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At least 307 records · Page 17
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NASA NTRS1987

Performance evaluation of fault tolerant systems represented by Markov models

A method to evaluate the performance of fault tolerant systems whose configuration can be represented by time-invariant, discrete-time, discrete-state Markov models is introduced. Each state is assumed to be associated with a constant qualitative measure of the system's performance. The method first computes the moments of the performance probability mass function (PMF) and then finds an approximating function that has the same moments. The form of this function is a maximum entropy solution of the moment matching problem. A simple algorithm for calculating the necessary moments is derived and a method for finding the approximate performance PMF is suggested. After some modification, the method is applied to an example, the Inertial Upper Stage navigation system.

Missana, Jean-Olivier A. A.↗
NASA NTRS1987

A laboratory demonstration of high-resolution hard X-ray and gamma-ray imaging using Fourier-transform techniques

A laboratory imaging system has been developed to study the use of Fourier-transform techniques in high-resolution hard X-ray and gamma-ray imaging, with particular emphasis on possible applications to high-energy astronomy. Considerations for the design of a Fourier-transform imager and the instrumentation used in the laboratory studies is described. Several analysis methods for image reconstruction are discussed including the CLEAN algorithm and maximum entropy methods. Images obtained using these methods are presented.

Palmer, David↗
NASA NTRS1987

Far-infrared detection of a circumstellar dust torus around Sharpless 106

High angular resolution scans at 42 and 95 microns have been obtained of the far-infrared source associated with S106. Using maximum entropy deconvolution techniques it is found that the far-IR-emitting dust appears to be distributed in a toroidal shape around the central star.This result is similar to the morphology implied to millimeter line and continuum mapping on a variety of size scales.

Harvey, Paul M.↗
NASA NTRS1987

A scanning modulation collimator observation of the high-energy X-ray source in the Crab Nebula

Two-dimensional maps of the 22-64 keV emission from the Crab Nebula with an angular resolution of 15 arcsec has been synthesized. The maps are generated by application of a maximum entropy method operating on a series of one-dimensional scans obtained with a balloon-borne modulation collimator telescope. The two-dimensional size, shape and orientation of the hard X-ray nebula relative to the pulsar have been measured for the first time. The implications of these results for models of electron transport in the Crab are discussed, and the geometry of the observed X-ray nebula is related to other features of the Crab Nebula system.

Pelling, R. M.↗
NASA NTRS1987

Aspects of interplanetary plasma turbulence

Data acquired in the free solar wind by the ISEE 1 and ISEE 2 propagation experiment were used to obtain the power spectra of fluctuations in electron density; a maximum-entropy technique has made it possible to exploit fully the intrinsic precision of the in situ measurements. The results have led to a new determination of the anisotropy of density fluctuations in the solar wind, which is in conflict with certain expectations based on the propagation of Langmuir waves, but is in agreement with multistation interplanetary-scintillation measurements: it could well be that Langmuir wave growth rates during type III solar bursts are much larger than has been inferred from the electron distribution function, whose measurement involves averaging over relatively long time intervals. The relation between the fluctuation level and the mean density is studied, and tentative evidence for a correlation between the relative fluctuation level and proton temperature is presented.

Celnikier, L. M.↗
NASA NTRS1986

Narrow band imagery in the 8-14 micron spectral region

The problem of deconvolution of the point source response function (PSF) for two-dimensional imaging infrared arrays is addressed. A variety of phenomena can lead to PSF degradation, ranging from atmospheric turbulence to the array itself. The maximum entropy algorithm is used to demonstrate that substantial image quality improvements are possible in the case of a PSF which is degraded by electronic rather than optical smearing. This result implies that IR arrays need not have perfect or even very good pixel-pixel crosstalk rejection ratios for use in astronomy.

Goebel, J. H.↗
NASA NTRS1987

Far-infrared continuum emission from the nucleus, starburst, and extended spiral arms of NGC 1068

Far-infrared slit scans across NGC 1068 are used to better define the contribution of the active nucleus, starburst disk, and extended spiral arms in the luminous output of this bright Seyfert 2 galaxy. The decomposition of the emission from the bright, inner 3 kpc of this galaxy suggests that at 50 microns, about half the emission is from star formation in a starburst disk and half from the compact, centrally heated circumnuclear cloud. The 100-micron profile of the starburst disk is asymmetric with respect to the nucleus. This appears to be the result of an extraordinary star-forming complex about a kiloparsec away from the center. With the application of maximum entropy deconvolution to the scans, evidence is found for 100-micron emission from the extended spiral arms of the galaxy. The far-infrared emission from the starburst region is distinguished from that of the extended spiral arms. In terms of both total luminosity and surface brightness, the extended spiral arms of NGC 1068 are at the bright end of the range of far-infrared emission from quiescent, noninteracting spirals.

Lester, D. F.↗
NASA NTRS1988

Performance evaluation of fault-tolerant systems with application to the IUS

A new method for quantitatively evaluating the performance of fault-tolerant systems is developed and applied to an example. The method assumes that the random failure and diagnostic decision behavior of the system can be modeled by a finite state Markov process. A performance value must be assigned to each of the states of the model. The method then generates the moments of the probability mass function of the cumulative performance and uses these to generate a maximum entropy approximation to the performance PMF. Some computational considerations are discussed. The method is applied to a typical mission for the Inertial Upper Stage to examine the attitude accuracy performance of the inertial system.

Missana, J.-O.↗
NASA NTRS1989

IRAS LRS spectra of extended objects: The Crab nebula

The spectra of extended sources from the raw Infrared Astronomy Satellite (IRAS) Low Resolution Spectrometer (LRS) data, is recovered by means of a nonlinear deconvolution technique built around a maximum entropy algorithm. The results are applied to the Crab Nebula. The deconvolved spectrum of the Crab shows significant departures from a blackbody spectrum, with emission features present at 10.2, 11.3, 12.3 and 12.8 microns.

Lynch, D. K.↗
NASA NTRS1989

High spatial resolution far-infrared scans of W3(OH)

The strong far-infrared continuum source associated with the OH maser-ultracompact H II region W3(OH) was observed from the Kuiper Airborne Observatory in order to determine its size at 50 and 100 microns. Maximum entropy deconvolutions of the slit scans made in orthogonal directions across the source show it to be approximately circular with FWHM sizes of 8 arcsec and 13 arcsec at 50 and 100 microns, respectively. For these sizes, W3(OH) is optically thick in the far-infrared. A spherically symmetric, dusty molecular cloud model is developed to fit the data.

Campbell, M. F.↗
NASA NTRS1989

Detection of elliptical polarization and mode splitting in discrete Schumann resonance excitations

Elliptical polarization and mode splitting have been detected in the magnetic component of discrete, well defined Schumann resonance excitations. These ELF excitations, which are large electromagnetic transients of approximately 1 s duration, are called Q-bursts and typically occur every few minutes. They are believed to be the signature of the impulsive excitation of the earth-ionosphere cavity by ultra-large lightning currents. In this paper the magnetic polarization and spectral characteristics of four large Q-bursts are examined in detail using a new analysis technique. Two events display right-hand polarization and two display left-hand polarization. The theoretical polarization properties of the central and side multiplets of the Schumann resonances are used to define a local orthogonal coordinate system in the measurement frame in which these components may be separated. Maximum entropy spectrums computed separately for what are identified to be the central and side multiplets in this coordinate system show distinctly different eigenfrequencies for the lowest mode near 7.5 Hz. For the limited number of cases examined the magnitude of the line splitting detected using this technique is roughly 1.4-1.8 Hz, larger by nearly a factor of two than theoretical or observed values of the splitting previously reported. The frequencies of the side multiplets may lie either above or below the frequency of the central multiplet.

Sentman, D. D.↗
NASA NTRS1989

Unstable vortices in the near region of an internal flow cavity

This paper presents experimental data taken in the forward region of a separated internal free-shear layer produced in an internal cavity flowfield. It has been found that in the region very near the forward restrictor, experimental velocity profiles agree closely with the exact Stuart instability velocity profile with various values of a steepness parameter. Reynolds shear-stress profiles suggest the presence of counter-rotating longitudinal vortices. Spectral analysis by the maximum entropy method of the time samples within the vortices indicates subharmonic and harmonic components of the fundamental frequency, with a weak indication of the fundamental frequency itself.

Isaacson, L. K.↗
NASA NTRS1990

Radio-source structure in astrometric and geodetic very long baseline interferometry

An algorithm to calculate source structure corrections for bandwidth synthesis delays and delay rates is used to refer the position of extragalacitc radio sources to a specific feature of each source's morphology. The delay and delay rate are obtained from VBLI observations. The algorithm and its theoretical basis are described, and simulations for a simple two-component source are discussed. VLBI data for the complex radio source NRAO 140 are analyzed. For this source, structure corrections are calculated with maps from three different VLBI imaging techniques: the hybrid-CLEAN algorithm, the maximum entropy method, and model fitting. The calculated structure corrections are compared with brightness distributions to the actual data. The results are used to map NRAO 140. It is found that the CLEAN map produces the most accurate structure corrections.

Charlot, P.↗
NASA NTRS1990

Four experimental demonstrations of active vibration control for flexible structures

Laboratory experiments designed to test prototype active-vibration-control systems under development for future flexible space structures are described, summarizing previously reported results. The control-synthesis technique employed for all four experiments was the maximum-entropy optimal-projection (MEOP) method (Bernstein and Hyland, 1988). Consideration is given to: (1) a pendulum experiment on large-amplitude LF dynamics; (2) a plate experiment on broadband vibration suppression in a two-dimensional structure; (3) a multiple-hexagon experiment combining the factors studied in (1) and (2) to simulate the complexity of a large space structure; and (4) the NASA Marshall ACES experiment on a lightweight deployable 45-foot beam. Extensive diagrams, drawings, graphs, and photographs are included. The results are shown to validate the MEOP design approach, demonstrating that good performance is achievable using relatively simple low-order decentralized controllers.

Phillips, Doug↗
NASA NTRS1989

Distribution of dust in W31 complex

W31 is a H II region/molecular cloud complex in the galactic plane at a distance of 6 Kpc. This complex consists of two prominent radio continuum sources (G10.2-0.3 and G10.3-0.1) representing H II regions. An extended region covering both these H II regions was mapped in the Far IR (FIR) using the TIFR 1 m balloon-borne telescope with an angular resolution of approx. 1 min and a dynamic range of 100. The resulting flux density distribution at an effective wavelength of 160 microns is presented. The coadded IRAS survey scan data at 60 and 100 microns were deconvolved using a maximum entropy method to generate the flux density maps of the same region. These 60 and 100 micron maps are given and are briefly discussed.

Ghosh, Swarna Kanti↗
NASA NTRS1989

Spectral and correlation analysis with applications to middle-atmosphere radars

The correlation and spectral analysis methods for uniformly sampled stationary random signals, estimation of their spectral moments, and problems arising due to nonstationary are reviewed. Some of these methods are already in routine use in atmospheric radar experiments. Other methods based on the maximum entropy principle and time series models have been used in analyzing data, but are just beginning to receive attention in the analysis of radar signals. These methods are also briefly discussed.

Rastogi, Prabhat K.↗
NASA NTRS1990

High spatial resolution mapping of the Cepheus A region at 20, 50, and 100 microns

The Cepheus A region at 20 microns has been mapped, and a bright, unresolved central source with a peak flux of 41 Jy into a 4.0 arcsec x 4.3 arcsec beam has been detected. One-dimensional slit scans have been obtained in two perpendicular directions across Cepheus A at 50 and 100 microns using a high spatial-resolution sampling technique. The temperature and optical depth profile of the far-IR emission are determined, and a peak temperature of 46 K and peak 100 micron optical depth of 0.4 are derived. Maximum entropy deconvolution of the far-IR scans indicate that there is a bright central core with clumpy extended emission. Within errors the position of the 50/100 micron peak is the same as the 20-micron source; this is interpreted as indicating that the 20 micron radiation represents the short-wavelength emission from the core of the dust cloud seen in the far-IR. It is concluded that an embedded source provides the major source of luminosity in the Cepheus A region.

Ellis, H. B., Jr.↗
NASA NTRS1990

Vertical structure and characteristics of 23-60 day (zonal) oscillations over the tropical latitudes during the winter months of 1986 - Results of equatorial wave campaign-II

Results are presented of the equatorial wave campaign-II, a meteorological rocket study which was part of the Indian Middle Atmosphere Program. The equatorial wave campaign-II was conducted from Shar, India (13.7 deg N, 80.2 deg E) from January 15-February 28, 1986. By means of high altitude balloon and the RH-200 meteorological rocket, winds were measured from ground level up to 60 km altitude once each day during the 45-day period. The oscillation frequencies of the deviations in the east-west component of the winds from their mean at each 1-km height interval are obtained by the maximum entropy method. The phases and amplitudes of these frequencies are determined by use of the least squares method on the wind variation time series. Enhanced wave activity is shown to take place in the troposphere and lower mesosphere. The tropospheric waves observed suggest themselves to be Rossby waves of extratropical origin penetrating to tropical latitudes. The observed stratospheric/mesospheric waves appear to emanate from a source around the stratopause.

Raghavarao, R.↗