406,658 research outputs found
Joint space aspect reconstruction of wide-angle SAR exploiting sparsity
In this paper we present an algorithm for wide-angle synthetic aperture radar (SAR) image formation. Reconstruction of wide-angle SAR holds a promise of higher resolution and better information about a scene, but it also poses a number of challenges when compared to the traditional narrow-angle SAR. Most prominently, the isotropic point scattering model is no longer valid. We present an algorithm capable of producing high resolution reïŹectivity maps in both space and aspect, thus accounting for the anisotropic scattering behavior of targets. We pose the problem as a non-parametric three-dimensional inversion problem, with two constraints: magnitudes of the backscattered power are highly correlated across closely spaced look angles and the backscattered power
originates from a small set of point scatterers. This approach considers jointly all scatterers in the scene across all azimuths, and exploits the sparsity of the underlying scattering ïŹeld. We implement the algorithm and present
reconstruction results on realistic data obtained from the XPatch Backhoe dataset
A High Precision, Optical Polarimeter to Measure Inclinations of High Mass X-Ray Binaries
We present commissioning data for the POLISH instrument obtained on the Hale
5-m telescope. The goal of this high precision polarimeter is to constrain
orbital inclination of high mass X-ray binaries and to therefore obtain
independent mass estimates for their black hole companions. We have obtained
photon shot noise limited precision on standard stars, and we have measured the
polarization of bright stars at the part per million level on a nightly basis.
Systematic effects have been reduced to less than 1% of the measured
polarization for polarized sources and to the part per million level for weakly
polarized sources. The high sensitivity of this instrument to asymmetry
suggests that valuable contributions will be made in many other fields,
including studies of extrasolar planets, debris disks, and stellar
astrophysics.Comment: 35 pages, 10 figures, accepted for publication in PAS
Detectability of Microwave Background Polarization
[NOTE: Previous versions of this paper (both on astro-ph and published in
Phys. Rev. D) contain results that are in error. The power spectra C_l were
normalized incorrectly by a factor of 2 pi. All observing times in
detector-years in those versions are too large by a factor of 2 pi. The main
place these numbers appear is on the vertical axes of Figures 4 and 5. Note
that because all calculations were based on the same power spectra, all
conclusions pertaining to comparisons of different techniques remain unchanged.
This error has been corrected in the present version of the paper. An erratum
is being sent to Phys. Rev. D. I apologize for the error.]
Using a Fisher-matrix formalism, we calculate the required sensitivities and
observing times for an experiment to measure the amplitudes of both E and B
components as a function of sky coverage, taking full account of the fact that
the two components cannot be perfectly separated in an incomplete sky map. We
also present a simple approximation scheme that accounts for mixing of E and B
components in computing predicted errors in the E-component power spectrum
amplitude. In an experiment with small sky coverage, mixing of the two
components increases the difficulty of detecting the subdominant B component by
a factor of two or more in observing time; however, for larger survey sizes the
effect of mixing is less pronounced. Surprisingly, mixing of E and B components
can enhance the detectability of the E component by increasing the effective
number of independent modes that probe this componentComment: Previous versions of this paper contained results that were in error.
The present version on astro-ph has been corrected, and an erratum is being
submitted. See abstract for detail
CLEAR: Covariant LEAst-square Re-fitting with applications to image restoration
In this paper, we propose a new framework to remove parts of the systematic
errors affecting popular restoration algorithms, with a special focus for image
processing tasks. Generalizing ideas that emerged for regularization,
we develop an approach re-fitting the results of standard methods towards the
input data. Total variation regularizations and non-local means are special
cases of interest. We identify important covariant information that should be
preserved by the re-fitting method, and emphasize the importance of preserving
the Jacobian (w.r.t. the observed signal) of the original estimator. Then, we
provide an approach that has a "twicing" flavor and allows re-fitting the
restored signal by adding back a local affine transformation of the residual
term. We illustrate the benefits of our method on numerical simulations for
image restoration tasks
A Robust Nonlinear Beamforming Assisted Receiver for BPSK Signalling
Nonlinear beamforming designed for wireless communications is investigated. We derive the optimal nonlinear beamforming assisted receiver designed for binary phase shift keying (BPSK) signalling. It is shown that this optimal Bayesian beamformer significantly outperforms the classic linear minimum mean square error (LMMSE) beamformer at the expense of an increased complexity. Hence the achievable user capacity of the wireless system invoking the proposed beamformer is substantially enhanced. In particular, when the angular separation between the desired and interfering signals is below a certain threshold, a linear beamformer will fail while a nonlinear beamformer can still perform adequately. Blockadaptive implementation of the optimal Bayesian beamformer can be realized using a Radial Basis Function network based on the Relevance Vector Machine (RVM) for classification, and a recursive sample-by-sample adaptation is proposed based on an enhanced ?-means clustering aided recursive least squares algorithm
Superconductivity emerging near quantum critical point of valence transition
The nature of the quantum valence transition is studied in the
one-dimensional periodic Anderson model with Coulomb repulsion between f and
conduction electrons by the density-matrix renormalization group method. It is
found that the first-order valence transition emerges with the quantum critical
point and the crossover from the Kondo to the mixed-valence states is strongly
stabilized by quantum fluctuation and electron correlation. It is found that
the superconducting correlation is developed in the Kondo regime near the sharp
valence increase. The origin of the superconductivity is ascribed to the
development of the coherent motion of electrons with enhanced valence
fluctuation, which results in the enhancement of the charge velocity, but not
of the charge compressibility. Statements on the valence transition in
connection with Ce metal and Ce compounds are given.Comment: 9 pages, 4 figure
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