SEARCH · Engineering Papers

Results for “measurement errors”

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.

SourceResearch connectionFrom yearThrough year

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

At least 487 records · Page 27

All sources NASA NTRS DOE OSTI

NASA NTRS1991

Relation between perception of vertical axis rotation and vestibulo-ocular reflex symmetry

Subjects seated in a vertical axis rotation chair controlled their rotational velocity by adjusting a potentiometer. Their goal was to null out pseudorandom rotational perturbations in order to remain perceptually stationary. Most subjects showed a slow linear drift of velocity (a constant acceleration) to one side when they were deprived of an earth-fixed visual reference. The amplitude and direction of this drift can be considered a measure of a static bias in the subject's perception of rotation. The presence of a perceptual bias is consistent with a small, constant imbalance of vestibular function which could be of either central or peripheral origin. Deviations from perfect vestibulocular reflex (VOR) symmetry are also assumed to be related to imbalances in either peripheral or central vestibular function. Researchers looked for correlations between perceptual bias and various measures of vestibular reflex symmetry that might suggest a common source for both reflective and perceptual imbalances. No correlations were found. Measurement errors could not account for these results since repeated tests on the same subjects of both perceptual bias and VOR symmetry were well correlated.

Peterka, Robert J.↗

NASA NTRS1991

Initial pointing calibrations for the DSS 13 34-meter beam-waveguide antenna

The beam pointing of the Deep Space Station (DSS) 13 beam-waveguide antenna at the Goldstone Venus site was calibrated during the postconstruction performance testing period from Jul. 1990 through Jan. 1991. The pointing calibrations were based on errors measured on radio sources at both the Cassegrain and centerline beam-waveguide focal points. The blind pointing performance goal of 5.0 mdeg, 3-sigma at Ka-band (32 GHz) was demonstrated to be met for low (less than 10 mph) wind conditions.

Alvarez, L. S.↗

NASA NTRS1990

Evaluation of discrete frequency sound in closed-test-section wind tunnels

The principal objective of this study is to assess the adequacy of linear acoustic theory with an impedance wall boundary condition for modeling the detailed sound field of an acoustic source in a duct. This study compares measurements and calculations of a simple acoustic source in a rectangular concrete duct lined with foam on the walls and anechoic end terminations. Measuring acoustic pressure for 12 wave numbers provides variation in frequency and absorption characteristics of the duct walls. The cases in this study contain low frequencies and low wall absorptions corresponding to measurements of low-frequency helicopter noise in a lined wind tunnel. This regime is particularly difficult to measure in wind tunnels due to high levels of the reverberant field relatively close to the source. Close to the source, where the interference of wall reflections is minimal, correlation is very good. Away from the source, correlation degrades, especially for the lower frequencies. Sensitivity studies show little effect on the predicted results for changes in impedance boundary condition values, source location, measurement location, temperature, and source model for variations spanning the expected measurement error.

Mosher, Marianne↗

NASA NTRS1990

Linear regression in astronomy. I

Five methods for obtaining linear regression fits to bivariate data with unknown or insignificant measurement errors are discussed: ordinary least-squares (OLS) regression of Y on X, OLS regression of X on Y, the bisector of the two OLS lines, orthogonal regression, and 'reduced major-axis' regression. These methods have been used by various researchers in observational astronomy, most importantly in cosmic distance scale applications. Formulas for calculating the slope and intercept coefficients and their uncertainties are given for all the methods, including a new general form of the OLS variance estimates. The accuracy of the formulas was confirmed using numerical simulations. The applicability of the procedures is discussed with respect to their mathematical properties, the nature of the astronomical data under consideration, and the scientific purpose of the regression. It is found that, for problems needing symmetrical treatment of the variables, the OLS bisector performs significantly better than orthogonal or reduced major-axis regression.

Isobe, Takashi↗

NASA NTRS1990

Potential, velocity, and density fields from sparse and noisy redshift-distance samples - Method

A method for recovering the three-dimensional potential, velocity, and density fields from large-scale redshift-distance samples is described. Galaxies are taken as tracers of the velocity field, not of the mass. The density field and the initial conditions are calculated using an iterative procedure that applies the no-vorticity assumption at an initial time and uses the Zel'dovich approximation to relate initial and final positions of particles on a grid. The method is tested using a cosmological N-body simulation 'observed' at the positions of real galaxies in a redshift-distance sample, taking into account their distance measurement errors. Malmquist bias and other systematic and statistical errors are extensively explored using both analytical techniques and Monte Carlo simulations.

Dekel, Avishai↗

NASA NTRS1989

Optical alignment equipment and methods for visible and IR imaging meteorological radiometer

This paper describes the test equipment required for precise IR and visible channel microradian alignments for geosynchronous orbiting meteorological instruments. The test equipment design criteria, design, function and use are explained in detail. Wave front error measurements, vibration isolation, thermal compensation and stress free mounting configurations are described.

Herendeen, R.↗

NASA NTRS1991

The structure of the Venus ionosphere

The morphology and temporal variability of the Venus ionosphere are characterized, reviewing the results of recent theoretical investigations, observations, and in situ measurements, especially by the Pioneer Venus Orbiter (PVO). Consideration is given to the Pioneer mission and orbit evolution, early radio occultation profiles of ionospheric N(e), the mean structure and thermal balance of the ionosphere, the ion composition and its dawn-dusk asymmetry, the small-scale spatial structure on the nightside, latitudinal and seasonal variations, solar-cycle effects, suprathermal electrons and superthermal ions, and the global configuration and stability of the ionopause. Data from a single PVO passage through the ionotail are discussed in detail, examining the implications for ion escape and the solar-cycle and short-term variability. The differences among the terrestrial, Martian, and Venusian ionospheres are outlined; the PVO data base is described; and the sources of measurement error are indicated.

Brace, L. H.↗

NASA NTRS1991

A signal model for GPS

As the development of GPS continues, there will be an increasing need for a software-centered signal model. This model must accurately generate the observed pseudorange that would typically be encountered. The observed pseudorange varies from the true geometric range because of range measurement errors, which stem from a variety of hardware and environmental factors. In this paper, these errors are classified as either deterministic or random, and, where appropriate, their models are summarized. Of particular interest is the model for Selective Availability, which was derived from actual GPS data. The procedure for determination of this model, known as system identification theory, is briefly outlined. The synthesis of these error sources into the final signal model is given, along with simulation results.

Braasch, Michael S.↗

NASA NTRS1991

A control network of Triton

A control network for Triton has been computed using a bundle-type analytical triangulation program. The network contains 105 points that were measured on 57 Voyager-2 pictures. The adjustment contained 1010 observation equations and 382 normal equations and resulted in a standard measurement error of 13.36 microns. The coordinates of the control points, the camera orientation angles at the times when the pictures were taken, and Triton's mean radius were determined. A separate statistical analysis confirmed Triton's radius to be 1352.6 + or - 2.4 km. Attempts to tie the control network around the satellite were unsuccessful because discontinuities exist in high-resolution coverage between 66 deg and 289 deg longitude, north of 38 deg latitude, and south of 78 deg latitude.

Davies, Merton E.↗

NASA NTRS1991

Evaluating the design of satellite scanning radiometers for earth radiation budget measurements with system simulations. Part 1: Instantaneous estimates

A set of system simulations was performed to evaluate candidate scanner configurations to fly as a part of the Earth Radiation Budget Instrument (ERBI) on the polar platforms during the 1990's. The simulation is considered of instantaneous sampling (without diurnal averaging) of the longwave and shortwave fluxes at the top of the atmosphere (TOA). After measurement and subsequent inversion to the TOA, the measured fluxes were compared to the reference fluxes for 2.5 deg lat/long resolution targets. The reference fluxes at this resolution are obtained by integrating over the 25 x 25 = 625 grid elements in each target. The differences between each of these two resultant spatially averaged sets of target measurements (errors) are taken and then statistically summarized. Five instruments are considered: (1) the Conically Scanning Radiometer (CSR); (2) the ERBE Cross Track Scanner; (3) the Nimbus-7 Biaxial Scanner; (4) the Clouds and Earth's Radiant Energy System Instrument (CERES-1); and (5) the Active Cavity Array (ACA). Identical studies of instantaneous error were completed for many days, two seasons, and several satellite equator crossing longitudes. The longwave flux errors were found to have the same space and time characteristics as for the shortwave fluxes, but the errors are only about 25 pct. of the shortwave errors.

Stowe, Larry↗

NASA NTRS1992

Turbulent heat flux measurements in a transitional boundary layer

During an experimental investigation of the transitional boundary layer over a heated flat plate, an unexpected result was encountered for the turbulent heat flux (bar-v't'). This quantity, representing the correlation between the fluctuating normal velocity and the temperature, was measured to be negative near the wall under certain conditions. The result was unexpected as it implied a counter-gradient heat transfer by the turbulent fluctuations. Possible reasons for this anomalous result were further investigated. The possible causes considered for this negative bar-v't' were: (1) plausible measurement error and peculiarity of the flow facility, (2) large probe size effect, (3) 'streaky structure' in the near wall boundary layer, and (4) contributions from other terms usually assumed negligible in the energy equation including the Reynolds heat flux in the streamwise direction (bar-u't'). Even though the energy balance has remained inconclusive, none of the items (1) to (3) appear to be contributing directly to the anomaly.

Sohn, K. H.↗

NASA NTRS1992

A Comparison Between Raman Lidar and Conventional Contact Measurements of Atmospheric Temperature

Described here are the results of comparison between lidar and conventional contact measurements of the vertical temperature profile of the atmosphere. The lidar measurements are based on the method of temperature dependence of pure rotational Raman scattering of nitrogen and oxygen molecules. The presented results show that, as a whole, the motion of the lidar and conventional profiles coincide in the confidence intervals. Still there are districts in which the differences lay out of the confidence intervals. In analysis and estimation of the coincidence and the distinction in moving the profiles, we must take into account the differences between the methods for measuring the temperature profiles by lidar and contact methods, i.e. the next additional factors. The tied balloon meter gives the momentary values of the temperature in particular points of the profile, as these momentary values are the results of consequent, but not simultaneous measurements. In the case of free flying balloon, the time of measuring the lower several hundred meters is little because of high vertical velocity (about 300 m/min). This leads to an indefinite increase of the measuring error. The complete space coincidence between the lidar's and conventional profiles isn't possible. The contribution of the factors mentioned above about the error of comparison could increase because of nonstable layers in the planetary boundary layer of the atmosphere as well as by the influence of the mountains and the city situated nearby.

Mitev, V. M.↗

NASA NTRS1992

Orbiter-orbiter and orbiter-lander tracking using same-beam interferometry

Two spacecraft orbiting Mars will subtend a small angle as viewed from Earth. This angle will usually be smaller than the beam width of a single radio antenna. Thus the two spacecraft may be tracked simultaneously by a single Earth-based antenna. The same-beam interferometry (SBI) technique involves using two widely separated antennas, each observing the two spacecraft, to produce a measurement of the angular separation of the two spacecraft in the plane of the sky. The information content of SBI data is thus complementary to the line-of-sight information provided by conventional Doppler data. The inclusion of SBI data with the Doppler data in a joint orbit estimation procedure can desensitize the solution to gravity mismodeling and result in improved orbit determination accuracy. This article presents an overview of the SBI technique, a measurement error analysis, and an error covariance analysis of some examples of the application of SBI to orbit determination. For hypothetical scenarios involving the Mars Observer and the Russian Mars '94 spacecraft, orbit determination accuracy improvements of up to an order of magnitude are predicted, relative to the accuracy that can be obtained by using only Doppler data acquired separately from each spacecraft. Relative tracking between a Mars orbiter and a lander fixed on the surface of Mars is also studied. Results indicate that the lander location may be determined to a few meters, while the orbiter ephemeris may be determined with accuracy similar to the orbiter-orbiter case.

Folkner, W. M.↗

NASA NTRS1991

In-flight demonstration of hybrid GPS/Loran RAIM

A receiver autonomous integrity monitoring (RAIM) scheme has been implemented in a real time, prototype hybrid GPS/LORAN receiver. A four-channel GPS receiver and an eight-channel LORAN-C receiver are used to obtain raw GPS and LORAN pseudorange measurements. The measurement data is sent to a microcomputer for position and integrity determination. Navigation data is presented to the pilot on a standard course deviation indicator with a data renewal rate of up to one update per second. Provisions are made to simulate signal malfunctions in real time by injecting step or ramp failures in the pseudorange measurements. A least-squares fault detection algorithm is presented for the detection of slowly growing measurement errors. The performance of the hybrid GPS/LORAN receiver has been demonstrated through actual flight tests. One of the flight tests is summarized in the paper.

Van Graas, Frank↗

NASA NTRS1992

A dual-frequency microwave technique for measuring rainwater content

This study shows that the dual-frequency (38, 25 GHz) differential attenuation (A(38-25)) coefficient can provide accurate estimates of rainwater content W potentially over a wide range of rainwater contents. While measurements along a microwave link are fairly easily implemented, radar estimates of A(38-25) can become clouded by differences between the radar reflectivity factors at the two frequencies (Z38, Z25). Root-mean-square deviations (epsilon) of the estimated W from the actual W are calculated for a wide variety of drop-size distributions and rainwater contents. The computed epsilon include the effects of standard measurement errors and differences between Z38 and Z25. Accurate estimates appear possible using a 38-25-GHz radar when W is not less than 1.5-2 g/cu m, depending upon the desired spatial resolution, and along a microwave link when W is not less than 0.5 g/cu m.

Jameson, A. R.↗

NASA NTRS1992

Structural damage detection using a subspace rotation algorithm

A computationally inexpensive algorithm is developed to provide an insight to the location of structural damage, using the original finite element model and a subset of measured eigenvalues and eigenvectors. The computational requirements of the algorithm may make the technique suitable for real-time implementation. With damage location determined, a second algorithm is developed to determine the extent of damage. The algorithms are demonstrated using two classes of structural models. The effects of eigenvector measurement error is demonstrated and techniques to overcome the effects of noise are discussed.

Zimmerman, David C.↗

NASA NTRS1991

Experimental balances for the second moments for a buoyant plume and their implication on turbulence modeling

The heat flux and Reynolds stress budgets are presented for a buoyant plume. The terms involving pressure correlations are obtained as the closing terms in these budgets. Despite certain measurement errors, these budgets provide useful information about how various phenomena contribute to the transport of second moments. These experimental results are used to assess the local equilibrium assumption and to investigate why the mechanical to thermal time scale ratio for a buoyant plume is different than the commonly recommended value. Analysis show that this departure is a consequence of the local equilibrium assumption being not satisfied in the present experiment.

Shabbir, Aamir↗

NASA NTRS1991

Navigation accuracy analysis for the TDRSS Onboard Navigation System (TONS) experiment on EP/EUVE

A navigation accuracy analysis of the TONS (TDRSS Onboard Navigation System) experiment on the Explorer Platform/Extreme Ultraviolet Explorer (EP/EUVE) is reported. The analysis process includes both sequential estimation covariance analysis and the processing of 'realistic' simulated tracking data. The sensitivity of the navigation accuracy to force modeling and measurement errors is evaluated as a function of the tracking schedule and the process noise model tuning parameters.

Gramling, C. J.↗