{"id":153,"date":"2010-11-18T11:05:24","date_gmt":"2010-11-18T10:05:24","guid":{"rendered":"https:\/\/samovar2022.int-evry.fr\/index.php\/2010\/11\/18\/soutenance-these-de-ihssan-ghannoum\/"},"modified":"2020-09-04T18:47:17","modified_gmt":"2020-09-04T16:47:17","slug":"soutenance-these-de-ihssan-ghannoum","status":"publish","type":"post","link":"https:\/\/samovar.telecom-sudparis.eu\/index.php\/2010\/11\/18\/soutenance-these-de-ihssan-ghannoum\/","title":{"rendered":"SOUTENANCE : th\u00e8se de Ihssan Ghannoum"},"content":{"rendered":"

mercredi 22<\/strong> septembre 2010
\n\u00ab\u00a0Etudes d’outils de calcul de propagation Radar en milieu complexe (milieu urbain, pr\u00e9sence de multi-trajets …) par des techniques de Lancer de Faisceaux Gaussiens\u00a0\u00bb<\/strong><\/p>\n

mercredi 22<\/strong> septembre 2010 \u00e0 14h00 sur le campus de T\u00e9l\u00e9com SudParis, en salle C06.<\/p>\n

Jury<\/strong> :<\/p>\n

– M. Rapha\u00ebl Gillard, IETR, INSA Rennes (rapporteur)<\/p>\n

– M. J\u00e9r\u00f4me Sokoloff, Universit\u00e9 Paul Sabatier, Toulouse (rapporteur) <\/p>\n

– M. Marc H\u00e9lier, Universit\u00e9 Paris 6 (examinateur)<\/p>\n

– M. Gilles Beauquet, Thal\u00e8s Air Systems, Limours (co-encadrant, examinateur)<\/p>\n

– Mme. Christine Letrou, T\u00e9l\u00e9com SudParis (directeur de th\u00e8se, examinateur).<\/p>\n

R\u00e9sum\u00e9<\/strong> :<\/p>\n

L’objectif de ce travail de th\u00e8se est d’enrichir la formulation du Lancer de Faisceaux Gaussiens (LFG) et de tester sa capacit\u00e9 \u00e0 r\u00e9pondre \u00e0 certains des besoins actuels en calculs de propagation dans le domaine du Radar terrestre. <\/p>\n

Le LFG est envisag\u00e9 comme une alternative possible aux m\u00e9thodes classiques (Equation Parabolique, m\u00e9thodes de rayons) en environnement complexe urbanis\u00e9, en particulier en pr\u00e9sence d’obstacles lat\u00e9raux, avec une cible situ\u00e9e en non visibilit\u00e9. <\/p>\n

La m\u00e9thode de LFG \u00ab\u00a0de base\u00a0\u00bb, qui utilise des expressions analytiques obtenues par approximation paraxiale, permet des calculs de propagation rapides en environnements complexes, sans probl\u00e8mes de caustiques.<\/p>\n

Elle conduit \u00e0 des r\u00e9sultats de pr\u00e9cision satisfaisante dans le domaine millim\u00e9trique, par exemple pour des calculs de champs intra-b\u00e2timents. <\/p>\n

Aux fr\u00e9quences plus basses comme celles utilis\u00e9es en Radar terrestre, elle est limit\u00e9e par une prise en compte trop approximative des effets de diffraction et par l’\u00e9largissement spatial des faisceaux gaussiens au regard des dimensions des obstacles. La th\u00e9orie des frames est utilis\u00e9e dans cette th\u00e8se pour d\u00e9passer ces limites.<\/p>\n

La th\u00e9orie des frames fournit un cadre rigoureux pour la d\u00e9composition initiale du champ rayonn\u00e9 en faisceaux gaussiens, et permet de calibrer le nombre et les directions des faisceaux \u00e0 lancer. <\/p>\n

Dans ce travail de th\u00e8se, l?emploi de frames de fen\u00eatres gaussiennes pour d\u00e9composer des distributions de champs ou de sources \u00e9quivalentes est g\u00e9n\u00e9ralis\u00e9 aux distributions de champs incidents sur des plans ou des portions de plans, choisis en fonction des obstacles rencontr\u00e9s et des distances parcourues. <\/p>\n

Les champs rayonn\u00e9s \u00e0 partir de ces plans sont alors obtenus par sommation des faisceaux gaussiens lanc\u00e9s depuis ces frames dits de \u00ab\u00a0re-d\u00e9composition\u00a0\u00bb.<\/p>\n

Les transformations de faisceaux gaussiens par des obstacles de taille limit\u00e9e sont ainsi trait\u00e9es par red\u00e9composition : les faisceaux incidents partiellement intercept\u00e9s par des surfaces limit\u00e9es sont \u00ab\u00a0re-d\u00e9compos\u00e9s\u00a0\u00bb
\nsuccessivement sur deux frames de re-d\u00e9composition, \u00e0 fen\u00eatres \u00ab\u00a0\u00e9troites\u00a0\u00bb puis \u00ab\u00a0larges\u00a0\u00bb, d\u00e9finis dans les plans de ces surfaces. <\/p>\n

Le frame \u00e0 fen\u00eatres \u00ab\u00a0\u00e9troites\u00a0\u00bb permet de traiter les discontinuit\u00e9s physiques, tandis que le frame \u00e0 fen\u00eatres \u00ab\u00a0larges\u00a0\u00bb permet de propager les champs transform\u00e9s sous la forme de faisceaux \u00ab\u00a0collimat\u00e9s\u00a0\u00bb.<\/p>\n

Dans cette th\u00e8se, nous pr\u00e9sentons une formulation de ces re-d\u00e9compositions permettant une mise en oeuvre num\u00e9riquement efficace, gr\u00e2ce \u00e0 des expressions analytiques approch\u00e9es des coefficients de frame pour la premi\u00e8re d\u00e9composition, et des \u00e9l\u00e9ments de la matrice de changement de frame pour la seconde. <\/p>\n

Cette formulation est mise en oeuvre num\u00e9riquement, et l’influence de diff\u00e9rents param\u00e8tres sur la pr\u00e9cision des re-d\u00e9compositions est analys\u00e9e. Finalement, l’algorithme de LFG enrichi de ces re-d\u00e9compositions successives est utilis\u00e9 dans un sc\u00e9nario simplifi\u00e9 proche de situations rencontr\u00e9es en propagation Radar terrestre.<\/p>\n

Abstract:<\/p>\n

In this work the Gaussian Beam Shooting (GBS) algorithm is complemented with new original formulations, and the ability of this \u00ab\u00a0augmented\u00a0\u00bb GBS algorithm to address specific problems encountered in electromagnetic field computations for ground-based Radar applications is tested. GBS is considered as an alternative to methods (Parabolic Equation, ray based methods) currently used for such computations in complex urban environments, especially when lateral obstacles and Non-Line-Of-Sight (NLOS) targets are involved. The \u00ab\u00a0basic\u00a0\u00bb
\nGBS algorithm makes use of analytical expressions obtained through paraxial approximations. It allows to perform fast computations in complex environments, without suffering from any caustics problems. Reasonably accurate results have been obtained with this method in the millimetric range, e.g. for indoor field calculations.
\nAt lower frequencies, such as used in ground Radar systems, \u00ab\u00a0basic\u00a0\u00bb GBS cannot model diffraction effects accurately enough, and Gaussian beam width with respect to obstacle dimensions becomes a problem after some propagation distance. Frame theory is used in this PhD to overcome these limitations.<\/p>\n

Frame theory provides a rigorous framework for the initial decomposition of radiated fields into a set of Gaussian beams, providing flexible rules to adjust the number and directions of the launched beams. In this thesis, frame theory is used to discretize not only the source field distribution but also incident field distributions over planes or parts of planes of interest, according to encountered obstacles and propagation distances. The radiated fields are then obtained by summation of Gaussian beams launched from these frames called \u00ab\u00a0reexpansion frames\u00a0\u00bb.<\/p>\n

Gaussian beam transformations by finite sized obstacles are addressed by this re-expansion scheme : the incident beams partially impinging on limited areas are successively \u00ab\u00a0re-expanded\u00a0\u00bb on two re-expansion frames, the first one composed of \u00ab\u00a0narrow\u00a0\u00bb windows and the second one of \u00ab\u00a0wide\u00a0\u00bb windows, both defined in the plane containing the limited area. Spatially narrow window frames allow to take into account abrupt transitions in space, and spatially wide window frames radiate in the form of collimated Gaussian beams.<\/p>\n

The re-expansion formulation proposed in this work is designed for efficient numerical implementation.
\nApproximate analytical expressions are established for expansion coefficients on narrow window frames, and for frame change matrix elements. This formulation has been implemented, and the influence of frame parameters on re-expansion accuracy is analyzed. Finally, the GBS algorithm augmented with successive re-expansions is used to compute fields in simplified scenarios similar to situations encountered in ground-based Radar propagation problems.<\/p>\n

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mercredi 22 septembre 2010 \u00ab\u00a0Etudes d’outils de calcul de propagation Radar en milieu complexe (milieu urbain, pr\u00e9sence de multi-trajets …) par des techniques de Lancer de Faisceaux Gaussiens\u00a0\u00bb mercredi 22 septembre 2010 \u00e0 14h00 sur le campus de T\u00e9l\u00e9com SudParis, en salle C06. Jury : – M. Rapha\u00ebl Gillard, IETR, INSA Rennes (rapporteur) – M. 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