14,145 research outputs found
Coal Seam Thickness Estimation Using GPR and Higher Order Statistics - The Near-Surface Case
A novel pattern recognition-based approach to detect near-surface interfaces using ground penetrating radar (GPR) has been reported in [1]. The approach was used to successfully detect interfaces within 5 cm of the ground surface. This technique has been adapted for the important task of layer thickness estimation in the near-surface range. This is inherently a difficult problem to solve in practice because the radar echo is often dominated by unwanted components such as antenna crosstalk and ring-down, ground reflection effects and clutter. Features derived from the bispectrum and a nearest-neighbour classifier have been utilized for this processing task. It is shown that unlike traditional second order correlation based methods such as matched filtering which can fail in known conditions, layer thickness estimation using this approach can be reliably extended to the near-surface region
In-Orbit Instrument Performance Study and Calibration for POLAR Polarization Measurements
POLAR is a compact space-borne detector designed to perform reliable
measurements of the polarization for transient sources like Gamma-Ray Bursts in
the energy range 50-500keV. The instrument works based on the Compton
Scattering principle with the plastic scintillators as the main detection
material along with the multi-anode photomultiplier tube. POLAR has been
launched successfully onboard the Chinese space laboratory TG-2 on 15th
September, 2016. In order to reliably reconstruct the polarization information
a highly detailed understanding of the instrument is required for both data
analysis and Monte Carlo studies. For this purpose a full study of the in-orbit
performance was performed in order to obtain the instrument calibration
parameters such as noise, pedestal, gain nonlinearity of the electronics,
threshold, crosstalk and gain, as well as the effect of temperature on the
above parameters. Furthermore the relationship between gain and high voltage of
the multi-anode photomultiplier tube has been studied and the errors on all
measurement values are presented. Finally the typical systematic error on
polarization measurements of Gamma-Ray Bursts due to the measurement error of
the calibration parameters are estimated using Monte Carlo simulations.Comment: 43 pages, 30 figures, 1 table; Preprint accepted by NIM
A High-resolution Scintillating Fiber Tracker With Silicon Photomultiplier Array Readout
We present prototype modules for a tracking detector consisting of multiple
layers of 0.25 mm diameter scintillating fibers that are read out by linear
arrays of silicon photomultipliers. The module production process is described
and measurements of the key properties for both the fibers and the readout
devices are shown. Five modules have been subjected to a 12 GeV/c proton/pion
testbeam at CERN. A spatial resolution of 0.05 mm and light yields exceeding 20
detected photons per minimum ionizing particle have been achieved, at a
tracking efficiency of more than 98.5%. Possible techniques for further
improvement of the spatial resolution are discussed.Comment: 31 pages, 27 figures, pre-print version of an article published in
Nuclear Instruments and Methods in Physics Research Section A, Vol. 62
Optimal Extraction of Fibre Optic Spectroscopy
We report an optimal extraction methodology, for the reduction of
multi-object fibre spectroscopy data, operating in the regime of tightly packed
(and hence significantly overlapping) fibre profiles. The routine minimises
crosstalk between adjacent fibres and statistically weights the extraction to
reduce noise. As an example of the process we use simulations of the numerous
modes of operation of the AAOmega fibre spectrograph and observational data
from the SPIRAL Integral Field Unit at the Anglo-Australian Telescope.Comment: Accepted for publication in PAS
Receptor crosstalk improves concentration sensing of multiple ligands
Cells need to reliably sense external ligand concentrations to achieve
various biological functions such as chemotaxis or signaling. The molecular
recognition of ligands by surface receptors is degenerate in many systems
leading to crosstalk between different receptors. Crosstalk is often thought of
as a deviation from optimal specific recognition, as the binding of non-cognate
ligands can interfere with the detection of the receptor's cognate ligand,
possibly leading to a false triggering of a downstream signaling pathway. Here
we quantify the optimal precision of sensing the concentrations of multiple
ligands by a collection of promiscuous receptors. We demonstrate that crosstalk
can improve precision in concentration sensing and discrimination tasks. To
achieve superior precision, the additional information about ligand
concentrations contained in short binding events of the non-cognate ligand
should be exploited. We present a proofreading scheme to realize an approximate
estimation of multiple ligand concentrations that reaches a precision close to
the derived optimal bounds. Our results help rationalize the observed ubiquity
of receptor crosstalk in molecular sensing
How reliable is Zeeman Doppler Imaging without simultaneous temperature reconstruction?
Aims: The goal of this study is to perform numerical tests of Zeeman Doppler
Imaging (ZDI) to asses whether correct reconstruction of magnetic fields is at
all possible without taking temperature into account for stars in which
magnetic and temperature inhomogeneities are spatially correlated. Methods: We
used a modern ZDI code employing a physically realistic treatment of the
polarized radiative transfer in all four Stokes parameters. We generated
artificial observations of isolated magnetic spots and of magnetic features
coinciding with cool temperature spots and then reconstructed magnetic and
temperature distributions from these data. Results: Using Stokes I and V for
simultaneous magnetic and temperature mapping for the star with a homogeneous
temperature distribution yields magnetic field strengths underestimated by
typically 10-15% relative to their true values. When temperature is kept
constant and Stokes I is not used for magnetic mapping, the underestimation is
30-60%. At the same time, the strength of magnetic field inside cool spots is
underestimated by as much as 80-95% and the spot geometry is also poorly
reconstructed when temperature variations are ignored. On the other hand, the
inversion quality is greatly improved when temperature variations are accounted
for in magnetic mapping. When using all four Stokes parameters the
reconstructed field strength inside cool spots is underestimated by 30-40% but
the spot geometry can be recovered very accurately compared to the experiments
with circular polarization alone. Conclusions: Reliable magnetic field
reconstruction for a star with high-contrast temperature spots is essentially
impossible if temperature inhomogeneities are ignored. A physically realistic
line profile modeling method, which simultaneously accounts for both types of
inhomogeneities, is required for meaningful ZDI of cool active stars
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