3,919 research outputs found
Circular polarization measurement in millimeter-wavelength spectral-line VLBI observations
This paper considers the problem of accurate measurement of circular
polarization in imaging spectral-line VLBI observations in the lambda=7 mm and
lambda=3 mm wavelength bands. This capability is especially valuable for the
full observational study of compact, polarized SiO maser components in the
near-circumstellar environment of late-type, evolved stars. Circular VLBI
polarimetry provides important constraints on SiO maser astrophysics, including
the theory of polarized maser emission transport, and on the strength and
distribution of the stellar magnetic field and its dynamical role in this
critical circumstellar region. We perform an analysis here of the data model
containing the instrumental factors that limit the accuracy of circular
polarization measurements in such observations, and present a corresponding
data reduction algorithm for their correction. The algorithm is an enhancement
of existing spectral line VLBI polarimetry methods using autocorrelation data
for calibration, but with innovations in bandpass determination,
autocorrelation polarization self-calibration, and general optimizations for
the case of low SNR, as applicable at these wavelengths. We present an example
data reduction at mm and derive an estimate of the predicted
accuracy of the method of m_c < 0.5% or better at lambda=7 mm and m_c < 0.5-1%
or better at lambda=3 mm. Both the strengths and weaknesses of the proposed
algorithm are discussed, along with suggestions for future work.Comment: 23 pages, 13 figure
Data-Driven Methods for the State of Charge Estimation of Lithium-Ion Batteries: An Overview
In recent years, there has been a noticeable shift towards electric mobility and an increasing emphasis on integrating renewable energy sources. Consequently, batteries and their management have been prominent in this context. A vital aspect of the BMS revolves around accurately determining the battery pack’s SOC. Notably, the advent of advanced microcontrollers and the availability of extensive datasets have contributed to the growing popularity and practicality of data-driven methodologies. This study examines the developments in SOC estimation over the past half-decade, explicitly focusing on data-driven estimation techniques. It comprehensively assesses the performance of each algorithm, considering the type of battery and various operational conditions. Additionally, intricate details concerning the models’ hyperparameters, including the number of layers, type of optimiser, and neuron, are provided for thorough examination. Most of the models analysed in the paper demonstrate strong performance, with both the MAE and RMSE for the estimation of SOC hovering around 2% or even lower
The Epoch of Reionization: Foregrounds and Calibration With Paper
Nearly half a billion years passed between the release of the now routinely observed Comic Microwave Backrgound and the formation of the first galaxies and black holes which reionized the ubiquitous hydrogen. This Epoch of Reionization (EoR) is the next major unexplored cosmological milestone. At the current time the space between galaxies is almost completely ionized, therefor we know that the universe must have undergone a global phase transition. The nature of the ionizing sources, whether young galaxies or accreting massive black holes is unknown. Neither do we know when this reionization occured or how long it took. Models suggest that we can detect fluctuations in the 21cm hydrogen emission line as ionization proceeds and high contrast ionized holes are carved in the neutral hydrogen. Detecting these fluctuations is one of the few direct probes of the reionization process but is a difficult task requiring a new generation of low frequency radio telescopes. Motivated by the breadth of unknowns, the Precision Array for Probing the Epoch of Reionization (PAPER) has been slowly building in complexity while folding the results of observations back into improving the design and operation of the telescope. As part of this process, this thesis analyzes early observations to explore three major areas of concern in detecting EoR: contamination by foreground sources, calibration stability and limiting sensitivity. Catalogs produced from this early data show good agreement with previous measurements. We conclude that the calibration is stable and sensitivity floors are close to the expected theoretical levels
Movements in Binaural Space: Issues in HRTF Interpolation and Reverberation, with applications to Computer Music
This thesis deals broadly with the topic of Binaural Audio. After reviewing the
literature, a reappraisal of the minimum-phase plus linear delay model for HRTF
representation and interpolation is offered. A rigorous analysis of threshold based
phase unwrapping is also performed. The results and conclusions drawn from these
analyses motivate the development of two novel methods for HRTF representation
and interpolation. Empirical data is used directly in a Phase Truncation method. A
Functional Model for phase is used in the second method based on the
psychoacoustical nature of Interaural Time Differences. Both methods are validated;
most significantly, both perform better than a minimum-phase method in subjective
testing.
The accurate, artefact-free dynamic source processing afforded by the above
methods is harnessed in a binaural reverberation model, based on an early reflection
image model and Feedback Delay Network diffuse field, with accurate interaural
coherence. In turn, these flexible environmental processing algorithms are used in
the development of a multi-channel binaural application, which allows the audition
of multi-channel setups in headphones. Both source and listener are dynamic in this
paradigm. A GUI is offered for intuitive use of the application.
HRTF processing is thus re-evaluated and updated after a review of accepted
practice. Novel solutions are presented and validated. Binaural reverberation is
recognised as a crucial tool for convincing artificial spatialisation, and is developed
on similar principles. Emphasis is placed on transparency of development practices,
with the aim of wider dissemination and uptake of binaural technology
On the Application of PSpice for Localised Cloud Security
The work reported in this thesis commenced with a review of methods for creating random binary sequences for encoding data locally by the client before storing in the Cloud. The first method reviewed investigated evolutionary computing software which generated noise-producing functions from natural noise, a highly-speculative novel idea since noise is stochastic. Nevertheless, a function was created which generated noise to seed chaos oscillators which produced random binary sequences and this research led to a circuit-based one-time pad key chaos encoder for encrypting data. Circuit-based delay chaos oscillators, initialised with sampled electronic noise, were simulated in a linear circuit simulator called PSpice. Many simulation problems were encountered because of the nonlinear nature of chaos but were solved by creating new simulation parts, tools and simulation paradigms. Simulation data from a range of chaos sources was exported and analysed using Lyapunov analysis and identified two sources which produced one-time pad sequences with maximum entropy. This led to an encoding system which generated unlimited, infinitely-long period, unique random one-time pad encryption keys for plaintext data length matching. The keys were studied for maximum entropy and passed a suite of stringent internationally-accepted statistical tests for randomness. A prototype containing two delay chaos sources initialised by electronic noise was produced on a double-sided printed circuit board and produced more than 200 Mbits of OTPs. According to Vladimir Kotelnikov in 1941 and Claude Shannon in 1945, one-time pad sequences are theoretically-perfect and unbreakable, provided specific rules are adhered to. Two other techniques for generating random binary sequences were researched; a new circuit element, memristance was incorporated in a Chua chaos oscillator, and a fractional-order Lorenz chaos system with order less than three. Quantum computing will present many problems to cryptographic system security when existing systems are upgraded in the near future. The only existing encoding system that will resist cryptanalysis by this system is the unconditionally-secure one-time pad encryption
LSST Science Book, Version 2.0
A survey that can cover the sky in optical bands over wide fields to faint
magnitudes with a fast cadence will enable many of the exciting science
opportunities of the next decade. The Large Synoptic Survey Telescope (LSST)
will have an effective aperture of 6.7 meters and an imaging camera with field
of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over
20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with
fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a
total point-source depth of r~27.5. The LSST Science Book describes the basic
parameters of the LSST hardware, software, and observing plans. The book
discusses educational and outreach opportunities, then goes on to describe a
broad range of science that LSST will revolutionize: mapping the inner and
outer Solar System, stellar populations in the Milky Way and nearby galaxies,
the structure of the Milky Way disk and halo and other objects in the Local
Volume, transient and variable objects both at low and high redshift, and the
properties of normal and active galaxies at low and high redshift. It then
turns to far-field cosmological topics, exploring properties of supernovae to
z~1, strong and weak lensing, the large-scale distribution of galaxies and
baryon oscillations, and how these different probes may be combined to
constrain cosmological models and the physics of dark energy.Comment: 596 pages. Also available at full resolution at
http://www.lsst.org/lsst/sciboo
Applications of Mathematical Models in Engineering
The most influential research topic in the twenty-first century seems to be mathematics, as it generates innovation in a wide range of research fields. It supports all engineering fields, but also areas such as medicine, healthcare, business, etc. Therefore, the intention of this Special Issue is to deal with mathematical works related to engineering and multidisciplinary problems. Modern developments in theoretical and applied science have widely depended our knowledge of the derivatives and integrals of the fractional order appearing in engineering practices. Therefore, one goal of this Special Issue is to focus on recent achievements and future challenges in the theory and applications of fractional calculus in engineering sciences. The special issue included some original research articles that address significant issues and contribute towards the development of new concepts, methodologies, applications, trends and knowledge in mathematics. Potential topics include, but are not limited to, the following: Fractional mathematical models; Computational methods for the fractional PDEs in engineering; New mathematical approaches, innovations and challenges in biotechnologies and biomedicine; Applied mathematics; Engineering research based on advanced mathematical tools
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