Best network chirplet-chain: Near-optimal coherent detection of unmodeled gravitation wave chirps with a network of detectors

The searches of impulsive gravitational waves (GW) in the data of the ground-based interferometers focus essentially on two types of waveforms: short unmodeled bursts and chirps from inspiralling compact binaries. There is room for other types of searches based on different models. Our objective is to fill this gap. More specifically, we are interested in GW chirps with an arbitrary phase/frequency vs. time evolution. These unmodeled GW chirps may be considered as the generic signature of orbiting/spinning sources. We expect quasi-periodic nature of the waveform to be preserved independent of the physics which governs the source motion. Several methods have been introduced to address the detection of unmodeled chirps using the data of a single detector. Those include the best chirplet chain (BCC) algorithm introduced by the authors. In the next years, several detectors will be in operation. The joint coherent analysis of GW by multiple detectors can improve the sight horizon, the estimation of the source location and the wave polarization angles. Here, we extend the BCC search to the multiple detector case. The method amounts to searching for salient paths in the combined time-frequency representation of two synthetic streams. The latter are time-series which combine the data from each detector linearly in such a way that all the GW signatures received are added constructively. We give a proof of principle for the full sky blind search in a simplified situation which shows that the joint estimation of the source sky location and chirp frequency is possible.Comment: 22 pages, revtex4, 6 figure

Estimation of the underwater acoustic channel

This paper deals with the processing of real data recorded during the Oceanic Acoustic Tomography experiments « PASSTIME » carried out by the SHOM in October 2005. The transmitted signal waveform of interest is assumed to be known at the receiver side, and we address the problem of the channel impulse response estimation in a bayesian framework : the channel is modeled via a Bernoulli-Gaussian process, and estimation is achieved via a Gibbs sampler. As the processing of PASSTIME data emphasizes distorsions of the transmitted signal, we add a preliminary processing step for estimating the distorded waveform by means of a simple iterative procedure that alternatively estimates the channel impulse response via a Matching Pursuit approach, and the waveform itself by maximizing the likelihood. The proposed algorithms are applied to PASSTIME records, and prove to be very satisfactory.Dans cet article, nous nous intéressons au traitement des données réelles enregistrées au cours de la campagne de Tomographie Acoustique Océanique « PASSTIME » effectuée par le SHOM en Octobre 2005. Pour une forme d'onde transmise connue du récepteur, nous proposons de présenter le problème de l'estimation de la réponse impulsionnelle du canal acoustique sous-marin sous la forme d'un problème bayésien. Le canal est alors modélisé par un a priori Bernoulli-Gaussien, et l'estimateur est évalué à partir de simulations générées au moyen d'un algorithme de Gibbs. Comme le traitement des données PASSTIME met en évidence d'importantes distorsions subies par le signal, nous effectuons l'estimation préalable de la forme d'onde distordue au moyen d'un algorithme alternant l'estimation du canal par une méthode de type Matching Pursuit, et l'estimation de la forme d'onde au sens du maximum de vraisemblance. L'application aux données PASSTIME conduit à des résultats très satisfaisants

Multitarget likelihood for Track-Before-Detect applications with amplitude fluctuations

International audienceTrack-Before-Detect methods jointly detect and track one or several targets from raw sensor measurements. They often require the computation of the measurement likelihood conditionally to the hidden state that depends on the complex amplitudes of the targets. Since these amplitudes are unknown and fluctuate over time this likelihood must be marginalized over the complex amplitude (i.e. phase and modulus). It has been demonstrated in [1] that this marginalization can be done analytically over the phase in the monotarget case. In this article, we first propose to extend the marginalization to the modulus in a monotarget setting, and we show that closed-forms can be obtained for fluctuations of type Swerling 1 and 3. Second, we demonstrate that, in a multitarget setting, a closed-form can be obtained for the Swerling 1 case. For Swerling 0 and 3 models, we propose some approximation to alleviate the computation. Since many articles consider the case of squared modulus measurements, we also consider this specific case in mono and multitarget settings with Swerling 0, 1 and 3 fluctuations. Finally, we compare the performance in estimation and detection for the different cases studied and we show the gain, both in detection and estimation, of the complex measurement method over the squared modulus method, for any fluctuation model

The effects of handling and group size on welfare of pigs in lairage and their influence on stomach weight, carcass microbial contamination and meat quality

At unloading and on the way to stunning, 800 barrows were exposed to either gentle handling (GH: slowly with a plastic board or whip) or rough handling (RH: quickly with an electric prod). Pigs were kept in large or small groups (30 or 10 pigs) during lairage. Compared with GH, RH increased climbing (P < 0.05), slipping (P < 0.01) and turning around (P < 0.001) behaviours during unloading, and climbing (P < 0.05) on the way to stunning. RH also reduced drinking behaviour during lairage (P < 0.01). Pigs kept in large groups were observed more often standing (P < 0.05) and fighting (P < 0.001) than pigs kept in small groups, but, in contrast, had a slightly lower level of urinary cortisol at slaughter. Stomach weight and microbial contamination at slaughter were not affected by treatments. RH tended to increase skin bruise score on the carcass (P < 0.06) and produced more exudative meat (P < 0.05). In conclusion, the response of pigs to the two specific stressors applied prior to slaughter in this study did not seem to contribute to stomach weight variation at slaughter, but it did influence pork quality. Key words: Pigs, pre-slaughter handling, group size, stress, stomach weight, microbial contamination, behaviour, meat qualit

Generalization and Improvement of the Levenshtein Lower Bound for Aperiodic Correlation

This paper deals with lower bounds on aperiodic correlation of sequences. It intends to solve two open questions. The first one is on the validity of the Levenshtein bound for a set of sequences other than binary sequences or those over the roots of unity. Although this result could be a priori extended to polyphase sequences, a formal demonstration is presented here, proving that it does actually hold for these sequences. The second open question is on the possibility to find a bound tighter than Welch’s, in the case of a set consisting of two sequences M = 2. By including the specific structure of correlation sequences, a tighter lower bound is introduced for this case. Besides, this method also provides in the cases M = 3 and M = 4 a tighter bound than the up-to-now tightest bound provided by Liu et al

A Joint Optimization for Coherent MIMO Radar

This paper deals with the optimization of a set of transmitted sequences and their associated mismatched filters, for the coherent MIMO radar. A minimization problem may be considered, with an objective function that measures for instance the correlations within the set. In the literature, it is usually solved alternatively, meaning that the optimization is performed on the sequences while the mismatched filters are set, and vice-versa. In this paper, an iterative method that proceeds jointly is introduced. Sequences are derived with a gradient descent, and mismatched filters are chosen in such a way that they are optimal in the ISL (Integrated Sidelobe Level) sense. Simulations show interesting results, as a joint optimization performs better than a separate one, even with relatively short sequences

Optimization Methods for Solving the Low Autocorrelation Sidelobes Problem

In this paper, a discussion is made on the optimization methods that can solve the low autocorrelation sidelobes problem for polyphase sequences. This paper starts with a description and a comparison of two algorithms that are commonly used in the literature: a stochastic method and a deterministic one (a gradient descent). Then, an alternative method based on the Random Walk Metropolis-Hastings algorithm is proposed, that takes the gradient as a search direction. It provides better results than a steepest descent alone. Finally, this autocorrelation question is handled differently, considering a mismatched filter. We will see that a mismatched filter performs impressively well on optimized sequences

Phase Code Optimization for Coherent MIMO Radar Via a Gradient Descent

In this paper, a gradient descent method is used to build radar waveform sequences with good autocorrelation and/or cross-correlation. The approach we propose is based on the energy, a function that measures the sidelobe level of a sequence, and its gradient. Then, we extend and apply it to the optimization of the coherent MIMO (Multiple Input Multiple Output) ambiguity function. We suggest to look for the transmitted signals that reduce the autocorrelation sidelobe level of the signal transmitted by the whole antenna. The obtained results, highlighted by the low sidelobe level of the ambiguity function, seem promising

Virgo calibration and reconstruction of the gravitational wave strain during VSR1

Virgo is a kilometer-length interferometer for gravitational waves detection located near Pisa. Its first science run, VSR1, occured from May to October 2007. The aims of the calibration are to measure the detector sensitivity and to reconstruct the time series of the gravitational wave strain h(t). The absolute length calibration is based on an original non-linear reconstruction of the differential arm length variations in free swinging Michelson configurations. It uses the laser wavelength as length standard. This method is used to calibrate the frequency dependent response of the Virgo mirror actuators and derive the detector in-loop response and sensitivity within ~5%. The principle of the strain reconstruction is highlighted and the h(t) systematic errors are estimated. A photon calibrator is used to check the sign of h(t). The reconstructed h(t) during VSR1 is valid from 10 Hz up to 10 kHz with systematic errors estimated to 6% in amplitude. The phase error is estimated to be 70 mrad below 1.9 kHz and 6 micro-seconds above.Comment: 8 pages, 8 figures, proceedings of Amaldi 8 conference, to be published in Journal of Physics Conference Series (JPCS). Second release: correct typo