1,131 research outputs found
A recurrent neural network for classification of unevenly sampled variable stars
Astronomical surveys of celestial sources produce streams of noisy time
series measuring flux versus time ("light curves"). Unlike in many other
physical domains, however, large (and source-specific) temporal gaps in data
arise naturally due to intranight cadence choices as well as diurnal and
seasonal constraints. With nightly observations of millions of variable stars
and transients from upcoming surveys, efficient and accurate discovery and
classification techniques on noisy, irregularly sampled data must be employed
with minimal human-in-the-loop involvement. Machine learning for inference
tasks on such data traditionally requires the laborious hand-coding of
domain-specific numerical summaries of raw data ("features"). Here we present a
novel unsupervised autoencoding recurrent neural network (RNN) that makes
explicit use of sampling times and known heteroskedastic noise properties. When
trained on optical variable star catalogs, this network produces supervised
classification models that rival other best-in-class approaches. We find that
autoencoded features learned on one time-domain survey perform nearly as well
when applied to another survey. These networks can continue to learn from new
unlabeled observations and may be used in other unsupervised tasks such as
forecasting and anomaly detection.Comment: 23 pages, 14 figures. The published version is at Nature Astronomy
(https://www.nature.com/articles/s41550-017-0321-z). Source code for models,
experiments, and figures at
https://github.com/bnaul/IrregularTimeSeriesAutoencoderPaper (Zenodo Code
DOI: 10.5281/zenodo.1045560
Group Iterative Spectrum Thresholding for Super-Resolution Sparse Spectral Selection
Recently, sparsity-based algorithms are proposed for super-resolution
spectrum estimation. However, to achieve adequately high resolution in
real-world signal analysis, the dictionary atoms have to be close to each other
in frequency, thereby resulting in a coherent design. The popular convex
compressed sensing methods break down in presence of high coherence and large
noise. We propose a new regularization approach to handle model collinearity
and obtain parsimonious frequency selection simultaneously. It takes advantage
of the pairing structure of sine and cosine atoms in the frequency dictionary.
A probabilistic spectrum screening is also developed for fast computation in
high dimensions. A data-resampling version of high-dimensional Bayesian
Information Criterion is used to determine the regularization parameters.
Experiments show the efficacy and efficiency of the proposed algorithms in
challenging situations with small sample size, high frequency resolution, and
low signal-to-noise ratio
Gap filling and noise reduction of unevenly sampled data by means of the Lomb-Scargle periodogram
International audienceThe Lomb-Scargle periodogram is widely used for the estimation of the power spectral density of unevenly sampled data. A small extension of the algorithm of the Lomb-Scargle periodogram permits the estimation of the phases of the spectral components. The amplitude and phase information is sufficient for the construction of a complex Fourier spectrum. The inverse Fourier transform can be applied to this Fourier spectrum and provides an evenly sampled series (Scargle, 1989). We are testing the proposed reconstruction method by means of artificial time series and real observations of mesospheric ozone, having data gaps and noise. For data gap filling and noise reduction, it is necessary to modify the Fourier spectrum before the inverse Fourier transform is done. The modification can be easily performed by selection of the relevant spectral components which are above a given confidence limit or within a certain frequency range. Examples with time series of lower mesospheric ozone show that the reconstruction method can reproduce steep ozone gradients around sunrise and sunset and superposed planetary wave-like oscillations observed by a ground-based microwave radiometer at Payerne
Analysis of surface atrial signals using spectral methods for time series with missing data
In this work, the analysis of atrial signals recorded during atrial fibrillation was pursued using two spectral estimators designed for series with missing data: the Lomb periodogram (LP) and the Iterative Singular Spectrum Analysis (ISSA). The main aim is to verify if subtraction ofthe ventricular activity might be avoided by performing spectral analysis on those ECG intervals where such activity is absent, (i.e. the T-Q intervals), at least to estimate the dominant atrial Fibrillatory Frequency (FF). Recordings coming from the 2004 Computers in Cardiology Termination Challenge Database were analyzed. Fibrillatory frequencies were then compared with those obtained from the analysis ofthe correspondent atrial signals extracted using a modified Average Beat Substraction (ABS) technique. We observed that the mean absolute difference was 0.42 \ub1 0.66 Hz for LP, (mean\ub1SD), and 0.39 \ub1 0.64 Hz for ISSA. We concluded that estimation of FF is feasible without applying QRS-T subtraction
Hierarchical Bayesian sparse image reconstruction with application to MRFM
This paper presents a hierarchical Bayesian model to reconstruct sparse
images when the observations are obtained from linear transformations and
corrupted by an additive white Gaussian noise. Our hierarchical Bayes model is
well suited to such naturally sparse image applications as it seamlessly
accounts for properties such as sparsity and positivity of the image via
appropriate Bayes priors. We propose a prior that is based on a weighted
mixture of a positive exponential distribution and a mass at zero. The prior
has hyperparameters that are tuned automatically by marginalization over the
hierarchical Bayesian model. To overcome the complexity of the posterior
distribution, a Gibbs sampling strategy is proposed. The Gibbs samples can be
used to estimate the image to be recovered, e.g. by maximizing the estimated
posterior distribution. In our fully Bayesian approach the posteriors of all
the parameters are available. Thus our algorithm provides more information than
other previously proposed sparse reconstruction methods that only give a point
estimate. The performance of our hierarchical Bayesian sparse reconstruction
method is illustrated on synthetic and real data collected from a tobacco virus
sample using a prototype MRFM instrument.Comment: v2: final version; IEEE Trans. Image Processing, 200
Surrogate time series
Before we apply nonlinear techniques, for example those inspired by chaos
theory, to dynamical phenomena occurring in nature, it is necessary to first
ask if the use of such advanced techniques is justified "by the data". While
many processes in nature seem very unlikely a priori to be linear, the possible
nonlinear nature might not be evident in specific aspects of their dynamics.
The method of surrogate data has become a very popular tool to address such a
question. However, while it was meant to provide a statistically rigorous,
foolproof framework, some limitations and caveats have shown up in its
practical use. In this paper, recent efforts to understand the caveats, avoid
the pitfalls, and to overcome some of the limitations, are reviewed and
augmented by new material. In particular, we will discuss specific as well as
more general approaches to constrained randomisation, providing a full range of
examples. New algorithms will be introduced for unevenly sampled and
multivariate data and for surrogate spike trains. The main limitation, which
lies in the interpretability of the test results, will be illustrated through
instructive case studies. We will also discuss some implementational aspects of
the realisation of these methods in the TISEAN
(http://www.mpipks-dresden.mpg.de/~tisean) software package.Comment: 28 pages, 23 figures, software at
http://www.mpipks-dresden.mpg.de/~tisea
Laser diffraction particle sizing : sampling and inversion
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 13, 1987The inverse problem of obtaining particle size distributions from observations
of the angular distribution of near forward scattered light is reexamined.
Asymptotic analysis of the forward problem reveals the information
content of the observations, and the sources of non-uniqueness and
instability in inverting them. A sampling criterion, such that the observations
uniquely specify the size distribution is derived, in terms of the
largest particle size, and an angle above which the intensity is indistinguishable
from an asymptote. The instability of inverting unevenly spaced
data is compared to that of super-resolving Fourier spectra. Resolution is
shown to be inversely proportional to the angular range of observations.
The problem is rephrased so that the size weighted number density is
sought from the intensity weighted by the scattering angle cubed. Algorithms
which impose positivity and bounds on particle size improve the
stability of inversions. The forward problem can be represented by an
over-determined matrix equation by choosing a large integration increment
in size dependent on the frequency content of the angular intensity, further
improving stability.
Experimental data obtained using a linear CCD array illustrates the theory, with standard polystyrene spheres as scatterers. The scattering
from single and tri-modal distributions is successfully inverted.I was supported by a NASA Technology
Transfer Traineeship grant (NGT-014-800, Supplement 5), and by
the Joint Program in Oceanographic Engineering between the Woods Hole
Oceanographic Institution and the Massachusetts Institute of Technology.
The experimental work was funded by the Coastal Research Laboratory of
the Woods Hole Oceanographic Institution, through the generosity of the
Mellon Foundation
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