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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.

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NASA NTRS1990

A high speed CMOS correlator

A full custom, 25 MHz, 1.6 microns CMOS Correlator chip is presented. The 5.15mm by 4.23mm chip performs either autocorrelation or crosscorrelation, consuming less than 10mW per channel. The correlator, designed for a space borne spectrometer, contains 32 channels. The 24 bit accumulator registers can be read independent of the input data path, in either 8 bit bytes, or 16 bit words. The device is cascadable and allows integration periods of up to 1.78 seconds, at 25 Megasamples/second. The controllers, for the input data path and the data output section, are implemented with Sequence Invariant State Machines.

Canaris, J.↗
NASA NTRS1992

Probabilistic evaluation of uncertainties and risks in aerospace components

This paper summarizes a methodology developed at NASA Lewis Research Center which computationally simulates the structural, material, and load uncertainties associated with Space Shuttle Main Engine (SSME) components. The methodology was applied to evaluate the scatter in static, buckling, dynamic, fatigue, and damage behavior of the SSME turbo pump blade. Also calculated are the probability densities of typical critical blade responses, such as effective stress, natural frequency, damage initiation, most probable damage path, etc. Risk assessments were performed for different failure modes, and the effect of material degradation on the fatigue and damage behaviors of a blade were calculated using a multi-factor interaction equation. Failure probabilities for different fatigue cycles were computed and the uncertainties associated with damage initiation and damage propagation due to different load cycle were quantified. Evaluations on the effects of mistuned blades on a rotor were made; uncertainties in the excitation frequency were found to significantly amplify the blade responses of a mistuned rotor. The effects of the number of blades on a rotor were studied. The autocorrelation function of displacements and the probability density function of the first passage time for deterministic and random barriers for structures subjected to random processes also were computed. A brief discussion was included on the future direction of probabilistic structural analysis.

Shah, A. R.↗
NASA NTRS1992

Applications of correlator chips in radio science

Spectral line observations in radio astronomy require simultaneous power estimation in many (often hundreds to thousands) frequency bins. Digital autocorrelation spectrometers, which appeared thirty years ago, are now being implemented in VLSI. The same architecture can be used to implement transversal digital filters. This was done at the Arecibo Observatory for pulse compression in radar observations of Venus.

Hagen, John B.↗
NASA NTRS1992

NASA SERC digital correlator projects

Interest in custom digital correlator processors is growing, in both the radio astronomy and earth sensing communities, as scientists realize that VLSI technology is available to them. This paper presents three digital correlator projects currently underway at the NASA SERC for VLSI Systems Design. The projects are as follows: a 60MHz chip for the ESTAR satellite; a 100MHz, 1024 channel autocorrelator; and a 64 MHz, VLSI system consisting of 8 32- channel complex crosscorrelators including phase rotators.

Canaris, John↗
NASA NTRS1992

Experiments with recursive estimation in astronomical image processing

Recursive estimation concepts were applied to image enhancement problems since the 70's. However, very few applications in the particular area of astronomical image processing are known. These concepts were derived, for 2-dimensional images, from the well-known theory of Kalman filtering in one dimension. The historic reasons for application of these techniques to digital images are related to the images' scanned nature, in which the temporal output of a scanner device can be processed on-line by techniques borrowed directly from 1-dimensional recursive signal analysis. However, recursive estimation has particular properties that make it attractive even in modern days, when big computer memories make the full scanned image available to the processor at any given time. One particularly important aspect is the ability of recursive techniques to deal with non-stationary phenomena, that is, phenomena which have their statistical properties variable in time (or position in a 2-D image). Many image processing methods make underlying stationary assumptions either for the stochastic field being imaged, for the imaging system properties, or both. They will underperform, or even fail, when applied to images that deviate significantly from stationarity. Recursive methods, on the contrary, make it feasible to perform adaptive processing, that is, to process the image by a processor with properties tuned to the image's local statistical properties. Recursive estimation can be used to build estimates of images degraded by such phenomena as noise and blur. We show examples of recursive adaptive processing of astronomical images, using several local statistical properties to drive the adaptive processor, as average signal intensity, signal-to-noise and autocorrelation function. Software was developed under IRAF, and as such will be made available to interested users.

Busko, I.↗
NASA NTRS1993

Fast simulation of packet loss rates in a shared buffer communications switch

This paper describes an efficient technique for estimating, via simulation, the probability of buffer overflows in a queueing model that arises in the analysis of ATM (Asynchronous Transfer Mode) communication switches. There are multiple streams of (autocorrelated) traffic feeding the switch that has a buffer of finite capacity. Each stream is designated as either being of high or low priority. When the queue length reaches a certain threshold, only high priority packets are admitted to the switch's buffer. The problem is to estimate the loss rate of high priority packets. An asymptotically optimal importance sampling approach is developed for this rare event simulation problem. In this approach, the importance sampling is done in two distinct phases. In the first phase, an importance sampling change of measure is used to bring the queue length up to the threshold at which low priority packets get rejected. In the second phase, a different importance sampling change of measure is used to move the queue length from the threshold to the buffer capacity.

Chang, Cheng-Shang↗
NASA NTRS1994

Numerical simulation of two-dimensional spatially-developing mixing layers

Two-dimensional, incompressible, spatially developing mixing layer simulations are performed at Re = 10(exp 2) and 10(exp 4) with two classes of perturbations applied at the inlet boundary; combinations of discrete modes from linear stability theory, and a broad spectrum of modes derived from experimentally measured velocity spectra. The effect of the type and strength of inlet perturbations on vortex dynamics and time-averaged properties are explored. Two-point spatial velocity and autocorrelations are used to estimate the size and lifetime of the resulting coherent structures and to explore possible feedback effects. The computed time-averaged properties such as mean velocity profiles, turbulent statistics, and spread rates show good agreement with experimentally measured values. It is shown that by forcing with a broad spectrum of modes derived from an experimental energy spectrum many experimentally observed phenomena can be reproduced by a 2-D simulation. The strength of the forcing merely affected the length required for the dominant coherent structures to become fully-developed. Thus intensities comparable to those of the background turbulence in many wind tunnel experiments produced the same results, given sufficient simulation length.

Wilson, R. V.↗
NASA NTRS1994

Dynamics of hard sphere colloidal dispersions

Our objective is to perform on homogeneous, fully equilibrated dispersions the full set of experiments characterizing the transition from fluid to solid and the properties of the crystalline and glassy solid. These include measurements quantifying the nucleation and growth of crystallites, the structure of the initial fluid and the fully crystalline solid, and Brownian motion of particles within the crystal, and the elasticity of the crystal and the glass. Experiments are being built and tested for ideal microgravity environment. Here we describe the ground based effort, which exploits a fluidized bed to create a homogeneous, steady dispersion for the studies. The differences between the microgravity environment and the fluidized bed is gauged by the Peclet number Pe, which measures the rate of convection/sedimentation relative to Brownian motion. We have designed our experiment to accomplish three types of measurements on hard sphere suspensions in a fluidized bed: the static scattering intensity as a function of angle to determine the structure factor, the temporal autocorrelation function at all scattering angles to probe the dynamics, and the amplitude of the response to an oscillatory forcing to deduce the low frequency viscoelasticity. Thus the scattering instrument and the colloidal dispersion were chosen such as that the important features of each physical property lie within the detectable range for each measurement.

Zhu, J. X.↗
NASA NTRS1994

Slant path L- and S-Band tree shadowing measurements

This contribution presents selected results from simultaneous L- and S-Band slant-path fade measurements through a pecan, a cottonwood, and a pine tree employing a tower-mounted transmitter and dual-frequency receiver. A single, circularly-polarized antenna was used at each end of the link. The objective was to provide information for personal communications satellite design on the correlation of tree shadowing between frequencies near 1620 and 2500 MHz. Fades were measured along 10 m lateral distance with 5 cm spacing. Instantaneous fade differences between L- and S-Band exhibited normal distribution with means usually near 0 dB and standard deviations from 5.2 to 7.5 dB. The cottonwood tree was an exception, with 5.4 dB higher average fading at S- than at L-Band. The spatial autocorrelation reduced to near zero with lags of about 10 lambda. The fade slope in dB/MHz is normally distributed with zero mean and standard deviation increasing with fade level.

Vogel, Wolfhard J.↗
NASA NTRS1994

Photogrammetric mobile satellite service prediction

Photographic images of the sky were taken with a camera through a fisheye lens with a 180 deg field-of-view. The images of rural, suburban, and urban scenes were analyzed on a computer to derive quantitative information about the elevation angles at which the sky becomes visible. Such knowledge is needed by designers of mobile and personal satellite communications systems and is desired by customers of these systems. The 90th percentile elevation angle of the skyline was found to be 10 deg, 17 deg, and 51 deg in the three environments. At 8 deg, 75 percent, 75 percent, and 35 percent of the sky was visible, respectively. The elevation autocorrelation fell to zero with a 72 deg lag in the rural and urban environment and a 40 deg lag in the suburb. Mean estimation errors are below 4 deg.

Akturan, Riza↗
NASA NTRS1995

The composite classification problem in optical information processing

Optical pattern recognition allows objects to be recognized from their images and permits their positional parameters to be estimated accurately in real time. The guiding principle behind optical pattern recognition is that a lens focusing a beam of coherent light modulated with an image produces the two-dimensinal Fourier transform of that image. When the resulting output is further transformed by the matched filter corresponding to the original image, one obtains the autocorrelation function of the original image, which has a peak at the origin. Such a device is called an optical correlator and may be used to recognize the locate the image for which it is designed. (From a practical perspective, an approximation to the matched filter must be used since the spatial light modulator (SLM) on which the filter is implemented usually does not allow one to independently control both the magnitude and phase of the filter.) Generally, one is not just concerned with recognizing a single image but is interested in recognizing a variety of rotated and scaled views of a particular image. In order to recognize these different views using an optical correlator, one may select a subset of these views (whose elements are called training images) and then use a composite filter that is designed to produce a correlation peak for each training image. Presumably, these peaks should be sharp and easily distinguishable from the surrounding correlation plane values. In this report we consider two areas of research regarding composite optical correlators. First, we consider the question of how best to choose the training images that are used to design the composite filter. With regard to quantity, the number of training images should be large enough to adequately represent all possible views of the targeted object yet small enough to ensure that the resolution of the filter is not exhausted. As for the images themselves, they should be distinct enough to avoid numerical difficulties yet similar enough to avoid gaps in which certain views of the target will be unrecognized. One method that we introduce to study this problem is called probing and involves the creation of the artificial imagery. The second problem we consider involves the clasification of the composite filter's correlation plane data. In particular, we would like to determine not only whether or not we are viewing a training image, but, in the former case, we would like to determine which training image is being viewed. This second problem is investigated using traditional M-ary hypothesis testing techniques.

Hall, Eric B.↗
NASA NTRS1994

Measurements of anisotropy in the cosmic microwave background radiation at 0.5 deg angular scales near the star gamma ursae minoris

We present results from a four-frequency observation of a 6 deg x 0.6 deg strip of the sky centered near the star Gamma Ursae Minoris (GUM) during the fourth flight of the Millimeter-wave Anistropy experiment(MAX). The observation was made with a 1.4 deg peak-to-peak sinusoidal chop in all bands. The FWHM beam sizes were 0.55 deg +/- 0.05 deg at 3.5 per cm and 0.75 deg +/- 0.05 deg at 6, 9, and 14 per cm. During this observation significant correlated structure was observed at 3.5, 6 and 9 per cm with amplitudes similar to those observed in the GUM region during the second and third fligts of MAX. The frequency spectrum is consistent with cosmic microwave background (CMB) and inconsistent with thermal emission from interstellar dust. The extrapolated amplitudes of synchrotron and free-free emission are too small to account for the amplitude of the observed structure, If all of the structure is attributed to CMB anisotropy with a Gaussian autocorrelation function and a coherence angle of 25 min, then the most probable values of delta T/T(sub CMB) in the 3.5, 6 and 9 per cm bads are (4.3 +2.7/-1.6) x 10(exp -5), 2.8 (+4.3/-1/1) x 10(exp -5), and 3.5 (+3.0/-1.6) x 10(exp -5) (95% confidence upper and lower limits), respectively.

Devlin, M. J.↗
NASA NTRS1994

Statistical association of QSO's with foreground galaxy clusters

We report a statistically significant overdensity of high redshift quasi-stellar objects (QSO's) in the directions of foreground galaxy clusters. QSO's are taken from the Large Bright QSO Survey (LBQS) between 1.4 less than or equal z less than or equal 2.2 with a limiting magnitude of m(sub B) = 18.5. Foreground clusters are regions within 6 Zwicky radii of small Zwicky clusters at a characteristic redshift of about z approximately = 0.2, covering about 40% of the total area surveyed (304 sq. deg). The overdensity, defined as the ratio of the number density of QSO's in the directions of clusters ('association QSO's) to that in the remainder of the fields ('background QSO's), is equal to 1.7, and formally differs from unity at 4.7 sigma significance. The observed overdensity probably is not due to statistical variation in QSO density, intrinsic QSO-QSO and/or cluster-cluster autocorrelations, or patchy Galactic obscuration. We thus interpret this observation as being due to statistical gravitational lensing of background QSO's by galaxy clusters. However, this amplitude of overdensity behind clusters cannot be accounted for in any cluster lensing model if the background QSO number-magnitude counts are similar to the intrinsic (unlensed) counts, and is implausible in any conventional model of cosmic mass distribution.

Rodrigues-Williams, Liliya L.↗
NASA NTRS1994

Correlated noise in the COBE DMR sky maps

The Cosmic Background Explorer Satellite Differential Radiometer (COBE DMR) sky maps contain low-level correlated noise. We obtain estimates of the amplitude and pattern of the correlated noise from three techniques: angular averages of the covariance matrix, Monte Carlo simulations of two-point correlation functions and direct analysis of the DMR maps. The results from the three methods are mutually consistent. The noise covariance matrix of a DMR sky maps is diagonal to an accuracy of better than 1%. For a given sky pixel, the dominant noise covariance occure with the ring of pixels at an angular separation of 60 deg due to the 60 deg separation of the DMR horns. The mean covariance at 60 deg is 0.45%((sup +0.18)(sub -0.14)) of the mean variance. Additionally, the variance in a given pixel is 0.7% greater than would be expected from a single beam experiment with the same noise properties. Autocorrelation functions suffer from a approximately 1.5 sigma positive bias at 60 deg while cross-correlations have no bias. Published COBE DMR results are not significantly affected by correlated noise.

Lineweaver, C. H.↗
NASA NTRS1994

Measurements of anisotropy in the cosmic microwave background radiation at degree angular scales near the stars Sigma Herculis and Iota Draconis

We present results from two four-frequency observations centered near the stars Sigma Herculis and Iota Draconis during the fourth flight of the Millimeter-wave Anisotropy eXperiment (MAX). The observations were made of 6 deg x 0.6 deg strips of the sky with a 1.4 deg peak to peak sinusoidal chop in all bands. The full width at half maximum (FWHM) beam sizes were calculated 0.55 deg +/- 0.05 deg at 3.5/cm and a 0.75 deg +/- 0.05 deg at 6, 9, and 14/cm. Significant correlated structures were observed at 3.5, 6, and 9/cm. The spectra of these signals are inconsistent with thermal emission from known interstellar dust populations. The extrapolated amplitudes of synchrotron and free-free emission are too small to account for the amplitude of the observed structures. If the observed structures are attributed to cosmic microwave background (CMB) anisotropy with a Gaussian autocorrelation function and a coherence angle of 25 min, then the most probable values at Delta T/T(sub CMB) = 3.1 (sup +1.7 sub -1.3) x 10(exp -5) for the Sigma Herculis scan, and Delta T/T(sub CMB) = 3.3(sup +1.1 sub -1.1) x 10(exp -5) for the Iota Draconis scan (95% confidence upper, lower limits).

Clapp, A. C.↗
NASA NTRS1994

The ensemble variability properties of faint QSOs

A refined sample of 64 variable objects with stellar image structure has been identified in SA 57 to B approximately = 22.5, over a time baseline of 15 yr, sampled at 11 distinct epochs. The photometric data typically have a root-mean-square (rms) error at B = 22 of only 0.05 mag. Thirty-five quasars in this field have already been spectroscopically confirmed, 34 of which are among the sample of variables. Of the other variables, six are known spectroscopically to be stars, 10 additional objects are stars based on reliable detection of proper motion, and one is spectroscopically a narrow emission line galaxy. Of the 13 remaining variables, it is argued that they are a mixture of distant halo subdwarfs and quasars with starlike colors. We compute the ensemble average structure function and autocorrelation function from the light curves in the respective quasar rest frames, which are used to investigate the general dependences on apparent magnitude, absolute magnitude, and redshift.

Trevese, Dario↗
NASA NTRS1995

Stochastic charging of dust grains in planetary rings: Diffusion rates and their effects on Lorentz resonances

Dust grains in planetary rings acquire stochastically fluctuating electric charges as they orbit through any corotating magnetospheric plasma. Here we investigate the nature of this stochastic charging and calculate its effect on the Lorentz resonance (LR). First we model grain charging as a Markov process, where the transition probabilities are identified as the ensemble-averaged charging fluxes due to plasma pickup and photoemission. We determine the distribution function P(t;N), giving the probability that a grain has N excess charges at time t. The autocorrelation function tau(sub q) for the strochastic charge process can be approximated by a Fokker-Planck treatment of the evolution equations for P(t; N). We calculate the mean square response to the stochastic fluctuations in the Lorentz force. We find that transport in phase space is very small compared to the resonant increase in amplitudes due to the mean charge, over the timescale that the oscillator is resonantly pumped up. Therefore the stochastic charge variations cannot break the resonant interaction; locally, the Lorentz resonance is a robust mechanism for the shaping of etheral dust ring systems. Slightly stronger bounds on plasma parameters are required when we consider the longer transit times between Lorentz resonances.

Schaffer, L.↗
NASA NTRS1993

Solar flux forecasting using mutual information with an optimal delay

Solar flux F(sub 10.7) directly affects the atmospheric density, thereby changing the lifetime and prediction of satellite orbits. For this reason, accurate forecasting of F(sub 10.7) is crucial for orbit determination of spacecraft. Our attempts to model and forecast F(sub 10.7) uncovered highly entangled dynamics. We concluded that the general lack of predictability in solar activity arises from its nonlinear nature. Nonlinear dynamics allow us to predict F(sub 10.7) more accurately than is possible using stochastic methods for time scales shorter than a characteristic horizon, and with about the same accuracy as using stochastic techniques when the forecasted data exceed this horizon. The forecast horizon is a function of two dynamical invariants: the attractor dimension and the Lyapunov exponent. In recent years, estimation of the attractor dimension reconstructed from a time series has become an important tool in data analysis. In calculating the invariants of the system, the first necessary step is the reconstruction of the attractor for the system from the time-delayed values of the time series. The choice of the time delay is critical for this reconstruction. For an infinite amount of noise-free data, the time delay can, in principle, be chosen almost arbitrarily. However, the quality of the phase portraits produced using the time-delay technique is determined by the value chosen for the delay time. Fraser and Swinney have shown that a good choice for this time delay is the one suggested by Shaw, which uses the first local minimum of the mutual information rather than the autocorrelation function to determine the time delay. This paper presents a refinement of this criterion and applies the refined technique to solar flux data to produce a forecast of the solar activity.

Ashrafi, S.↗