1,978 research outputs found
Hidden Markov model tracking of continuous gravitational waves from a neutron star with wandering spin
Gravitational wave searches for continuous-wave signals from neutron stars
are especially challenging when the star's spin frequency is unknown a priori
from electromagnetic observations and wanders stochastically under the action
of internal (e.g. superfluid or magnetospheric) or external (e.g. accretion)
torques. It is shown that frequency tracking by hidden Markov model (HMM)
methods can be combined with existing maximum likelihood coherent matched
filters like the F-statistic to surmount some of the challenges raised by spin
wandering. Specifically it is found that, for an isolated, biaxial rotor whose
spin frequency walks randomly, HMM tracking of the F-statistic output from
coherent segments with duration T_drift = 10d over a total observation time of
T_obs = 1yr can detect signals with wave strains h0 > 2e-26 at a noise level
characteristic of the Advanced Laser Interferometer Gravitational Wave
Observatory (Advanced LIGO). For a biaxial rotor with randomly walking spin in
a binary orbit, whose orbital period and semi-major axis are known
approximately from electromagnetic observations, HMM tracking of the
Bessel-weighted F-statistic output can detect signals with h0 > 8e-26. An
efficient, recursive, HMM solver based on the Viterbi algorithm is
demonstrated, which requires ~10^3 CPU-hours for a typical, broadband (0.5-kHz)
search for the low-mass X-ray binary Scorpius X-1, including generation of the
relevant F-statistic input. In a "realistic" observational scenario, Viterbi
tracking successfully detects 41 out of 50 synthetic signals without spin
wandering in Stage I of the Scorpius X-1 Mock Data Challenge convened by the
LIGO Scientific Collaboration down to a wave strain of h0 = 1.1e-25, recovering
the frequency with a root-mean-square accuracy of <= 4.3e-3 Hz
Kalman-filter control schemes for fringe tracking. Development and application to VLTI/GRAVITY
The implementation of fringe tracking for optical interferometers is
inevitable when optimal exploitation of the instrumental capacities is desired.
Fringe tracking allows continuous fringe observation, considerably increasing
the sensitivity of the interferometric system. In addition to the correction of
atmospheric path-length differences, a decent control algorithm should correct
for disturbances introduced by instrumental vibrations, and deal with other
errors propagating in the optical trains. We attempt to construct control
schemes based on Kalman filters. Kalman filtering is an optimal data processing
algorithm for tracking and correcting a system on which observations are
performed. As a direct application, control schemes are designed for GRAVITY, a
future four-telescope near-infrared beam combiner for the Very Large Telescope
Interferometer (VLTI). We base our study on recent work in adaptive-optics
control. The technique is to describe perturbations of fringe phases in terms
of an a priori model. The model allows us to optimize the tracking of fringes,
in that it is adapted to the prevailing perturbations. Since the model is of a
parametric nature, a parameter identification needs to be included. Different
possibilities exist to generalize to the four-telescope fringe tracking that is
useful for GRAVITY. On the basis of a two-telescope Kalman-filtering control
algorithm, a set of two properly working control algorithms for four-telescope
fringe tracking is constructed. The control schemes are designed to take into
account flux problems and low-signal baselines. First simulations of the
fringe-tracking process indicate that the defined schemes meet the requirements
for GRAVITY and allow us to distinguish in performance. In a future paper, we
will compare the performances of classical fringe tracking to our Kalman-filter
control.Comment: 17 pages, 8 figures, accepted for publication in A&
Geometric Cross-Modal Comparison of Heterogeneous Sensor Data
In this work, we address the problem of cross-modal comparison of aerial data
streams. A variety of simulated automobile trajectories are sensed using two
different modalities: full-motion video, and radio-frequency (RF) signals
received by detectors at various locations. The information represented by the
two modalities is compared using self-similarity matrices (SSMs) corresponding
to time-ordered point clouds in feature spaces of each of these data sources;
we note that these feature spaces can be of entirely different scale and
dimensionality. Several metrics for comparing SSMs are explored, including a
cutting-edge time-warping technique that can simultaneously handle local time
warping and partial matches, while also controlling for the change in geometry
between feature spaces of the two modalities. We note that this technique is
quite general, and does not depend on the choice of modalities. In this
particular setting, we demonstrate that the cross-modal distance between SSMs
corresponding to the same trajectory type is smaller than the cross-modal
distance between SSMs corresponding to distinct trajectory types, and we
formalize this observation via precision-recall metrics in experiments.
Finally, we comment on promising implications of these ideas for future
integration into multiple-hypothesis tracking systems.Comment: 10 pages, 13 figures, Proceedings of IEEE Aeroconf 201
Approximate Dynamic Programming via Sum of Squares Programming
We describe an approximate dynamic programming method for stochastic control
problems on infinite state and input spaces. The optimal value function is
approximated by a linear combination of basis functions with coefficients as
decision variables. By relaxing the Bellman equation to an inequality, one
obtains a linear program in the basis coefficients with an infinite set of
constraints. We show that a recently introduced method, which obtains convex
quadratic value function approximations, can be extended to higher order
polynomial approximations via sum of squares programming techniques. An
approximate value function can then be computed offline by solving a
semidefinite program, without having to sample the infinite constraint. The
policy is evaluated online by solving a polynomial optimization problem, which
also turns out to be convex in some cases. We experimentally validate the
method on an autonomous helicopter testbed using a 10-dimensional helicopter
model.Comment: 7 pages, 5 figures. Submitted to the 2013 European Control
Conference, Zurich, Switzerlan
Integrated Approach to Airborne Laser Communication
Lasers offer tremendous advantages over RF communication systems in bandwidth and security, due to their ultra-high frequency and narrow spatial beamwidth. Atmospheric turbulence causes severe received power variations and high bit error rates (BERs) in airborne laser communication. Airborne optical communication systems require special considerations in size, complexity, power, and weight. Conventional adaptive optics systems correct for the phase only and cannot correct for strong scintillation, but here the two transmission paths are separated sufficiently so that the strong scintillation is \averaged out by incoherently summing up the two beams in the receiver. This requisite separation distance is derived for multiple geometries, turbulence conditions, and turbulence effects. Integrating multiple techniques into a system alleviates the deleterious effects of turbulence without bulky adaptive optics systems. Wave optics simulations show multiple transmitters, receiver and transmitter trackers, and adaptive thresholding significantly reduce the BER (by over 10,000 times)
Fusion of Head and Full-Body Detectors for Multi-Object Tracking
In order to track all persons in a scene, the tracking-by-detection paradigm
has proven to be a very effective approach. Yet, relying solely on a single
detector is also a major limitation, as useful image information might be
ignored. Consequently, this work demonstrates how to fuse two detectors into a
tracking system. To obtain the trajectories, we propose to formulate tracking
as a weighted graph labeling problem, resulting in a binary quadratic program.
As such problems are NP-hard, the solution can only be approximated. Based on
the Frank-Wolfe algorithm, we present a new solver that is crucial to handle
such difficult problems. Evaluation on pedestrian tracking is provided for
multiple scenarios, showing superior results over single detector tracking and
standard QP-solvers. Finally, our tracker ranks 2nd on the MOT16 benchmark and
1st on the new MOT17 benchmark, outperforming over 90 trackers.Comment: 10 pages, 4 figures; Winner of the MOT17 challenge; CVPRW 201
Real-time data analysis at the LHC: present and future
The Large Hadron Collider (LHC), which collides protons at an energy of 14
TeV, produces hundreds of exabytes of data per year, making it one of the
largest sources of data in the world today. At present it is not possible to
even transfer most of this data from the four main particle detectors at the
LHC to "offline" data facilities, much less to permanently store it for future
processing. For this reason the LHC detectors are equipped with real-time
analysis systems, called triggers, which process this volume of data and select
the most interesting proton-proton collisions. The LHC experiment triggers
reduce the data produced by the LHC by between 1/1000 and 1/100000, to tens of
petabytes per year, allowing its economical storage and further analysis. The
bulk of the data-reduction is performed by custom electronics which ignores
most of the data in its decision making, and is therefore unable to exploit the
most powerful known data analysis strategies. I cover the present status of
real-time data analysis at the LHC, before explaining why the future upgrades
of the LHC experiments will increase the volume of data which can be sent off
the detector and into off-the-shelf data processing facilities (such as CPU or
GPU farms) to tens of exabytes per year. This development will simultaneously
enable a vast expansion of the physics programme of the LHC's detectors, and
make it mandatory to develop and implement a new generation of real-time
multivariate analysis tools in order to fully exploit this new potential of the
LHC. I explain what work is ongoing in this direction and motivate why more
effort is needed in the coming years.Comment: Contribution to the proceedings of the HEPML workshop NIPS 2014. 20
pages, 5 figure
Improved detection limits using a hand-held optical imager with coregistration capabilities
Optical imaging is emerging as a non-invasive and non-ionizing method for breast cancer diagnosis. A hand-held optical imager has been developed with coregistration facilities towards flexible imaging of different tissue volumes and curvatures in near real-time. Herein, fluorescence-enhanced optical imaging experiments are performed to demonstrate deeper target detection under perfect and imperfect (100:1) uptake conditions in (liquid) tissue phantoms and in vitro. Upon summation of multiple scans (fluorescence intensity images), fluorescent targets are detected at greater depths than from single scan alone
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