45,801 research outputs found
High Speed Dim Air Target Detection Using Airborne Radar under Clutter and Jamming Effects
The challenging potential problems associated with using airborne radar in detection of high Speed Maneuvering Dim Target (HSMDT) are the highly noise, jamming and clutter effects. The problem is not only how to remove clutter and jamming as well as the range migration and Doppler ambiguity estimation problems due to high relative speed between the targets and airborne radar. Some of the recently published works ignored the range migration problems, while the others ignored the Doppler ambiguity estimation. In this paper a new hybrid technique using Optimum Space Time Adaptive Processing (OSTAP), Second Order Keystone Transform (SOKT), and the Improved Fractional Radon Transform (IFrRT) was proposed. The OSTAP was applied as anti-jamming and clutter rejection method, the SOKT corrects the range curvature and part of the range walk, then the IFrRT estimates the target’ radial acceleration and corrects the residual range walk. The simulation demonstrates the validity and effectiveness of the proposed technique, and its advantages over the previous researches by comparing its probability of detection with the traditional methods. The new approach increases the probability of detection, and also overcomes the limitation of Doppler frequency ambiguity
Universality and correlations in individuals wandering through an online extremist space
The 'out of the blue' nature of recent terror attacks and the diversity of
apparent motives, highlight the importance of understanding the online
trajectories that individuals follow prior to developing high levels of
extremist support. Here we show that the physics of stochastic walks, with and
without temporal correlation, provides a unifying description of these online
trajectories. Our unique dataset comprising all users of a global social media
site, reveals universal characteristics in individuals' online lifetimes. Our
accompanying theory generates analytical and numerical solutions that describe
the characteristics shown by individuals that go on to develop high levels of
extremist support, and those that do not. The existence of these temporal and
also many-body correlations suggests that existing physics machinery can be
used to quantify and perhaps mitigate the risk of future events
Possible Weyl fermions in the magnetic Kondo system CeSb
Materials where the electronic bands have unusual topologies allow for the
realization of novel physics and have a wide range of potential applications.
When two electronic bands with linear dispersions intersect at a point, the
excitations could be described as Weyl fermions which are massless particles
with a particular chirality. Here we report evidence for the presence of Weyl
fermions in the ferromagnetic state of the low-carrier density, strongly
correlated Kondo lattice system CeSb, from electronic structure calculations
and angle-dependent magnetoresistance measurements. When the applied magnetic
field is parallel to the electric current, a pronounced negative
magnetoresistance is observed within the ferromagnetic state, which is
destroyed upon slightly rotating the field away. These results give evidence
for CeSb belonging to a new class of Kondo lattice materials with Weyl fermions
in the ferromagnetic state.Comment: 18 pages, 4 figures, Supplementary Information available from journal
link (open access
Mode Repulsion and Mode Coupling in Random Lasers
We studied experimentally and theoretically the interaction of lasing modes
in random media. In a homogeneously broadened gain medium, cross gain
saturation leads to spatial repulsion of lasing modes. In an inhomogeneously
broadened gain medium, mode repulsion occurs in the spectral domain. Some
lasing modes are coupled through photon hopping or electron absorption and
reemission. Under pulsed pumping, weak coupling of two modes leads to
synchronization of their lasing action. Strong coupling of two lasing modes
results in anti-phased oscillations of their intensities.Comment: 13 pages, 4 figure
Calibration of LAMOST Stellar Surface Gravities Using the Kepler Asteroseismic Data
Asteroseismology is a powerful tool to precisely determine the evolutionary
status and fundamental properties of stars. With the unprecedented precision
and nearly continuous photometric data acquired by the NASA Kepler mission,
parameters of more than 10 stars have been determined nearly consistently.
However, most studies still use photometric effective temperatures (Teff) and
metallicities ([Fe/H]) as inputs, which are not sufficiently accurate as
suggested by previous studies. We adopted the spectroscopic Teff and [Fe/H]
values based on the LAMOST low-resolution spectra (R~1,800), and combined them
with the global oscillation parameters to derive the physical parameters of a
large sample of stars. Clear trends were found between {\Delta}logg(LAMOST -
seismic) and spectroscopic Teff as well as logg, which may result in an
overestimation of up to 0.5 dex for the logg of giants in the LAMOST catalog.
We established empirical calibration relations for the logg values of dwarfs
and giants. These results can be used for determining the precise distances to
these stars based on their spectroscopic parameters.Comment: 22 pages, 13 figures and 3 tables, accepted for publication in
Astronomical Journal. Table 3 is available at
http://lwang.info/research/kepler_lamost
An Efficient Algorithm by Kurtosis Maximization in Reference-Based Framework
This paper deals with the optimization of kurtosis for complex-valued signals in the independent component analysis (ICA) framework, where source signals are linearly and instantaneously mixed. Inspired by the recently proposed reference-based contrast schemes, a similar contrast function is put forward, based on which a new fast fixed-point (FastICA) algorithm is proposed. The new optimization method is similar in spirit to the former classical kurtosis-based FastICA algorithm but differs in the fact that it is much more efficient than the latter in terms of computational speed, which is significantly striking with large number of samples. The performance of this new algorithm is confirmed through computer simulations
Generalized r-matrix structure and algebro-geometric solution for integrable systems
The purpose of this paper is to construct a generalized r-matrix structure of
finite dimensional systems and an approach to obtain the algebro-geometric
solutions of integrable nonlinear evolution equations (NLEEs). Our starting
point is a generalized Lax matrix instead of usual Lax pair. The generalized
r-matrix structure and Hamiltonian functions are presented on the basis of
fundamental Poisson bracket. It can be clearly seen that various nonlinear
constrained (c-) and restricted (r-) systems, such as the c-AKNS, c-MKdV,
c-Toda, r-Toda, c-Levi, etc, are derived from the reduction of this structure.
All these nonlinear systems have {\it r}-matrices, and are completely
integrable in Liouville's sense. Furthermore, our generalized structure is
developed to become an approach to obtain the algebro-geometric solutions of
integrable NLEEs. Finally, the two typical examples are considered to
illustrate this approach: the infinite or periodic Toda lattice equation and
the AKNS equation with the condition of decay at infinity or periodic boundary.Comment: 41 pages, 0 figure
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