According to the Mars Express mission, the MARSIS primary scientific objectives are to map the distribution of water, both liquid and solid, in the upper portions of the crust of Mars. Three secondary objectives are also defined subsurface geologic probing, surface characterization, and ionosphere sounding. In order to obtain the primary objectives the Radar Sounder design was based on the Ice/water interface and Dry/ice interface scenario: defining the material composition of the first layers and porosity and the pore filling materials. Concerning the surface, we have characterized the geometric structure in terms of a large-scale morphology, on which a small-scale geometric structure, due to rocks, is superimposed, taking into account also that recently the structure of the planets surface was described by means of fractals and in particular the new MARS surface models obtained by processing of the MOLA data. According to these models, this paper provides a description of the operational planning approach and expected performances of MARSIS.
INTRODUCTIONMARSIS is a low-frequency nadir-looking pulse limited radar sounder and altimeter with ground penetration capabilities, this radar can effectively operate at any altitude lower than 800 km. In order to maximize the penetration capabilities of the transmitted pulse MARSIS must operate at a frequency as low as possible as carrier frequencies taking into account the expected values of the plasma frequency in the Martian ionosphere: 1.8, 3, 4 and 5 MHz was selected. Moreover the requirement of fine range resolution entails a relatively large transmitted bandwidth (1 MHz), so that MARSIS will operate with a very high fractional bandwidth: a 1 MHz bandwidth allows a vertical resolution of 150 m in vacuum which corresponds to 50-100 m in the subsurface, depending on the e.m. wave propagation speed in the crust. According to the well known principle of operation of a subsurface radar, a short pulse of e.m. energy transmitted by the antenna impinging on the top of the Mars surface produce a first reflection echo, which propagates backward to the radar, moreover, thanks to the long wavelengths employed, a significant fraction of the e.m. energy impinging on the surface is transmitted into the crust and propagates downward. Additional reflections, due to the subsurface dielectric discontinuities would occur and the
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