19,371 research outputs found
Surface topography of the Greenland Ice Sheet from satellite radar altimetry
Surface elevation maps of the southern half of the Greenland subcontinent are produced from radar altimeter data acquired by the Seasat satellite. A summary of the processing procedure and examples of return waveform data are given. The elevation data are used to generate a regular grid which is then computer contoured to provide an elevation contour map. Ancillary maps show the statistical quality of the elevation data and various characteristics of the surface. The elevation map is used to define ice flow directions and delineate the major drainage basins. Regular maps of the Jakobshavns Glacier drainage basin and the ice divide in the vicinity of Crete Station are presented. Altimeter derived elevations are compared with elevations measured both by satellite geoceivers and optical surveying
Multispace and Multilevel BDDC
BDDC method is the most advanced method from the Balancing family of
iterative substructuring methods for the solution of large systems of linear
algebraic equations arising from discretization of elliptic boundary value
problems. In the case of many substructures, solving the coarse problem exactly
becomes a bottleneck. Since the coarse problem in BDDC has the same structure
as the original problem, it is straightforward to apply the BDDC method
recursively to solve the coarse problem only approximately. In this paper, we
formulate a new family of abstract Multispace BDDC methods and give condition
number bounds from the abstract additive Schwarz preconditioning theory. The
Multilevel BDDC is then treated as a special case of the Multispace BDDC and
abstract multilevel condition number bounds are given. The abstract bounds
yield polylogarithmic condition number bounds for an arbitrary fixed number of
levels and scalar elliptic problems discretized by finite elements in two and
three spatial dimensions. Numerical experiments confirm the theory.Comment: 26 pages, 3 figures, 2 tables, 20 references. Formal changes onl
College Readiness Initiative: AVID and Navigation 101
The purpose of this report is to provide summative feedback to personnel at the Office of Superintendent of Public Instruction (OSPI) and at the College Spark Washington regarding evidence of implementation and impact of the Advancement via Individual Determination (AVID) and Navigation 101 programs in schools funded by the College Readiness Initiative (CRI) in Washington State. The report, while addressing the effects of both programs, is also designed to provide formative feedback to assist in ongoing program development
Global mean sea surface computation based upon a combination of SEASAT and GEOS-3 satellite altimeter data
A mean sea surface map was computed for the global ocean areas between 70 deg N latitude and 62 deg S latitude based upon the 70 day SEASAT and 3.5 year GEOS-3 altimeter data sets. The mean sea surface is presented in the form of a global contour map and a 0.25 deg x 0.25 deg grid. A combination of regional adjustments based upon crossover techniques and the subsequent adjustment of the regional solutions into a global reference system was employed in order to minimize the effects of radial orbit error. A global map of the crossover residuals after the crossover adjustments are made is in good agreement with earlier mesoscale variability contour maps based upon the last month of SEASAT collinear data. This high level of agreement provides good evidence that relative orbit error was removed to the decimeter level on a regional basis. This represents a significant improvement over our previous maps which contained patterns, particularly in the central Pacific, which were due to radial orbit error. Long wavelength, basin scale errors are still present with a submeter amplitude due to errors in the PGS-S4 gravity model. Such errors can only be removed through the improvement of the Earth's gravity model and associated geodetic parameters
Satellite radar altimetry over ice. Volume 4: Users' guide for Antarctica elevation data from Seasat
A gridded surface-elevation data set and a geo-referenced data base for the Seasat radar altimeter data over Greenland are described. This is a user guide to accompany the data provided to data centers and other users. The grid points are on a polar stereographic projection with a nominal spacing of 20 km. The gridded elevations are derived from the elevation data in the geo-referenced data base by a weighted fitting of a surface in the neighborhood of each grid point. The gridded elevations are useful for the creating of large-scale contour maps, and the geo-referenced data base is useful for regridding, creating smaller-scale contour maps, and examinating individual elevation measurements in specific geographic areas. Tape formats are described, and a FORTRAN program for reading the data tape is listed and provided on the tape
Satellite radar altimetry over ice. Volume 1: Processing and corrections of Seasat data over Greenland
The data-processing methods and ice data products derived from Seasat radar altimeter measurements over the Greenland ice sheet and surrounding sea ice are documented. The corrections derived and applied to the Seasat radar altimeter data over ice are described in detail, including the editing and retracking algorithm to correct for height errors caused by lags in the automatic range tracking circuit. The methods for radial adjustment of the orbits and estimation of the slope-induced errors are given
Experimental and numerical study of error fields in the CNT stellarator
Sources of error fields were indirectly inferred in a stellarator by
reconciling computed and numerical flux surfaces. Sources considered so far
include the displacements and tilts (but not the deformations, yet) of the four
circular coils featured in the simple CNT stellarator. The flux surfaces were
measured by means of an electron beam and phosphor rod, and were computed by
means of a Biot-Savart field-line tracing code. If the ideal coil locations and
orientations are used in the computation, agreement with measurements is poor.
Discrepancies are ascribed to errors in the positioning and orientation of the
in-vessel interlocked coils. To that end, an iterative numerical method was
developed. A Newton-Raphson algorithm searches for the coils' displacements and
tilts that minimize the discrepancy between the measured and computed flux
surfaces. This method was verified by misplacing and tilting the coils in a
numerical model of CNT, calculating the flux surfaces that they generated, and
testing the algorithm's ability to deduce the coils' displacements and tilts.
Subsequently, the numerical method was applied to the experimental data,
arriving at a set of coil displacements whose resulting field errors exhibited
significantly improved quantitative and qualitative agreement with experimental
results.Comment: Special Issue on the 20th International Stellarator-Heliotron
Worksho
Satellite radar altimetry over ice. Volume 2: Users' guide for Greenland elevation data from Seasat
A gridded surface-elevation data set and a geo-referenced data base for the Seasat radar altimeter data over Antarctica are described. It is intended to be a user's guide to accompany the data provided to data centers and other users. The grid points are on a polar stereographic projection with a nominal spacing of 20 km. The gridded elevations are derived from the elevation data in the geo-referenced data base by a weighted fitting of a surface in the neighborhood of each grid point. The gridded elevations are useful for the creating smaller-scale contour maps, and examining individual elevation measurements in specific geographic areas. Tape formats are described, and a FORTRAN program for reading the data tape is listed and provided on the tape
Self-assembling DNA-caged particles: nanoblocks for hierarchical self-assembly
DNA is an ideal candidate to organize matter on the nanoscale, primarily due
to the specificity and complexity of DNA based interactions. Recent advances in
this direction include the self-assembly of colloidal crystals using DNA
grafted particles. In this article we theoretically study the self-assembly of
DNA-caged particles. These nanoblocks combine DNA grafted particles with more
complicated purely DNA based constructs. Geometrically the nanoblock is a
sphere (DNA grafted particle) inscribed inside a polyhedron (DNA cage). The
faces of the DNA cage are open, and the edges are made from double stranded
DNA. The cage vertices are modified DNA junctions. We calculate the
equilibriuim yield of self-assembled, tetrahedrally caged particles, and
discuss their stability with respect to alternative structures. The
experimental feasability of the method is discussed. To conclude we indicate
the usefulness of DNA-caged particles as nanoblocks in a hierarchical
self-assembly strategy.Comment: v2: 21 pages, 8 figures; revised discussion in Sec. 2, replaced 2
figures, added new reference
Raman modes of the deformed single-wall carbon nanotubes
With the empirical bond polarizability model, the nonresonant Raman spectra
of the chiral and achiral single-wall carbon nanotubes (SWCNTs) under uniaxial
and torsional strains have been systematically studied by \textit{ab initio}
method. It is found that both the frequencies and the intensities of the
low-frequency Raman active modes almost do not change in the deformed
nanotubes, while their high-frequency part shifts obviously. Especially, the
high-frequency part shifts linearly with the uniaxial tensile strain, and two
kinds of different shift slopes are found for any kind of SWCNTs. More
interestingly, new Raman peaks are found in the nonresonant Raman spectra under
torsional strain, which are explained by a) the symmetry breaking and b) the
effect of bond rotation and the anisotropy of the polarizability induced by
bond stretching
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