Optimization of the Antennas of the EISS Radar Designed to Perform Deep Martian Subsurface Sounding
Résumé
The Electromagnetic Investigation of SubSurface (EISS) instrument has been designed to get information on the morphology and geological structure of the Martian subsurface and search for potential liquid water reservoirs. EISS is an impulse radar operating from the Martian surface at central frequencies of a few megahertz in order to perform deep sounding of the subsurface. Particular attention has been paid to the design of the instrument's electrical antennas that need to transmit and receive the electromagnetic waves without distortion over the instrument's full frequency bandwidth (i.e., from 1 to 7 MHz). To fit into the mass and volume allocation, wire antennas made of identical loaded dipoles have been selected; they will be deployed on the surface by a mechanical mortarlike device. The resistive profile along the monopoles needs to be carefully chosen to ensure a proper behavior of the antenna once they are deployed on the surface. This paper deals with the performance of these electrical antennas and the optimization of their resistive profile. The study of the impact of the subsurface is performed on simulated data obtained by a numerical finite difference in time domain code. When available, experimental data show the good agreement with the theory. The studies performed show that a slightly suboptimal resistive profile will allow a correct operation of the instrument at the surface of Mars for any landing site.