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Results for “goodness of fit”

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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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

A cumulus cloud field observed by Landsat Thematic Mapper

The development of a spatial coherence scene model which is to be utilized to determine cloud properties from TM data is described. The observed radiances are modeled in terms of ocean and cloud spectra, the cloud fraction, and a geometric factor. Consideration is given to saturation effects and the need to constrain the model. The conjugate gradient algorithm is utilized to fit the model. A one-dimensional simulation was performed in order to evaluate the model; it is observed that the model provides good fit.

Hoffman, Ross N.↗
NASA NTRS1980

Cosmic ray studies with a gas Cerenkov counter in association with an ionization spectrometer

The results from a balloon-borne gas Cerenkov counter (threshold 16.5 GeV/nuc) and an ionization spectrometer are presented. The gas Cerenkov counter provides an absolute energy calibration for the response of the calorimeter for the Z range of 5-26 nuclei of cosmic rays. The contribution of scintillation to the gas Cerenkov pulse height has been obtained by independently selecting particles below the gas Cerenkov threshold using the ionization spectrometer. Energy spectra were derived by minimizing the chi-squared between a Monte Carlo simulated data and flight data. Best fit power laws were determined for C, N, O, Ne, Mg, and Si. The power laws, all consistent with E exp-2.7, are not good fits to the data. A better fit is obtained using the spectrum derived from the spectrometer.

Balasubrahmanyan, V. K.↗
NASA NTRS1982

Direct comparison of Viking 2.3-GHz signal phase fluctuation and columnar electron density between 2 and 160 solar radii

The relationship between solar wind induced signal phase fluctuation and solar wind columnar electron density has been the subject of intensive analysis during the last two decades. In this article, a sizeable volume of 2.3-GHz signal phase fluctuation and columnar electron density measurements separately and concurrently inferred from Viking spacecraft signals are compared as a function of solar geometry. These data demonstrate that signal phase fluctuation and columnar electron density are proportional over a very wide span of solar elongation angle. A radially dependent electron density model which provides a good fit to the columnar electron density measurements and, when appropriately scaled, to the signal phase fluctuation measurements, is given. This model is also in good agreement with K-coronameter observations at 2 solar radii (2r0), with pulsar time delay measurements at 10r0, and with spacecraft in situ electron density measurements at 1 AU.

Berman, A. L.↗
NASA NTRS1995

Spectrophotometry and organic matter on Iapetus. 1: Composition models

Iapetus shows a greater hemispheric albedo asymmetry than any other body in the solar system. Hapke scattering theory and optical constants measured in the laboratory are used to identify possible compositions for the dark material on the leading hemisphere of Iapetus. The materials considered are poly-HCN, kerogen, Murchison organic residue, Titan tholin, ice tholin, and water ice. Three-component mixtures of these materials are modeled in intraparticle mixture of 25% poly-HCN, 10% Murchison residue, and 65% water ice is found to best fit the spectrum, albedo, and phase behavior of the dark material. The Murchison residue and/or water ice can be replaced by kerogen and ice tholin, respectively, and still produce very good fits. Areal and particle mixtures of poly-HCN, Titan tholin, and either ice tholin or Murchison residue are also possible models. Poly-HCN is a necessary component in almost all good models. The presence of poly-HCN can be further tested by high-resolution observations near 4.5 micrometers.

Wilson, Peter D.↗
NASA NTRS1974

Investigation of nitric oxide decomposition in the temperature range 2500-4100 K

The decomposition of nitric oxide has been studied in the temperature range 2500-4100 K using a shock-tube technique. The principle result of the study was the determination of the rate constant (k) for the reaction O + NO yields N + O2. Mixtures of NO and N2O diluted in argon or krypton were heated by incident shock waves and the infrared emission from the fundamental vibration-rotation band of NO at 5.3 microns was used to monitor the time-varying NO concentration. The experimental values of k are fit closely by the expression k = (2.36 times 10 to the 9th power) x T x exp(-38,640/RT) cm cu/mole-sec, which also provides a good fit to previous data at higher and lower temperatures.

Flower, W. L.↗
NASA NTRS2007

Payload Processing for Mice Drawer System

Experimental payloads flown to the International Space Station provide us with valuable research conducted in a microgravity environment not attainable on earth. The Mice Drawer System is an experiment designed by Thales Alenia Space Italia to study the effects of microgravity on mice. It is designed to fly to orbit on the Space Shuttle Utilization Logistics Flight 2 in October 2008, remain onboard the International Space Station for approximately 100 days and then return to earth on a following Shuttle flight. The experiment apparatus will be housed inside a Double Payload Carrier. An engineering model of the Double Payload Carrier was sent to Kennedy Space Center for a fit check inside both Shuttles, and the rack that it will be installed in aboard the International Space Station. The Double Payload Carrier showed a good fit quality inside each vehicle, and Thales Alenia Space Italia will now construct the actual flight model and continue to prepare the Mice Drawer System experiment for launch.

Brown, Judy↗
NASA NTRS2012

A Search Model for Imperfectly Detected Targets

Under the assumptions that 1) the search region can be divided up into N non-overlapping sub-regions that are searched sequentially, 2) the probability of detection is unity if a sub-region is selected, and 3) no information is available to guide the search, there are two extreme case models. The search can be done perfectly, leading to a uniform distribution over the number of searches required, or the search can be done with no memory, leading to a geometric distribution for the number of searches required with a success probability of 1/N. If the probability of detection P is less than unity, but the search is done otherwise perfectly, the searcher will have to search the N regions repeatedly until detection occurs. The number of searches is thus the sum two random variables. One is N times the number of full searches (a geometric distribution with success probability P) and the other is the uniform distribution over the integers 1 to N. The first three moments of this distribution were computed, giving the mean, standard deviation, and the kurtosis of the distribution as a function of the two parameters. The model was fit to the data presented last year (Ahumada, Billington, & Kaiwi, 2 required to find a single pixel target on a simulated horizon. The model gave a good fit to the three moments for all three observers.

Ahumada, Albert↗
NASA NTRS1980

Cosmic ray studies with a gas Cherenkov counter in association with an ionization spectrometer

The results from a balloon-borne gas Cherenkov counter (threshold 16.5 GeV/nucleon) and an ionization spectrometer are presented. The gas Cherenkov counter provides an absolute energy distribution for the response of the calorimeter for 5 or = Z 26 nuclei of cosmic rays. The contribution of scintillation to the gas Cherenkov pulse height was obtained by independently selecting particles below the gas Cherenkov threshold using the ionization spectrometer. Energy spectra were derived by minimizing the chi squared between Monte Carlo simulted data and flight data. Best fit power laws, dN/dE = AE-gamma, were determined for C, N, O, Ne, Mg, and Si. The power laws, all consistent with E (-2.7) are not good fits to the data. A better fit is obtained using the spectrum derived from the spectrometer. The data from the ionization calorimeter and the gas Cherenkov are thus completely self-consistent.

Balasubrahmanyan, V. K.↗
NASA NTRS2006

X-ray and Sunyaev-Zel'dovich Effect Measurements of the Gas Mass Fraction in Galaxy Clusters

We present gas mass fractions of 38 massive galaxy clusters spanning redshifts from 0.14 to 0.89, derived from Chandra X-ray data and OVRO/BIMA interferometric Sunyaev-Zel' dovich Effect (SZE) measurements. We use three models for the gas distribution: (1) an isothermal Beta-model fit jointly to the X-ray data at radii beyond 100 kpc and to all of the SZE data, (2) a nonisothermal double Beta-model fit jointly to all of the X-ray and SZE data, and (3) an isothermal Beta-model fit only to the SZE spatial data. We show that the simple isothermal model well characterizes the intracluster medium (ICM) outside of the cluster core, and provides consistently good fits to clusters spanning a wide range of morphological properties. The agreement in the results shows that the core can be satisfactorily accounted for by either excluding the core in fits to the X-ray data (the 100 kpc-cut model) or modeling the intracluster gas with a non-isothermal double Beta-model. We find that the SZE is largely insensitive to structure in the core.

LaRoque, Samuel J.↗
NASA NTRS1976

Apollo 17 far-ultraviolet spectra in the Large Magellanic Cloud

During the Apollo 17 mission, low-resolution far-ultraviolet spectra were obtained for two parts of the Large Magellanic Cloud. The spectra are reasonably well fit by a model of a 30,000-K star reddened by a (B-V) color excess of 0.3 magnitude, but other combinations of temperature and reddening provide equally good fits.

Henry, R. C.↗
NASA NTRS2009

Complexity of Fit, with Application to Space Suits

Although fitting a garment is often considered more of an art than a science, experts suggest that a subjectively poor fit is a symptom of inappropriate ease, the space between the wearer and the garment. The condition of poor suit fit is a unique problem for the space program and it can be attributed primarily to: a) NASA s policy to accommodate a wide variety of people (males and females from 1st to 99th percentile range and with various shapes and sizes) and b) its requirement to deploy a minimum number of suit sizes for logistical reasons. These factors make the space suit fit difficult to assess, where a wide range of people must be fit by the minimum possible number of suits, and yet, fit is crucial for operability and safety. Existing simplistic sizing scheme do not account for wide variations in shape within a diverse population with very limited sizing options. The complex issue of fit has been addressed by a variety of methods, many of which have been developed by the military, which has always had a keen interest in fitting its diverse population but with a multitude of sizing options. The space program has significantly less sizing options, so a combination of these advanced methods should be used to optimize space suit size and assess space suit fit. Multivariate methods can be used to develop sizing schemes that better reflect the wearer population, and integrated sizing systems can form a compromise between fitting men and women. Range of motion and operability testing can be combined with subjective feedback to provide a comprehensive evaluation of fit. The amount of ease can be tailored using these methods, to provide enough extra room where it is needed, without compromising mobility and comfort. This paper discusses the problem of fit in one of its most challenging applications: providing a safe and comfortable spacesuit that will protect its wearer from the extreme environment of space. It will discuss the challenges and necessity of closely fitting its potential wearers, a group of people from a broad spectrum of the population, and will detail some of the methods that can be employed to ensure and validate a good fit.

Rajulu, Sudhakar↗
NASA NTRS2017

Cloudless Atmospheres for Young Low-Gravity Substellar Objects

Atmospheric modeling of low-gravity (VL-G) young brown dwarfs remains challenging. The presence of very thick clouds is a possible source of this challenge, because of their extremely red near-infrared (NIR) spectra, but no cloud models provide a good fit to the data with a radius compatible with the evolutionary models for these objects. We show that cloudless atmospheres assuming a temperature gradient reduction caused by fingering convection provide a very good model to match the observed VL-G NIR spectra. The sequence of extremely red colors in the NIR for atmospheres with effective temperatures from approx. 2000 K down to approx. 1200 K is very well reproduced with predicted radii typical of young low-gravity objects. Future observations with NIRSPEC and MIRI on the James Webb Space Telescope (JWST) will provide more constraints in the mid-infrared, helping to confirm or refute whether or not the NIR reddening is caused by fingering convection. We suggest that the presence or absence of clouds will be directly determined by the silicate absorption features that can be observed with MIRI. JWST will therefore be able to better characterize the atmosphere of these hot young brown dwarfs and their low-gravity exoplanet analogs.

Tremblin, P.↗
NASA NTRS2017

Cloudless Atmospheres for Young Low-Gravity Substellar Objects

Atmospheric modeling of low-gravity (VL-G) young brown dwarfs remains challenging. The presence of very thick clouds is a possible source of this challenge, because of their extremely red near-infrared (NIR) spectra, but no cloud models provide a good fit to the data with a radius compatible with the evolutionary models for these objects. We show that cloudless atmospheres assuming a temperature gradient reduction caused by fingering convection provide a very good model to match the observed VL-G NIR spectra. The sequence of extremely red colors in the NIR for atmospheres with effective temperatures from ∼2000 K down to ∼1200 K is very well reproduced with predicted radii typical of young low-gravity objects. Future observations with NIRSPEC and MIRI on the James Webb Space Telescope (JWST) will provide more constraints in the mid-infrared, helping to confirm or refute whether or not the NIR reddening is caused by fingering convection. We suggest that the presence or absence of clouds will be directly determined by the silicate absorption features that can be observed with MIRI. JWST will therefore be able to better characterize the atmosphere of these hot young brown dwarfs and their low-gravity exoplanet analogs.

Tremblin, P.↗
NASA NTRS1995

The spectrum and pulses of 1E 2259+586 from ASCA and BBXRT observations

The 7 s X-ray pulsator 1E 2259+586 was observed for approximately 1 day in 1993 with the Advanced Satellite for Cosmology and Astrophysics (ASCA). Observations were also obtained with Broad Band X-ray Telescope (BBXRT) in 1990 a few months after Ginga had observed 1E 2259+586 to be brighter than normal and the BBXRT data show 1E 2259+586 to be at an intermediate brightness level. By contrast, the ASCA data appear to have been obtained during a more common lower luminosity state. The pulse profiles we obtain are consistent with a connection between flux and pulse shape reported from Ginga data, and the pulsator continues to spin down. We use our high spectral resolution data to search for cyclotron lines in the spectrum that were claimed from observations made with other satellites. We find that the ASCA spectra of 1E 2259+586 cannot be satisfactorily fitted with either a single power law or a combination of two power laws, and that significant residuals occur around 1.5 and 5 keV. However, a combination of a power law and blackbody gives a good fit over the entire ASCA energy band with no evidence of spectral features. We have reanalyzed a Ginga LAC spectrum and find that this is also significantly better fitted by this two-component spectrum than a single power law. A possible explanation for such a two-component spectrum is that the blackbody emission comes from a neutron star and that the power-law component comes, at least in part, from a surrounding nebula. As there has, so far, no direct evidence that 1E 2259+586 is a binary system we consider whether there are other plausible mechanisms that might power the observed X-ray emission.

Corbet, R. H. D.↗
NASA NTRS1973

The magnetospheric structure of pulsars

A model of pulsar magnetospheres is described which has evolved inductively from the work of Sturrock, where the radiation is produced near the surface of a neutron star. Some of the theoretical ideas of others, particularly those of Sturrock, are discussed. The braking index n and period-pulse-width distribution of pulsars are first reinvestigated by relaxing the conventional assumption that R sub Y = R sub L, where R sub Y is the radius of the neutral points marking the transition from closed to open magnetic field lines, and R sub L is the radius of the light cylinder. This is replaced by the parameterization R sub Y = R sub * (1- eta )power R sub L (eta), where R sub * is the neutron star radius. If the ratio frequency radiation is created near the surface and beamed along open field lines, it is found that a good fit to the period-pulse-width distribution can be obtained for eta in the range 0.5 = or eta = or 0.7. The relation n = 1 + 2 eta then gives n = 2.2 + or - 0.2, which is in good agreement with the values measured for the Crab pulsar.

Roberts, D. H.↗
NASA NTRS1984

A complete sample of quasars at B = 19.80

A quasar candidate spectroscopic UV-excess survey with B less than 19.80 has yielded 35 quasars in the 1.75 sq deg region studied. The X-ray data for the 35 quasars are used to show that quasars with B lower than 19.80 and z lower than 2.2 constitute 24 + 10, -8 percent of the observed X-ray background at 2 keV. This sample is combined with one containing 22 quasars with B values lower than 18.25, and it is noted that fits of evolution and luminosity functions are substantially similar to previous results, although the confidence limits for the model parameters are reduced by a factor of 2. Pure luminosity evolution models are found to produce very good fits to the observed N(less than B) data, and are used to estimate the overall contribution of quasars to the X-ray background.

Marshall, H. L.↗
NASA NTRS1997

Model of visual contrast gain control and pattern masking

We have implemented a model of contrast gain and control in human vision that incorporates a number of key features, including a contrast sensitivity function, multiple oriented bandpass channels, accelerating nonlinearities, and a devisive inhibitory gain control pool. The parameters of this model have been optimized through a fit to the recent data that describe masking of a Gabor function by cosine and Gabor masks [J. M. Foley, "Human luminance pattern mechanisms: masking experiments require a new model," J. Opt. Soc. Am. A 11, 1710 (1994)]. The model achieves a good fit to the data. We also demonstrate how the concept of recruitment may accommodate a variant of this model in which excitatory and inhibitory paths have a common accelerating nonlinearity, but which include multiple channels tuned to different levels of contrast.

NASA Discipline Space Human Factors↗
DOE OSTI2024

Limits on non-relativistic matter during Big-bang nucleosynthesis

Big-bang nucleosynthesis (BBN) probes the cosmic mass-energy density at temperatures ~10 MeV to ~100 keV. Here, we consider the effect of a cosmic matter-like species that is non-relativistic and pressureless during BBN. Such a component must decay; doing so during BBN can alter the baryon-to-photon ratio, η, and the effective number of neutrino species. We use light element abundances and the cosmic microwave background (CMB) constraints on η and N ν to place constraints on such a matter component. We find that electromagnetic decays heat the photons relative to neutrinos, and thus dilute the effective number of relativistic species to N eff < 3 for the case of three Standard Model neutrino species. Intriguingly, likelihood results based on Planck CMB data alone find N ν = 2.800 ± 0.294, and when combined with standard BBN and the observations of D and 4 He give N ν = 2.898 ± 0.141. While both results are consistent with the Standard Model, we find that a nonzero abundance of electromagnetically decaying matter gives a better fit to these results. Our best-fit results are for a matter species that decays entirely electromagnetically with a lifetime τ X = 0.89 sec and pre-decay density that is a fraction ξ = (ρ X /ρ rad |10 MeV = 0.0026 of the radiation energy density at 10 MeV; similarly good fits are found over a range where ξτ X 1/2 is constant. On the other hand, decaying matter often spoils the BBN+CMB concordance, and we present limits in the (τ X ,ξ) plane for both electromagnetic and invisible decays. For dark (invisible) decays, standard BBN (i.e. ξ = 0) supplies the best fit. We end with a brief discussion of the impact of future measurements including CMB-S4.

79 ASTRONOMY AND ASTROPHYSICS↗