Laurent Féral

A Large-Scale Space-Time Stochastic Simulation Toolof Rain Attenuation for the Design and Optimization of Adaptive Satellite Communication Systems Operating between 10 and 50 GHz

The design and optimization of propagation impairment techniques for space telecommunication systems operating at frequencies above 20 GHz require a precise knowledge of the propagation channel both in space and time. For that purpose, space-time channel models have to be developed. In this paper the description of a model for the simulation of long-term rain attenuation time series correlated both in space and time is described. It relies on the definition of a stochastic rain field simulator constrained by the rain amount outputs of the ERA-40 reanalysis meteorological database. With this methodology, realistic propagation conditions can be generated at the scale of satellite coverage (i.e., over Europe or USA) for many years. To increase the temporal resolution, a stochastic interpolation algorithm is used to generate spatially correlated time series sampled at 1 Hz, providing that way valuable inputs for the study of the performances of propagation impairment techniques required for adaptive SatCom systems operating at Ka band and above.

Degradation of Radar Reflectivity by Cloud Attenuation at Microwave Frequency

Abstract The main object of this paper is to emphasize that clouds—the nonprecipitating component of condensed atmospheric water—can produce a strong attenuation at operational microwave frequencies, although they present a low reflectivity preventing their radar detection. By way of a simple and realistic model, simulations of radar observations through warm precipitating targets are thus presented in order to quantify cloud attenuation. Simulations concern an airborne radar oriented downward and observing precipitation at four frequencies: 3, 10, 35, and 94 GHz. Two cases are first considered: a convective cell (vigorous cumulus congestus plus rain) and a stratiform one (nimbostratus plus drizzle) superimposed on the previous one. Other simulations are then performed on different types of cumulus (congestus, mediocris, and humilis) with various thicknesses characterized, in a microphysical sense, by their maximum liquid water content. Simulations confirm the low cumulus reflectivity ranging from −45 dBZ...

Hail Detection Using S- and C-Band Radar Reflectivity Difference

Abstract In reflectivity fields observed with conventional radar networks, hailstorm identification is not easy. In the present paper, a hailstorm detection method using two single-wavelength radars located far from each other is discussed. The two radars, C and S bands, respectively, are about 200 km apart. Because large hailstones are non-Rayleigh scatterers, the distribution of the dual-wavelength reflectivity ratio is shown to display an identifying signal for hail-bearing storms. The relevance and sensitivity of the proposed hail detection algorithm is first analyzed from a simulation of 10.7- and 5.3-cm radar observations. Various convective storm conditions, differing by the horizontal shape and the nature of precipitation, are considered, notably rain and hail with hailstones of different diameters, dry or coated with liquid water. The attenuation for the radar–target propagation path and inside the target is also taken into account. The dual-wavelength reflectivity ratio for all the simulated rad...

Comparison of 2D and 3D Electromagnetic Approaches to Predict Tropospheric Turbulence Effects in Clear Sky Conditions

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.

Inter-annual variability, risk and confidence intervals associated with propagation statistics. Part I: theory of estimation

SUMMARY This set of two companion papers aims at providing a statistical framework to quantify the inter-annual variability observed on the statistics of rain attenuation or rainfall rate derived from Earth-space propagation measurements. This part I is more specifically devoted to the theoretical study of the variance of estimation of empirical complementary cumulative distribution functions (ECCDFs) derived from Earth-space rain attenuation or rainfall rate time series. To focus the analysis on the statistical variability but without loss of generality, synthetic rain attenuation time series are considered. A large variability on the ECCDFs, which depends on the duration of the synthetic data, is first put into evidence. The variance of estimation is then derived from the properties of the statistical estimator. The formulation is validated numerically, by comparison with the ECCDF variances derived from the synthetic data. The variance of the fluctuations around the CCDF is then shown to be dependent on the average of the correlation function of the time series, on the probability level and on the measurement duration. This variance of estimation is needed as a prerequisite in conjunction with the knowledge of the climatic variability to characterize the yearly fluctuations of propagation statistics computed from experimental time series. The extensions from simulations to experiments as well as the application to system planning are detailed in part II. Copyright

Statistical Distribution of Integrated Liquid Water and Water Vapor Content From Meteorological Reanalysis

A worldwide modelling of integrated liquid water and water vapor content distributions is proposed and evaluated. The knowledge of those distributions is valuable to predict attenuation for Earth-space communication systems operating at frequencies higher than 10 GHz.

A Rain Attenuation Time-Series Synthesizer Based on a Dirac and Lognormal Distribution

In Recommendation ITU-R P.1853-1, a stochastic approach is proposed to generate long-term rain attenuation time series , including rain and no rain periods anywhere in the world. Nevertheless, its dynamic properties have been validated so far from experimental rain attenuation time series collected at mid-latitudes only. In the present paper, an effort is conducted to derive analytically the first- and second-order statistical properties of the ITU rain attenuation time-series synthesizer. It is then shown that the ITU synthesizer does not reproduce the first-order statistics (particularly the rain attenuation cumulative distribution function CDF), however, given as input parameters. It also prevents any rain attenuation correlation function other than exponential to be reproduced, which could be penalizing if a worldwide synthesizer that accounts for the local climatology has to be defined. Therefore, a new rain attenuation time-series synthesizer is proposed. It assumes a mixed Dirac-lognormal modeling of the absolute rain attenuation CDF and relies on a stochastic generation in the Fourier plane. It is then shown analytically that the new synthesizer reproduces much better the first-order statistics given as input parameters and enables any rain attenuation correlation function to be reproduced. The ability of each synthesizer to reproduce absolute rain attenuation CDFs given by Recommendation ITU-R P.618 is finally compared on a worldwide basis. It is then concluded that the new rain attenuation time-series synthesizer reproduces the rain attenuation CDF much better, preserves the rain attenuation dynamics of the current ITU synthesizer for simulations at mid-latitudes, and, if it proves to be necessary for worldwide applications, is able to reproduce any rain attenuation correlation function.

A large scale, high resolution channel model for propagation impairment techniques design and optimization

As the impairments due to rain on the propagation channel for frequency bands such as Ka or Q/V have to be compensated by adaptive fade mitigation techniques, optimized radio resource management needs to be implemented, which requires a coarse knowledge of spatio-temporal dynamic of the attenuation due to rain. In this paper a model able to emulate the space-time dynamic of the attenuation due to rain on a satellite coverage is presented. It consists of a stochastic model that is constrained by the outputs of a reanalysis model. The spatial resolution of the modeling is of 1 km and the temporal one of 0.1 h. A stochastic interpolation model is then used to get a temporal resolution of 1s more suitable to study fade dynamic.

Inter‐annual variability, risk and confidence intervals associated with propagation statistics. Part II: parameterization and applications

SUMMARY This set of two companion papers aims at providing a model for the inter-annual variability of earth-space propagation statistics and for the inherent risk and CIs. In part I, it was proposed to model the yearly variance σ² of empirical complementary CDFs so that where is the variance of estimation, the inter-annual climatic variance and p the long-term probability. Particularly, an analytical formulation of was derived and parameterized from synthetic rain attenuation data. Considering the statistical framework developed in part I, this part II is specifically devoted to the parameterization of the variance of estimation from experimental data of rain attenuation and rainfall rate. Then, a methodology to model and parameterize worldwide the inter-annual climatic variance is presented. The model of yearly variance of the empirical complementary CDFs is finally compared against yearly experimental variances derived from data collected worldwide. The knowledge of this variability is very useful for system design as it allows the risk on a required availability and associated with a given propagation margin to be quantified. Copyright

Short- And Mid-Term Prediction Techniques Of Propagation Conditions From Weather Radar Observations. Application to resource management in SATCOM systems

This paper discusses possible scenarios for inclusion of forecasting of rain fields in satellite communication system for the purposes of optimizing the use of layer 1 resources (EIRP, frequencies)