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NASA NTRS2004

Discrete X-Ray Source Populations and Star-Formation History in Nearby Galaxies

This program aims in understanding the connection between the discrete X-ray source populations observed in nearby galaxies and the history of star-formation in these galaxies. The ultimate goal is to use this knowledge in order to constrain X-ray binary evolution channels. For this reason although the program is primarily observational it has a significant modeling component. During the first year of this study we focused on the definition of a pilot sample of galaxies with well know star-formation histories. A small part of this sample has already been observed and we performed initial analysis of the data. However, the majority of the objects in our sample either have not been observed at all, or the detection limit of the existing observations is not low enough to probe the bulk of their young X-ray binary populations. For this reason we successfully proposed for additional Chandra observations of three targets in Cycle-5. These observations are currently being performed. The analysis of the (limited) archival data for this sample indicated that the X-ray luminosity functions (XLF) of the discrete sources in these galaxies may not have the same shape as is widely suggested. However, any solid conclusions are hampered by the small number of detected sources. For this reason during the second year of this study, we will try to extend the sample in order to include more objects in each evolutionary stage. In addition we are completing the analysis of the Chandra monitoring observations of the Antennae galaxies. The results from this work, apart from important clues on the nature of the most luminous sources (Ultra-luminous X-ray sources; ULXs) provide evidence that source spectral and/or temporal variability does not significantly affect the shape of their X-ray luminosity functions. This is particularly important for comparisons between the XLFs of different galaxies and comparisons with predictions from theoretical models. Results from this work have been presented in several conferences. Refereed journal papers presenting these conclusions are currently in preparation. An important part of this study is the Chandra survey of the Small Magellanic Cloud, our second nearest star- forming galaxy. So far we have been awarded 5 Chandra observations of the central youngest part of the galaxy. These observations will help to study the very faint end of the young X-ray binary populations which is not possible to probe in more distant objects. Results from this study have been presented in several conferences and two papers are in preparation. In addition during year-2 we are planning of undertaking the task of identifying optical counterparts to the X-ray sources, which will help us to isolate interlopers (sources not associated with the SMC) and classify the X-ray binaries which are found to be associated with the SMC. In the theoretical front, the Star-Track X-ray binary population synthesis code which will be used for the modeling of the X-ray binary populations (led by co-I V. Kalogera and C. Belczynski), is complete. A first test using the XLF of the star-forming galaxy NGC-1569 showed remarkable agreement between the observed and the modeled XLF. These results are presented in an ApJ. Letters paper (Belczynski et al, 2004, 601, 147). During year-2 of this study we are planning of performing a parameter study in order to investigate which parameters are most important for the shape of the XLF. In addition we will perform comparisons with observations of other galaxies from our sample as they become available.

Zezas, Andreas↗
NASA NTRS1994

Performance of mean-frequency estimators for Doppler radar and lidar

The performance of mean-frequency estimators for Doppler radar and lidar measurements of winds is presented in terms of two basic parameters: Phi, the ratio of the average signal energy per estimate to the spectral noise level; and Omega, which is proportional to the number of independent samples per estimate. For fixed Phi and Omega, the Cramer-Rao bound (CRB) (theoretical best performance) for unbiased estimators of mean frequency (normalized by the spectral width of the signal), signal power, and spectral width are essentially independent of the number of data samples M. For large Phi, the estimators of mean frequency are unbiased and the performance is independent of M. The spectral domain estimators and covariance based estimators are bounded by the approximate period of M. The spectral domain estimators and covariance based estimators are bounded by the approximate periodogram CRB. The standard deviation of the maximum-likelihood estimator approaches the exact CRB, which can be more than a factor of 2 better than the performance of the spectral domain estimators or covariance-based estimators for typical Omega. For small Phi, the estimators are biased due to the effects of the uncorrelated noise (white noise), which results in uniformly distributed 'bad' estimates. The fraction of bad estimates is a function of Phi and M with weak dependence on the parameter Omega. Simple empirical models describe the standard deviation of the good estimates and the fraction of bad estimates. For Doppler lidar and for large Phi, better performance is obtained by using many low-energy pulses instead of one pulse with the same total energy. For small Phi, the converse is true.

Frehlich, R. G.↗
NASA NTRS2019

Cellular Statistical Models of Broken Cloud Fields. Part IV: Effects of Pixel Size on Idealized Satellite Observations

In the fourth part of our “Cellular Statistical Models of Broken Cloud Fields” series we use the binary Markov processes framework for quantitative investigation of the effects of low resolution of idealized satellite observations on the statistics of the retrieved cloud masks. We assume that the cloud fields are Markovian and are characterized by the “actual” cloud fraction (CF) and scale length. We use two different models of observations: a simple discrete-point sampling and a more realistic “pixel” protocol. The latter is characterized by a state attribution function (SAF) which has the meaning of the probability that the pixel with a certain CF is declared cloudy in the observed cloud mask. The stochasticity of the SAF means that the cloud/clear attribution is not ideal and can be affected by external or unknown factors. We show that the observed cloud masks can be accurately described as Markov chains of pixels and use the master-matrix formalism (introduced in Part III of the series) for analytical computation of their parameters: the “observed” CF and scale length. This procedure allows us to establish a quantitative relationship (which is pixel-size dependent) between the actual and the observed cloud-field statistics. The feasibility of restoring the former from the latter is considered. The adequacy of our analytical approach to idealized observations is evaluated using numerical simulations. Comparison of the observed parameters of the simulated datasets with their theoretical expectations showed an agreement within 0.005 for the CF, while for the scale length it is within 1% in the sampling case and within 4% in the pixel case.

Mikhail D. Alexandrov↗
NASA NTRS1980

A hill-sliding strategy for initialization of Gaussian clusters in the multidimensional space

A hill sliding technique was devised to extract Gaussian clusters from the multivariate probability density estimate of sample data for the first step of iterative unsupervised classification. Each cluster was assumed to posses a unimodal normal distribution. A clustering function proposed distinguished elements of a cluster under formation from the rest in the feature space. Initial clusters were extracted one by one according to the hill sliding tactics. A dimensionless cluster compactness parameter was proposed as a universal measure of cluster goodness and used satisfactorily in test runs with LANDSAT multispectral scanner data. The normalized divergence, defined by the cluster divergence divided by the entropy of the entire sample data, was utilized as a general separability measure between clusters. An overall clustering objective function was set forth in terms of cluster covariance matrices, from which the cluster compactness measure could be deduced. Minimal improvement of initial data partitioning was evaluated by this objective function in eliminating scattered sparse data points. The hill sliding clustering technique developed herein has the potential applicability to decomposition any multivariate mixture distribution into a number of unimodal distributions when an appropriate distribution function to the data set is employed.

Park, J. K.↗
NASA NTRSDate not supplied

Comparing Acoustic Prediction Methods for Additively Manufactured Porous Strutures

While macroscale methods for predicting the acoustic properties of porous structures have been popular in the past, they often require time-consuming manufacturing and testing workflows. Meanwhile, microscale approaches allow the prediction of transport parameters based exclusively on a periodic structure’s unit cell geometry. Here, we compare these methods to predict the characteristic impedance of additively manufactured porous structures. We use the microscale approach to estimate the geometry’s transport parameters, then predict the characteristic properties using the Johnson-Champoux-Allard (JCA) model. We measure the acoustic properties of the printed structures using a normal incidence impedance tube and estimate the transport parameters using an inverse characterization approach. We use the two-thickness method as a macroscale approach to predict the characteristic properties from the measured surface impedances of two sample thicknesses. Finally, we compare these characteristic prediction methods. Our results show that the inverse characterization and two-thickness methods offer the closest match to the measured values at low frequencies.

impedance↗
NASA NTRS1971

The minimum distance approach to classification

The work to advance the state-of-the-art of miminum distance classification is reportd. This is accomplished through a combination of theoretical and comprehensive experimental investigations based on multispectral scanner data. A survey of the literature for suitable distance measures was conducted and the results of this survey are presented. It is shown that minimum distance classification, using density estimators and Kullback-Leibler numbers as the distance measure, is equivalent to a form of maximum likelihood sample classification. It is also shown that for the parametric case, minimum distance classification is equivalent to nearest neighbor classification in the parameter space.

Wacker, A. G.↗
NASA NTRS1981

The Dynamics Explorer Langmuir probe instrument

The Dynamics Explorer Langmuir probe instrument (DE-LANG), a spare unit from the Pioneer Venus mission that has been modified to work with the DE spacecraft, is described. Two independent sensors are connected to individual adaptive sweep voltage circuits that continuously track the changing electron temperature and spacecraft potential while autoranging electrometers adjust their gain in response to the changing plasma density. The control signals used in achieving this automatic tracking provide a continuous monitor of the ionospheric parameters without telemetering each volt-ampere curve. In addition, internal data storage circuits permit high-resolution, high-data-rate sampling of selected volt-ampere curves for transmission to the ground to verify or correct the inflight processed data.

Krehbiel, J. P.↗
NASA NTRS1984

Measurement results from a balloon experiment simulating land mobile satellite transmissions

A transmitter operating at 869.525 MHz was twice carried by a stratospheric balloon to an altitude of about 40 km. A motor vehicle was driven within the line-of-sight from the transmitter. Measurements of the received signal strength were made every 1/8 wavelength for an overall travelling distance of about seven hundred kilometers. This scenario was to simulate a satellite system providing mobile communications to rural areas. The statistics of the sampled field, consisting of a combination of direct wave, specular reflection and diffuse components, are presented as a function of elevation angle. Parameters such as type of road driven (mostly 2 lane) or type of landscape (rolling to flat) and vegetation (pine and mixed forest) encountered are described where possible. The power distribution function for all the data, at elevation angles from 10 to 35 degrees, is 1 dB below the free space mean at the 50% level, 7 dB below at the 90% level, and 18 dB below at the 99% level. In the elevation angle range of 30 to 35 degrees the corresponding values were found to be .5, 1.2, and 4.5 dB. The conditional fade duration and level crossing rate distribution functions are also presented. The former shows some dependence on the threshold level, the latter almost none.

Vogel, W. J.↗
NASA NTRS1987

Infrared spectroscopic measurements of halogenated source gases in the stratosphere with the ATMOS instrument

The volume mixing ratios of the six most important halogenated source species (CH3Cl, CF2Cl2, CFCl3, CHF2Cl, CCl4, and CF4) have been retrieved over the 10- to 30-km altitude range from the analysis of 0.01/cm resolution infrared solar occultation spectra recorded near 30 deg N and 47 deg S latitudes with the ATMOS (Atmospheric Trace Molecule Spectroscopy) instrument, operating from on board Spacelab 3 (April-May 1985). The results reported here, although in satisfactory agreement with recent in situ values obtained from air sampling techniques, are limited in accuracy by the limited absorption representative of most of the species and by uncertainties in the spectroscopic parameters currently available for these gases. They demonstrate, however, the power of the IR remote sensing approach for evaluating on a global scale the total chlorine budget of the atmosphere, and they provide an independent set of simultaneous data acquired for the important source and reservoir halogenated molecular species in the upper atmosphere.

Zander, R.↗
NASA NTRS1988

Development of remote sensing techniques capable of delineating soils as an aid to soil survey

Eighty-one benchmark soils from Alabama, Georgia, Florida, Tennessee, and Mississippi were evaluated to determine the feasibility of spectrally differentiating among soil categories. Relationships among spectral properties that occur between soils and within soils were examined, using discriminant analysis. Soil spectral data were obtained from air-dried samples using an Exotech Model 20C field spectroradiometer (0.37 to 2.36 microns). Differentiating among the orders, suborders, great groups, and subgroups using reflectance spectra achieved varying percentages of accuracy. Six distinct reflectance curve forms were developed from the air-dried samples based on the shape and presence or absence of adsorption bands. Iron oxide and organic matter content were the dominant soil parameters affecting the spectral characteristics for differentiating among and between these soils.

Coleman, T. L.↗
NASA NTRS1987

Adaptive control of dual-arm robots

Three strategies for adaptive control of cooperative dual-arm robots are described. In the position-position control strategy, the adaptive controllers ensure that the end-effector positions of both arms track desired trajectories in Cartesian space despite unknown time-varying interaction forces exerted through the load. In the position-hybrid control strategy, the adaptive controller of one arm controls end-effector motions in the free directions and applied forces in the constraint directions, while the adaptive controller of the other arm ensures that the end-effector tracks desired position trajectories. In the hybrid-hybrid control strategy, the adaptive controllers ensure that both end-effectors track reference position trajectories while simultaneously applying desired forces on the load. In all three control strategies, the cross-coupling effects between the arms are treated as disturbances which are rejected by the adaptive controllers while following desired commands in a common frame of reference. The adaptive controllers do not require the complex mathematical model of the arm dynamics or any knowledge of the arm dynamic parameters or the load parameters such as mass and stiffness. The controllers have simple structures and are computationally fast for on-line implementation with high sampling rates.

Seraji, H.↗
NASA NTRS1988

Digital robust active control law synthesis for large order flexible structure using parameter optimization

A generic procedure for the parameter optimization of a digital control law for a large-order flexible flight vehicle or large space structure modeled as a sampled data system is presented. A linear quadratic Guassian type cost function was minimized, while satisfying a set of constraints on the steady-state rms values of selected design responses, using a constrained optimization technique to meet multiple design requirements. Analytical expressions for the gradients of the cost function and the design constraints on mean square responses with respect to the control law design variables are presented.

Mukhopadhyay, V.↗
NASA NTRS1988

Evaluation of concepts and methods for extraction of flow parameters from laser transit anemometry data

A Monte-Carlo type simulation system was developed to test concepts and associated algorithms proposed by Mayo and Smart (1984) for the extraction of two-dimensional flow parameters from laser transit anemometry data sets. The Monte-Carlo LTA simulation program models a flow field as well as the sample volume geometry of the LTA system, thereby permitting simulation of two-dimensional data acquisition in a three-dimensional flow field. The results of these tests on the proposed processing concepts suggest that the instrument is capable of measuring mean velocities to less than +0.15 percent error and flow angles to less then +0.3 percent for turbulence intensities of up to 15.0 percent.

Humphreys, W. M., Jr.↗
NASA NTRS1992

Microwave radiometry for monitoring the diverse cloudiness regimes on Earth: A review

Recent work concerning the use of microwave radiometers in space to provide views of the water content of clouds is reported. Recently, theoretical algorithms were refined and the sampling by microwave radiometers for polar orbiting satellite became more commensurate in resolution and coverage with the scale of the meteorological phenomena. Parameters available from the Special Sensor Microwave/Imager (SSM/I) include total integrated water vapor (kg/sq m), integrated cloud liquid water (kg/sq m), rain rate (mm/hr), and an index for the presence and concentration of large ice particles; this so called scattering index can also be converted to a rain rate, although conversion is still rather uncertain. The hydrologic aspects of climatically important cloud systems on the Earth such as subtropical stratus decks, tropical convection, and midlatitude and polar cyclone can thus be monitored with the SSM/I parameters. The diagnostic as well as prognostic potential of these satellite derived water content properties of cloud systems are also discussed.

Katsaros, Kristina B.↗
NASA NTRS1993

A preliminary error analysis of the gravity field recovery from a lunar Satellite-to-Satellite mission

A low cost lunar Satellite-to-Satellite radio tracking mission in a low-low configuration could considerably improve the existing knowledge about the lunar gravity field. The impact of various mission parameters that may contribute to the recovery of the gravity field, such as satellite altitude, satellite separation mission duration, measurement precision and sampling interval were quantified using the Jekeli-Rapp algorithm. Preliminary results indicate that the gravity field resolution up to harmonic degree 40 to 80 is feasible depending on various mission configurations. Radio tracking data from a six-month mission with a precision of 1 mm/s every 10 s and 300 km satellite separation at 150 km altitude will permit the determination of 5 deg x 5 deg mean gravity anomalies with an error of approximately 15 mgals. Consideration of other unaccounted error sources of instrumental, operational, and environmental nature may lower this resolution.

Iz, Huseyin B.↗
NASA NTRS1995

Influence of the vapor flux on temperature, density, and abundance distributions in a multicomponent, porous, icy body

We calculated the vapor flux of the icy components in the surface layer of a porous, short-period, Jupiter-class comet, in order to investigate the relationship of the observed relative molecular abundances in the coma with those in the nucleus. The model assumes a body containing one major ice component (H20) and up to three minor components of higher volatility (e.g., CO, CO2, CH3OH). The body's porous structure is modeled as a bundle of tubes with a given tortuosity and initially a constant pore diameter. The mass and energy equations for the different volatiles are solved simultaneously under appropiate boundary conditions. Heat is conducted by the matrix and carried by the vapors. The one-dimensional model includes radially inward and outward flowing vapor within the body, complete depletion of less volatile ices in outer layers, and recondensation of vapor in deeper, coller layers. As a result, we obtain the temperature and abundance distribution in the nucleus and the gas flux into the interior and into the coma for each of the volatiles at various positions in the orbit. The ratio of the gas flux of minor volatiles to that of H2) in the coma varies by several orders of magnitude throughout the orbit. Thus, the relative abundances of species observed in the coma are in most cases not the same as those in the nucleus. Results also indicate that it will be impossible to determine the relative abundances of ices more volatile than water from samples taken a few meters below the surface during a comet rendezvous mission. We made calculations for a wide range of different parameters, such as porosity, pore radius, and thermal conductivity of the matrix. To introduce the model we present typical results for a dust-free comet.

Benkhoff, J.↗
NASA NTRS1998

Northern and Southern Hemisphere Ground-Based Infrared Spectroscopic Measurements of Tropospheric Carbon Monoxide and Ethane

Time series of CO and C2H, measurements have been derived from high-resolution infrared solar spectra recorded in Lauder, New Zealand (45.0 degrees S, 169.7 degrees E, altitude 0.37 km), and at the U.S. National Solar Observatory (31.9 degrees N, 11, 1.6 degrees W, altitude 2.09 km) on Kitt Peak. Lauder observations were obtained between July 1993 and November 1997, while the Kitt Peak measurements were recorded between May 1977 and December 1997. Both databases were analyzed with spectroscopic parameters that included significant improvements for C2H6 relative to previous studies. Target CO and C2H6 lines were selected to achieve similar vertical samplings based on averaging kernels. These calculations show that partial columns from layers extending from the surface to the mean tropopause and from the mean tropopause to 100 km are nearly independent. Retrievals based on a semiempirical application of the Rodgers optimal estimation technique are reported for the lower layer, which has a broad maximum in sensitivity in the upper troposphere. The Lauder CO and C2H, partial columns exhibit highly asymmetrical seasonal cycles with minima in austral autumn and sharp peaks in austral spring. The spring maxima are the result of tropical biomass burning emissions followed by deep convective vertical transport to the upper troposphere and long-range horizontal transport. Significant year-to-year variations are observed for both CO and C2H6, but the measured trends, (+0.37 +/- 0.57)% yr(exp -1) and (-0.64 +/- 0.79)% yr(exp -1), I sigma, respectively, indicate no significant long-term changes. The Kitt Peak data also exhibit CO and C2H6, seasonal variations in the lower layer with trends equal to (-0.27 +/- 0.17)% yr(exp -1) and (-1.20 +/- 0.35)% yr(exp -1), 1 sigma, respectively. Hence a decrease in the Kitt Peak tropospheric C2H6 column has been detected, though the CO trend is not significant. Both measurement sets are compared with previous observations, reported trends, and three-dimensional model calculations.

Rinsland, Curtis P.↗
NASA NTRS1998

Northern and Southern Hemisphere Ground-Based Infrared Spectroscopic Measurements of Tropospheric Carbon Monoxide and Ethane

Time series of CO and C2H6 measurements have been derived from high resolution infrared solar spectra recorded in Lauder, New Zealand (45.0 deg S, 169.7 deg E, altitude 0.37 km) and at the U. S. National Solar Observatory (31.90 deg N, 111.6 deg W, altitude 2.09 km) on Kitt Peak. Lauder observations were obtained between July 1993 and November 1997 while the Kitt Peak measurements were recorded between May 1977 and December 1997. Both databases were analyzed with spectroscopic parameters that included significant improvements for C2H6 relative to previous studies. Target CO and C2H6 lines were selected to achieve similar vertical samplings based on averaging kernels. These calculations show that partial columns from layers extending from the surface to the mean tropopause and from the mean tropopause to 100 km are nearly independent. Retrievals based on a semiempirical application of the Rodgers optimal estimation technique are reported for the lower layer, which has a broad maximum in sensitivity in the upper troposphere. The Lauder CO and C2H6 partial columns exhibit highly asymmetrical seasonal cycles with minima in austral autumn and sharp peaks in austral spring. The spring maxima are the result of tropical biomass burning emissions followed by deep convective vertical transport to the upper troposphere and long-range horizontal transport. Significant year-to-year variations are observed for both CO and C2H6, but the measured trends, (+0.37 +/- 0.57)%/ yr and (-0.64 +/- 0.79)%/ yr, 1 sigma, respectively, indicate no significant long-term changes. The Kitt Peak data also exhibit CO and C2H6 seasonal variations in the lower layer with trends equal to (-0.27 +/- 0.17)%/ yr and (-1.20 +/- 0.35)%/ yr, 1 sigma, respectively. Hence, a decrease in the Kitt Peak tropospheric C2H6 column has been detected, though the CO trend is not significant. Both measurement sets are compared with previous observations, reported trends, and three-dimensional model calculations.

Rinsland, Curtis P.↗