Retrieval of the ground dielectric permittivity by planetary GPR accommodated on a rover: Application to the estimation of the reflectors' depth by the WISDOM/ExoMars radar
Résumé
Sounding radars, also called Ground Penetrating Radars (GPR), have unlocked a third dimension in planetary exploration, offering the possibility to remotely investigate the subsurface of a planetary object. In the last decade, the increasing developments in the robotic exploration of the Moon and Mars have allowed the accommodation of GPR on rovers. As such, they can perform soundings at the local scale, directly from the surface. This led to the selection of two Lunar and three Martian GPR instruments on board 5 different rovers. The interpretation of GPR soundings in terms of distances between buried interfaces/structures, and the nature of subsurface materials requires the knowledge of the velocity of the electromagnetic wave in the subsurface. This latter depends on the ground dielectric permittivity with is commonly derived from the shape of diffraction curves, if any, in the acquired radargrams. In this work we propose an automatized technique based on the Hough transform designed to detect and characterize diffraction curves. All planetary GPR to this day being “air-coupled” (with antennas a few decimeters above the surface for safety and mobility reasons), the proposed technique accounts for refraction at the surface. It is now included in the data processing and interpretation chain of WISDOM, the GPR of the Rosalind Franklin ExoMars rover. The technique was validated on both synthetic and experimental WISDOM radargrams.
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