WIRE-GRID SIMULATIONS OF THE MARS EXPRESS/MARSIS ANTENNA SYSTEM W. Macher ∗ , D. Plettemeier † , H.O. Rucker ∗ , and G. Fischer ∗ Abstract The MARSIS experiment (Mars Advanced Radar for Subsurface and Ionosphere Sounding) onboard Mars Express is a ground penetrating radar for the investiga- tion of the Martian surface and subsurface structure. A particular objective is the search for underground water and ice, which is thought to be essential in the evo- lution of microbial life. The MARSIS antenna system consists of a primary dipole and a secondary monopole. The dipole transmits radar pulses and receives their echoes from the surface. The monopole is only used as a receiving antenna, serving for the cancellation of surface clutter echoes. An exact knowledge of the recep- tion properties of the antenna system is essential for the evaluation of the sounding measurements. These properties, in particular the monopole gain and the effect of polarization, are perturbed by the spacecraft body. Numerical simulations are per- formed to determine the deviation from the idealized dipole and monopole behavior, respectively. The influences of the antenna feed configuration and various spacecraft parts are illustrated. Furthermore, the effect of the dipole matching network and the consequences of wave polarization are discussed. 1 Introduction The primary scientific objective of MARSIS is the analysis of the subsurface layers of the Martian crust and the search for possible underground water reservoirs, detected as liquid- water- or ice-interfaces. For that purpose the MARSIS experiment uses a low-frequency nadir-looking radar sounder in the frequency range 0.1–5.5 MHz with ground penetrating capabilities, depending on the frequency of the transmitted pulse [Picardi et al., 2004]. Short pulses released from the dipole antenna travel to the Martian surface where they suffer a partial reflection. A small fraction of energy is transmitted into lower Martian surface layers, with additional potential reflections at subsurface dielectric discontinuities. This results into echoes with different time delays when arriving at the MARSIS antennas, depending on the depth of penetration and the wave propagation speed in the subsurface ∗ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria † Electrotechnical Institute, University of Dresden, Germany. 483