[1] This paper introduces a ''refractivity from clutter'' (RFC) approach with an inversion method based on a pregenerated database. The RFC method exploits the information contained in the radar sea clutter return to estimate the refractive index profile. Whereas initial efforts are based on algorithms giving a good accuracy involving high computational needs, the present method is based on a learning machine algorithm in order to obtain a real-time system. This paper shows the feasibility of a RFC technique based on the least squares support vector machine inversion method by comparing it to a genetic algorithm on simulated and noise-free data, at 1 and 5 GHz. These data are simulated in the presence of ideal trilinear surface-based ducts. The learning machine is based on a pregenerated database computed using Latin hypercube sampling to improve the efficiency of the learning. The results show that little accuracy is lost compared to a genetic algorithm approach. The computational time of a genetic algorithm is very high, whereas the learning machine approach is real time. The advantage of a real-time RFC system is that it could work on several azimuths in near real time.Citation: Douvenot, R., V. Fabbro, P. Gerstoft, C. Bourlier, and J. Saillard (2008), A duct mapping method using least squares support vector machines, Radio Sci., 43, RS6005,

Abstract-In this paper, a fast method is presented to model the forward propagation above Gaussian rough surfaces and taking into account atmospheric refraction. The method is based on the Discrete Mixed Fourier Transform (DMFT) solved by the Parabolic Wave Equation, in which the Ament boundary condition with shadowing effect is used at grazing angle.In this model, for a bistatic configuration, the surface height PDF of the illuminated points is derived and it is introduced in the boundary condition. Examples demonstrate the capacities of the method to compute propagation factor above rough surfaces following Gaussian statistics and Gaussian height correlation and the proposed method is validated by comparison to a Monte Carlo approach.

[1] Refractivity from clutter (RFC) retrieves the radio frequency refractive conditions along a propagation path by inverting the measured radar sea clutter return. In this paper, a real-time RFC technique is proposed called ''Improved Best Fit'' (IBF). It is based on finding the environment with best fit to one of many precomputed, modeled radar returns for different environments in a database. The method is improved by considering the mean slope of the propagation factor, and physical considerations are added: smooth variations of refractive conditions with azimuth and smooth variations of duct height with range. The approach is tested on data from 1998 Wallops Island, Virginia, measurement campaign with good results on most of the data, and questionable results are detected with a confidence criterion. A comparison between the refractivity structures measured during the measurement campaign and the ones retrieved by inversion shows a good match. Radar coverage simulations obtained from these inverted refractivity structures demonstrate the potential utility of IBF.Citation: Douvenot, R., V. Fabbro, P. Gerstoft, C. Bourlier, and J. Saillard (2010), Real time refractivity from clutter using a best fit approach improved with physical information, Radio Sci., 45, RS1007,

Two-dimensional electromagnetic simulations are often used to evaluate the atmospheric turbulence effects on radiowave propagation in clear sky conditions. However, turbulence is clearly a three-dimensional atmospheric process. Therefore, errors potentially introduced by 2D propagation schemes to predict 3D scintillation effects have to be quantitatively assessed. On the one hand, as part of an analytical approach and starting from the Kolmogorov-von Karman turbulent spectrum, 2D formulations for log-amplitude and phase variances and for log-amplitude and phase temporal power spectra are derived from the 2D scalar propagation equation. They are compared asymptotically to their classical 3D counterparts. On the other hand, as part of a numerical approach, the scintillation effects are evaluated from 3D and 2D parabolic wave equation (PWE) approaches associated with 2D and 1D multiple phase screen (MPS), respectively. It is then shown that 2D propagation schemes underestimate by a factor 1.86 the log-amplitude variances in Fresnel regime and can lead to significant errors in predicting log-amplitude and phase temporal spectra at low frequencies. It is then suggested that the dimensional reduction should be limited to the prediction of log-amplitude and phase variances in Fraunhofer configurations, or to the evaluation of log-amplitude and phase power spectra at high frequencies.Index Terms-Multiple phase screen (MPS), parabolic wave equation (PWE), radiowave propagation, scintillation, weak scattering.

SUMMARYTo compensate propagation impairments on Earth-space communication links, a specific fade mitigation technique to make up for rain propagation impairments is studied in this paper: the time diversity. This process consists in sending the information when the propagation channel allows to get it through. Here the time diversity technique is applied to different experimental attenuation time series collected in Europe: Spino d'Adda (Italy), Louvain-la-Neuve and Lessive (Belgium). These propagation measurements have been collected from 12 to 50 GHz and the performance of time diversity technique is assessed from the generation of time diversity statistics conditioned to the time delay. A prediction method of these statistics is also described. The proposed model relies on the time correlation of the attenuation time series. The model is validated and its accuracy analysed in terms of prediction error calculated using the different databases.

Abstract-Classical assesssment of the received power by a radar leads to a decorrelation of many relevant phenomena (i.e. propagation, backscattering), which may introduce modelling errors notably in the presence of large target with respect to the wavelength. To overcome this limitation, a new hybrid approach is proposed. It combines a method of propagation calculation (the parabolic wave equation) with a method of scattering calculation (the EFIE solved by a method of moment approach) and an application of the reciprocity principle (the power coupling factor). Each method constituting the hybrid approach is described; the example of a large cargo is chosen and its apparent RCS is evaluated above the sea at low frequency. The results are discussed, studying the influence of the different parts of the boat on the apparent RCS. † Also with UPS-AD2M-IGEEP, 118 route de Narbonne,

The work presented here aims at interpreting the pattern observed at low frequencies in wavenumber-frequency (k, ) representations (dispersion diagrams) of radar returns from sea surface. This pattern is sometimes referred to as the "group line" in the literature and is supposed to result from the nonlinear behavior of the signal with respect to surface profile. To confirm this assumption, we have calculated the patterns generated by various nonlinear terms of second order in sea surface height. It is shown that the resolution in frequency of most data is not sufficient to allow for an accurate representation of the group line through an FFT. A methodology is proposed to define an average slope of the group line, which can be interpreted in terms of velocity.

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