Search NASA⌕ Search

SEARCH · Search NASA

Results for “ABSTRACT”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

(abstract) Galileo PPR Observations of Shoemaker-Levy 9

The Galileo spacecraft Photopolarimeter Radiometer (PPR), a hybrid visual/thermal IR instrument designed primarily to measure properties of the Jovian atmosphere, was employed for SL9 as a staring high-speed photometer at 945 and 678 nm, taking advantage of Galileo's direct view of the impact point.The PPR was able to acquire data at times when no other Galileo optical instruments could operate.

Galileo Jupiter Shoemaker-Levy 9 IR observations p↗
NASA NTRS1994

(abstract) Venus Gravity Data Reduction

The Magellan spacecraft has provided high resolution gravity data to its very end, October 13, 1994, when it was consumed by the Venusian atmosphere. After aerobraking in August of 1993 to attain a near circular orbit, excellent high latitude data were acquired which previously were very weak during the elliptical orbit coverage. There are 1500 orbits during the near circular orbit, supplying redundant coverage at different geometries over many features. This allowed the relaxation of apriori constraints, so true amplitudes are being extracted from the data. In this paper we present the results of a 75(sup th) degree and order field that incorporates all the old Pioneer Venus Orbiter data as well as all the Magellan data to September 1994. The new results reflect even higher correlation with topography, higher amplitude values for the highs and lows, and global results that have essentially very little apriori constraint on the solution parameters. We also correlate our new model with the earlier ones based on 60(sup th) and 40(sup th) degree and order presentations.

Magellan Venus gravity orbits topography surface f↗
NASA NTRS1994

(abstract) A Miniature, High-Sensitivity, Electron-Tunneling Accelerometer

A prototype low-noise accelerometer has been fabricated with an electron-tunneling transducer. By measuring the tunneling current between an electrode on the proof mass and a feedback-controlled monitor electrode, very small accelerations can be detected with high responsivity. This particular prototype (10x10x1.5 mm) was designed for underwater acoustic measurement from a few hertz to 1 kHz. The measured responsivity below the fundamental device resonance at 100 Hz is roughly 1500 volts per m/s(sup 2) with a measured noise spectral density of 10(sup -6) m/s(sup 2) per root hertz or less between 30 and 300 Hz. The noise floor is controlled primarily by 1/f noise in the tunneling current although the noise floor reaches the theoretical molecular-agitation limit at 100 hertz. The responsivity and directivity of the device were measured in a standard gradient-hydrophone calibrator; the noise floor was determined in a vacuum-ionization chamber assembled from commercial off-the-shelf components; and the detailed dynamics of the proof-mass motion were examined using a heterodyne laser interferometer that was scanned across the surface and synchronously detected with respect to the excitation.

accelerometer electron tunneling acoustic measurem↗
NASA NTRS1994

(abstract) Modeling Protein Families and Human Genes: Hidden Markov Models and a Little Beyond

We will first give a brief overview of Hidden Markov Models (HMMs) and their use in Computational Molecular Biology. In particular, we will describe a detailed application of HMMs to the G-Protein-Coupled-Receptor Superfamily. We will also describe a number of analytical results on HMMs that can be used in discrimination tests and database mining. We will then discuss the limitations of HMMs and some new directions of research. We will conclude with some recent results on the application of HMMs to human gene modeling and parsing.

Hidden Markov Models HMMs proteins computational m↗
NASA NTRS1995

(abstract) Variations in Polarimetric Backscatter of Saline Ice Grown Under Diurnal Thermal Cycling Condition

An experiment was carried out in January 1994 at the Geophysical Research Facility in the Cold Regions Research and Engineering Laboratory. To investigate effects on polarimetric scattering signatures of sea ice growth under diurnal temperature variations, an ice sheet was grown for 2.5 days for the thickness of 10 cm and a polarimetric radar operating at C-band was used to obtain backscattering data in conjunction with ice-characterization measurements. The ice sheet was grown in the late morning of January 19, 1994. The initial growth rate was slow due to high insolation and temperature. As the air temperature dropped during the night, the growth rate increased significantly. The air temperature changed drastically from about -10(deg)C to -35(deg)C between day and night. The temperature cycle was repeated during the next day and the growth rate varied in the same manner. The surface of the ice was partially covered by frost flowers and the areal coverage increased as the ice became thicker. Throughout the ice growth duration of 2.5 days, polarimetric backscatter data were collected at roughly every centimeter of ice growth. For each set of radar measurements of saline ice, a set of calibration measurements was carried out with trihedrial corner reflectors and a metallic sphere. Measured polarimetric backscattering coefficients of the ice sheet reveal a strong correlation between radar data and temperature variations. As the temperature increased (decreased), the backscatter increased (decreased) correspondingly. From the ice-characterization data, temperatures of the air, at the ice-air interface, and in the ice layer had the same variation trend. Another interesting experimental observation is that the salinity measured as a function of ice depth from a sample of 10-cm thich ice indicated that the salinity variations had a similar cycle as the temperature; i.e., the salinity profile recorded the history of the temperature variations. Characterization data of the ice sheet are used in a theoretical model for scattering from saline ice with frost cover to explain the observed polarimetric signatures.

characterization polarimetric radar salinity tempe↗
NASA NTRS1995

(abstract) A Polarimetric Model for Effects of Brine Infiltrated Snow Cover and Frost Flowers on Sea Ice Backscatter

A polarimetric scattering model is developed to study effects of snow cover and frost flowers with brine infiltration on thin sea ice. Leads containing thin sea ice in the Artic icepack are important to heat exchange with the atmosphere and salt flux into the upper ocean. Surface characteristics of thin sea ice in leads are dominated by the formation of frost flowers with high salinity. In many cases, the thin sea ice layer is covered by snow, which wicks up brine from sea ice due to capillary force. Snow and frost flowers have a significant impact on polarimetric signatures of thin ice, which needs to be studied for accessing the retrieval of geophysical parameters such as ice thickness. Frost flowers or snow layer is modeled with a heterogeneous mixture consisting of randomly oriented ellipsoids and brine infiltration in an air background. Ice crystals are characterized with three different axial lengths to depict the nonspherical shape. Under the covering multispecies medium, the columinar sea-ice layer is an inhomogeneous anisotropic medium composed of ellipsoidal brine inclusions preferentially oriented in the vertical direction in an ice background. The underlying medium is homogeneous sea water. This configuration is described with layered inhomogeneous media containing multiple species of scatterers. The species are allowed to have different size, shape, and permittivity. The strong permittivity fluctuation theory is extended to account for the multispecies in the derivation of effective permittivities with distributions of scatterer orientations characterized by Eulerian rotation angles. Polarimetric backscattering coefficients are obtained consistently with the same physical description used in the effective permittivity calculation. The mulitspecies model allows the inclusion of high-permittivity species to study effects of brine infiltrated snow cover and frost flowers on thin ice. The results suggest that the frost cover with a rough interface significantly increases the backscatter from thin saline ice and the polarimetric signature becomes closer to the isotropic characteristics. The snow cover also modifies polarimetric signatures of thin sea ice depending on the snow mixture and the interface condition.

thin sea ice radar backscatter frost flowers snow ↗
NASA NTRS1995

(abstract) Unidirectional Carbon/Carbon for Ion Engine Optics

Conventional ion engine optical grids are made from hydroformed molybdenum. Carbon/carbon has been utilized in place of molybdenum because of its lower sputter yield, which contributes a greatly increased engine life, and for its low cte, which allows more efficient engine operation. The requirements for this material are that it must have high stiffness, very tight dimensional tolerances, and can be optimized for an hexagonal hole pattern with a very high open area friction. The carbon/carbon for this application was fabricated from unidirectional tape prepreg, using pitch fiber, and was processed to a very high temperature. The use of unidirectional tape allowed for a sufficient number of plies to be used to generate a balanced three directional layup within the thickness constraints of the material, as well as providing strength and stiffness over that normally seen with fabric based carbon/carbons.

ion engine carbon/carbon unidirectional tape prepr↗
NASA NTRS1995

(abstract) Experimental Results From Internetworking Data Applications Over Various Wireless Networks Using a Single Flexible Error Control Protocol

This paper describes results from several wireless field trials in New Jersey, California, and Colorado, conducted jointly by researchers at Bellcore, JPL, and US West over the course of 1993 and 1994. During these trials, applications communicated over multiple wireless networks including satellite, low power PCS, high power cellular, packet data, and the wireline Public Switched Telecommunications Network (PSTN). Key goals included 1) designing data applications and an API suited to mobile users, 2) investigating internetworking issues, 3) characterizing wireless networks under various field conditions, and 4) comparing the performance of different protocol mechanisms over the diverse networks and applications. We describe experimental results for different protocol mechanisms and parameters, such as acknowledgment schemes and packet sizes. We show the need for powerful error control mechanisms such as selective acknowledgements and combining data from multiple transmissions. We highlight the possibility of a common protocol for all wireless networks, from micro-cellular PCS to satellite networks.

wireless networks field trials mobile users protoc↗
NASA NTRS1995

(abstract) Mars Global Surveyor: A Global Mapping Mission

The Mars Global Surveyor Mission will launch a single spacecraft to Mars in November 1996. After arrival at the planet in September 1997 aerobraking will be used to lower the spacecraft into a low, sun-synchronous, polar-mapping orbit over several months. Early in 1998 mapping observations will begin and continue for a Mars year (687 days). Following completion of this first Mars year of mapping the spacecraft will continue to act as a relay orbiter for an additional three Earth years. The five instruments carried involve magnetometry, surface and atmosphere imaging, atmospheric sounding, laser altimetry, gravity and thermal emission spectroscopy. In addition, the spacecraft carries a Mars relay receiver/transmitter which will operate over the entire five year orbital mission ending in January of 2003. The Mars Global Surveyor mission is intended to accomplish a portion of the scientific objectives of the Mars Observer mission which was lost in 1993 three days before entering Mars orbit. The instrumentation to be used for the magnetometers, cameras, laser altimeter, emission spectrometer and radio science are very nearly the same as those carried by Mars Observer. The spacecraft design will be new but will use spare equipment from Mars Observer and has a mass about two and one-half time smaller. All experiments will be controlled remotely from their investigators home installation. The long planned period of observation supports the mission's global and seasonal objectives.

Mars Global Surveyor Mars Observer mapping spectro↗
NASA NTRS1995

(abstract) Application of Non-coherent Data Types for Deep Space Navigation

Several options are being examined to reduce the costs of spacecraft and deep space missions. One such option is to fly spacecraft in a non-coherent mode, that is, the spacecraft does not carry a transponder and cannot coherently return a Doppler signal. Historically, such one-way data has not been used as the sole data type due to the instability of the onboard oscillator, the use of S-band frequencies, and the corresponding larger error sources which could not be modeled. However, with the advent of high-speed work stations and more sophisticated modeling ability, the possibility of using one-way data is being re-examined. This paper addresses the navigation performance of various one-way data types for use in interplanetary missions.

deep space navigation non-coherent data spacecraft↗
NASA NTRS1995

(abstract) Global Gravity and Topography

This paper will discuss global gravity and topography, pole orientation, rotation, and a geodetic control network. The gravity reductions produced two products for geophysical modeling. They are line-of-sight acceleration profiles and spherical harmonic coefficients. The acceleration profiles were generated from the raw Doppler residual on a single orbit of Magellan (MGN) radio tracking data. There are over 2500 profiles from excellent X-band Doppler tracking, producing over three million individual observations. The topography data acquired by the radar altimeter on MGN were reduced and archived as three different products. The Venus spin pole orientation, rotation rate and geodetic control network were obtained by processing the SAR imaging data independently and also by incorporating Doppler radio tracking and radar altimetry. Some data from Pioneer Venus Orbiter and Venera were used also.

Venus Magellan Venera Pioneer Venus Orbiter gravit↗
NASA NTRS1995

(abstract) All Epitaxial Edge-geometry SNS Devices with Doped PBCO and YBCO Normal Layers

We will present our results on tapered-edge-geometry SNS weak link fabricated from c-axis oriented base-, counterelectrode and normal layers using a variety of processing conditions. To date, we have employed a variety of different normal materials (Co-doped YBCO, Y-doped PBCO, Ca-doped PBCO). We have been examining the junction fabrication process in detail and we will present our methods. In particular, we have been examining both epitaxial and non-epitaxial milling mask overlayers and we will present a comparison of both methods. These devices behave similar to the expectations of the resisively shunted junction model and conventional SNS proximity effect models but with some differences which will be discussed. We will present the detailed systematics of our junctions including device parameters versus temperature, rf and dc magnetic response for the various processing conditions.

epitaxial doped PBCO YBCO junction fabrication mil↗
NASA NTRS1995

(abstract) Microwave Induced Combustion Synthesis of Ceramic and Ceramic-Metal Composites

We have used microwaves to induce self-propagating high temperature synthesis (SHS) in the reaction 3TiO(sub 2)+3C+(4+x)Al ---> 3TiC+2Al(sub 2)O(sub 3)+xAl. The SHS process was studied for x = 0 and 4, with and without the application of uniaxial pressure. A TE102 microwave cavity mode internally ignited the samples using less than 50 watts. The resultant microstructure of the microwave processed samples were compared to SHS products obtained using a conventional hot wire ignition technique. Uniaxial stresses along the vertical axis of approximately 1400 and 200 psi were applied simultaneously as the SHS reactions were initiated in some of the conventional and microwave processed samples, respectively. SEM photomicrographs will be presented that show the different microstructural features of the products using the microwave and hot wire techniques.

ceramic microwave ceramic-metal composites microwa↗
NASA NTRS1995

(abstract) Venus Gravity Field

A global gravity field model of Venus to degree and order 75 (5772 spherical harmonic coefficients) has been estimated from Doppler radio tracking of the orbiting spacecraft Pioneer Venus Orbiter (1979-1992) and Magellan (1990-1994). After the successful aerobraking of Magellan, a near circular polar orbit was attained and relatively uniform gravity field resolution (approximately 200 km) was obtained with formal uncertainties of a few milligals. Detailed gravity for several highland features are displayed as gravity contours overlaying colored topography. The positive correlation of typography with gravity is very high being unlike that of the Earth, Moon, and Mars. The amplitudes are Earth-like, but have significantly different gravity-topography ratios for different features. Global gravity, geoid, and isostatic anomaly maps as well as the admittance function are displayed.

Venus gravity field model gravity-topography ratio↗
NASA NTRS1994

(abstract) PV Technology for Low Intensity, Low Temperature (LILT) Applications

As a result of the recent NASA emphasis on smaller, lower cost space missions, PV is now being considered for a number of missions operating at solar distances of 3 AU or greater. In the past, many of these missions would utilize an RTG (radioisotope thermoelectric generator). Historically, silicon solar cell behavior at these distances has been compromised by a number of mechanisms including shunting, nonohmic back contacts, and the 'broken knee' curve shape. The former two can usually be neglected for modern silicon cells, but the latter has not been eliminated. This problem has been identified with localized diffusion at the top contact/silicon interface which leads to structural changes at the local junction. This is believed to create a resistive metal-semiconductor-like (MSL) interface in parallel with the junction which results in the characteristic forms of the LILT (low intensity, low temperature) 'broken knee'. This paper discusses a TaSiN contact barrier that will prevent the MSL structure in the junction.

spacecraft power LILT low temperature low intensit↗
NASA NTRS1995

(abstract) ARGOS: a System to Monitor Ulysses Nutation and Thruster Firings from Variations of the Spacecraft Radio Signal

Ulysses is a spin-stabilized spacecraft that experienced significant nutation after its launch in October 1990. This was due to the Sun-spacecraft-Earth geometry, and a study of the phenomenon predicted that the nutation would again be a problem during 1994-95. The difficulty of obtaining nutation estimates in real time from the spacecraft telemetry forced the ESA/NASA Ulysses Team to explore alternative information sources. The work performed by the ESA Operations Team provided a model for a system that uses the radio signal strength measurements to monitor the spacecraft dynamics. These measurements (referred to as AGC) are provided once per second by the tracking stations of the DSN. The system was named ARGOS (Attitude Reckoning from Ground Observable Signals) after the ever-vigilant, hundred-eyed giant of Greek Mythology. The ARGOS design also included Doppler processing, because Doppler shifts indicate thruster firings commanded by the active nutation control carried out onboard the spacecraft. While there is some visibility into thruster activity from telemetry, careful processing of the high-sample-rate Doppler data provides an accurate means of detecting the presence and time of thruster firings. DSN Doppler measurements are available at a ten-per-second rate in the same tracking data block as the AGC data.

Ulysses nutation Doppler radio signal strength mea↗
NASA NTRS1994

(abstract) Neptune: CO and HCN Distributions from Observations at the CSO

We used the Caltech Submillimeter Observatory (CSO ) to make spatially unresolved observations of the 230.538 GHz 2-1 transition of CO and the 265.886 GHz 3-2 transition of HCN at Neptune. All observations used the 10.4 meter antenna with a double sideband SIS junction heterodyne receiver. Spectra were observed with parallel acousto-optic spectrometers simultaneously providing 580 MHz bandwidth at 0.57 MHz resolution and 50 MHz bandwidth at 0.049 MHz resolution. Observed line shapes agree with prior observations by Rosenqvist et al and Marten et al. Analysis of the line shapes and intensities will yield information about the distributions of CO and HCN in the stratosphere of Neptune, and these results will be compared with previous results. The data imply that the mixing ratios of CO and HCN must decrease with altitude somewhere within the pressure range from 1 to 0.001 mbar. HCN data will also address the sharp saturation-induced decrease in the HCN mixing ratio at the lower levels in the stratosphere.

Caltech Submillimeter Observatory SRO CO carbon mo↗