456 research outputs found
The averaged control system of fast oscillating control systems
For control systems that either have a fast explicit periodic dependence on
time and bounded controls or have periodic solutions and small controls, we
define an average control system that takes into account all possible
variations of the control, and prove that its solutions approximate all
solutions of the oscillating system as oscillations go faster. The dimension of
its velocity set is characterized geometrically. When it is maximum the average
system defines a Finsler metric, not twice differentiable in general. For
minimum time control, this average system allows one to give a rigorous proof
that averaging the Hamiltonian given by the maximum principle is a valid
approximation.Comment: (2012
A conjugate gradient algorithm for the astrometric core solution of Gaia
The ESA space astrometry mission Gaia, planned to be launched in 2013, has
been designed to make angular measurements on a global scale with
micro-arcsecond accuracy. A key component of the data processing for Gaia is
the astrometric core solution, which must implement an efficient and accurate
numerical algorithm to solve the resulting, extremely large least-squares
problem. The Astrometric Global Iterative Solution (AGIS) is a framework that
allows to implement a range of different iterative solution schemes suitable
for a scanning astrometric satellite. In order to find a computationally
efficient and numerically accurate iteration scheme for the astrometric
solution, compatible with the AGIS framework, we study an adaptation of the
classical conjugate gradient (CG) algorithm, and compare it to the so-called
simple iteration (SI) scheme that was previously known to converge for this
problem, although very slowly. The different schemes are implemented within a
software test bed for AGIS known as AGISLab, which allows to define, simulate
and study scaled astrometric core solutions. After successful testing in
AGISLab, the CG scheme has been implemented also in AGIS. The two algorithms CG
and SI eventually converge to identical solutions, to within the numerical
noise (of the order of 0.00001 micro-arcsec). These solutions are independent
of the starting values (initial star catalogue), and we conclude that they are
equivalent to a rigorous least-squares estimation of the astrometric
parameters. The CG scheme converges up to a factor four faster than SI in the
tested cases, and in particular spatially correlated truncation errors are much
more efficiently damped out with the CG scheme.Comment: 24 pages, 16 figures. Accepted for publication in Astronomy &
Astrophysic
Aider les élèves à produire des textes
Souvent délaissée par beaucoup d'enseignants, malgré son importance capitale pour l'élève et le développement de son savoir et de ses compétences, la production de texte est un défi à l'école primaire. C'est pourquoi nous nous sommes demandé comment aider les élèves à écrire, notamment en nous questionnant sur les outils à convoquer et sur la manière de les utiliser. Après une analyse du cadre théorique et une description de notre séquence d'expérimentation, nous analyserons les données récoltées afin de mettre en avant une pratique orientée des outils décrits. Inducteurs, pédagogie de projet, tableau de critère, grille de relecture sont les media dont nous essayerons de démêler les influences sur les capacités rédactionnelles des élèves
Les transferts orbitaux à faible poussée : optimalité et stabilisation
This thesis presents a study of the low thrust two body control system and in particular orbital transfer problems. After a controllability study, we focus on the link between the optimal control in minimum time and the feedbacks built from the Jurdjevic-Quinn method. Numerical simulations show that the Jurdjevic-Quinn feedbacks can be close to the optimal command. In order to understand this property we study an average control system whose trajectories approximate these of low thrust systems. This technique allows us to give a positive answer to a conjecture on the asymptotic behavior of the minimum time. Moreover it is a promising way to build an efficient Lyapunov function associated to the Jurdjevic-Quinn method: a feedback close to the optimal controlCette thèse présente une étude du système à deux corps contrôlé en poussée faible, et en particulier, des problèmes de transferts orbitaux. Après une étude de contrôlabilité, nous nous focalisons sur le lien entre la commande optimale en temps minimum et les contrôles en boucle fermée construits à partir de la méthode de Jurdjevic-Quinn. Des simulations numériques montrent que les commandes Jurdjevic-Quinn peuvent être proches de la commande temps minimum. Pour comprendre cette propriété nous étudions un système contrôlé moyen dont les trajectoires approchent celles des systèmes à faible poussée. Cette technique nous permet de répondre à une conjecture concernant le comportement asymptotique du temps minimum quand la poussée tends vers zéro. D'autre part elle constitue une piste prometteuse pour construire une fonction de Lyapunov associée la méthode de Jurdjevic-Quinn efficace: un contrôle en boucle fermée proche de la commande optimale
A census of Oph candidate members from Gaia DR2
The Ophiuchus cloud complex is one of the best laboratories to study the
earlier stages of the stellar and protoplanetary disc evolution. The wealth of
accurate astrometric measurements contained in the Gaia Data Release 2 can be
used to update the census of Ophiuchus member candidates. We seek to find
potential new members of Ophiuchus and identify those surrounded by a
circumstellar disc. We constructed a control sample composed of 188 bona fide
Ophiuchus members. Using this sample as a reference we applied three different
density-based machine learning clustering algorithms (DBSCAN, OPTICS, and
HDBSCAN) to a sample drawn from the Gaia catalogue centred on the Ophiuchus
cloud. The clustering analysis was applied in the five astrometric dimensions
defined by the three-dimensional Cartesian space and the proper motions in
right ascension and declination. The three clustering algorithms systematically
identify a similar set of candidate members in a main cluster with astrometric
properties consistent with those of the control sample. The increased
flexibility of the OPTICS and HDBSCAN algorithms enable these methods to
identify a secondary cluster. We constructed a common sample containing 391
member candidates including 166 new objects, which have not yet been discussed
in the literature. By combining the Gaia data with 2MASS and WISE photometry,
we built the spectral energy distributions from 0.5 to 22\microm for a subset
of 48 objects and found a total of 41 discs, including 11 Class II and 1 Class
III new discs. Density-based clustering algorithms are a promising tool to
identify candidate members of star forming regions in large astrometric
databases. If confirmed, the candidate members discussed in this work would
represent an increment of roughly 40% of the current census of Ophiuchus.Comment: A&A, Accepted. Abridged abstrac
Heterogeneous Clutter Models for Change Detection in PolSAR Imagery
International audienceThe new generation of Synthetic Aperture Radar (RADARSAT-2, TerraSAR-X, ALOS, . . . ) allows us to capture Earth surface images with very high resolution. Therefore the possibility to characterize objects has become more and more attainable. Moreover, the short revisit time propertie of these satellites enables the development of techniques of change detection and their applications. Spherically Invariant Random Vector (SIRV) model was designed specifically for the analysis of heterogeneous clutters in high resolution radar images. In this paper, we propose four algorithms of change detection based on different criteria including: Gaussian (sample covariance matrix estimator), Gaussian (fixed point estimator), Fisher texture-based and KummerU-based (Fisher distributed texture)
Hierarchical Segmentation of Polarimetric SAR Images Using Heterogeneous Clutter Models
International audienceIn this paper, heterogeneous clutter models are used to describe polarimetric synthetic aperture radar (PolSAR) data. The KummerU distribution is introduced to model the PolSAR clutter. Then, a detailed analysis is carried out to evaluate the potential of this new multivariate distribution. It is implemented in a hierarchical maximum likelihood segmentation algorithm. The segmentation results are shown on both synthetic and high-resolution PolSAR data at the X- and L-bands. Finally, some methods are examined to determine automatically the "optimal" number of segments in the final partition
Performance of the maximum likelihood estimators for the parameters of multivariate generalized Gaussian distributions
International audienceThis paper studies the performance of the maximum likelihood estimators (MLE) for the parameters of multivariate generalized Gaussian distributions. When the shape parameter belongs to ]0,1[, we have proved that the scatter matrix MLE exists and is unique up to a scalar factor. After providing some elements about this proof, an estimation algorithm based on a Newton-Raphson recursion is investigated. Some experiments illustrate the convergence speed of this algorithm. The bias and consistency of the scatter matrix estimator are then studied for different values of the shape parameter. The performance of the shape parameter estimator is finally addressed by comparing its variance to the Cramér-Rao bound
- …