26,946 research outputs found
Azimuth sidelobes suppression using multi-azimuth angle synthetic aperture radar images
A novel method is proposed for azimuth sidelobes suppression using multi-pass squinted (MPS) synthetic aperture radar (SAR) data. For MPS SAR, the radar observes the scene with different squint angles and heights on each pass. The MPS SAR mode acquisition geometry is given first. Then, 2D signals are focused and the images are registered to the master image. Based on the new signal model, elevation processing and incoherent addition are introduced in detail, which are the main parts for azimuth sidelobes suppression. Moreover, parameter design criteria in incoherent addition are derived for the best performance. With the proposed parameter optimization step, the new method has a prominent azimuth sidelobes suppression effect with a slightly better azimuth resolution, as verified by experimental results on both simulated point targets and TerraSAR-X data
Detection of leaf structures in close-range hyperspectral images using morphological fusion
Close-range hyperspectral images are a promising source of information in plant biology, in particular, for in vivo study of physiological changes. In this study, we investigate how data fusion can improve the detection of leaf elements by combining pixel reflectance and morphological information. The detection of image regions associated to the leaf structures is the first step toward quantitative analysis on the physical effects that genetic manipulation, disease infections, and environmental conditions have in plants. We tested our fusion approach on Musa acuminata (banana) leaf images and compared its discriminant capability to similar techniques used in remote sensing. Experimental results demonstrate the efficiency of our fusion approach, with significant improvements over some conventional methods
LAMOST observations in the Kepler field. Analysis of the stellar parameters measured with the LASP based on the low-resolution spectra
All of the 14 subfields of the Kepler field have been observed at least once
with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST,
Xinglong Observatory, China) during the 2012-2014 observation seasons. There
are 88,628 reduced spectra with SNR (signal-to-noise ratio in g band)
6 after the first round (2012-2014) of observations for the
LAMOST-Kepler project (LK-project). By adopting the upgraded version of the
LAMOST Stellar Parameter pipeline (LASP), we have determined the atmospheric
parameters ( , , and ) and heliocentric radial
velocity for 51,406 stars with 61,226 spectra. Compared with
atmospheric parameters derived from both high-resolution spectroscopy and
asteroseismology method for common stars in Huber et al. (2014), an external
calibration of LASP atmospheric parameters was made, leading to the
determination of external errors for the giants and dwarfs, respectively.
Multiple spectroscopic observations for the same objects of the LK-project were
used to estimate the internal uncertainties of the atmospheric parameters as a
function of SNR with the unbiased estimation method. The LASP atmospheric
parameters were calibrated based on both the external and internal
uncertainties for the giants and dwarfs, respectively. A general statistical
analysis of the stellar parameters leads to discovery of 106 candidate
metal-poor stars, 9 candidate very metal-poor stars, and 18 candidate
high-velocity stars. Fitting formulae were obtained segmentally for both the
calibrated atmospheric parameters of the LK-project and the KIC parameters with
the common stars. The calibrated atmospheric parameters and radial velocities
of the LK-project will be useful for studying stars in the Kepler field.Comment: 53 pages, 21 figures, 5 tables, Accepted for publication by ApJ
Chebyshev expansion for Impurity Models using Matrix Product States
We improve a recently developed expansion technique for calculating real
frequency spectral functions of any one-dimensional model with short-range
interactions, by postprocessing computed Chebyshev moments with linear
prediction. This can be achieved at virtually no cost and, in sharp contrast to
existing methods based on the dampening of the moments, improves the spectral
resolution rather than lowering it. We validate the method for the exactly
solvable resonating level model and the single impurity Anderson model. It is
capable of resolving sharp Kondo resonances, as well as peaks within the
Hubbard bands when employed as an impurity solver for dynamical mean-field
theory (DMFT). Our method works at zero temperature and allows for arbitrary
discretization of the bath spectrum. It achieves similar precision as the
dynamical density matrix renormalization group (DDMRG), at lower cost. We also
propose an alternative expansion, of 1-exp(-tau H) instead of the usual H,
which opens the possibility of using established methods for the time evolution
of matrix product states to calculate spectral functions directly.Comment: 13 pages, 9 figure
MPI Phantom Study with A High-Performing Multicore Tracer Made by Coprecipitation
Magnetic particle imaging (MPI) is a new imaging technique that detects the spatial distribution of magnetic nanoparticles (MNP) with the option of high temporal resolution. MPI relies on particular MNP as tracers with tailored characteristics for improvement of sensitivity and image resolution. For this reason, we developed optimized multicore particles (MCP 3) made by coprecipitation via synthesis of green rust and subsequent oxidation to iron oxide cores consisting of a magnetite/maghemite mixed phase. MCP 3 shows high saturation magnetization close to that of bulk maghemite and provides excellent magnetic particle spectroscopy properties which are superior to Resovist® and any other up to now published MPI tracers made by coprecipitation. To evaluate the MPI characteristics of MCP 3 two kinds of tube phantoms were prepared and investigated to assess sensitivity, spatial resolution, artifact severity, and selectivity. Resovist® was used as standard of comparison. For image reconstruction, the regularization factor was optimized, and the resulting images were investigated in terms of quantifying of volumes and iron content. Our results demonstrate the superiority of MCP 3 over Resovist® for all investigated MPI characteristics and suggest that MCP 3 is promising for future experimental in vivo studies
Applying autonomy to distributed satellite systems: Trends, challenges, and future prospects
While monolithic satellite missions still pose significant advantages in terms of accuracy and
operations, novel distributed architectures are promising improved flexibility, responsiveness,
and adaptability to structural and functional changes. Large satellite swarms, opportunistic satellite
networks or heterogeneous constellations hybridizing small-spacecraft nodes with highperformance
satellites are becoming feasible and advantageous alternatives requiring the adoption
of new operation paradigms that enhance their autonomy. While autonomy is a notion that
is gaining acceptance in monolithic satellite missions, it can also be deemed an integral characteristic
in Distributed Satellite Systems (DSS). In this context, this paper focuses on the motivations
for system-level autonomy in DSS and justifies its need as an enabler of system qualities. Autonomy
is also presented as a necessary feature to bring new distributed Earth observation functions
(which require coordination and collaboration mechanisms) and to allow for novel structural
functions (e.g., opportunistic coalitions, exchange of resources, or in-orbit data services). Mission
Planning and Scheduling (MPS) frameworks are then presented as a key component to implement
autonomous operations in satellite missions. An exhaustive knowledge classification explores the
design aspects of MPS for DSS, and conceptually groups them into: components and organizational
paradigms; problem modeling and representation; optimization techniques and metaheuristics;
execution and runtime characteristics and the notions of tasks, resources, and constraints.
This paper concludes by proposing future strands of work devoted to study the trade-offs of
autonomy in large-scale, highly dynamic and heterogeneous networks through frameworks that
consider some of the limitations of small spacecraft technologies.Postprint (author's final draft
Spinon confinement in a quasi one dimensional anisotropic Heisenberg magnet
Confinement is a process by which particles with fractional quantum numbers
bind together to form quasiparticles with integer quantum numbers. The
constituent particles are confined by an attractive interaction whose strength
increases with increasing particle separation and as a consequence, individual
particles are not found in isolation. This phenomenon is well known in particle
physics where quarks are confined in baryons and mesons. An analogous
phenomenon occurs in certain magnetic insulators; weakly coupled chains of
spins S=1/2. The collective excitations in these systems is spinons (S=1/2). At
low temperatures weak coupling between chains can induce an attractive
interaction between pairs of spinons that increases with their separation and
thus leads to confinement. In this paper, we employ inelastic neutron
scattering to investigate the spinon confinement in the quasi-1D S=1/2 XXZ
antiferromagnet SrCo2V2O8. Spinon excitations are observed above TN in
quantitative agreement with established theory. Below TN the pairs of spinons
are confined and two sequences of meson-like bound states with longitudinal and
transverse polarizations are observed. Several theoretical approaches are used
to explain the data. A new theoretical technique based on Tangent-space Matrix
Product States gives a very complete description of the data and provides good
agreement not only with the energies of the bound modes but also with their
intensities. We also successfully explained the effect of temperature on the
excitations including the experimentally observed thermally induced resonance
between longitudinal modes below TN ,and the transitions between thermally
excited spinon states above TN. In summary, our work establishes SrCo2V2O8 as a
beautiful paradigm for spinon confinement in a quasi-1D quantum magnet and
provides a comprehensive picture of this process.Comment: 17 pages, 18 figures, submitted to PR
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