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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AThe NIARSIS primary scientific objectives are to map the distribution ofwater, both liquid and solid, in the upper portions of the crust of Mars. Detection of such reservoirs of water will address key issues in the hydrologic, geologic, climatic and possible biologic evolution of Mars, including the current and past global inventory of water, mechanisms of transport and storage of water.Three secondary objectives are defined for the MARSIS experiment: subsurface geologic probing, surface characterization, and ionosphere sounding. According to the previous scientific objectives, this paper provides a description of the design approach, expected performances and first science results of the MARSIS. In order to assess the performances, taking into account of Mars Orbital Laser Altimeter (MOLA) data, some models, either dielectric and geometric, of the Martian crust have been worked out, being the related structure the result of many different processes and validate by the preliminary results. Moreover the two most likely scenarios representing the relevant interfaces MARSIS are: Icelwater(IIW) interface -in this scenario the pores are filled with ice from the surface down to a depth below which liquid water is stable and becomes the pore-filling material. Drylice(DII) interface -here the pore-filling material is considered to be gas or some other vacuum-equivalent material up to a depth, below which ice fills the pores. Hence the interface to detect is between dry regolith and ice-filled regolith. Taking into account the previous models, the MARSIS instrument was designed as a lowfrequency nadir-looking pulse limited radar sounder and altimeter with ground penetration capabilities of the order of some kilometers; this radar can be effectively operated at any altitude lower than 800 km. Moreover several factors that can strongly reduce the subsurface detection dynamic mainly the noise and the surface clutter. Three techniques are used to increase the detection performance against surface clutter: Doppler beam sharpening: the Doppler azimuth processing significantly reduces the surface echoes coming from along track off nadir reflections. A secondary monopole antenna, oriented along the nadir axis will receive the off-nadir surface returns, that could be thus subtracted by the primary antenna composite signal, further reducing the surface clutter level . Finally, the Marsis frequency-agile design will allow to tune the sounding parameters in response to changes in sun illumination condition, latitude etc.. The best penetration capabilities will be obtained during night side observation, when also the longest wavelengths can be operated. The multi frequency observation will allow the estimation of the material attenuation in the crust and will give significant indications on the dielectric properties of the detected interfaces. Finally echo profiles collected at two different frequencies can be processed to separate the subsurface reflections, which are strongly dependent on the frequency, from the surface reflections...