Search NASA⌕ Search

SEARCH · Search NASA

Results for “Variance to Mean”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Additional studies of forest classification accuracy as influenced by multispectral scanner spatial resolution

First, an analysis of forest feature signatures was used to help explain the large variation in classification accuracy that can occur among individual forest features for any one case of spatial resolution and the inconsistent changes in classification accuracy that were demonstrated among features as spatial resolution was degraded. Second, the classification rejection threshold was varied in an effort to reduce the large proportion of unclassified resolution elements that previously appeared in the processing of coarse resolution data when a constant rejection threshold was used for all cases of spatial resolution. For the signature analysis, two-channel ellipse plots showing the feature signature distributions for several cases of spatial resolution indicated that the capability of signatures to correctly identify their respective features is dependent on the amount of statistical overlap among signatures. Reductions in signature variance that occur in data of degraded spatial resolution may not necessarily decrease the amount of statistical overlap among signatures having large variance and small mean separations. Features classified by such signatures may thus continue to have similar amounts of misclassified elements in coarser resolution data, and thus, not necessarily improve in classification accuracy.

Sadowski, F. E.↗
NASA NTRS1978

Parameter estimation applied to Nimbus 6 wide-angle longwave radiation measurements

A parameter estimation technique was used to analyze the August 1975 Nimbus 6 Earth radiation budget data to demonstrate the concept of deconvolution. The longwave radiation field at the top of the atmosphere is defined from satellite data by a fifth degree and fifth order spherical harmonic representation. The variations of the major features of the radiation field are defined by analyzing the data separately for each two-day duty cycle. A table of coefficient values for each spherical harmonic representation is given along with global mean, gradients, degree variances, and contour plots. In addition, the entire data set is analyzed to define the monthly average radiation field.

Green, R. N.↗
NASA NTRS1978

Fine structure and time variation of the quiet sun at 1.3 cm

The Hat Creek two-element interferometer has been used to study the quiet sun at 22 GHz. A statistical analysis of output of the interferometer clearly shows the existence of time variations on the quiet sun with time scales of approximately 180 s. The observations suggest that the fine structure on the quiet sun might consist of two components - one which varies with the time scales of approximately 3 min and the other being relatively stable. The average visibility amplitude indicates that the fine structure on the sun has a typical angular size of approximately 6 sec. The observation that the variance and the mean of the visibility amplitudes depend in the same way on the projected baseline suggests that the transient sources have angular size similar to the average size of the fine structure on the quiet sun. Power spectra of the output of the interferometer show no significant periodicity.

Rao, A. P.↗
NASA NTRS1983

Airborne remote sensing measurements with a pulsed CO2 DIAL system

The present investigation is concerned with an optical radar instrument which has been developed for airborne remote sensing of atmospheric trace species at infrared wavelengths. The instrument makes use of compact, pulsed carbon dioxide lasers. It was designed for conducting differential absorption lidar (DIAL) measurements. The backscatter of laser pulse energy from the ocean and terrain surface is utilized to determine trace species column content. The instrument, which is carried by the NASA P3 aircraft, is used in a flight test program started in July 1981. The results obtained with the instrument in this program are discussed, taking into account ocean surface backscatter statistics, the ratio of ocean to terrain backscatter, the dual-wavelength correlation coefficient, the dual-wavelength cross-covariance, and the dependence of un-correlated noise variance on the mean backscatter signal.

Bufton, J. L.↗
NASA NTRS1984

Design and evaluation of a fault-tolerant multiprocessor using hardware recovery blocks

Attention is given to the design and evaluation of a fault-tolerant multiprocessor whose rollback recovery mechanism is based on hardware recovery blocks constructed by consecutive state-save operations and several state-save units in every processor and memory module. Upon detection of failure, the multiprocessor reconfigures itself to replace the faulty module and the process originally assigned to the faulty module retreats to one of the previously saved states, in order to resume fault-free execution. A mathematical model is proposed for the calculation of multistep rollback recovery coverage and the risk of restart when all available saved states are exhausted. The mean and the variance of task execution time with the occurrence of rollbacks and/or restarts are evaluated.

Lee, Y.-H.↗
NASA NTRS1987

The Portable Instant Display and Analysis Spectrometer (PIDAS)

A field spectrometer covering the range 0.4 to 2.5 microns was developed that acquires spectra in 2 seconds at 872 points within the spectrum. The Portable Instant Display and Analysis Spectrometer (PIDAS) can acquire spectra every 8 seconds and stores up to 288 spectra in bubble memory. A hand held display unit allows for display of the current spectrum acquired and superimposed on one of 128 permanently stored library spectra. PIDAS represents a major advance in the technology of field spectral data acquisition and for the first time makes possible the acquisition of enough spectra to characterize the mean and intraclass variance within a LANDSAT MSS or TM pixel.

Goetz, Alexander F. H.↗
NASA NTRS1989

Performance of the all-digital data-transition tracking loop in the advanced receiver

The performance of the all-digital data-transition tracking loop (DTTL) with coherent or noncoherent sampling is described. The effects of few samples per symbol and of noncommensurate sampling rates and symbol rates are addressed and analyzed. Their impacts on the loop phase-error variance and the mean time to lose lock (MTLL) are quantified through computer simulations. The analysis and preliminary simulations indicate that with three to four samples per symbol, the DTTL can track with negligible jitter because of the presence of earth Doppler rate. Furthermore, the MTLL is also expected to be large engough to maintain lock over a Deep Space Network track.

Cheng, U.↗
NASA NTRS1990

Flux estimation of the FIFE planetary boundary layer (PBL) with 10.6 micron Doppler lidar

A method is devised for calculating wind, momentum, and other flux parameters that characterize the planetary boundary layer (PBL) and thereby facilitate the calibration of spaceborne vs. in situ flux estimates. Single Doppler lidar data are used to estimate the variance of the mean wind and the covariance related to the vertically pointing fluxes of horizontal momentum. The skewness of the vertical velocity and the range of kinetic energy dissipation are also estimated, and the surface heat flux is determined by means of a statistical Navier-Stokes equation. The conclusion shows that the PBL structure combines both 'bottom-up' and 'top-down' processes suggesting that the relevant parameters for the atmospheric boundary layer be revised. The conclusions are of significant interest to the modeling techniques used in General Circulation Models as well as to flux estimation.

Gal-Chen, Tzvi↗
NASA NTRS1990

Modeling bidirectional reflectance of forests and woodlands using Boolean models and geometric optics

Geometric-optical discrete-element mathematical models for forest canopies have been developed using the Boolean logic and models of Serra. The geometric-optical approach is considered to be particularly well suited to describing the bidirectional reflectance of forest woodland canopies, where the concentration of leaf material within crowns and the resulting between-tree gaps make plane-parallel, radiative-transfer models inappropriate. The approach leads to invertible formulations, in which the spatial and directional variance provides the means for remote estimation of tree crown size, shape, and total cover from remotedly sensed imagery.

Strahler, Alan H.↗
NASA NTRS1992

Estimations of atmospheric boundary layer fluxes and other turbulence parameters from Doppler lidar data

Methods for extraction of boundary layer parameters from measurements of a short pulse CO2 Doppler lidar are presented. Assuming that the turbulence is horizontally homogeneous, the mean wind, its standard deviations, and the momentum fluxes are estimated. Regarding the analysis method, it is shown that for a horizontally homogeneous turbulence the mean wind, its variance and the vertical momentum fluxes can be determined from single Doppler lidar data.

Gal-Chen, Tzvi↗
NASA NTRS1992

Detector noise statistics in the non-linear regime

The statistical behavior of an idealized linear detector in the presence of threshold and saturation levels is examined. It is assumed that the noise is governed by the statistical fluctuations in the number of photons emitted by the source during an exposure. Since physical detectors cannot have infinite dynamic range, our model illustrates that all devices have non-linear regimes, particularly at high count rates. The primary effect is a decrease in the statistical variance about the mean signal due to a portion of the expected noise distribution being removed via clipping. Higher order statistical moments are also examined, in particular, skewness and kurtosis. In principle, the expected distortion in the detector noise characteristics can be calibrated using flatfield observations with count rates matched to the observations. For this purpose, some basic statistical methods that utilize Fourier analysis techniques are described.

Shopbell, P. L.↗
NASA NTRS1995

Crossflow Mixing of Noncircular Jets

An experimental investigation has been conducted of the isothermal mixing of a turbulent jet injected perpendicular to a uniform crossflow through several different types of sharp-edged orifices. Jet penetration and mixing was studied using planar Mie scattering to measure time-averaged mixture fraction distributions of circular, square, elliptical, and rectangular orifices of equal geometric area injected into a constant velocity crossflow. Hot-wire anemometry was also used to measure streamwise turbulence intensity distributions at several downstream planes. Mixing effectiveness was determined using (1) a spatial unmixedness parameter based on the variance of the mean jet concentration distributions and (2) by direct comparison of the planar distributions of concentration and of turbulence intensity. No significant difference in mixing performance was observed for the six configurations based on comparison of the mean properties.

Liscinsky, D. S.↗
NASA NTRS1995

Effects of Initial Conditions on a Single Jet in Crossflow

An experimental investigation of the effects of jet inlet flow conditions has been conducted for the isothermal mixing of a single jet injected into a crossflow. Jet penetration and mixing was studied using planar Mie scattering to measure time-averaged jet mixture fraction distributions. The effects of 'passive' control methods such as jet 'tabs' and jet swirl are reported. Mixing effectiveness, determined using a spatial unmixedness parameter based on the variance of the mean jet concentration distributions, was compared to a baseline case of a round jet injected into a uniform crossflow. All results are compared at a jet-to-mainstream momentum-flux ratio of 8.5. In the near-field, the mixing rates are similar to, or less than, the baseline configuration using this measure of mixedness. None of the tested configurations appear to significantly augment mixing within a downstream distance of 3 diameters of an equivalent-area round orifice.

Liscinsky, D. S.↗
NASA NTRS1993

Transport and residence times of tropospheric aerosols inferred from a global three-dimensional simulation of Pb-210

A global three-dimensional model is used to investigate the transport and tropospheric residence time of Pb-210, an aerosol tracer produced in the atmosphere by radioactive decay of Rn-222 emitted from soils. The model uses meteorological input with 4 deg x 5 deg horizontal resolution and 4-hour temporal resolution from the Goddard Institute for Space Studies general circulation model (GCM). It computes aerosol scavenging by convective precipitation as part of the wet convective mass transport operator in order to capture the coupling between vertical transport and rainout. Scavenging in convective precipitation accounts for 74% of the global Pb-210 sink in the model; scavenging in large-scale precipitation accounts for 12%, and scavenging in dry deposition accounts for 14%. The model captures 63% of the variance of yearly mean Pb-210 concentrations measured at 85 sites around the world with negligible mean bias, lending support to the computation of aerosol scavenging. There are, however, a number of regional and seasonal discrepancies that reflect in part anomalies in GCM precipitation. Computed residence times with respect to deposition for Pb-210 aerosol in the tropospheric column are about 5 days at southern midlatitudes and 10-15 days in the tropics; values at northern midlatitudes vary from about 5 days in winter to 10 days in summer. The residence time of Pb-210 produced in the lowest 0.5 km of atmosphere is on average four times shorter than that of Pb-210 produced in the upper atmosphere. Both model and observations indicate a weaker decrease of Pb-210 concentrations between the continental mixed layer and the free troposphere than is observed for total aerosol concentrations; an explanation is that Rn-222 is transported to high altitudes in wet convective updrafts, while aerosols and soluble precursors of aerosols are scavenged by precipitation in the updrafts. Thus Pb-210 is not simply a tracer of aerosols produced in the continental boundary layer, but also of aerosols derived from insoluble precursors emitted from the surface of continents. One may draw an analogy between Pb-210 and nitrate, whose precursor NO(sub x) is sparingly soluble, and explain in this manner the strong correlation observed between nitrate and Pb-210 concentrations over the oceans.

Balkanski, Yves J.↗
NASA NTRS1996

Effects of Inlet Flow Conditions on Crossflow Jet Mixing

An experimental investigation of the effects of mainstream turbulence, mainstream swirl and non-symmetric mass addition has been conducted for the isothermal mixing of multiple jets injected into a confined rectangular crossflow. Jet penetration and mixing in the near field was studied using planar Mie scattering to measure time-averaged mixture fraction distributions. Orifice configurations were used that were optimized for mixing performance based on previous experimental and computational results for a homogeneous approach flow. Mixing effectiveness, determined using a spatial unmixedness parameter based on the variance of the mean jet concentration distributions, was found to be minimally affected by free-stream turbulence but significantly influenced by the addition of swirl to the mainstream. The results for non-symmetric mass addition indicate that the concentration distribution of the flowfield can be tailored if desired.

Liscinsky, D. S.↗
NASA NTRS1996

A Velocity Distribution Model for Steady State Heat Transfer

Consider a box that is filled with an ideal gas and that is aligned along Cartesian coordinates (x, y, z) having until length in the 'y' direction and unspecified length in the 'x' and 'z' directions. Heat is applied uniformly over the 'hot' end of the box (y = 1) and is removed uniformly over the 'cold' end (y = O) at a constant rate such that the ends of the box are maintained at temperatures T(sub 0) at y = O and T(sub 1) at y = 1. Let U, V, and W denote the respective velocity components of a molecule inside the box selected at some random time and at some location (x, y, z). If T(sub 0) = T(sub 1), then U, Y, and W are mutually independent and Gaussian, each with mean zero and variance RT(sub 0), where R is the gas constant. When T(sub 0) does not equal T(sub 1) the velocity components are not independent and are not Gaussian. Our objective is to characterize the joint distribution of the velocity components U, Y, and W as a function of y, and, in particular, to characterize the distribution of V given y. It is hoped that this research will lead to an increased physical understanding of the nature of turbulence.

Hall, Eric B.↗
NASA NTRS2001

The AutoBayes Program Synthesis System: System Description

AUTOBAYES is a fully automatic program synthesis system for the statistical data analysis domain. Its input is a concise description of a data analysis problem in the form of a statistical model; its output is optimized and fully documented C/C++ code which can be linked dynamically into the Matlab and Octave environments. AUTOBAYES synthesizes code by a schema-guided deductive process. Schemas (i.e., code templates with associated semantic constraints) are applied to the original problem and recursively to emerging subproblems. AUTOBAYES complements this approach by symbolic computation to derive closed-form solutions whenever possible. In this paper, we concentrate on the interaction between the symbolic computations and the deductive synthesis process. A statistical model specifies for each problem variable (i.e., data or parameter) its properties and dependencies in the form of a probability distribution, A typical data analysis task is to estimate the best possible parameter values from the given observations or measurements. The following example models normal-distributed data but takes prior information (e.g., from previous experiments) on the data's mean value and variance into account.

Fischer, Bernd↗