No abstract

[1] We report on a new category of long lasting meteor echoes observed occasionally with HF and VHF radars. These meteoric returns, which have lifetimes from many seconds to a few minutes, are characterized by a distinct Doppler spectral signature showing a pronounced Doppler bifurcation which includes narrow bands of discrete Doppler velocities, often of opposite polarity. The large signal to noise ratios and the narrowness of the spectra imply that coherent or Bragg scattering is not of relevance here, therefore these echoes do not associate with the long living meteor-induced backscatter (MIB) from the lower E region. A reasonable interpretation needs to explain both the Doppler spectrum bifurcation and the long echo duration. As such, we propose the idea of a structured vertical wind shear in the lower E region which traps different fragments of a meteor trail plasma in the same way that sporadic E layers form. These trail parts inside the shear-related wind profile may act as relatively long-lasting meteoric reflectors moving with different Doppler velocities, also of opposite polarity. Citation: Bourdillon, A., C. Haldoupis, C. Hanuise, Y. Le Roux, and J. Menard (2005), Long duration meteor echoes characterized by Doppler spectrum bifurcation, Geophys. Res. Lett., 32, L05805,

The improvement of maritime radio links often requires an increase of emitted power or receiver sensitivity. Another way is to replace the poor antenna gains of traditional surface ship whips by novel antenna structures with directive properties. However, ship motions, especially when launches are involved, may affect the radio link quality. Therefore, a tool for modelling the impact of ship motions on such antenna structures was developed. It helps the specification of antenna radiating parameters and improves the radio link performance evaluation. This tool includes a deterministic two-ray model for radio-wave propagation over the sea surface. The geometrical 3D engine for ship motions has the advantage to be compatible with any propagation model and antenna structure.

This paper presents studies, carried out by Télécom Bretagne, concerning new radio technologies (mainly WiMAX, 802.16e) in the maritime environment. The aim of these studies is to reinforce the quality and the robustness of such transmissions. Several measurements were set up to characterize the propagation channel in maritime environment in order to study the communication performances of WiMAX. This characterization was obtained by specific experimental devices, developed and implemented by Télécom Bretagne. The experimentations were carried out at frequencies of 3.5 GHz (Licensed WiMAX Band in France) and 5.8 GHz (Free Band in France). Similar studies [1] and [2] were already carried out, but they did not take into account all the environment characteristics, namely tides and coast relief masking. Our study looks at these parameters for a better understanding of the propagation in operational transmission environments. In this paper, measurements were carried out by measuring the mean received power both for several distances over the sea and for a coastal point-to-point link during long periods. Part of the work presented in this paper has been obtained during the Ex'treme project.