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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 NTRS2010

VLBI2020: From Reality to Vision

The individual apparent motions of distant radio sources are believed to be caused by the effect of intrinsic structure variations of the active galactic nuclei (AGN). However, some cosmological models of the expanded Universe predict that systematic astrometric proper motions of distant quasars do not vanish as the radial distance from the observer to the quasar grows. These systematic effects can even increase with the distance, making it possible to measure them with high-precision astrometric techniques like VLBI. The Galactocentric acceleration of the Solar System barycenter may cause a secular aberration drift with a magnitude of 4 uas/yr. The Solar System motion relative to the cosmic microwave background produces an additional dipole effect, proportional to red shift. We analyzed geodetic VLBI data spanning from 1979 until 2009 to estimate the vector spherical harmonics in the expansion of the vector field of the proper motion of 687 radio sources. The dipole and quadrupole vector spherical harmonics were estimated with an accuracy of 1-5 as/yr. We have shown that over the next decade the geodetic VLBI may approach the level of accuracy on which the cosmological models of the Universe could be tested. Hence, it is important to organize a dedicated observational program to increase the number of measured proper motions to 3000.

Titov, Oleg↗
NASA NTRS1984

Earth Orientation Effects on Mobile VLBI Baselines

Improvements in data quality for the mobile VLBI systems have placed higher accuracy requirements on Earth orientation calibrations. Errors in these calibrations may give rise to systematic effects in the nonlength components of the baselines. Various sources of Earth orientation data were investigated for calibration of Mobile VLBI baselines. Significant differences in quality between the several available sources of UT1-UTC were found. It was shown that the JPL Kalman filtered space technology data were at least as good as any other and adequate to the needs of current Mobile VLBI systems and observing plans. For polar motion, the values from all service suffice. The effect of Earth orientation errors on the accuracy of differenced baselines was also investigated. It is shown that the effect is negligible for the current mobile systems and observing plan.

Allen, S. L.↗
NASA NTRS1991

The TOPSAR interferometric radar topographic mapping instrument

We have augmented the NASA DC-8 Airborne Synthetic Aperture Radar (AIRSAR) instrument with a pair of C-band antennas displaced across the track to form an interferometer sensitive to topographic variations of the Earth's surface. The antennas were developed by Alenia Spazio under the sponsorship of the Italian Space Agency (ASI), and the AIRSAR instrument and modifications to it supporting TOPSAR were sponsored by NASA. A new data processor was developed at JPL for producing the topographic maps. As of May 1991, one engineering flight line over San Francisco, CA was reduced to a cartographically rectified topographic map. Analysis of the results indicates that statistical errors are in the range of 2 to 4 m, while systematic effects due to aircraft motion are in the range of 6 to 12 m. Future aircraft motion compensation algorithms should reduce the systematic variations to near zero, while the statistical errors could likely be reduced to 2 m or less with some processor improvements.

Zebker, Howard A.↗
NASA NTRS1993

Measuring rapid ocean tidal earth orientation variations with very long baseline interferometry

Ocean tidal effects on universal time and polar motion (UTPM) are investigated at four nearly diurnal (K(sub 1), P(sub 1), O(sub 1), and Q(sub 1)) and four nearly semidiurnal (K(sub 2), S(sub 2), M(sub 2), and N(sub 2)) frequencies by analyzing very long baseline interferometry (VLBI) data extending from 1978 to 1992. We discuss limitations of comparisons between experiment and theory for the retograde nearly diurnal polar motion components due to their degeneracy with prograde components of the nutation model. Estimating amplitudes of contributions to the modeled VLBI observables at these eight frequencies produces a statistically highly significant improvement of 7 mm to the residuals of a fit to the observed delays. Use of such an improved UTPM model also reduces the 14-30 mm scatter of baseline lengths about a time-linear model of tectonic motion by 3-14 mm, also withhigh significance levels. A total of 28 UTPM ocean tidal amplitudes can be unambiguously estimated from the data, with resulting UTI and PM magnitudes as large as 21 micro secs and 270 microarc seconds and formal uncertainties of the order of 0.3 micro secs and 5 microarc secs for UTI and PM, respectively. Empirically determined UTPM amplitudes and phases are com1pared to values calculated theoretically by Gross from Seiler's global ocean tide model. The discrepancy between theory and experiment is larger by a factor of 3 for UTI amplitudes (9 micro secs) than for prograde PM amplitudes (42 microarc secs). The 14-year VLBI data span strongly attenuates the influence of mismodeled effects on estimated UTPM amplitudes and phases that are not coherent with the eight frequencies of interest. Magnitudes of coherent and quasi-coherent systematic errors are quantified by means of internal consistency tests. We conclude that coherent systematic effects are many times larger than the formal uncertainties and can be as large as 4 micro secs for UTI and 60 microarc secs for polar motion. On the basis of such ealistic error estimates, 22 of the 31 fitted UTPM ocean tidal amplitudes differ from zero by more than 2 sigma.

Sovers, O. J.↗
NASA NTRS1993

A laser-cooled cesium fountain frequency standard and a measurement of the frequency shift due to ultra-cold collisions

A frequency standard based on an atomic fountain of cesium atoms may have an accuracy of 10(exp -16) due to longer interaction times and smaller anticipated systematic errors. All of the known systematic effects that now limit the accuracy of the Cs frequency standard increase either linearly or as some higher power of the atom's velocity. The one systematic frequency shift which is dramatically different is the frequency shift due to the collisions between the laser cooled atoms. At a temperature of a few micro-K, the de Broglie wavelength (lambda(sub deB) = h/p, where h is Planck's constant and p is the momentum of the atom) is much larger than the scale of the interatomic potential. Under these conditions the collision cross sections can be as large as (lambda(sub deB)(sup 2))/Pi and the frequency shift due to these collisions was recently calculated. In our Cs atomic fountain, we laser cooled and trapped 10(exp 10) Cs atoms in 0.4 s. By shifting the frequencies of the laser beams, the atoms were launched upwards at 2.5 m/s and a fraction of the atoms were optically pumped into the F=3 ground state. The unwanted atoms in the F=4 ground state were removed from the fountain with radiation pressure from a laser beam tuned to excite only those atoms. The Cs atoms in the F=3 state traveled ballistically upwards, were excited by the microwave cavity, and then returned back through the same cavity in the atomic fountain configuration. By varying the cold atom density, a density dependent shift of -12.9 +/- 0.7 mHz or -1.4 x 10-12 for an average fountain density of (2.7 +/- 1.5) 10(exp 9) atoms/cm(sup 3) was measured.

Gibble, Kurt↗
NASA NTRS2021

Cosmology with the Roman Space Telescope - Multiprobe Strategies

We simulate the scientific performance of the Nancy Grace Roman Space Telescope High Latitude Survey (HLS) on dark energy and modified gravity. The 1.6-yr HLS Reference survey is currently envisioned to image 2000 deg2 in multiple bands to a depthof∼26.5 in Y, J, H and to cover the same area with slit-less spectroscopy beyond z=3. The combination of deep, multiband photometry and deep spectroscopy will allow scientists to measure the growth and geometry of the Universe through a variety of cosmological probes (e.g. weak lensing, galaxy clusters, galaxy clustering, BAO, Type Ia supernova) and, equally, it will allow an exquisite control of observational and astrophysical systematic effects. In this paper, we explore multiprobe strategies that can be implemented, given the telescope’s instrument capabilities. We model cosmological probes individually and jointly and account for correlated systematics and statistical uncertainties due to the higher order moments of the density field. We explore different levels of observational systematics for the HLS survey (photo-z and shear calibration) and ultimately run a joint likelihood analysis in N-dim parameter space. We find that the HLS reference survey alone can achieve a standard dark energy FoM of>300 when including all probes. This assumes no information from external data sets, we assume a flat universe however, and includes realistic assumptions for systematics. Our study of the HLS reference survey should be seen as part of a future community-driven effort to simulate and optimize the science return of the Roman Space Telescope.

Cosmological parameters↗
NASA NTRS1992

The TOPSAR interferometric radar topographic mapping instrument

The NASA DC-8 AIRSAR instrument has been augmented with a pair of C-band antennas displaced across track to form an interferometer sensitive to topographic variations of the earth's surface. During the 1991 DC-8 flight campaign, data were acquired over several sites in the U.S. and Europe, and topographic maps were produced from several of these flight lines. Analysis of the results indicate that statistical errors are in the 2-4 m range, while systematic effects due to aircraft motion are in the 10-20 m range. Initial results from development of a second generation processor at JPL show that aircraft motion compensation algorithms reduce the systematic variations to 2 m, while the statistical errors are reduced to 2-3 m.

Zebker, Howard A.↗
NASA NTRS2016

The Atacama Cosmology Telescope: Dynamical Masses for 44 SZ-Selected Galaxy Clusters over 755 Square Degrees

We present galaxy velocity dispersions and dynamical mass estimates for 44 galaxy clusters selected via the Sunyaev-Zeldovich (SZ) effect by the Atacama Cosmology Telescope. Dynamical masses for 18 clusters are reported here for the first time. Using N-body simulations, we model the different observing strategies used to measure the velocity dispersions and account for systematic effects resulting from these strategies. We find that the galaxy velocity distributions may be treated as isotropic, and that an aperture correction of up to 7 per cent in the velocity dispersion is required if the spectroscopic galaxy sample is sufficiently concentrated towards the cluster centre. Accounting for the radial profile of the velocity dispersion in simulations enables consistent dynamical mass estimates regardless of the observing strategy. Cluster masses M200 are in the range (1 - 15) times 10 (sup 14) Solar Masses. Comparing with masses estimated from the SZ distortion assuming a gas pressure profile derived from X-ray observations gives a mean SZ-to-dynamical mass ratio of 1:10 plus or minus 0:13, but there is an additional 0.14 systematic uncertainty due to the unknown velocity bias; the statistical uncertainty is dominated by the scatter in the mass-velocity dispersion scaling relation. This ratio is consistent with previous determinations at these mass scales.

Sunyaev-Zeldovich (SZ)↗
NASA NTRS1993

Latitudinal variation of solar wind corotating stream interaction regions: Ulysses

Ulysses' initial transit tot high heliographic latitudes at a heliocentric distance of approximately 5 AU has revealed systematic effects in the latitudinal evolution of corotating interaction regions (CIRs). At a latitude corresponding roughly to, but slightly less than, the inferred tilt of the coronal streamer belt and embedded heliospheric current sheet, the strong forward shocks commonly associated with CIRs at lower latitudes disappeared almost entirely; however, the reverse shocks associated with these CIRs persisted to latitudes approximately 10 deg above the streamer belt. Systematic meridional flow deflections observed in association with the forward and reverse waves bounding the CIRs demonstrate that the above effect is the result of the fact that the forward waves propagate to lower latitudes and the reverse waves to higher latitudes with increasing heliocentric distance. These observational results are in excellent agreement with the predictions of a three-dimensional model of corotating solar wind flows that originate in a tilted dipole geometry back at the Sun.

Gosling, J. T.↗
NASA NTRS1971

Flow field measurements for cylindrical configurations in a hypersonic wind tunnel: Windward and leeward flow fields

Windward and leeward measurements were made for a variety of simulated infinite cylinders exposed to hypersonic streams over an angle of attack from 30 deg to 90 deg. For the range of conditions included in the study, the following conclusions are made: (1) Swept cylinder theory provides a reasonable correlation of the measured laminar heat transfer rates from the plane of symmetry. (2) The boundary layer transition criteria in the plane of symmetry are a function of the transverse curvature. (3) Relaminarization of the circumferential boundary layer for a right circular cylinder was observed at the highest Reynolds number tested. (4) The effect of leeside geometry on the average heat transfer rate can be correlated with a single geometric parameter which is dependent on the location of separation. (5) The relationship of leeward heating to angle of attack is virtually linear for each cross section. (6) No systematic effect of free stream Reynolds number was observed.

Bertin, J. J.↗
NASA NTRS1992

Investigations on entropy layer along hypersonic hyperboloids using a defect boundary layer

A defect approach coupled with matched asymptotic expansions is used to derive a new set of boundary layer equations. This method ensures a smooth matching of the boundary layer with the inviscid solution. These equations are solved to calculate boundary layers over hypersonic blunt bodies involving the entropy gradient effect. Systematic comparisons are made for both axisymmetric and plane flows in several cases with different Mach and Reynolds numbers. After a brief survey of the entropy layer characteristics, the defect boundary layer results are compared with standard boundary layer and full Navier-Stokes solutions. The entropy gradient effects are found to be more important in the axisymmetric case than in the plane one. The wall temperature has a great influence on the results through the displacement effect. Good predictions can be obtained with the defect approach over a cold wall in the nose region, with a first order solution. However, the defect approach gives less accurate results far from the nose on axisymmetric bodies because of the thinning of the entropy layer.

Brazier, J. P.↗
NASA NTRS1985

The inference of vector magnetic fields from polarization measurements with limited spectral resolution

A method is presented for recovery of the vector magnetic field and thermodynamic parameters from polarization measurement of photospheric line profiles measured with filtergraphs. The method includes magneto-optic effects and may be utilized on data sampled at arbitrary wavelengths within the line profile. The accuracy of this method is explored through inversion of synthetic Stokes profiles subjected to varying levels of random noise, instrumental wave-length resolution, and line profile sampling. The level of error introduced by the systematic effect of profile sampling over a finite fraction of the 5 minute oscillation cycle is also investigated. The results presented here are intended to guide instrumental design and observational procedure.

Lites, B. W.↗
NASA NTRS1992

Physics of systematic frequency variations in hydrogen masers

The frequency stability of hydrogen masers for intervals longer than 10 exp 4 s is currently limited by systematic processes. The physics of frequency-determining mechanisms internal to the maser that are susceptible to systematic variations, and the connections between these internal mechanisms and external environmental factors are discussed. From estimates of the magnitudes of systematic effects, it is found that the primary internal mechanisms limiting long-term maser frequency stability are cavity pulling, at the level of parts in 1015 per day, and wall shift variations, at the level of parts in 10 exp 16 to parts in 10 exp 15 per day. Strategies for reducing systematic frequency variations are discussed.

Mattison, Edward M.↗
NASA NTRS1990

Physics of systematic frequency variations in hydrogen masers

The frequency stability of hydrogen masers for intervals longer that 10(exp 4) seconds is limited at present by systematic processes. Researchers discuss the physics of frequency-determining mechanisms internal to the maser that are susceptible to systematic variations, and the connections between these internal mechanisms and external environmental factors. Based upon estimates of the magnitudes of systematic effects, they find that the primary internal mechanisms currently limiting long-term maser frequency stability are cavity pulling, at the level parts in 10(exp 15) per day, and wall shift variations, at the level of parts in 10(exp 16) to parts in 10(exp 15) per day. They discuss strategies for reducing systematic frequency variations.

Mattison, Edward M.↗
NASA NTRS2026

Atacama Cosmology Telescope A Measurement of Galaxy Cluster Temperatures Through Relativistic Corrections to the Thermal Sunyaev-Zeldovich Effect

The high electron temperature in galaxy clusters (> 1 keV or > 107 K) leads to corrections at the level of a few percent in their thermal Sunyaev-Zeldovich effect signatures. Both the size and frequency dependence of these corrections, which are known as relativistic temperature corrections, depend upon the temperature of the objects. In this work we exploit this effect to measure the average temperature of a stack of Compton-y selected clusters. Specifically, we apply the “spectroscopic method” and search for the temperature that best fits the clusters’ signal measured at frequencies from 30 to 545 GHz by the Atacama Cosmology Telescope and Planck satellite. We measure the average temperature of clusters detected in the Atacama Cosmology Telescope maps to be 8.5 2.4 keV, with an additional systematic error of comparable amplitude dominated by passband uncertainty. Upcoming surveys, such as the Simons Observatory and CMB-S4, have the potential to dramatically improve upon these measurements and thereby enable precision studies of cluster temperatures with millimeter observations. The key challenge for future observations will be mitigating instrumental systematic effects, which already limit this analysis.

William R Coulton↗
NASA NTRS2016

Far Sidelobe Effects from Panel Gaps of the Atacama Cosmology Telescope

The Atacama Cosmology Telescope is a 6 meter diameter CMB telescope located at 5200 meters in the Chilean desert. ACT has made arc-minute scale maps of the sky at 90 and 150 GHz which have led to precise measurements of the fine angular power spectrum of the CMB fluctuations in temperature and polarization. One of the goals of ACT is to search for the B-mode polarization signal from primordial gravity waves, and thus extending ACT's data analysis to larger angular scales. This goal introduces new challenges in the control of systematic effects, including better understanding of far sidelobe effects that might enter the power spectrum at degree angular scales. Here we study the effects of the gaps between panels of the ACT primary and secondary reflectors in the worst case scenario in which the gaps remain open. We produced numerical simulations of the optics using GRASP up to 8 degrees away from the main beam and simulated timestreams for observations with this beam using real pointing information from ACT data. Maps from these simulated timestreams showed leakage from the sidelobes, indicating that this effect must be taken into consideration at large angular scales.

The Atacama Cosmology Telescope↗
NASA NTRS1988

Stellar winds in the Small Magellanic Cloud

According to the predictions of line-driven stellar wind theory, the mass loss from luminous stars should be proportional to some power of the metallicity, yet previous studies have not shown this effect clearly. The authors have observed 22 main-sequence and giant O-type stars n the SMC with the low-resolution mode of the IUE in order to study the stellar winds in a particularly metal-poor galaxy. These stars are compared with carefully selected Galactic counterparts for wind terminal velocities and line strengths of C IV and N v. While the wind velocities in the SMC stars are confirmed to be systematically lower than in Galactic stars, as theory predicts, the mass-loss rates which are inferred from these data are comparable to those in the Galactic stars. The possible reasons for this result are discussed, including systematic effects in the data, assumed metal abundance, different ionization structure, and different physical parameters for the SMC stars.

Garmany, Catharine D.↗
NASA NTRS1986

The composition of solar energetic particles

The present measurements of elemental abundances in 15 large solar energetic particle events confirm the existence of two major effects: systematic differences between solar energetic particle abundances and photospheric abundances that are approximately correlated with first ionization potential (implying that the solar corona is the likely source population for these particles), and an enhancement of heavy ion abundances relative to a baseline of solar energetic particle abundances whose magnitude increases with rising atomic number. These data suggest the possibility that the degree of heavy ion enhancement has some correlation with spectral slope, and confirm that the aforementioned effects are not due to time or energy variations within individual events. The maximum column density above the solar acceleration region is less than 0.1 g/sq cm, on the basis of these data.

Mcguire, R. E.↗