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At least 631 records · Page 35

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NASA NTRS1986

Effects of noise on ERA-identified modal parameters

The basic concept of the Eigensystem Realization Algorithm (ERA) for modal parameter identification and model reduction is extended to minimize the distortion of the identified parameters caused by noise. The mathematical foundation for the properties of accuracy indicators such as the singular values of the data matrix and modal amplitude coherence is provided based on knowledge of the noise characteristics. These indicators quantitatively discriminate noise from system information and are used to reduce the realized system model to a better approximation of the true model. Monte Carlo simulations are included to support the analytical studies.

Juang, J.-N.↗

NASA NTRS1987

Identifying Vibration Parameters in Large Structures

Analytical method separately examines low-frequency and highfrequency behavior. Report discusses techniques for identifying rigid-body parameters, flexible-body parameters, and quasi-static disturbances in large structures. Techniques developed to aid in planning space station. Though effort directed towards space station dynamic model and environment, proposed methodology for systems identification is generic enough to allow application to arbitrarily large space structures and spacecraft configurations.

Bayard, D. S.↗

NASA NTRS1987

Structural Error and Identifiability in Mathematical Models

Errors and previously implicit assumptions treated explicitly. Paper discusses errors in mathematical models of physical systems and problem of identifying model from system input and output. Problem approached by explicitly taking account of erroneous choices of model structure. Paper concerned specifically with linear or weakly nonlinear models.

Hadaegh, F. Y.↗

NASA NTRS1986

Parameter identifiability under approximation

The problem of injectivity of the parameter-to-state map is discussed for Galerkin approximations of a linear parabolic equation. A necessary and sufficient condition is derived and illustrated by means of simple examples. Finally, output least squares identifiability of the Galerkin approximations is discussed.

Kunisch, K.↗

NASA NTRS1987

Distinct meteoroid families identified on the lunar seismograms

The meteoroid impact-seismic activity data recorded by the Apollo lunar seismic network is examined. The study investigates the difference in temporal distribution between large and small impacts, clustering of impacts in a two-dimensional space of the time of the year and the time of the month, and the relationship of these observations with terrestrial observations. Several distinct families of meteoroids impacting the moon are identified. Most meteoroids producing small impact-seismic events, including ones associated with cometary showers, appear to approach from retrograde heliocentric orbits. In contrast, most meteoroids associated with large impact-seismic events appear to approach from prograde orbits; the observation is consistent with a hypothesis that many of them represent stony asteroidal material. It is suggested that the previously reported discrepancy between lunar and terrestrial meteoroid-flux estimates may be due to the differences in lunar and terrestrial detection efficiency among various families of meteoroids.

Oberst, Jurgen↗

NASA NTRS1987

First polymer in space identified in Comet Halley

The heavy-ion mass spectrum obtained in the inner coma of Comet Halley with the PICCA instrument on the Giotto spacecraft has been examined. Short polymer chains of polyoxymethylene and their decay products are identified as the source for the spectrum with six mass peaks between about 45 and 120 atomic mass units. The properties of polyoxymethylene are consistent with many of the unexpected observations in the coma.

Huebner, W. F.↗

NASA NTRS1988

On identifying parent plutonic rocks from lunar breccia and soil fragments

Breccia fragments expected from a well-studied boulder of Stillwater anorthosite have been modeled to test the ability to identify parental rock types from examination of breccia and soil fragments. Depending on their size, the boulder fragments give distributions that suggest mixtures of rock types, including monominerallic anorthosite with subordinant amounts of more gabbroic anorthosite, anorthosite, and gabbro for small fragments. The distribution of FeO in samples of lunar ferroan anorthosite (FAN) indicates that FAN has a heterogeneous distribution of mafic minerals like the boulder.

Haskin, Larry A.↗

NASA NTRS1989

Observational goals for Max '91 to identify the causative agent for impulsive bursts

Recent studies of impulsive hard x ray and microwave bursts suggest that a propagating causative agent with a characteristic velocity of the order of 1000 km/s is responsible for these bursts. The results of these studies are summarized and observable distinguishing characteristics of the various possible agents are highlighted, with emphasis on key observational goals for the Max '91 campaigns. The most likely causative agents suggested by the evidence are shocks, thermal conduction fronts, and propagating modes of magnetic reconnection in flare plasmas. With new instrumentation planned for Max '91, high spatial resolution observations of hard x ray sources have the potential to identify the agent by revealing detailed features of source spatial evolution. Observations with the Very Large Array and other radio imaging instruments are of great importance, as well as detailed modeling of coronal loop structures to place limits on their density and temperature profiles. With the combined hard x ray and microwave imaging observations, aided by loop model results, the simplest causative agent to rule out would be the propagating modes of magnetic reconnection. To fit the observational evidence, reconnection modes would need to travel at approximately the same velocity (the Alfven velocity) in different coronal structures that vary in length by a factor of 10(exp 3). Over such a vast range in loop lengths, it is difficult to believe that the Alfven velocity is constant. Thermal conduction fronts would be suggested by sources that expand along the direction of B and exhibit relatively little particle precipitation. Particle acceleration due to shocks could produce more diverse radially expanding source geometries with precipitation at loop footprints.

Batchelor, D. A.↗

NASA NTRS1989

Identifying atmospheric monitoring needs for Space Station Freedom

The atmospheric monitoring needs for Space Station Freedom were identified by examining the following from an industrial hygiene perspective: the experiences of past missions; ground based tests of proposed life support systems; the unique experimental and manufacturing facilities; the contaminant load model; metabolic production; and a fire. A target list of compounds to be monitored is presented and information is provided relative to the frequency of analysis, concentration ranges, and locations for monitoring probes.

Casserly, Dennis M.↗

NASA NTRS1990

Identifying decohering paths in closed quantum systems

A specific proposal is discussed for how to identify decohering paths in a wavefunction of the universe. The emphasis is on determining the correlations among subsystems and then considering how these correlations evolve. The proposal is similar to earlier ideas of Schroedinger and of Zeh, but in other ways it is closer to the decoherence functional of Griffiths, Omnes, and Gell-Mann and Hartle. There are interesting differences with each of these which are discussed. Once a given coarse-graining is chosen, the candidate paths are fixed in this scheme, and a single well defined number measures the degree of decoherence for each path. The normal probability sum rules are exactly obeyed (instantaneously) by these paths regardless of the level of decoherence. Also briefly discussed is how one might quantify some other aspects of classicality. The important role that concrete calculations play in testing this and other proposals is stressed.

Albrecht, Andreas↗

NASA NTRS1989

New spectral features of stratospheric trace gases identified from high-resolution infrared balloon-borne and laboratory spectra

A new Michelson-type interferometer system operating in the infrared at very high resolution has been used to record numerous balloon-borne solar absorption spectra of the stratosphere, ground-based solar absorption spectra, and laboratory spectra of molecules of atmospheric interest. In the present work results obtained for several important stratospheric trace gases, HNO3, CIONO2, HO2NO2, NO2, and COF2, in the 8- to 12-micron spectral region are reported. Many new features of these gases have been identified in the stratospheric spectra. Comparison of the new spectra with line-by-line simulations shows that previous spectral line parameters are often inadequate and that new analysis of high-resolution laboratory and atmospheric spectra and improved theoretical calculations will be required for many bands. Preliminary versions of several sets of improved line parameters under development are discussed.

Goldman, A.↗

NASA NTRS1990

Identifying atmospheric monitoring needs for Space Station Freedom

The monitoring needs for Space Station Freedom were identified by examining: the experiences of past missions; ground based tests of proposed life support systems; a contaminant load model; metabolic production from an 8-person crew; and a fire scenario. Continuous monitoring is recommended for components critical for life support, and that intermittent analysis be provided for all agents that may exceed one-half the spacecraft maximum allowable concentration. The minimum monitoring effort recommended includes continuous monitoring for: N2, O2, CO2, CO, H2O, H2, CH4, nonmethane hydrocarbons, aromatic hydrocarbons, refrigerants, and halons. Information on over 70 compounds is presented on the rationale for monitoring the frequency of analysis, and concentration ranges.

Casserly, Dennis M.↗

NASA NTRS1990

Localize and identify the gravity sensing mechanism of plants

The machinery by which a plant transduces the gravity stimulus into a growth response is localized and identified at the cellular level. The fact that a plant grows unequally on the lower side of a horizontally placed stem implies that there must be an asymmetric distribution of some of the chemical substances involved in the growth response. The three most likely chemicals to cause this growth were determined to be potassium, calcium, or the growth hormone, indole-3-acetic acid (IAA). IAA was chosen for this study and the results present a fairly complete understanding of the transduction of the gravity stimulus.

Bandurski, Robert S.↗

NASA NTRS1990

O(-) identified at high temperatures in CaO-based catalysts for oxidative methane dimerization

A technique called charge-distribution analysis (CDA) is employed to study mobile charge carriers in the oxidation catalysts CaO, CaO with 11 percent Na2O, and CaO with 10 percent La2O3. A threshold temperature of about 550-600 C is identified at which highly mobile charge carriers are present, and the CDA studies show that they are O(-) states. The present investigation indicates the usefulness of CDA in catalysis research with pressed powder samples and gas/solid reactions.

Freund, F.↗

NASA NTRS1992

Newly identified main-sequence A stars with circumstellar dust

The IRAS Faint Source Survey data base and the ADDSCAN/SCANPI software are used to search for systems with circumstellar dust in two samples: all of the 62 A stars in Woolley's catalog, which lie near the main sequence and are typically within 25 pc of the sun, and all A stars in the Bright Star Catalogue with mV in the range 4-5 and v sin i not less than 100 km/s. In the first sample, 11 nearby A stars having (12)-(25) and (25)-(60) colors consistent with circumstellar dust are found. The analysis of the second sample demonstrates that the use of the FSS data base increases the likelihood of identifying A stars with circumstellar dust fainter than mV = 4. Three more new dusty systems are found in this sample. Beta Pictoris (A5 IV-V) is the only star within 25 pc of the sun that displays IR colors indicative of both warm and cool dust. It is inferred from the absence of cool dust in the vast majority of the systems that the dust disks of these stars are much less extended than that seen around Beta Pictoris.

Cheng, K.-P.↗

NASA NTRS1993

The sensitivity of identified modal parameters to sensor placement errors and construction tolerances

This paper examines the sensitivity of experimentally measured modal frequencies and mode shapes to structural reassembly and sensor placement errors on a suspended three-bay truss. The statistical variations of identified mode shapes and frequencies are measured by repeated experiments. Both parameters are shown to vary measurably more with reassembly than sensor placement errors. Also, a directional stiffness in the truss joints was found to cause a parameter dependence on member orientation during reconstruction.

Hinkle, Jason↗

NASA NTRS1992

Techniques used to identify tornado producing thunderstorms using geosynchronous satellite data

Satellite imagery in the outbreak region in the time prior to and during tornado occurrence was examined in detail to obtain descriptive characteristics of the anvil plume. These characteristics include outflow strength (UMAX), departure of anvil centerline from the storm relative ambient wind (MDA), storm relative ambient wind (SRAW), and maximum surface vorticity (SFCVOR). It is shown that by using satellite derived parameters which characterize the flow field in the anvil region, the occurrence and intensity of tornadoes, which the parent thunderstorm produces, can be identified. Analysis of the censored regression models revealed that the five explanatory variables (UMAX, MDA, SRAW, UMAX-2, and SFCVOR) were all significant predictors in the identification of tornadic intensity of a particular thunderstorm.

Schrab, Kevin J.↗

NASA NTRS1993

Physiologic mechanisms of circulatory and body fluid losses in weightlessness identified by mathematical modeling

Central volume expansion due to fluid shifts in weightlessness is believed to activate adaptive reflexes which ultimately result in a reduction of the total circulating blood volume. However, the flight data suggests that a central volume overdistention does not persist, in which case some other factor or factors must be responsible for body fluid losses. We used a computer simulation to test the hypothesis that factors other than central volume overdistention are involved in the loss of blood volume and other body fluid volumes observed in weightlessness and in weightless simulations. Additionally, the simulation was used to identify these factors. The results predict that atrial volumes and pressures return to their prebedrest baseline values within the first day of exposure to head down tilt (HDT) as the blood volume is reduced by an elevated urine formation. They indicate that the mechanisms for large and prolonged body fluid losses in weightlessness is red cell hemoconcentration that elevates blood viscosity and peripheral resistance, thereby lowering capillary pressure. This causes a prolonged alteration of the balance of Starling forces, depressing the extracellular fluid volume until the hematocrit is returned to normal through a reduction of the red cell mass, which also allows some restoration of the plasma volume. We conclude that the red cell mass becomes the physiologic driver for a large 'undershoot' of the body fluid volumes after the normalization of atrial volumes and pressures.

Simanonok, K. E.↗