Rayonnement en ondes métriques et décimétriques des orages

This article presents the result of the current knowledge on the very high frequency electromagnetic radiation of lightning. A phenomenology analysis of the new results obtained from the study of triggered lightning in New Mexico (TRIP 82) and at Saint-Privat-d’ Allier in Haute-Loire, France (1983) is first described. The authors then present the spectral and temporal characteristics of the VHF/UHF radiation which can be applied to the electromagnetic compatibility of systems. From a synthesis of the experimental and theoretical contributions, they propose a reflection on the lightning initiation process and on the mechanisms generating lightning radiation.     

Recherche locale à voisinage variable pour le routage optique dans des réseaux utilisant des routeurs latins

This paper proposes an approach based on Variable Neighbourhood Search (vns) to solve the Routing and Wavelength Assignment (rwa) problem in optical networks including latin routers. It can be summed up as establishing routing (finding intermediate routers on optical paths between each origin/destination pair) and wavelength assignment of these optical paths in such networks. The problem will be tackled according to two scenarii: to optimize the number of established connections (scenario 1) or to answer as well as possible an accurate traffic array (scenario 2). In traditional optical networks (without latin routers), one often separates routing and wavelength assignment because of the general problem complexity (each of the two sub-problems isnp-hard). However, latin routers introduce constraints connecting routing and assignment, this requires a simultaneous treatment of the two problems. A local search heuristic,vnsfor, based on the definition of different neighbourhoods (with simple and double moves) is proposed in this paper to provide us with solutions close to the optimum. Experimental results show howvnsfor leads to better solutions, according to both scenarii, in comparison with the reference algorithmlonca.     

Classification de données SAR multifréquences polarimétriques

Multi frequency polarimetric sar data classification requires to consider the polarimetrie information from each data set simultaneously. Dual data sets are classified using an iterative k-mean algorithm based on a maximum likelihood decision rule evaluated from the Wishart polarimetrie density of the 6×6 coherency matrix. An alternative technique is also proposed which introduces the polarimetrie cross-correlation information to refine the results of classification to a small number of clusters using the conditional probability of the cross-correlation matrix. Both new multi frequency polarimetric sar data classification methods are applied to sar data acquired over the Nezer forest (nasa /jpl AirSAR database (1989)).     

Technique de multiplexage pour réseau optique de vidéocommunication

The paper describes the building of the subscriber equipment for a fiber optic local cable network. Three signals are multiplexed on the user access: video, digital audio and anIsdn channel. The multiplexing procedure is matched to a video signal in an analogue format likely to evolve towards the digital format of the future. This procedure is based on the properties of the video signal: the periodicity of a line structure and the availability of a time slot between active sections of the video lines.     

Utilisation de la théorie géométrique de la diffraction pour l’étude du rayonnement des ouvertures circulaires dans leur zone de champ proche

The aim of this article is to evaluate the precision which can be given by the geometrical theory of diffraction (GTD)when it is used to calculate the field radiated by a circular aperture in its near zone. Both versions of the GTD are used, that due to Keller (classical GTD)and that due to Kouyoumjian (modified GTD).The results obtained with the latter are far better than those obtained with the first one, since the divergence disappears in the neighborhood of the shadow boundary. Moreover, with times of calculation shorter by several tens of times, they can well be compared to the reference results obtained with Kottler’s formulas.     

Détermination du facteur de multiplication M pour un fonctionnement optimal des photodétecteurs à avalanche

The purpose of this paper is relative to the determination of the multiplication factor Mfor an optimum behaviour of the avalanche photodetector. Two types of avalanche photodetector s are compared: a silicon structure n ⁺ πpπp ⁺ Read diode and a GaAlAs heterostructure. Calculating the Noise Equivalent Power (nep)of the whole system, the law S_i(f) =2q I_phoM_x is used, where the parameter x is determined experimentally in both avalanche photodetectors. The exponent x (between 2 and 4) dominates in the nep expression and the ideal nep for the avalanche photodetector system is obtained when x equals 2.     

Étude et réalisation de modulateurs et de démodulateurs directs quadriphases pour la régénération à bord des satellites relais

This paper presents the study of a four phase remodulation demodulator which is part of a microwave signal regenerator for communications payload or data relay satellites. At first, 3.87 GHz biphase (bpsk) balanced modulator has been produced, using planar technology with slotlines and microstrip lines engraved on an Epsilam-type substrate, and two beam-lead Schottky diodes. Under modulation, carrier and even harmonic rejection of better than 50 dB is obtained, with insertion losses lower than 2 dB. A fourphase (qpsk) modulator is also produced using two bpsk modulators. Each phase state is achieved at ± 1.3° with 5.2 dB of insertion losses and a residual amplitude modulation of ± 0.14 dB. Carrier and even harmonic rejection of 45 dB is obtained.