Filtrage dans le plan tempsfréquence. Caractérisation de signaux UBF et du milieu magnétosphérique

The authors are concerned in studying some signals whose energetic frequency-time representation (given for example by a sonagram) exhibits several distinct patterns. A method is proposed in order to isolate each pattern from the others. The principle of the method is a variable frequency demodulation. The resolving power of this processing and its properties are evaluated with synthetic signals. Then this method is applied to Ulf signals of Pc1type. The Pc1are usually first approximated by a gliding tone with numerous echoes. The proposed method enables one to isolate the echoes and to compare them one to another. A characterization of the magnetospheric medium is made from an interspectral analysis between any two isolated echoes. It is then possible to avoid the averaging effects of classical methods. Thus one may follow the evolution of the propagating medium from one wave train to another.     

Modélisation semi-empirique de la couche limite nocturne. Application au calcul du profil d’indice de réfraction

Electromagnetic propagation depends on the refraction index profile, which is itself a function of pressure, temperature and water vapor pressure. The authors propose here a method to calculate the evolution of the refraction index profile during the night in order to predict when and how the multipath propagation will occur. The data collected during the 1982 « pacem 1 »campaign are used in the calculations. The present semi-empirical model of the nocturnal boundary layer requires short computer calculations.     

Mesure du Débit d’Absorption Spécifique (das) des téléphones mobiles — contribution du projetcomobio à la normalisation internationale

Human exposure to electromagnetic fields emitted by mobile phones is characterised by the Specific Absorption Rate (sar). This value corresponds to therf power absorbed by unitary mass of the equivalent tissue. It represents the basic restriction used to define limit of human exposure torf electromagnetic fields. Academic and industrial labs have contributed through thecomobio project to define international standards for mobile phones certification.