Search

The study of asteroids interior is crucial to the understanding of their accretion in the early Solar System. Furthermore, space-borne sounding radar is the most mature technique to image asteroid’s internal structure. The low-frequency radar is a bistatic radar measuring the radar wave propagation between a Lander and an Orbiter through the asteroid. Time accuracy and frequency stability are fundamental for the success of such an instrument. Furthermore, accuracy in the propagation delay measurement is a way to increase science return. This work studies the impact the drift of the clocks located on the two platforms has on the accuracy of the time measurement while taking into account the on-board processing. We present the time analysis of such a radar by developing a behavior model of the system for a long time scale which is implemented in a software simulator. A clock test bench is then developed to provide simulator inputs from true oven controlled crystal oscillators clocks under selection for the mission. The simulator coupled to the test bench allows us to better quantify the impact of the time errors on the measurement, to support clock selection and to improve science return.

We present orbital bistatic radar observations of a small-body, acquired during occultation by the Dawn spacecraft at asteroid Vesta. The radar forward-scattering properties of different reflection sites are used to assess the textural properties of Vesta's surface at centimeter-to-decimeter scales and are compared to subsurface hydrogen concentrations observed by Dawn's Gamma Ray and Neutron Detector to assess potential volatile occurrence in the surface and shallow subsurface. We observe significant differences in surface radar reflectivity, implying substantial spatial variations in centimeter-to-decimeter-scale surface roughness. Our results suggest that unlike the Moon, Vesta's surface roughness variations cannot be explained by cratering processes only. In particular, the occurrence of heightened hydrogen concentrations within large smoother terrains (over hundreds of square kilometers) suggests that potential ground-ice presence may have contributed to the formation of Vesta's current surface texture. Our observations are consistent with geomorphological evidence of transient water flow from Dawn Framing Camera images.The Dawn spacecraft has provided orbital bistatic radar observations of a small body in the solar system. Here, the authors present results from Vesta suggesting that smooth terrains with heightened hydrogen concentrations indicate that ground-ice presence potentially helped shape Vesta's current surface texture.

In this paper we describe the existing and planned radar measurements of the planetary bodies. The dielectric properties of water ice and other potential surface and subsurface materials are discussed, as well as their dependency on temperature and structure. We then evaluate the performance of subsurface sounding radars using these parameters. Finally we describe some laboratory technique to help interpret the radar data, presenting some results obtained using dielectric spectroscopy methods.

In an effort to understand the mechanisms which give rise to the enhanced ion acoustic and Langmuir waves in HF modification experiments, we perform measurements with the EISCAT Tromso heater and the two incoherent scatter radars to locate the enhancement regions in space. Simultaneous measurements of the enhancements were made with both the VHF (224 MHz) and the UHF (933 MHz) radars, the latter being scanned in the magnetic meridian plane between vertical and field aligned through theSpitze angle. The results show that enhanced bottomside and topside ion acoustic and plasma waves at UHF and VHF are a common phenomena in these data. UHF topside enhanced plasma waves were not observed, possibly due to lack of radar sensitivity. At UHF the enhanced ion waves were strongest when the radar was pointed between theSpitze angle and geomagnetic field aligned direction. The topside features we believe give evidence for the production and propagation of Z-mode waves during this experiment. There is no evidence in these data for the decay of Z-mode waves into Bernstein and lower hybrid waves inside the overdense F layer as we believe to have observed previously.

The Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment is one of five experiments which together comprise the Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, and a spectrum analyser to calculate in near-real time aboard the spacecraft, the complete auto- and cross-spectral matrices using the three magnetic and two electric components of the electromagnetic field. The magnetic waveform at frequencies below either 10 Hz or 180 Hz is also transmitted. The sensitivity of the search coil is adapted to the phenomena theo be studied: the values 3 × 10-3 nT Hz-1/2 and 3 × 10-5 nT Hz-1/2 are achieved respectively at 1 Hz and 100 Hz. The dynamic range of the STAFF instruments is about 96 dB in both waveform and spectral power, so as to allow the study of waves near plasma boundaries. Scientific objectives of the STAFF investigations, particularly those requiring four point measurements, are discussed. Methods by which the wave data will be characterised are described with emphasis on those specific to four-point measurements, including the use of the Field Energy Distribution function.

The incoherent scatter technique has been applied since 1965 to study the ionosphere and thermosphere in different regions of the Earth. The analysis of the received signal gives access to several ionospheric parameters as a function of height: electron density, electron and ion temperatures and ion velocity. The derivation of these parameters is usually a complicated mathematical procedure that requires a non-linear regression program. A lot of research has been done in the ionospheric and atmospheric science using this technique. In this paper we describe how one derives the ion-neutral collision frequency and the ion composition parameters. It is usually difficult to retrieve these parameters with the incoherent scatter technique; as a result, in the standard data analysis procedure, an ionospheric model is used instead. However the numerical values chosen in the model have an influence on the other derived parameters. For instance the choice of a wrong ion composition leads to erroneous plasma temperatures. It is therefore important to assess by how much the standard procedure deviates from reality. For this reason we compare the ion composition and collision frequency retrieved from a sophisticated analysis scheme with the values that are derived from models under similar geophysical conditions. It also possible to derive from the observed ionospheric parameters the neutral concentrations, temperatures and winds, by using the energy and momentum equations for the ions and the neutrals. In this paper the different methods and the corresponding assumptions involved in the data analysis are discussed. We describe the influence of the frictional heating, of the vertical neutral wind and of the ionospheric perturbations on the derivation of the neutral atmospheric parameters. Our discussion of the processes involved are drawn from results obtained by Chatanika, Sondrestrom and EISCAT radars.

On etudie un systeme adaptatif non boucle permettant d’estimer en temps reel un signal perturbe additivement par un bruit. Le bruit perturbateur du signal est une transformation lineaire du bruit sourcedont revolution temporelle est accessible. La transformation n’etant pas connue, il est necessaire de l’estimer pour pouvoir obtenir le bruit perturbateur a partir du bruit source,puis le signal. Les resultats de l’etude theorique des caracteristiques de ce systeme sont compares aux resultats d’experiences sur un correlofiltre realisees en laboratoire. On observe une bonne concordance entre la theorie et la pratique. Enfin, une discussion des limites theoriques du systeme adaptables a des processus qui peuvent etre consideres comme stationnaires sur un temps fini et de sa complexite en fonction des performances desirees est presentee.