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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

Coupled Inertial Navigation and Flush Air Data Sensing Algorithm for Atmosphere Estimation

This paper describes an algorithm for atmospheric state estimation that is based on a coupling between inertial navigation and flush air data sensing pressure measurements. In this approach, the full navigation state is used in the atmospheric estimation algorithm along with the pressure measurements and a model of the surface pressure distribution to directly estimate atmospheric winds and density using a nonlinear weighted least-squares algorithm. The approach uses a high fidelity model of atmosphere stored in table-look-up form, along with simplified models of that are propagated along the trajectory within the algorithm to provide prior estimates and covariances to aid the air data state solution. Thus, the method is essentially a reduced-order Kalman filter in which the inertial states are taken from the navigation solution and atmospheric states are estimated in the filter. The algorithm is applied to data from the Mars Science Laboratory entry, descent, and landing from August 2012. Reasonable estimates of the atmosphere and winds are produced by the algorithm. The observability of winds along the trajectory are examined using an index based on the discrete-time observability Gramian and the pressure measurement sensitivity matrix. The results indicate that bank reversals are responsible for adding information content to the system. The algorithm is then applied to the design of the pressure measurement system for the Mars 2020 mission. The pressure port layout is optimized to maximize the observability of atmospheric states along the trajectory. Linear covariance analysis is performed to assess estimator performance for a given pressure measurement uncertainty. The results indicate that the new tightly-coupled estimator can produce enhanced estimates of atmospheric states when compared with existing algorithms.

Karlgaard, Christopher D.↗
NASA NTRS1987

The abundance of interstellar CO

Data from recent observations of interstellar CO column densities are compiled in tables and graphs and compared with the predictions of theoretical models. The energy-level structure of CO and the excitation of interstellar CO are discussed; UV-absorption and IR line observations are characterized; and techniques used to estimate the CO column density from mm-wavelength observations (based on calibration of the CO line intensity with the column density of H2) are described in detail. Consideration is then given to CO chemistry under interstellar-cloud conditions (isotope fractionation and photodissociation) and models for diffuse, translucent, and dense clouds. A number of unsolved problems requiring additional observations or laboratory investigations are identified.

Van Dishoeck, Ewine F.↗
NASA NTRS1977

A Fourier approach to cloud motion estimation

A Fourier technique is described for estimating cloud motion from pairs of pictures using the phase of the cross spectral density. The method allows motion estimates to be made for individual spatial frequencies, which are related to cloud pattern dimensions. Results obtained are presented and compared with the results of a Fourier domain cross correlation scheme. Using both artificial and real cloud data show that the technique is relatively sensitive to the presence of mixtures of motions, changes in cloud shape, and edge effects.

Arking, A.↗
NASA NTRS1988

Canopy reflectance modeling in a tropical wooded grassland

The Li-Strahler canopy reflectance model, driven by LANDSAT Thematic Mapper (TM) data, provided regional estimates of tree size and density in two bioclimatic zones in Africa. This model exploits tree geometry in an inversion technique to predict average tree size and density from reflectance data using a few simple patameters measured in the field and in the imagery. Reflectance properties of the trees were measured in the study sites using a pole-mounted radiometer. The measurements showed that the assumptions of the simple Li-Strahler model are reasonable for these woodlands. The field radiometer measurements were used to calculate the normalized difference vegetation index (NDVI), and the integrated NDVI over the canopy was related to crown volume. Predictions of tree size and density from the canopy model were used with allometric equations from the literature to estimate woody biomass and potential foliar biomass for the sites and for the regions. Estimates were compared with independent measurements made in the Sahelian sites, and to typical values from the literature for these regions and for similar woodlands. In order to apply the inversion procedure regionally, an area must first be stratified into woodland cover classes, and dry-season TM data were used to generate a stratum map of the study areas with reasonable accuracy. The method used was unsupervised classification of multi-data principal components images.

Simonett, David↗
NASA NTRS1997

A study of the physics and chemistry of TMC-1

We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.

NASA Discipline Exobiology↗
NASA NTRS1998

Optical Data Storage Capabilities of Bacteriorhodopsin

We present several measurements of the data storage capability of bacteriorhodopsin films to help establish the baseline performance of this material as a medium for holographic data storage. In particular, we examine the decrease in diffraction efficiency with the density of holograms stored at one location in the film, and we also analyze the recording schedule needed to produce a set of equal intensity holograms at a single location in the film. Using this information along with the assumptions about the performance of the optical system, we can estimate potential data storage densities in bacteriorhodopsin.

Gary, Charles↗
NASA NTRS2004

A Plasmaspheric Mass Density Model and Constraints on its Heavy Ion Concentration

The first empirical model of the equatorial mass density of the plasmasphere is constructed using ground-based ULF wave diagnostics. Plasmaspheric mass density between L=l.7 and L=3.2 has been determined using over 5200 hours of data from pairs of stations in the MEASURE array of ground magnetometers. The least-squares fit to the data as a function of L shows that mass density falls logarithmically with L. Average ion mass as a function of L is also estimated by combining the mass density model with plasmaspheric electron density profiles determined from the IMAGE Radio Plasma Imager (RPI). Additionally, we use the RPI electron density database to examine how the average ion mass changes under different levels of geomagnetic activity. We find that average ion mass is greatest under the most disturbed conditions. This result indicates that heavy ion concentrations are enhanced during large geomagnetic disturbances, and therefore play an important role in storm-time plasmaspheric dynamics. The average ion mass is also used to constrain the concentrations of He(+) and O(+). Estimates of the He(+) concentration determined this way can be useful for interpreting IMAGE Extreme Ultraviolet Imager (EUV) images.

Berube, D.↗
NASA NTRS1992

Bulk parameters of water group ions at Comet Giacobini-Zinner

Measurements of water group ions at Comet Giacobini-Zinner are analyzed which have been obtained with the EPAS instrument on the ICE spacecraft, in order to determine the bulk parameters of these ions. The measured ion velocity distributions are extrapolated to zero energy and integrated to obtain estimates of both number and thermal energy density. The number density is compared to independent observational and theoretical estimates. Combining the EPAS results with magnetic field and plasma data, the beta and Mach numbers of the flow have been obtained in the region surrounding the comet.

Staines, K.↗
NASA NTRS1997

Nonstationary Transient Vibroacoustic Response of a Beam Structure

This study consists of an investigation into the nonstationary transient response of the Verification Test Article (VETA) when subjected to random acoustic excitation. The goal is to assess excitation models that can be used in the design of structures and equipment when knowledge of the structure and the excitation is limited. The VETA is an instrumented cantilever beam that was exposed to acoustic loading during five Space Shuttle launches. The VETA analytical structural model response is estimated using the direct averaged power spectral density and the normalized pressure spectra methods. The estimated responses are compared to the measured response of the VETA. These comparisons are discussed with a focus on prediction conservatism and current design practice.

Caimi, R. E.↗
NASA NTRS1981

Ultraviolet absorption studies of H2O and other species in comet comae with satellite telescope-spectrometers

It is noted that ultraviolet absorption by H2O and other species in the comae of comets could be detected by studying, with satellite telescope-spectrometers, the occultation of hot stars by comets. Observations of this type could produce the first detection of H2O, the fundamental parent molecule in comet comae, and give measures of molecular level populations. The capabilities of the High Resolution Spectrograph on Space Telescope, the first instrument suitable for such observations, are discussed. A Haser model is used to estimate the molecular column densities and to predict equivalent widths for lines of H2O, OH, CO, C, and O as functions of time and angular distance from a comet with a high H2O production rate. The minimum detectable equivalent widths are determined, thereby giving the maximum angular separation from such a comet at which H2O, OH, and CO could be studied. Estimated equivalent widths for CO, OH, and the resonance lines of C and O suggest that these species may also be detected.

Smith, P. L.↗
NASA NTRS1998

Pulsations, Shocks, and Mass Loss

This grant provided long-term support for my investigation of the outflows powered by young stars. Several major research results emerged during the course of this research, including: (1) The discovery of giant Herbig-Haro outflows from young stars that can extend for many parsecs from their sources. The first parsec-scale outflow to be recognized led to the realization that Herbig-Haro outflows, even those produced by low mass young stellar objects, can extend orders of magnitude farther from their sources than previously thought. Our preconceptions were to a large extent driven by the narrow fields-of-view then provided by CCD detectors. With the recent advent of large format CCDs and CCD mosaics, we have come to realize that most outflows attain parsec-scale dimensions. Even at the distance of the Orion star forming clouds, such flows can subtend a degree on the sky. Our work has led to the recognition of over two dozen giant. (2) The discovery that outflows are highly clustered. Even regions of relatively isolated star formation such as those in Taurus frequently produce multiple outflows. (3) The discovery of a new family of externally irradiated jets. During the last year of support from this grant, we made the startling discovery that there is a class of jets from young stars that are illuminated by the ionizing radiation field of nearby massive stars. The first four examples were discovered in the vicinity of the a Orionis sub-group of the Orion OB Association which is believed to be at least 2 million years old. Since the jets are photo-ionized, their densities can be reliably estimated. Most HH jets are shock excited, and are therefore notoriously difficult to characterize since their visibility and observed properties depend on the complex and highly non-linear processes associated with shocks. Furthermore, many irradiated jets are one sided rather than bipolar. Thus, irradiated jets may for the first time be used to accurately diagnose jet densities and mass loss rates, and to probe the physics of jet collimation, and may indicate that the jet production phase of certain young stars may last more than a million years. These three discoveries provide us with fundamental new insights into the star formation process, into the physical conditions inside and near star forming clouds, and into new ways to probe the physics and chemistry of such clouds.

Bally, John↗
NASA NTRS1993

Estimation of random bending strain using a scanning laser vibrometer

Results are presented which were obtained from vibration measurements at discrete locations on a randomly excited structure to estimate the power spectral density of bending strain. The technique is intended to be applied using a scanning laser vibrometer to allow a non-contacting measurement of random bending strain over the surface of a structure. The experimental setup is described along with the data analysis procedure. The results presented here indicate that the method is practical and can lead to reliable estimates.

Miles, R. N.↗
NASA NTRS1994

(Fe II) 1.53 and 1.64 micron emission from pre-main-sequence stars

We present flux-calibrated profiles of the (Fe II) 1.53 and 1.64 micron lines in five pre-main-sequence stars, PV Cep, V1331 Cyg, R Mon, and DG and HL Tau. The line centroids are blueshifted in all five sources, and four of the five have only blueshifted flux. In agreement with previous studies, we attribute the line asymmetries to local obscuration by dusty circumstellar disks. The absence of redshifted flux implies a minimum column density of obscuring material. The largest limit, N(sub H) greater than 3 x 10(exp 22)/sq cm, derived for V1331 Cyg, suggests disk surface densities greater than 0.05 g/sq cm and disk masses greater than 0.001 solar mass within a radius of approximately 200 AU. The narrow high-velocity lines in PV Cep, V1331 Cyg, and HL Tau require formation in well collimated winds. The maximum full opening angles of their winds range from less than 20 deg in V1331 Cyg to less than 40 deg in HL Tau. The (Fe II) data also yield estimates of the electron densities (n(sub e) approximately 10(exp 4)/cu cm), hydrogen ionization fractions (f(sub H(+)) approximately 1/3), mass-loss rates (approximately 10(exp -7) to 2 x 10(exp -6) solar mass/yr), and characteristic radii of the emitting regions (approximately 32 to approximately 155 AU). The true radial extents will be larger, and the mass-loss rates smaller, by factors of a few for the outflows with limited opening angles. In our small sample the higher mass stars have stronger lines, larger emitting regions, and greater mass-loss rates. These differences are probably limited to the scale and energetics of the envelopes, because the inferred geometries, kinematics and physical conditions are similar. The measured (Fe II) profiles samples both 'high'- and 'low'-velocity environments. Recent studies indicate that these regions have some distinct physical properties and may be spatially separate. The (Fe II) data show that similar sizes and densities can occur in both environments.

Hamann, Fred↗
NASA NTRS1975

A search for H/-/ in the shell surrounding chi Ophiuchi

Assuming that chi Oph is an extreme Be star, some consequences are investigated for the interpretation that the IR excesses in such stars are produced by H(-) free-bound emission. The column density of H(-) in the shell surrounding chi Oph is estimated from the observed IR excess, and Copernicus UV spectrophotometric data are used to search for a predicted absorption feature at 1129.54 A due to H(-) and to derive an upper limit to the H(-) column density. No feature resembling the predicted H(-) line is seen in the spectrum of chi Oph, and a marked discrepancy is noted between the estimated column density and the derived upper limit. Possible causes of these discrepancies are discussed, including incorrect interpretation of the IR excess in extreme Be stars, incorrect theoretical calculations describing the predicted H(-) UV absorption line, and the possibility that the line of sight through the shell does not intersect the regions where the IR excess originates.

Snow, T. P., Jr.↗
NASA NTRS1986

Electron density diagnostics in the 10-100 A interval for a solar flare

Electron density measurements from spectral-line diagnostics are reported for a solar flare on July 13, 1982, 1627 UT. The spectrogram, covering the 10-95 A interval, contained usable lines of helium-like ions C V, N VI, O VII, and Ne IX which are formed over the temperature interval 0.7-3.5 x 10 to the 6th K. In addition, spectral-line ratios of Si IX, Fe XIV, and Ca XV were compared with new theoretical estimates of their electron density sensitivity to obtain additional electron density diagnostics. An electron density of 3 x 10 to the 10th/cu cm was obtained. The comparison of these results from helium-like and other ions gives confidence in the utility of these tools for solar coronal analysis and will lead to a fuller understanding of the phenomena observed in this flare.

Brown, W. A.↗
NASA NTRS1993

The ultraviolet absorption spectrum of the quasar PKS 0405-12 and the local density of Lyman-alpha absorption systems

A sample of 32 absorption lines has been identified in the ultraviolet spectrum of the z = 0.57 quasar PKS 0405-12. Data cover the wavelength range 1190-3260 A. There are 10 extragalactic Ly-alpha absorption lines in the complete sample, all with observed equivalent widths greater than or equal to 0.40 A; three of the Ly-alpha lines have Ly-beta counterparts. The number of Ly-alpha lines observed in the spectrum of PKS 0405-12 is within 1 sigma of the number predicted on the basis of previous HST observations of 3C 273 and of H1821 + 643. Combining the HST observations of 3C 273, H1821 + 643, and PKS 0405-12, we estimate the local number density of Ly-alpha systems with rest equivalent widths larger than 0.32 A to be about 15 +/- 4 Ly-alpha lines per unit redshift. Ground-based images reveal a rich field of galaxies in the direction of PKS 0405-12, including many galaxies with the brightnesses and sizes expected if they belong to a cluster associated with the quasar. The quasar spectrum does not show any evidence for absorption at the redshift of the emission lines, indicating a covering factor of less than unity for the halos of galaxies in the cluster around PKS 0405 - 12.

Bahcall, John N.↗
NASA NTRS1973

Possibility of Saturnian synchrotron radiation

Several synchrotron spectra are computed for a hypothetical Saturnian radiation belt. A monoenergetic distribution of relativistic electrons is assumed to exist in a toroidal volume in a dipole magnetic field aligned with the rotation axis. When synchrotron emission is added to the thermal component implied by observations at wavelengths shorter than 50 cm, the upturn in the total emission spectrum can become gradual to quite sharp depending on the critical wavelength and the number density of radiating electrons. As an example, the ranges of magnetic field intensity, and electron energy and density are tabulated for an estimate of the nonthermal component at the longest decimetric wavelength observation. The present observations are insufficient to determine the critical wavelength; but, for several estimates of the magnetic field loading factor, the minimum surface magnetic field intensity and minimum electron number density are computed.

Luthey, J. L.↗
NASA NTRS1994

An IUE survey of interstellar H I Ly alpha absorption. 2: Interpretations

We present an analysis of interstellar neutral hydrogen column densities obtained from archival Ly alpha absorption line data toward the complete sample of B2 and hotter stars observed at high spectral resolution with the IUE satellite. The full sample includes 554 stars and more than doubles the number of lines of sight previously observed. Many of the B2 and B1.5 stars exhibit contamination from stellar Ly alpha absorption, and were excluded from the subsequent analysis. The final working sample includes 393 stars. We present statistical averages for a number of quantities derived for the sample. The stars range in distance from 0.12 to 11 kpc with an average distance of 2.1 kpc. Values for average sight-line density range from 0.017 to 8.62 atoms cm(exp -3), with an overall average of 0.23 atoms cm(exp -3). Neutral hydrogen and dust are well correlated, with N(H I)/E(B - V) = 4.93 x 10(exp 21) cm(exp -2) mag(exp -1), and N(H I)/E(Bump) = 2.17 x 10(exp 21) cm(exp -2) mag(exp -1). There is an increase in the H I to dust ratios for the densest sight lines. Evidently, the H I to dust ratios in dense clouds are higher than in the lower density medium between the clouds and in interarm directions. The effect is probably an indication of dust modification in the dense clouds. The H I to dust ratios are approx. 17% smaller for the sight lines to O stars compared to the ratios found toward B stars. We have identified sight lines with anomalously large and small values of N(H I)/E(B - V) and H(H I)/E(Bump). Some of these sight lines are well-known examples of peculiar extinction (i.e., HD 147933 and HD 37061 (NU Ori)). Others are less well known and may provide important new examples of regions with highly modified dust. The number of stars with anomalously low H I to dust ratios is disproportionately larger than the number of stars with anomalously high H I to dust ratios. The distribution of the gas away form the Galactic plane for the sample of objects is complex. For the subset of 375 stars whose lines of sight have a statistically low probablility of intersecting a large cloud, the distribution may be roughly approximated by an exponential density distribution, with a midplane density of 0.366 atoms cm(exp -3), a scale height of 195 pc, and a random logarithmic scattering parameter sigma (sub p) of 0.159 dex. For the same sample of stars, interstellar dust is found to be confined closer to the plane than the H I, with (E(B - V)/r)(sub 0) = 0.257 mag kpc(exp -1), scale height h = 152 pc, and sigma(sub p) = 0.263 dex. The dust distribution is more inhomogeneous than the gas distribution. The observed distribution of H I away from the Galactic plane can also be fitted with a two-component exponential model including the effects of patchiness. Unfortunately, the foreground contamination produced by the cloudy and more confined component of H I makes it difficult to estimate the scale height of the extended component without invoking an independent estimate of its midplane density. Assuming n(sub 2)(0) = 0.16 cm(exp -3) from the Copernicus satellite H I/H(sub 2) survey, we obtain n(sub 1)(0) = 0.247 cm(exp -3), h(sub 1) = 73 PC, and h(sub 2) + 357 pc.

Diplas, Athanassios↗