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Results for “systematic 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.

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NASA NTRS1992

A measurement of the cosmic microwave background temperature at 7.5 GHz

The temperature of the cosmic microwave background (CMB) radiation at a frequency of 7.5 GHz (4 cm wavelength) is measured, obtaining a brightness temperature of T(CMB) = 2.70 +/- 0.08 K (68 percent confidence level). The measurement was made from a site near the geographical South Pole during the austral spring of 1989 and was part of an international collaboration to measure the CMB spectrum at low frequencies with a variety of radiometers from several different sites. This recent result is in agreement with the 1988 measurement at the same frequency, which was made from a different site with significantly different systematic errors. The combined result of the 1988 and 1989 measurements is 2.64 +/- 0.06 K.

Levin, S.↗
NASA NTRS1992

Instrumentation of the variable-angle magneto-optic ellipsometer and its application to M-O media and other non-magnetic films

A new and comprehensive dielectric tensor characterization instrument is presented for characterization of magneto-optical recording media and non-magnetic thin films. Random and systematic errors of the system are studied. A series of TbFe, TbFeCo, and Co/Pt samples with different composition and thicknesses are characterized for their optical and magneto-optical properties. The optical properties of several non-magnetic films are also measured.

Zhou, Andy F.↗
NASA NTRS1992

COBE Differential Microwave Radiometer (DMR) data processing techniques

The purpose of the Differential Microwave Radiometer (DMR) experiment on the Cosmic Background Explorer (COBE) satellite is to make whole-sky maps, at frequencies of 31.5, 53, and 90 GHz, of any departures of the Cosmic Microwave Background (CMB) from its mean value of 2.735 K. An elaborate software system is necessary to calibrate and invert the differential measurements, so as to make sky maps free from large scale systematic errors to levels less than a millionth of the CMB.

Jackson, P. D.↗
NASA NTRS1992

The shape of asteroid 1917 Cuyo

Lightcurves obtained for 1917 Cuyo at solar phase angles near 54 degrees have an amplitude delta m = 0.44 mag. However, convex-profile inversion of the lightcurves yields an estimate of the asteroid's mean cross section (C, a 2-D average of the 3-D shape) that is only slightly noncircular, with an elongation approximately 1.15. The estimate of C undoubtedly contains systematic errors, the most severe of which could arise from non-equatorial viewing/illumination geometry. However, Cuyo's radar echo shows very little variation in bandwidth vs. rotation phase, supporting the hypothesis that this asteroid's elongation is rather modest.

Ostro, Steven J.↗
NASA NTRS1991

COBE - New sky maps of the early universe

This paper presents early results obtained from the first six months of measurements of the cosmic microwave background (CMB) by instruments aboard NASA's Cosmic Background Explorer (COBE) satellite and discusses the implications for cosmology. The three instruments: FIRAS, DMR, and DIRBE have operated well and produced significant new results. The FIRAS measurement of the CMB spectrum supports the standard big bang nucleosynthesis model. The maps made from the DMR instrument measurements show a surprisingly smooth early universe. The measurements are sufficiently precise that we must pay careful attention to potential systematic errors. The maps of galactic and local emission produced by the DIRBE instrument will be needed to identify foregrounds from extragalactic emission and thus to interpret the terms of events in the early universe.

Smoot, G. F.↗
NASA NTRS1991

Gravitational lens determinations of cosmological parameters

Currently available attempts to determine the three classical cosmological parameters (H(0), Omega(0), and Lambda), using gravitational lens techniques and arguments are reviewed. These suggest that H(0) greater than about 75 km/s/Mpc, that contributions to Omega(0) by certain hypothetical types of dark matter are less than 1, and that Lambda is considerably smaller than 3H(0)-squared if a flat cosmological model is assumed. Unfortunately, none of these three conclusions is satisfactorily free of model dependence, possible systematic error, and caveats.

Turner, Edwin L.↗
NASA NTRS1992

A parameterization for longwave surface radiation from satellite data - Recent improvements

Several improvements have been made recently to the parameterization for surface longwave radiation described by Gupta (1989). Model constants have been modified in order to use meteorological data from the International Satellite Cloud Climatology Project instead of from the TIROS Operational Vertical Sounder data, primarily to take advantage of the vastly superior cloud information available from the former. Additional modifications were made to improve the estimation of cloud effect in the presence of low-level clouds. The latter modifications reduced the systematic error of the overcast-sky fluxes from 10.0 to 1.7 W/sq m and the random error from +/- 18.9 to +/- 6.3 W/sq m when compared to the fluxes computed with a detailed radiative transfer model.

Gupta, Shashi K.↗
NASA NTRS1993

Temporal averaging of phase measurements in the presence of spurious phase drift - Application to phase-stepped real-time holographic interferometry

A technique that compensates for low spatial frequency spurious phase changes during an interference experiment is developed; it permits temporal averaging of multiple-phase measurements, made before and after object displacement. The method is tested with phase-stepped real-time holographic interferometry applied to cantilever bending of a piezoelectric bimorph ceramic. Results indicate that temporal averaging of the corrected data significantly reduces the white noise in a phase measurement without incurring systematic errors or sacrificing spatial resolution. White noise is reduced from 3 deg to less than 1 deg using these methods.

Ovryn, B.↗
NASA NTRS1993

Second-order solution for determining density and velocity fields of galaxies

In this Letter, I use second-order Lagrangian perturbation theory to calculate an analytical expression relating density to velocity in a gravitating system. This solution can be used to compare peculiar velocity field measurements with observations of large-scale structure. The predictions of both linear theory and second-order theory are compared with the results of N-body simulations. While linear theory systematically overestimates the velocity flows in high-density regions, the second-order corrections calculated herein remove this systematic error.

Gramann, Mirt↗
NASA NTRS1993

A model for the beam-filling effect associated with the microwave retrieval of rain

Estimating rain rate from environmental microwave emissions is hampered by several difficulties. One of these difficulties is known as the beam-filling effect. Beam filling is the systematic error introduced when the microwave radiometer's field of view is not filled with uniform rain. Beam filling can have dramatic effects on rain-rate estimation, causing rain rates to be underestimated by as much as a factor of 2. The present study derives an approximate expression for beam filling that provides, in principle, a way to estimate this effect. In addition, this study deals only with single-channel microwave rain estimation over the ocean. The final results reveal that beam filling is essentially determined by the freezing level, the mean fraction of the footprint raining, and the footprint-averaged rain rate. Also, the numerical results appear to agree with other empirical studies. Furthermore, the analysis brings to light an interesting connection with rain threshold techniques for estimating area-averaged rain rates.

Graves, Charles E.↗
NASA NTRS1993

Planet X - No dynamical evidence in the optical observations

It is shown that the alleged 'unexplained anomalies in the motion of Uranus' disappear when one properly accounts for the correct value of the mass of Neptune and properly adjusts the orbit of Uranus to the observational data. Also, it is shown that each of the 'irregularities in the measured positions of Neptune' has a complete explanation within the framework of the presently known solar system. As a check of certainty, an actual planetary ephemeris is integrated which well fits the observations of Uranus. Minor systematic errors do remain in the data, but they are very small; they are easily explained by a number of uncertainties in the observations themselves. There is now known to be a mass concentration of significant size in the outer solar system - 1992 QB1. In comparison to any of the major planets, though, this object is miniscule. For the meridian circle observations, there is still no evidence which requires or even indicates the existence of any planet-sized object; there remains no need to hypothesize the existence of a tenth planet in the solar system.

Standish, E. M., Jr.↗
NASA NTRS1992

Gamma-ray burst locations from the Burst and Transient Source Experiment

The Burst and Transient Source Experiment (BATSE) consists of eight anisotropic gamma-ray spectrometers at the corners of the Compton Gamma Ray Observatory. BATSE monitors the full sky from a fixed orientation and determines the direction of gamma-ray bursts with an accuracy appropriate for studying the bursts' celestial distribution. We describe the calculation of gamma-ray burst directions from measurements made by BATSE. We present a sample of calculated directions from BATSE's measurement of solar flaxes and compare the calculated directions with the solar direction. We describe the systematic errors apparent in these data and discuss ongoing efforts to correct them.

Brock, M. N.↗
NASA NTRS1992

Preliminary burst location calibration results for the BATSE instrument on CGRO

Preliminary results pertaining to burst location using BATSE Large Area Detector measurements of solar flares are presented. These solar flare measurements are currently being used to fine tune the calibration of our data analysis software. The current status of techniques for identifying and eliminating systematic errors from the data analysis tools is discussed. Data revealing the effects of the atmospheric scattering algorithm that will be implemented in the data analysis software are presented.

Pendleton, G. N.↗
NASA NTRS1993

Asymmetries of solar oscillation line profiles

Asymmetries of the power spectral line profiles of solar global p-modes are detected in full-disk intensity observations of the Ca II K Fraunhofer line. The asymmetry is a strong function of temporal frequency being strongest at the lowest frequencies observed and vanishing near the peak of the power distribution. The variation with spherical harmonic degree is small. The asymmetry is interpreted in terms of a model in which the solar oscillation cavity is compared to a Fabry-Perot interferometer with the source slightly outside the cavity. A phase difference between an outward direct wave and a corresponding inward wave that passes through the cavity gives rise to the asymmetry. The asymmetry is different in velocity and intensity observations. Neglecting the asymmetry when modeling the power spectrum can lead to systematic errors in the measurement of mode frequencies of as much as 10 exp -4 of the mode frequency. The present observations and interpretation locate the source of the oscillations to be approximately 60 km beneath the photosphere, the shallowest position suggested to date.

Duvall, T. L., Jr.↗
NASA NTRS1993

Observations of intermediate degree solar oscillations - 1989 April-June

Frequencies, splittings, and line widths from 85 d of full disk Doppler observations of solar p-modes taken between April 4 and June 30, 1989 are presented. Comparison of the present mode parameters with published Big Bear Solar Observatory (BBSO) results yields good agreement in general and is thus a confirmation of their work using an independent instrument and set of analysis routines. Average differences in p-mode frequencies measured by the two experiments in spring-summer 1989 are explained as a result of differences in the exact periods of data collection during a time of rapidly changing solar activity. It is shown that the present a(1) splitting coefficients for p-modes with nu/L less than 45 micro-Hz suffer from a significant systematic error. Evidence is presented to the effect that a detector distortion or alignment problem, not a problem with the power spectra analysis, is the most likely explanation of this a(1) anomaly.

Bachmann, Kurt T.↗
NASA NTRS1992

COBE DMR results and implications

This lecture presents early results obtained from the first six months of measurements of the Cosmic Microwave Background (CMB) by Differential Microwave Radiometers (DMR) aboard COBE and discusses significant cosmological implications. The DMR maps show the dipole anisotropy and some galactic emission but otherwise a spatially smooth early universe. The measurements are sufficiently precise that we must pay careful attention to potential systematic errors. Maps of galactic and local emission such as those produced by the FIRAS and DIRBE instruments will be needed to identify foregrounds from extragalactic emission and thus to interpret the results in terms of events in the early universe. The current DMR results are significant for Cosmology.

Smoot, George F.↗
NASA NTRS1993

A degree scale anisotropy measurement of the cosmic microwave background near the star Gamma Ursae Minoris

Results from a search for anisotropy in the cosmic microwave background (CMB) are presented from the third flight of the Millimeter-wave Anisotropy experiment. The CMB observation occurred over 1.37 hours and covered a 6.24 sq deg area of the sky where very little foreground emission is expected. Significant correlated structure is observed at 6 and 9/cm. At 12/cm we place an upper limit on the structure. The relative amplitudes at 6, 9, and 12/cm are consistent with a CMB spectrum. The spectrum of the structure is inconsistent with thermal emission from known forms of interstellar dust. Synchrotron and free-free emission would both require unusually flat spectral indices at cm wavelengths in order to account for the amplitude of the observed structure. Although known systematic errors are not expected to contribute significantly to any of the three optical channels, excess sidelobe contamination cannot be definitively ruled out. If all the structure is attributed to CMB anisotropy, a value of the weighted rms of the 6 and 9/cm channels of Delta T/T(CMB) = 4.7 +/- 0.8 x 10 exp -5 (+/- 1 sigma) was measured. If the CMB anisotropy is assumed to have a Gaussian autocorrelation function with a coherence angle of 25 arcmin, then the most probable value is Delta T/T(CMB) = 4.2 +1.7 or -1.1 x 10 exp -5, where the +/- refers to the 95 percent confidence limits.

Gundersen, J. O.↗