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At least 1,441 records · Page 80
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NASA NTRS1994

Comparison of methods of predicting community response to impulsive and nonimpulsive noise

Several scientific, regulatory, and policy-coordinating bodies have developed methods for predicting community response to sonic booms. The best known of these is the dosage-response relationship of Working Group 84 of the National Academy of Science's Committee on Hearing, Bioacoustics and Biomechanics. This dosage-response relationship between C-weighted DayNight Average Sound Level and the prevalence of annoyance with high energy impulsive sounds was derived from limited amounts of information about community response to regular, prolonged, and expected exposure to artillery and sonic booms. U.S. Army Regulation 201 adapts this approach to predictions of the acceptability of impulsive noise exposure in communities. This regulation infers equivalent degrees of effect with respect to a well known dosage-response relationship for general (nonimpulsive) transportation noise. Differences in prevalence of annoyance predicted by various relationships lead to different predictions of the compatibility of land uses with sonic boom exposure. An examination of these differences makes apparent several unresolved issues in current practice for predicting and interpreting the prevalence of annoyance due to sonic boom exposure.

Fidell, Sanford↗
NASA NTRS1994

Does training-induced orthostatic hypotension result from reduced carotid baroreflex responsiveness?

As manned space travel has steadily increased in duration and sophistication, the answer to a simple, relevant question remains elusive. Does endurance exercise training - high intensity rhythmic activity, performed regularly for extended periods of time - alter the disposition to, or severity of, postflight orthostatic hypotension? Research results continue to provide different views; however, data are difficult to compare because of the following factors that vary between investigations: the type of orthostatic stress imposed (+Gz, lower body negative pressure (LBNP), head-up tilt); pretest perturbations used (exercise, heat exposure, head-down tilting, bed rest, water immersion, hypohydration, pharmacologically-induced diuresis); the length of the training program used in longitudinal investigations (days versus weeks versus months); the criteria used to define fitness; and the criteria used to define orthostatic tolerance. Generally, research results indicate that individuals engaged in aerobic exercise activities for a period of years have been reported to have reduced orthostatic tolerance compared to untrained control subjects, while the results of shorter term longitudinal studies remain equivocal. Such conclusions suggest that chronic athletic training programs reduce orthostatic tolerance, whereas relatively brief (days to weeks) training programs do not affect orthostatic tolerance to any significant degree (increase or decrease). A primary objective was established to identify the alterations in blood pressure control that contribute to training-induced orthostatic hypotension (TIOH). Although any aspect of blood pressure regulation is suspect, current research has been focused on the baroreceptor system. Reductions in carotid baroreflex responsiveness have been documented in exercise-trained rabbits, reportedly due to an inhibitory influence from cardiac afferent, presumably vagal, nerve fibers that is abolished with intrapericardiac denervation. The purpose of this investigation was to attempt to determine if similar relationships existed in men with varied levels of fitness, using maximal aerobic power, VO2 max, as the marker of fitness.

Pawelczyk, James A.↗
NASA NTRS1994

The use of biomechanics in the study of movement in microgravity

As biomechanists interested in the adaptability of the human body to microgravity conditions, it appears that our job is not only to make sure that the astronauts can function adequately in space but also that they can function upon their return to Earth. This is especially significant since many of the projects now being designed at NASA concern themselves with humans performing for up to 3 years in microgravity. While the Extended Duration Orbiter flights may last 30 to 60 days, future flights to Mars using current propulsion technology may last from 2 to 3 years. It is for this range of time that the adaptation process must be studied. Specifically, biomechanists interested in space travel realize that human performance capabilities will change as a result of exposure to microgravity. The role of the biomechanist then is to first understand the nature of the changes realized by the body. These changes include adaptation by the musculoskeletal system, the nervous system, cardiorespiratory system, and the cardiovascular system. As biomechanists, it is also our role to take part in the development of countermeasure programs that involve some form of regular exercise. Exercise countermeasure programs should include a variety of modalities with full knowledge of the loads imposed on the body by these modalities. Any exercise programs that are to be conducted by the astronauts during space travel must consider the fact that the musculoskeletal and neuromuscular systems degrade as a function of flight duration. Additionally, it must be understood that the central nervous system modifies its output in the control of the human body during space flight and most importantly, we must prepare the astronauts for their return to one g.

Gregor, R. J.↗
NASA NTRS1994

The two-way time synchronization system via a satellite voice channel

A newly developed two-way time synchronization system is described in this paper. The system uses one voice channel at a SCPC satellite digital communication earth station, whose bandwidth is only 45 kHz, thus saving satellite resources greatly. The system is composed of one master station and one or several, up to sixty-two, secondary stations. The master and secondary stations are equipped with the same equipment, including a set of timing equipment, a synthetic data terminal for time synchronizing, and a interface unit between the data terminal and the satellite earth station. The synthetic data terminal for time synchronization also has an IRIG-B code generator and a translator. The data terminal of master station is the key part of whole system. The system synchronization process is full automatic, which is controlled by the master station. Employing an autoscanning technique and conversational mode, the system accomplishes the following tasks: linking up liaison with each secondary station in turn, establishing a coarse time synchronization, calibrating date (years, months, days) and time of day (hours, minutes, seconds), precisely measuring the time difference between local station and the opposite station, exchanging measurement data, statistically processing the data, rejecting error terms, printing the data, calculating the clock difference and correcting the phase, thus realizing real-time synchronization from one point to multiple points. We also designed an adaptive phase circuit to eliminate the phase ambiguity of the PSK demodulator. The experiments have shown that the time synchronization accuracy is better than 2 mu S. The system has been put into regular operation.

Heng-Qiu, Zheng↗
NASA NTRS1993

Geoid-to-topography ratios on Venus: A global perspective

Recently available spherical harmonic solutions for the geoid and topography of Venus are sufficiently high resolution that they can be used to address questions concerning the relationship between geoid and topography on a regional scale. We have approached this question by mapping the geoid-to-topography ratio (GTR) on a systematic global basis. For a given point on the surface, we consider the geoid and elevation values at all points on a gridded representation of those fields located within a specified distance of the reference point. From the set of paired values, we determine the correlation coefficient and the best-fitting straight line. The latter is the GTR at that position, and the former is a measure of the significance of the derived ratio. This procedure is then repeated for all points on the global grid, yielding maps of the GTR and the correlation coefficient. Unlike previous studies of the GRT on Venus, this apprach permits us to make an objective and systematic search for regions with anomalous GTR's as well as areas that do not demonstrate any strong correlation between geoid and topography. These maps can be updated regularly as new harmonic models of the Venus geoid are produced from new Magellan tracking data. This procedure permits the development of a global perspective on the relationship between GTR and venusian surface tectonics.

Simons, Mark↗
NASA NTRS1994

Renormalization Group Theory of Bolgiano Scaling in Boussinesq Turbulence

Bolgiano scaling in Boussinesq turbulence is analyzed using the Yakhot-Orszag renormalization group. For this purpose, an isotropic model is introduced. Scaling exponents are calculated by forcing the temperature equation so that the temperature variance flux is constant in the inertial range. Universal amplitudes associated with the scaling laws are computed by expanding about a logarithmic theory. Connections between this formalism and the direct interaction approximation are discussed. It is suggested that the Yakhot-Orszag theory yields a lowest order approximate solution of a regularized direct interaction approximation which can be corrected by a simple iterative procedure.

Rubinstein, Robert↗
NASA NTRS1994

Probabilistic assessment of uncertain adaptive hybrid composites

Adaptive composite structures using actuation materials, such as piezoelectric fibers, were assessed probabilistically utilizing intraply hybrid composite mechanics in conjunction with probabilistic composite structural analysis. Uncertainties associated with the actuation material as well as the uncertainties in the regular (traditional) composite material properties were quantified and considered in the assessment. Static and buckling analyses were performed for rectangular panels with various boundary conditions and different control arrangements. The probability density functions of the structural behavior, such as maximum displacement and critical buckling load, were computationally simulated. The results of the assessment indicate that improved design and reliability can be achieved with actuation material.

Shiao, Michael C.↗
NASA NTRS1994

An HP Adaptive Discontinuous Galerkin Method for Hyperbolic Conservation Laws

This dissertation addresses various issues for model classes of hyperbolic conservation laws. The basic approach developed in this work employs a new family of adaptive, hp-version, finite element methods based on a special discontinuous Galerkin formulation for hyperbolic problems. The discontinuous Galerkin formulation admits high-order local approximations on domains of quite general geometry, while providing a natural framework for finite element approximations and for theoretical developments. The use of hp-versions of the finite element method makes possible exponentially convergent schemes with very high accuracies in certain cases; the use of adaptive hp-schemes allows h-refinement in regions of low regularity and p-enrichment to deliver high accuracy, while keeping problem sizes manageable and dramatically smaller than many conventional approaches. The use of discontinuous Galerkin methods is uncommon in applications, but the methods rest on a reasonable mathematical basis for low-order cases and has local approximation features that can be exploited to produce very efficient schemes, especially in a parallel, multiprocessor environment. The place of this work is to first and primarily focus on a model class of linear hyperbolic conservation laws for which concrete mathematical results, methodologies, error estimates, convergence criteria, and parallel adaptive strategies can be developed, and to then briefly explore some extensions to more general cases. Next, we provide preliminaries to the study and a review of some aspects of the theory of hyperbolic conservation laws. We also provide a review of relevant literature on this subject and on the numerical analysis of these types of problems.

Bey, Kim S.↗
NASA NTRS1994

Observations of loops and prominences

We review recent observations by the Yohkoh-SXT (Soft X-ray Telescope) in collaboration with other spacecraft and ground-based observatories of coronal loops and prominences. These new results point to problems that SoHO will be able to address. With a unique combination of rapid-cadence digital imaging (greater than or equal to 32 s full-disk and greater than or equal to 2 s partial-frame images), high spatial resolution (greater than or equal to 2.5 arcsec pixels), high sensitivity (EM less than or equal to 10(exp 42) cm(exp -3)), a low-scatter mirror, and large dynamic range, SXT can observe a vast range of targets on the Sun. Over the first 21 months of Yohkoh operations SXT has taken over one million images of the corona and so is building up an invaluable long-term database on the large-scale corona and loop geometry. The most striking thing about the SXT images is the range of loop sizes and shapes. The active regions are a bright tangle of magnetic field lines, surrounded by a network of large-scale quiet-Sun loops stretching over distances in excess of 105 km. The cross-section of most loops seems to be constant. Loops displaying significant Gamma's are the exception, not the rule, implying the presence of widespread currents in the corona. All magnetic structures show changes. Time scales range from seconds to months. The question of how these structures are formed, become filled with hot plasma, and are maintained is still open. While we see the propagation of brightenings along the length of active-region loops and in X-ray jets with velocities of several hundred km/s, much higher velocities are seen in the quiet Sun. In XBP flares, for example, velocities of over 1000 km/s are common. Active-region loops seem to be in constant motion, moving slowly outward, carrying plasma with them. During flares, loops often produce localized brightenings at the base and later at the apex of the loop. Quiescent filaments and prominences have been observed regularly. Their coronal manifestation seems to be an extended arcade of loops overlying the filament. Reliable alignment of the ground-based data with the X-ray images make it possible to make a detailed intercomparison of the hot and cold plasma structures over extended periods. Hence we are able to follow the long-term evolution of these structures and see how they become destabilized and erupt.

Strong, Keith T.↗
NASA NTRS1994

Sea ice lead dynamics from ERS-1 SAR

The use of ERS-1 SAR (Synthetic Aperture Radar) images to measure ice lead changes and ice deformation, and thus to determine their relationships and compare them to theory, is reported. ERS-1 SAR data collected at the Alaska (U.S.) SAR facility are processed into low resolution geocoded images which are used as input to the Geophysical Processor System (GPS). The GPS uses cross correlation and edge detection on sequential pairs of images to track a regular 5 km array of grid points. The result for each sequential image pair is a field of ice displacement that contains, on average, more than 200 vectors. The GPS also classifies all the pixels of the images into one of four ice types based on the normalized backscatter coefficients and a seasonal look-up table. This allows all pixels to be labeled as either multiyear ice or as the first year ice and open water that comprise leads.

Stern, Harry L.↗
NASA NTRS1994

Automatic generation of efficient array redistribution routines for distributed memory multicomputers

Appropriate data distribution has been found to be critical for obtaining good performance on Distributed Memory Multicomputers like the CM-5, Intel Paragon and IBM SP-1. It has also been found that some programs need to change their distributions during execution for better performance (redistribution). This work focuses on automatically generating efficient routines for redistribution. We present a new mathematical representation for regular distributions called PITFALLS and then discuss algorithms for redistribution based on this representation. One of the significant contributions of this work is being able to handle arbitrary source and target processor sets while performing redistribution. Another important contribution is the ability to handle an arbitrary number of dimensions for the array involved in the redistribution in a scalable manner. Our implementation of these techniques is based on an MPI-like communication library. The results presented show the low overheads for our redistribution algorithm as compared to naive runtime methods.

Ramaswamy, Shankar↗
NASA NTRS1994

Fixed-head star tracker attitude updates on the Hubble Space Telescope

The Hubble Space Telescope (HST) was launched in April 1990 to begin observing celestial space to the edge of the universe. National Aeronautics and Space Administration (NASA) standard fixed-head star trackers (FHST's) are used operationally onboard the HST to regularly adjust ('update') the spacecraft attitude before the acquisition of guide stars for science observations. During the first 3 months of the mission, the FHST's updated the spacecraft attitude successfully only 85 percent of the time. During the other periods, the trackers were unable to find the selected stars -- either they failed to find any star, or worse, they selected incorrect stars and produced erroneous attitude updates. In July 1990, the HST project office at Goddard Space Flight Center (GSFC) requested that Computer Sciences Corporation (CSC) form an investigative 'tiger' team to examine these FHST update failures. This paper discusses the work of the FHST tiger team, describes the investigations that led the team to identify the sources of the errors, and defines the solutions that were subsequently developed, which ultimately increased the success rate of FHST updates to approximately 98 percent.

Nadelman, Matthew S.↗
NASA NTRS1994

A long-baseline method for HST gyro drift rate bias calibration

The routine on-orbit calibration of the Hubble Space Telescope (HST) rate gyro assemblies (RGA's) has depended on several related algorithms for drift rate bias calibration. The gyros have exhibited time-varying biases, which must be regularly corrected to maintain pointing stability. Currently, gyro drift parameters are uplinked to the spacecraft every 1-2 days for low rate mode and every 7 days for high rate mode. In order to minimize the impact of frequent calibrations on the HST science schedule, we have refined the gyro calibration algorithms and data collection schemes to reduce the amounts of telemetry data and processing time required for accurate bias calibration. We present a review of the evolution of the gyro calibration algorithms, with particular attention to what we have called the long-baseline bias (LBBIAS) technique, and describe the relative success of these methods in maintaining spacecraft stability.

Boia, John J.↗
NASA NTRS1994

The in-flight calibration of the Hubble Space Telescope fine guidance sensors, 2 (a success story)

The Hubble Space Telescope's fine guidance sensors (FGS's) are unique in the performance levels being attempted; spacecraft control and astrometric research with accuracies better than 3 milli-arcseconds (mas) are the ultimate goals. This paper presents a review of the in-flight calibration of the sensors, describing both the algorithms used and the results achieved to date. The work was done primarily in support of engineering operations related to spacecraft pointing and control and secondarily in support of the astrometric science calibration effort led by the Space Telescope Astrometry Team. Calibration items of principal interest are distortion, sensor magnification, and relative alignment. An initial in-flight calibration of the FGS's was performed in December 1990; this calibration has been used operationally over the past few years. Followup work demonstrated that significant, unexpected temporal variations in the calibration parameters are occurring; provided good characterization of the variation; and set the stage for a distortion calibration designed to achieve the full design accuracy for one of the FGS's. This full distortion calibration, using data acquired in January 1993, resulted in a solution having single-axis residuals with a standard deviation of 2.5 mas. Scale and alignment calibration results for all of the FGS's have been achieved commensurate with the best ground-based astrometric catalogs (root-mean-square error approximately 25 mas). A calibration monitoring program has been established to allow regular updates of the calibration parameters as needed.

Welter, Gary L.↗
NASA NTRS1994

Patterns of brittle deformation under extension on Venus

The development of fractures at regular length scales is a widespread feature of Venusian tectonics. Models of lithospheric deformation under extension based on non-Newtonian viscous flow and brittle-plastic flow develop localized failure at preferred wavelengths that depend on lithospheric thickness and stratification. The characteristic wavelengths seen in rift zones and tessera can therefore provide constraints on crustal and thermal structure. Analytic solutions were obtained for growth rates in infinitesimal perturbations imposed on a one-dimensional, layered rheology. Brittle layers were approximated by perfectly-plastic, uniform strength, overlying ductile layers exhibiting thermally-activated power-law creep. This study investigates the formation of faults under finite amounts of extension, employing a finite-element approach. Our model incorporates non-linear viscous rheology and a Coulomb failure envelope. An initial perturbation in crustal thickness gives rise to necking instabilities. A small amount of velocity weakening serves to localize deformation into planar regions of high strain rate. Such planes are analogous to normal faults seen in terrestrial rift zones. These 'faults' evolve to low angle under finite extension. Fault spacing, orientation and location, and the depth to the brittle-ductile transition, depend in a complex way on lateral variations in crustal thickness. In general, we find that multiple wavelengths of deformation can arise from the interaction of crustal and mantle lithosphere.

Neumann, G. A.↗
NASA NTRS1993

Applying formal methods and object-oriented analysis to existing flight software

Correctness is paramount for safety-critical software control systems. Critical software failures in medical radiation treatment, communications, and defense are familiar to the public. The significant quantity of software malfunctions regularly reported to the software engineering community, the laws concerning liability, and a recent NRC Aeronautics and Space Engineering Board report additionally motivate the use of error-reducing and defect detection software development techniques. The benefits of formal methods in requirements driven software development ('forward engineering') is well documented. One advantage of rigorously engineering software is that formal notations are precise, verifiable, and facilitate automated processing. This paper describes the application of formal methods to reverse engineering, where formal specifications are developed for a portion of the shuttle on-orbit digital autopilot (DAP). Three objectives of the project were to: demonstrate the use of formal methods on a shuttle application, facilitate the incorporation and validation of new requirements for the system, and verify the safety-critical properties to be exhibited by the software.

Cheng, Betty H. C.↗
NASA NTRS1993

Distribution and motions of atomic hydrogen in lenticular galaxies. X - The blue S0 galaxy NGC 5102

Results of the mapping of the blue gas-rich S0 galaxy NGC 5102 in the 21-cm H I line with a spatial resolution of 34 x 37 arcsec (delta(alpha) x Delta(delta)) and a velocity resolution of 12 km/s are presented. The H I distribution has a pronounced central depression of 1.9 kpc radius, and most of the H I is concentrated in a 3.6 kpc wide ring with an average radius of 3.7 kpc, assuming a distance of 4 Mpc for NGC 5102. The maximum azimuthally averaged H I surface density in the ring is 1.4 solar mass/sq pc, comparable to that found in other S0 galaxies. The HI velocity field is quite regular, showing no evidence for large-scale deviations from circular rotation, and the H I is found to rotate in the plane of the stellar disk. Both the H I mass/blue luminosity ratio and the radial H I distribution are similar to those in early-type spirals. The H I may be an old disk or it may have been acquired through capture of a gas-rich smaller galaxy. The recent starburst in the nuclear region, which gave the galaxy its blue color, may have been caused by partial radial collapse of the gas disk, or by infall of a gas-rich dwarf galaxy.

Van Woerden, H.↗
NASA NTRS1994

Development of human epithelial cell systems for radiation risk assessment

The most important health effect of space radiation for astronauts is cancer induction. For radiation risk assessment, an understanding of carcinogenic effect of heavy ions in human cells is most essential. In our laboratory, we have successfully developed a human mammary epithelial cell system for studying the neoplastic transformation in vitro. Growth variants were obtained from heavy ion irradiated immortal mammary cell line. These cloned growth variants can grow in regular tissue culture media and maintain anchorage dependent growth and density inhibition property. Upon further irradiation with high-Linear Energy Transfer (LET) radiation, transformed foci were found. Experimental results from these studies suggest that multiexposure of radiation is required to induce neoplastic tranformation of human epithelial cells. This multihits requirement may be due to high genomic stability of human cells. These growth variants can be useful model systems for space flight experiments to determine the carcinogenic effect of space radiation in human epithelial cells.

Yang, C. H.↗