Yves Barbin

Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar

The Philae lander provides a unique opportunity to investigate the internal structure of a comet nucleus, providing information about its formation and evolution in the early solar system. We present Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) measurements of the interior of Comet 67P/Churyumov-Gerasimenko. From the propagation time and form of the signals, the upper part of the “head” of 67P is fairly homogeneous on a spatial scale of tens of meters. CONSERT also reduced the size of the uncertainty of Philae’s final landing site down to approximately 21 by 34 square meters. The average permittivity is about 1.27, suggesting that this region has a volumetric dust/ice ratio of 0.4 to 2.6 and a porosity of 75 to 85%. The dust component may be comparable to that of carbonaceous chondrites.

Comet nucleus sounding experiment by radiowave transmission

Abstract We describe the radio science experiment proposed for the Rosetta cometary mission. The experiment consists in the transmission of electromagnetic waves between the landers and the orbiter through the comet to study its internal structure. In the paper, the electromagnetic model of the comet is presented and used to evaluate the potentiality of the experiment. Various modellings of the radio wave propagation are discussed. Finally, a description of the experiment and the instrument is made.

Toward an integrated HF radar network in the Mediterranean sea to improve search and rescue and oil spill response: The TOSCA project experience

High-frequency (HF) coastal radars measure current velocity at the ocean surface with a 30–100 km range and 1–3 km resolution, every 0.25–1 h. HF radars are well suited to many applications, such as search and rescue (SaR), oil-spill mitigation and ecosystem management. Here we present a first organized core of 12 HF radars installed in five sites in four countries (Greece, Italy, France and Spain) within the European MED project, the Tracking Oil Spill and Coastal Awareness (TOSCA) network. Dedicated experiments tested radar capabilities to estimate transport driven by currents, which is the key feature for all the above applications. Experiments involved the deployment of drifters, i.e., floating buoys, acting as proxies for substances passively advected by currents. Using HF radars the search range is reduced by a factor of 1.6 to 5.3 after 24 h. The paper also underlines the importance of sharing common tools for HF radar data processing and the need to mitigate radio frequency interference. The effort can be regarded as an initial step toward the creation of a Mediterranean or European HF radar network, crucial for any European integrated ocean observing system (IOOS).

The CONSERT instrument for the ROSETTA mission

Abstract In this paper, we describe the CONSERT experiment for the international ROSETTA mission (Verdant and Schwehm, 1998). CONSERT aims at obtaining radio images of the nucleus of comet 46P/Wirtanen by transmitting radio waves through the comet nucleus, between the Lander and the Orbiter (Kofman et al. , 1998). We describe the principle of the instrument, its main parameters, its modes of operation, as well as technical implications and trade-off. The compatibility of the science requirements and the technical solutions is shown.

HF Bistatic Ocean Doppler Spectra: Simulation Versus Experimentation

We simulate the Doppler spectra that can be obtained under experimental conditions from bistatic high-frequency oceanic radar. For this, we combine the bistatic second-order theory with the characteristics of the full radar system, such as antenna patterns, range attenuation, and postprocessing of the received signal. Because of the sharp variation of the bistatic geometry at short range, we show that it is crucial to take these effects into account for the correct interpretation of the first-order Bragg peaks. The second-order spectrum is more robust to the system characteristics but can under some conditions also suffer from its artifacts, especially in the vicinity of the secondary peaks. A comparison is made with experimental spectra acquired recently with a Wellen radar system on the Mediterranean coast. The Wave Watch III model is used to simulate directional wave height spectra after a preliminary validation with in situ buoy measurements. Experimental and simulated Doppler spectra agree generally well, except in the case where the directional wave spectrum has little energy in the line of sight of the radar.

An HF bi-phase shift keying radar: application to ice sounding in Western Alps and Spitsbergen glaciers

A low-frequency ground penetrating radar, developed for martian subsurface sounding (Mars 96 Mission) is presented. During the preliminary campaigns, its performances have been studied by ice thickness determination on various glaciers. This radar is a pulse-compression system designed in order to limit the necessary peak power. It involves the phase modulation of the transmitted pulse by coded sequences. By moving the radar along a profile on the surface, a continuous section of the reflected energy associated with dielectric permittivity contrasts is obtained. Modulating the transmitted pulse and matched filtering the received signal provide a significant depth penetration while preserving a satisfactory range resolution. This processing is efficient even if the echo is partially shortened or if the signal is modified by differential attenuation. A multichannel filtering based on the spectral properties of the signal is then applied to the section. Experimental results of ice sounding on glaciers show that the ice-bedrock interface can be identified at depths between 30 and 120 m. >

Generation of 3-D synthetic data for the modeling of the CONSERT experiment (the radiotomography of Comet 67P/Churyumov-Gerasimenko)

This paper presents a 3-D simulation of the COmet Nucleus Sounding Experiment by Radiowave Propagation (CONSERT) radiotomography experiment of Comet 67P/Churyumov-Gerasimenko. This experiment is part of the ROSETTA space mission. Our simulation is based on a ray tracing algorithm and takes into account multiple internal reflections, power losses, phase delays and polarization distortions. We emulate as closely as possible the true CONSERT data and discuss the results of the simulation.

Empirical Forecasting of HF-Radar Velocity Using Genetic Algorithms

We present a coastal ocean current forecasting system using exclusively past observations of a high-frequency radar (HF-Radar). The forecast is made by developing a new approach based on physical and mathematical results of the nonlinear dynamical systems theory that allows to obtain a predictive equation for the currents. Using radial velocities from two HF-Radar stations, the spatiotemporal variability of the fields is first decomposed using the empirical orthogonal functions. The amplitudes of the most relevant modes representing their temporal evolution are then approximated with functions obtained through a genetic algorithm. These functions will be then combined to obtain the hourly currents at the area for the next 36 h. The results indicate that after 4 h and for a horizon of 24 h, the computed predictions provide more accurate current fields than the latest available field (i.e., persistent field).

Monitoring of Surface Ocean Circulation in the Gulf of Lions (North-West Mediterranean Sea) Using WERA HF Radars

This paper presents the main results obtained by a long term experiment using HF radars which was conducted in the eastern part of the Gulf of Lions. The analysis focuses on statistical observations of the North Current, a large scale current vein occurring in the North of the Western Mediterranean, of current motions at diurnal and quasi-inertial periods and of submeso-mesoscale eddies. The study uses high resolution modellings of wind and current fields.

L-band doppler radar echoes of the sea surface in coastal zone

A remote sensing experiment using an L-Band radar was conducted in a nearshore environment. The results obtained contribute to identify some aspects of the coherent Lband response of the sea, specified by the Doppler spectrum, in neargrazing conditions. At low winds the radar response is of Bragg type. The asymmetry of Doppler spectra varies with wind direction. Surface current measurements were performed from Doppler spectra and assessed by in situ measurements. At high winds the radar response is of Bragg type for VV polarization but not for HH. We show that fast scatterers detected in HH are linked to the dominant waves, which do not propagate in direction of the wind in the complex coastal area considered. Measurements of attenuation with distance of radar signals are also presented.