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Heuristic Methods for the Antenna-Constrained Beam Layout Optimization on Multibeam Broadcasting Mission
No full textMeilleur papier étudiant de la conférence ICORES-2024International audienceIn this paper, we tackle a payload design problem for a broadcasting mission where a telecommunication satellite must provide television services to distinct regions defined as polygons. To cover these polygons, several telecommunication beams are emitted by the satellite, with the risk that they mutually degrade their performance while also being hard to accommodate mechanically on the spacecraft. The problem is to determine a set of non-conflicting beams that cover all the regions and optimize a performance metric related to the sizes of the beams used. The first method is a matheuristic exploiting iterative solv of an ILP model. The second method, called the merge-and-split heuristic, is inspired by Iterated Local Search and reuses a fast graph coloring algorithm to analyze conflicts among selected beams. These two methods are evaluated on realistic instances, the largest one involving more than one hundred regions to cover.De nos jours, les satellites de télécommunication deviennent de plus en plus complexes afin d'augmenter la quantité et la qualité des services fournis par les opérateurs. Cela rend la phase de conception de ces satellites très difficile et les fabricants doivent faire face à des problèmes hautement combinatoires pour atteindre les performances requises. Dans cet article, nous examinons l'un de ces problèmes combinatoires. Ce dernier concerne un satellite qui doit fournir des services de télévision à des régions distinctes. Pour cela, le satellite doit émettre un ensemble de faisceaux pour couvrir un ensemble de polygones à la surface de la Terre, sachant que deux faisceaux utilisant la même bande de fréquence doivent être suffisamment séparés pour éviter tout conflit. Nous proposons ici deux méthodes pour résoudre ce problème, c'est-à-dire pour déterminer un ensemble de faisceaux non conflictuels qui couvrent tous les polygones et optimisent une mesure de performance liée à la taille des faisceaux utilisés. La première méthode est une matheuristique exploitant des résolutions itératives d'un modèle ILP. La seconde méthode, appelée heuristique 'merge-and-split', s'inspire de la recherche locale itérée et utilise un algorithme rapide de coloration de graphe pour analyser les conflits entre les faisceaux sélectionnées. Ces deux méthodes sont évaluées sur des instances réalistes, la plus grande impliquant plus d'une centaine de polygones à couvrir
The MICROSCOPE space mission to test the Equivalence Principle
No full textThe MICROSCOPE mission tested the Weak Equivalence Principle (WEP) with an unprecedented precision of order 10-15 , two orders of magnitude better than the previous best lab experiments. While the WEP, the cornerstone of General Relativity (GR), does not sway, the decade-long problems faced by fundamental physics stay still: how can we unify GR with the Standard Model, and how can we explain the acceleration of the cosmological expansion? As most beyond-GR models predict a violation of the WEP, albeit at an unknown level, it remains critical to even better test the WEP. In this paper, we review the MICROSCOPE mission, give its final constraint on the WEP, and build on its experimental limitations to show how we could improve them by a further two-order of magnitude in the precision of the test of the WEP
Wigner measures of electromagnetic waves in bianisotropic heterogeneous media
No full textInternational audienceWe study the propagation of high-frequency electromagnetic waves in randomly heterogeneous bianisotropic media with dissipative properties. For that purpose we consider randomly fluctuating optical responses of such media with correlation lengths comparable to the typical wavelength of the waves. Although the fluctuations are weak, they induce multiple scattering over long propagation times and/or distances such that the waves end up travelling in many different directions with mixed polarizations. We derive the dispersion and evolution properties of the Wigner measure of the electromagnetic fields, which describes their angularly-resolved energy density in this propagation regime. The analysis starts from Maxwell's equations with general constitutive equations. We first ignore the random fluctuations of the optical response and obtain uncoupled transport equations for the components of the Wigner measure on the different propagation modes (polarizations). Then we use a multi-scale expansion of the Wigner mesure to obtain the radiative transfer equations satisfied by these components when the fluctuations are no longer ignored. The radiative transfer equations are coupled through their collisional parts, which account for the scattering of waves by the random fluctuations and their possible changes in polarization. The collisional kernels describing these processes depend on the power and cross-power spectral densities of the fluctuations at the wavelength scale. The overall derivation is based on the interpretation of Wigner transforms and Wigner measures in terms of semiclassical pseudo-differential operators in their standard quantization
Modèle d'ordre réduit de structures flexibles non-linéaires géométriques pour des applications d'interaction fluide-structure
No full textInternational audienceThis paper deals with the numerical computation, via a reduced order models (ROM), of the vibrations of geometrically nonlinear structures triggered by the aeroelastic coupling with a fluid flow. The formulation of the ROM proposed in this paper is based on the projection on a basis of reduced dimension enhanced with dual modes. An explicit expression of the projected nonlinear forces is computed in a non-intrusive way based on the Implicit Condensation method. The resulting ROM is an improvement of the classical ICE method since the effects of membrane stretching are taken into account in the resolution of the dynamic equation of motion. Such a ROM aims to be adapted to follower aerodynamic unsteady loads. The construction of the ROM is first detailed and validated under several load cases on a Euler-Bernoulli beam with von Kármán hypothesis. Then a fluid-structure partitioned coupling on a two-dimensional example involving vortex-induced vibrations is considered to demonstrate the capability of such ROM to replace a nonlinear FE solver. In this paper, the limitations of the ICE method are highlighted in the examples treated, while the ROM proposed overcomes such limitations and captures accurately the dynamics.Cet article traite du calcul numérique, via un modèle d'ordre réduit (ROM), des vibrations de structures avec non-linéarités géométriques sollicitées par le couplage aéroélastique avec un écoulement de fluide. La formulation du ROM proposée dans cet article est basée sur la projection sur une base de dimension réduite enrichie avec des modes duaux. Une expression explicite des forces non linéaires projetées est calculée de manière non intrusive sur la base de la méthode de condensation implicite. Le ROM qui en résulte est une amélioration de la méthode ICE classique puisque les effets de l'étirement de la membrane sont pris en compte dans la résolution de l'équation dynamique du mouvement. Un tel ROM est adapté aux charges aérodynamiques instationnaires et suiveuses. Dans cet article, la construction du ROM est d'abord détaillée et validée dans plusieurs cas de charge sur une poutre d'Euler-Bernoulli avec l'hypothèse de von Kármán. Ensuite, un couplage fluide-structure sur un exemple bidimensionnel impliquant des vibrations induites par des vortex est considéré pour démontrer la capacité de ce ROM à remplacer un solveur d'éléments finis non linéaires. Dans cet article, les limitations de la méthode ICE sont mises en évidence dans les exemples traités, tandis que le ROM proposé surmonte ces limitations et capture avec précision la dynamique
Localization and Monitoring of workload, fatigue and their combined effects in the brain
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Réanalyse climatique de la ceinture de radiation électronique dynamique a long terme, a l'aide d'un schéma numérique spécifique
No full textInternational audienceClimate reanalysis represents a valuable tool to improve radiation belt specification models such as ONERA’s global radiation Earth environment (GREEN) model. Generally, it consists in the elaboration of a long-term database of best estimates by the assimilation of in situ data in radiation belt theoretical models such as ONERA’s Salammbô code. However, their construction needs strong confidence in the code’s physical description, but also in its numerical resolution. We present in this article the new numerical solver of Salammbô. The latter provides stronger accuracy, robustness, and efficiency arguments that will ensure more reliable estimations from Salammbô. In addition, we will show how these improvements will open the path to the improvement of the GREEN model, through climate reanalysis.La réanalyse climatique représente un outil précieux pour améliorer les modèles de spécification de la ceinture de radiation tels que le modèle GREEN (Global Radiation Earth Environment) de l'ONERA. En général, elle consiste en l'élaboration d'une base de données à long terme des meilleures estimations par l'assimilation de données insitu dans des modèles théoriques de la ceinture de radiation tels que le code Salammbô de l'ONERA. Cependant, leur construction nécessite une grande confiance dans la description physique du code, mais aussi dans sa résolution numérique. C'est pourquoi nous présentons dans cet article le nouveau solveur numérique de Salammbô. Ce dernier fournit des arguments de précision et de robustesse plus solides qui assureront la fondation d'une base de données de réanalyse fiable pour améliorer le modèle GREEN
Effects of the LGS geometry on the Shack-Hartmann wavefront sensor and the Pyramid wavefront sensor
No full textInternational audienceIn this work we study the effects of the laser guide star (LGS) on the measurements of both the Shack-Hartmann wavefront sensor (SHWFS) and the Pyramid wavefront sensor (PWFS). We started by describing the LGS geometry and the general effects on each wavefront sensor. Then, we introduced a statistical analysis to predict the centroiding variance for the SHWFS when using an LGS, which we tested for read-out noise and photon noise. We found good agreement between end-to-end simulations and the predictions of the model. We found that the centroiding variance, as expected, follows closely the elongation of the LGS, with the X and Y centroiding evolving each according to the LGS geometry. For the PWFS, we used a convolutional model to compute sensitivity maps. With these maps we could observe that the size of the LGS greately decreases the sensitivity in the low frequencies. We could also obtain a better definition of the size of the LGS, which takes into account the depth of field of the telescope, which can be used to predict the sensitivity of the instrument by computing an equivalent modulation radius equivalent to the LGS size
High-performance AO systems: impact of the chromatic pupil shift in the case of RISTRETTO
No full textInternational audienceRISTRETTO is an instrument developed by the University of Geneva and the Laboratoire d’Astrophysique de Marseille, designed to characterise temperate rocky planets around M-dwarfs, such as Proxima b, using reflected light. Achieving this requires a contrast of approximately 10−7, necessitating a highly efficient extreme adaptive optics (AO) system. The high performance targeted by the RISTRETTO instrument translates into a very restrictive error budget for the AO system. New error terms that used to be neglected for previous AO systems must be carefully evaluated such as the Chromatic Pupil Shift (CPS). In this proceeding, we introduce the concept of CPS and we propose a method to compute it. Then, we inject this errors into-end to-end simulation to obtain its value. We found that the chromatic shift creates, for a science wavelength of 750nm and a zenith angle of 38°, a wavefront error of 5.24nm RMS at a 1400nm wavefront sensing wavelength and 2.53nm at 925nm. We conclude that the chromatic pupil shift is twice lower than the temporal error for a close-loop at 5kHz with a gain of 0.4 and an one frame delay but is in the order of other lower error terms such as aliasing and we must consider it in RISTRETTO’s error budget
Compared Influence of a Lubricant on Mode I and Mode II Fatigue Crack Growth Kinetics in a Gear Steel
No full textInternational audienceThe influence of a helicopter gearbox lubricant on Mode I or Mode II fatigue crack growth in 16NCD13 steel was characterized through tests performed on single-edge notched tensile samples loaded in tension–compression and on cruciform samples submitted to reversed shear plus static biaxial compression, respectively. In Mode I, the lubricant reduced the growth rate at low ΔKI and increased the threshold , while in Mode II, it accelerated crack growth at low ΔKIIeffective, which was not only due to a reduction in crack face friction. The upward convective flow of lubricant carrying debris exuding from the crack, a modification in oil aspect and properties, and chemical analyses near the crack front suggest that a temperature-induced degradation of the lubricant leads to a corrosive attack of the metal, which accelerates crack growth. A tribologically transformed structure is observed along the lips of cracks grown in Mode II with normal compression in oil
Aspect ratio-dependent etching in silicon using XeF<sub>2</sub>: experimental investigation and comparative analysis with dry etching methods
No full textInternational audienceSilicon machining plays a crucial role in shaping three-dimensional structures for micro-electro-mechanical systems applications. This study investigates aspect ratio dependent etching (ARDE) across various silicon etching processes, with a particular focus on Xenon Difluoride etching, in comparison to reactive ion etching (RIE) and Deep RIE . By exploring different etching parameters, the study highlights the presence of ARDE in both plasma and non-plasma etching processes. Additionally, it is demonstrated that ARDE can be modeled by a saturating exponential function through experimental adjustment of parameters, enabling the estimation of etching profiles