside shadowing (ERS), attenuation frequency scaling, fade duration distribution, non-fade duration distribution, and fading due to multipath. Because the above prediction models have been empirically derived using a limited amount of data, these schemes work only for restricted ranges of link parameters. With the first two models, for example, the frequency and elevation angle parameters are restricted to 0.8 to 2.7 GHz and 20 to 60 degrees, respectively. Recently measured data have enabled us to enhance the range of the first two schemes. Moreover, for convenience, they have been combined into a single scheme named the extended empirical roadside shadowing (EERS) model.

Davarian, F.↗
NASA NTRS1997

Total Solar Eclipse of 1999 August 11

On 1999 August 11, a total eclipse of the Sun will be visible from within a narrow corridor which traverses the Eastern Hemisphere. The path of the Moon's umbral shadow begins in the Atlantic and crosses central Europe, the Middle East, and India, where it ends at sunset in the Bay of Bengal. A partial eclipse will be seen within the much broader path of the Moon's penumbral shadow, which includes northeastern North America, all of Europe, northern Africa, and the western half of Asia. Detailed predictions for this event are presented and include besselian elements, geographic coordinates of the path of totality, physical ephemeris of the umbra, topocentric limb profile corrections, local circumstances for approximately 1400 cities, maps of the eclipse path, weather prospects, the lunar limb profile, and the sky during totality. Tips and suggestions are also given on how to safely view and photograph the eclipse.

Espenak, Fred↗
NASA NTRS1996

The Clementine Bistatic Radar Experiment

During the Clementine 1 mission, a bistatic radar experiment measured the magnitude and polarization of the radar echo versus bistatic angle, beta, for selected lunar areas. Observations of the lunar south pole yield a same-sense polarization enhancement around beta = 0. Analysis shows that the observed enhancement is localized to the permanently shadowed regions of the lunar south pole. Radar observations of periodically solar-illuminated lunar surfaces, including the north pole, yielded no such enhancement. A probable explanation for these differences is the presence of low-loss volume scatterers, such as water ice, in the permanently shadowed region at the south pole.

Nozette, S.↗
NASA NTRS1996

The interstellar medium and the soft X-ray background

The soft X-ray background (SXRB) surface brightness provides data to study the approximately 10(exp 6) K plasma of the local interstellar medium of our Galaxy. Various studies were carried out in order to search for negative correlation, or shadowing, of the SXRB, and were coupled with interstellar medium absorption line studies. The purpose was to determine whether the distances to the shadowing material will lead to a three dimensional mapping of the X-ray emitting, and X-ray absorbing components.

Snowden, S. L.↗
NASA NTRS1996

TRMM Solar Array

Basic requirement of 978.59 watts per Panel output @ 58.9 volts B.O.L. was met on an average basis per agreement with NASA. Lower grade Cells were used on the shadowed Panel (Boom shadow) to maximize available power to the Spacecraft. The average output @ 58.9 volts was 991 watts. The outputs of the four t4) Panels ranged from 960 to 1,022 watts. The Panels successfully passed environmental testing at TRW to the contract specification and subsequent testing at NASA which involved output measurements at elevated temperatures. As this type of Array had never previously been built by TRW (aluminum Substrate with 4 cm x 4.4 cm GaAs Cells), the TRMM Program was a development effort combined with a Qual and Flight production effort. The most significant technical problem was Cell cracking during Qual thermal cycling. The cracking problem was determined to be generic within our Solar Array factory in the application of GaAs Cells to our designs. As a result, a TRW funded manufacturing process verification panel (known as the Manufacturing Verification Panel) was built to demonstrate our ability to properly apply GaAs Cells. The original Qual Panel comprised three (3) design variations with respect to Coverglass-to-Cell and Cell-to-Substrate adhesives. The intent was to qualify multiple designs in case one or more failed. When two of the three combinations failed due to excessive Cell breakage during thermal cycling, NASA was reluctant to allow Flight production based on the one remaining good Qual Panel Quadrant. This issue was pivotal for continuing the contract. Facts and recommendations are as follows: (1) The cause of the excessive cracking was never determined. and (2) The areas where the excessive cracking occurred utilized DC93-500 glassing adhesive which was NASA approved, and had been widely used by TRW on a multitude of projects.

Source record↗
NASA NTRS1998

Lunar Prospector Orbit Determination Results

The orbit support for Lunar Prospector (LP) consists of three main areas: (1) cislunar orbit determination, (2) rapid maneuver assessment using Doppler residuals, and (3) routine mapping orbit determination. The cislunar phase consisted of two trajectory correction maneuvers during the translunar cruise followed by three lunar orbit insertion burns. This paper will detail the cislunar orbit determination accuracy and the real-time assessment of the cislunar trajectory correction and lunar orbit insertion maneuvers. The non-spherical gravity model of the Moon is the primary influence on the mapping orbit determination accuracy. During the first two months of the mission, the GLGM-2 lunar potential model was used. After one month in the mapping orbit, a new potential model was developed that incorporated LP Doppler data. This paper will compare and contrast the mapping orbit determination accuracy using these two models. LP orbit support also includes a new enhancement - a web page to disseminate all definitive and predictive trajectory and mission planning information. The web site provides definitive mapping orbit ephemerides including moon latitude and longitude, and four week predictive products including: ephemeris, moon latitude/longitude, earth shadow, moon shadow, and ground station view periods. This paper will discuss the specifics of this web site.

Beckman, Mark↗
NASA NTRS1998

Impact of Radiation Hardness and Operating Temperatures of Silicon Carbide Electronics on Space Power System Mass

The effect of silicon carbide (SiC) electronics operating temperatures on Power Management and Distribution (PMAD), or Power Conditioning (PC), subsystem radiator size and mass requirements was evaluated for three power output levels (100 kW(e) , 1 MW(e), and 10 MW(e)) for near term technology ( i.e. 1500 K turbine inlet temperature) Closed Cycle Gas Turbine (CCGT) power systems with a High Temperature Gas Reactor (HTGR) heat source. The study was conducted for assumed PC radiator temperatures ranging from 370 to 845 K and for three scenarios of electrical energy to heat conversion levels which needed to be rejected to space by means of the PC radiator. In addition, during part of the study the radiation hardness of the PC electronics was varied at a fixed separation distance to estimate its effect on the mass of the instrument rated reactor shadow shield. With both the PC radiator and the conical shadow shield representing major components of the overall power system the influence of the above on total power system mass was also determined. As expected, results show that the greatest actual mass savings achieved by the use of SiC electronics occur with high capacity power systems. Moreover, raising the PC radiator temperature above 600 K yields only small additional system mass savings. The effect of increased radiation hardness on total system mass is to reduce system mass by virtue of lowering the shield mass.

Juhasz, Albert J.↗
NASA NTRS1999

The Origin of the EUV Emission in Her X-1

Her X-1 exhibits a strong orbital modulation of its EUV (Extreme Ultraviolet Radiation) flux with a large decrease around time of eclipse of the neutron star, and a significant dip which appears at different orbital phases at different 35-day phases. We consider observations of Her X-1 in the EUVE by the Extreme Ultraviolet Explorer (EUVE), which includes data from 1995 near the end of the Short High state, and date from 1997 at the start of the Short High state. The observed EUV lightcurve has bright and faint phases. The bright phase can be explained as the low energy tail of the soft x-ray pulse. The faint phase emission has been modeled to understand its origin. We find: the x-ray heated surface of HZ Her is too cool to produce enough emission; the accretion disk does not explain the orbital modulation; however, reflection of x-rays off of HZ Her can produce the observed lightcurve with orbital eclipses. The dip can be explained by shadowing of the companion by the accretion disk. We discuss the constraints on the accretion disk geometry derived from the observed shadowing.

Leahy, D. A.↗