812 research outputs found
Subtraction of temperature induced phase noise in the LISA frequency band
Temperature fluctuations are expected to be one of the limiting factors for
gravitational wave detectors in the very low frequency range. Here we report
the characterisation of this noise source in the LISA Pathfinder optical bench
and propose a method to remove its contribution from the data. Our results show
that temperature fluctuations are indeed limiting our measurement below one
millihertz, and that their subtraction leads to a factor 5.6 (15 dB) reduction
in the noise level at the lower end of the LISA measurement band 10^{-4} Hz,
which increases to 20.2 (26 dB) at even lower frequencies, i.e., 1.5x10^{-5}
Hz. The method presented here can be applied to the subtraction of other noise
sources in gravitational wave detectors in the general situation where multiple
sensors are used to characterise the noise source.Comment: 8 pages, 6 figure
Why are some South African children with Down syndrom not being offered cardiac surgery?
About 1 in 1 000 children has Down syndrome. Extra chromosomal material results in a myriad of potential problems for the affected individual. About 40% of Down syndrome children will have cardiac abnormalities, ranging from the simple arterial duct to the complex atrioventricular septal defect. Virtually all these defects are amenable to surgical correction and extended survival is possible. In South Africa many of these children do not undergo cardiac surgery
A closed-loop EKF and multi-failure diagnosis approach for cooperative GNSS positioning
Current cooperative positioning with Global Navigation Satellite System (GNSS) for connected vehicle application mainly uses pseudorange measurements. However the positioning accuracy offered cannot meet the requirements for lane-level positioning, collision avoidance and future automatic driving, which needs real-time positioning accuracy of better than 0.5m. Furthermore, there is an apparent lack of research into the integrity issue for these new applications under emerging driverless vehicle applications. In order to overcome those problems, a new Extended Kalman Filter (EKF) and a multi-failure diagnosis algorithm are developed to process both GNSS pseudorange and carrier phase measurements. We first introduce a new closed-loop EKF with partial ambiguity resolution (PAR) as feedback to address the low accuracy issue. Then a multi-failure diagnosis algorithm is proposed to improve integrity and reliability. The core of this new algorithm includes using Carrier phase based Receiver Autonomous Integrity Monitoring (CRAIM) method for failure detection, and the double extended w-test detectors to identify failure. A cooperative positioning experiment was carried out to validate the proposed method. The results show that the proposed closed-loop EKF can provide highly accurate positioning, and the multi-failure diagnosis method is effective in detecting and identifying failures for both code and carrier phase measurements
Photon pressure induced test mass deformation in gravitational-wave detectors
A widely used assumption within the gravitational-wave community has so far
been that a test mass acts like a rigid body for frequencies in the detection
band, i.e. for frequencies far below the first internal resonance. In this
article we demonstrate that localized forces, applied for example by a photon
pressure actuator, can result in a non-negligible elastic deformation of the
test masses. For a photon pressure actuator setup used in the gravitational
wave detector GEO600 we measured that this effect modifies the standard
response function by 10% at 1 kHz and about 100% at 2.5 kHz
A template bank for gravitational waveforms from coalescing binary black holes: non-spinning binaries
Gravitational waveforms from the inspiral and ring-down stages of the binary
black hole coalescences can be modelled accurately by
approximation/perturbation techniques in general relativity. Recent progress in
numerical relativity has enabled us to model also the non-perturbative merger
phase of the binary black-hole coalescence problem. This enables us to
\emph{coherently} search for all three stages of the coalescence of
non-spinning binary black holes using a single template bank. Taking our
motivation from these results, we propose a family of template waveforms which
can model the inspiral, merger, and ring-down stages of the coalescence of
non-spinning binary black holes that follow quasi-circular inspiral. This
two-dimensional template family is explicitly parametrized by the physical
parameters of the binary. We show that the template family is not only
\emph{effectual} in detecting the signals from black hole coalescences, but
also \emph{faithful} in estimating the parameters of the binary. We compare the
sensitivity of a search (in the context of different ground-based
interferometers) using all three stages of the black hole coalescence with
other template-based searches which look for individual stages separately. We
find that the proposed search is significantly more sensitive than other
template-based searches for a substantial mass-range, potentially bringing
about remarkable improvement in the event-rate of ground-based interferometers.
As part of this work, we also prescribe a general procedure to construct
interpolated template banks using non-spinning black hole waveforms produced by
numerical relativity.Comment: A typo fixed in Eq.(B11
Bayesian inference on compact binary inspiral gravitational radiation signals in interferometric data
Presented is a description of a Markov chain Monte Carlo (MCMC) parameter
estimation routine for use with interferometric gravitational radiational data
in searches for binary neutron star inspiral signals. Five parameters
associated with the inspiral can be estimated, and summary statistics are
produced. Advanced MCMC methods were implemented, including importance
resampling and prior distributions based on detection probability, in order to
increase the efficiency of the code. An example is presented from an
application using realistic, albeit fictitious, data.Comment: submitted to Classical and Quantum Gravity. 14 pages, 5 figure
Bayesian Model Selection for LISA Pathfinder
The main goal of the LISA Pathfinder (LPF) mission is to fully characterize
the acceleration noise models and to test key technologies for future
space-based gravitational-wave observatories similar to the eLISA concept. The
data analysis team has developed complex three-dimensional models of the LISA
Technology Package (LTP) experiment on-board LPF. These models are used for
simulations, but more importantly, they will be used for parameter estimation
purposes during flight operations. One of the tasks of the data analysis team
is to identify the physical effects that contribute significantly to the
properties of the instrument noise. A way of approaching this problem is to
recover the essential parameters of a LTP model fitting the data. Thus, we want
to define the simplest model that efficiently explains the observations. To do
so, adopting a Bayesian framework, one has to estimate the so-called Bayes
Factor between two competing models. In our analysis, we use three main
different methods to estimate it: The Reversible Jump Markov Chain Monte Carlo
method, the Schwarz criterion, and the Laplace approximation. They are applied
to simulated LPF experiments where the most probable LTP model that explains
the observations is recovered. The same type of analysis presented in this
paper is expected to be followed during flight operations. Moreover, the
correlation of the output of the aforementioned methods with the design of the
experiment is explored
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