829 research outputs found
A Study of Linear Approximation Techniques for SAR Azimuth Processing
The application of the step transform subarray processing techniques to synthetic aperture radar (SAR) was studied. The subarray technique permits the application of efficient digital transform computational techniques such as the fast Fourier transform to be applied while offering an effective tool for range migration compensation. Range migration compensation is applied at the subarray level, and with the subarray size based on worst case range migration conditions, a minimum control system is achieved. A baseline processor was designed for a four-look SAR system covering approximately 4096 by 4096 SAR sample field every 2.5 seconds. Implementation of the baseline system was projected using advanced low power technologies. A 20 swath is implemented with approximately 1000 circuits having a power dissipation of from 70 to 195 watts. The baseline batch step transform processor is compared to a continuous strip processor, and variations of the baseline are developed for a wide range of SAR parameters
Advanced digital SAR processing study
A highly programmable, land based, real time synthetic aperture radar (SAR) processor requiring a processed pixel rate of 2.75 MHz or more in a four look system was designed. Variations in range and azimuth compression, number of looks, range swath, range migration and SR mode were specified. Alternative range and azimuth processing algorithms were examined in conjunction with projected integrated circuit, digital architecture, and software technologies. The advaced digital SAR processor (ADSP) employs an FFT convolver algorithm for both range and azimuth processing in a parallel architecture configuration. Algorithm performace comparisons, design system design, implementation tradeoffs and the results of a supporting survey of integrated circuit and digital architecture technologies are reported. Cost tradeoffs and projections with alternate implementation plans are presented
Soliton approximation in continuum models of leader-follower behavior
Complex biological processes involve collective behavior of entities
(bacteria, cells, animals) over many length and time scales and can be
described by discrete models that track individuals or by continuum models
involving densities and fields. We consider hybrid stochastic agent-based
models of branching morphogenesis and angiogenesis (new blood vessel creation
from pre-existing vasculature), which treat cells as individuals that are
guided by underlying continuous chemical and/or mechanical fields. In these
descriptions, leader (tip) cells emerge from existing branches and follower
(stalk) cells build the new sprout in their wake. Vessel branching and fusion
(anastomosis) occur as a result of tip and stalk cell dynamics. Coarse-graining
these hybrid models in appropriate limits produces continuum partial
differential equations (PDEs) for endothelial cell densities that are more
analytically tractable. While these models differ in nonlinearity, they produce
similar equations at leading order when chemotaxis is dominant. We analyze this
leading order system in a simple quasi-one-dimensional geometry and show that
the numerical solution of the leading order PDE is well described by a soliton
wave that evolves from vessel to source. This wave is an attractor for
intermediate times until it arrives at the hypoxic region releasing the growth
factor. The mathematical techniques used here thus identify common features of
discrete and continuum approaches and provide insight into general biological
mechanisms governing their collective dynamics.Comment: 13 pages, 9 figure
Mortality associated with delays between clinic entry and ART initiation in resource-limited settings: results of a transition-state model.
OBJECTIVE: To estimate the mortality impact of delay in antiretroviral therapy (ART) initiation from the time of entry into care. DESIGN: A state-transition Markov process model. This technique allows for assessing mortality before and after ART initiation associated with delays in ART initiation among a general population of ART-eligible patients without conducting a randomized trial. METHODS: We used patient-level data from 3 South African cohorts to determine transition probabilities for pre-ART CD4 count changes and pre-ART and on-ART mortality. For each parameter, we generated probabilities and distributions for Monte Carlo simulations with 1-week cycles to estimate mortality 52 weeks from clinic entry. RESULTS: We estimated an increase in mortality from 11.0% to 14.7% (relative increase of 34%) with a 10-week delay in ART for patients entering care with our pre-ART cohort CD4 distribution. When we examined low CD4 ranges, the relative increase in mortality delays remained similar; however, the absolute increase in mortality rose. For example, among patients entering with CD4 count 50-99 cells per cubic millimeter, 12-month mortality increased from 13.3% with no delay compared with 17.0% with a 10-week delay and 22.9% with a 6-month delay. CONCLUSIONS: Delays in ART initiation, common in routine HIV programs, can lead to important increases in mortality. Prompt ART initiation for patients entering clinical care and eligible for ART, especially those with lower CD4 counts, could be a relatively low-cost approach with a potential marked impact on mortality
Superconducting-coil--resistor circuit with electric field quadratic in the current
It is shown for the first time that the observed [Phys. Lett. A 162 (1992)
105] potential difference Phi_t between the resistor and the screen surrounding
the circuit is caused by polarization of the resistor because of the kinetic
energy of the electrons of the superconducting coil. The proportionality of
Phi_t to the square of the current and to the length of the superconducting
wire is explained. It is pointed out that measuring Phi_t makes it possible to
determine the Fermi quasimomentum of the electrons of a metal resistor.Comment: 2 pages, 1 figur
3D characterization of CdSe nanoparticles attached to carbon nanotubes
The crystallographic structure of CdSe nanoparticles attached to carbon
nanotubes has been elucidated by means of high resolution transmission electron
microscopy and high angle annular dark field scanning transmission electron
microscopy tomography. CdSe rod-like nanoparticles, grown in solution together
with carbon nanotubes, undergo a morphological transformation and become
attached to the carbon surface. Electron tomography reveals that the
nanoparticles are hexagonal-based with the (001) planes epitaxially matched to
the outer graphene layer.Comment: 7 pages, 8 figure
Relativistic calculations of the lifetimes and hyperfine structure constants in Zn
This work presents accurate {\it ab initio} determination of the magnetic
dipole (M1) and electric quadrupole (E2) hyperfine structure constants for the
ground and a few low-lying excited states in Zn, which is one of
the interesting systems in fundamental physics. The coupled-cluster (CC) theory
within the relativistic framework has been used here in this calculations. Long
standing demands for a relativistic and highly correlated calculations like CC
can be able to resolve the disagreements among the lifetime estimations
reported previously for a few low-lying states of Zn. The role of
different electron correlation effects in the determination of these quantities
are discussed and their contributions are presented.Comment: 9 pages, 1 figure. submitted to J. Phys. B Fast Trac
Combined CI+MBPT calculations of energy levels and transition amplitudes in Be, Mg, Ca, and Sr
Configuration interaction (CI) calculations in atoms with two valence
electrons, carried out in the V(N-2) Hartree-Fock potential of the core, are
corrected for core-valence interactions using many-body perturbation theory
(MBPT). Two variants of the mixed CI+MBPT theory are described and applied to
obtain energy levels and transition amplitudes for Be, Mg, Ca, and Sr
Artificial Intelligence
Contains research objectives and reports on eight research projects.Computation Center, M.I.T
Electronic and nuclear contributions to time-resolved optical and X-ray absorption spectra of hematite and insights into photoelectrochemical performance
Ultrafast time-resolved studies of photocatalytic thin films can provide a wealth of information crucial for understanding and thereby improving the performance of these materials by directly probing electronic structure, reaction intermediates, and charge carrier dynamics. The interpretation of transient spectra, however, can be complicated by thermally induced structural distortions, which appear within the first few picoseconds following excitation due to carrier–phonon scattering. Here we present a comparison of ex situ steady-state thermal difference spectra and transient absorption spectra spanning from NIR to hard X-ray energies of hematite thin films grown by atomic layer deposition. We find that beyond the first 100 picoseconds, the transient spectra measured for all excitation wavelengths and probe energies are almost entirely due to thermal effects as the lattice expands in response to the ultrafast temperature jump and then cools to room temperature on the microsecond timescale. At earlier times, a broad excited state absorption band that is assigned to free carriers appears at 675 nm, and the lifetime and shape of this feature also appear to be mostly independent of excitation wavelength. The combined spectroscopic data, which are modeled with density functional theory and full multiple scattering calculations, support an assignment of the optical absorption spectrum of hematite that involves two LMCT bands that nearly span the visible spectrum. Our results also suggest a framework for shifting the ligand-to-metal charge transfer absorption bands of ferric oxide films from the near-UV further into the visible part of the solar spectrum to improve solar conversion efficiency
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