It is well known that the bistatic radar altimeter have some differences with respect to a monostatic one: indeed the spatial resolution is achieved over elliptical areas. These ellipses are in general tilted with respect to the isodoppler line, as these depend on the relative direction of motion of both transmitter and receiver. We can notice that also in the bistatic altimeter the spatial resolution can be improved at least in along track direction, by synthetic aperture approach: the instruments based on this concept are called Doppler Beam Sharpening (DBS) altimeters. Moreover under usual sea conditions, the geometric optics backscattering model appears applicable and the first point of our task appears the slope "m" measurement. We can estimate the slope by considering the backscattering in the specular or reflection central point, or/and by estimation of the decreasing of the backscattering versus observation angle. The second approach appears the best (in order to leave aside from sea surface pollution), which entails the backscattering estimation in the same region from many observation angle (multilook), with uncoherent processing . Therefore a new analytical closed-form model is derived for the average power echo received by a bistatic altimeter from oceanic surface, taking into account the previous requirement not usual in Altimetry. We can notice that, if the aim of our investigation is the wind measurement, due to the models available in literature, the accuracy required in the slope measurement is the order of 10%. Because of the nonlinear interaction at the interface between the water and the air, the wave crests are more peaked than the wave troughs and this effect can be taken into account by given to the surface height probability density function a non symmetric behaviour that is a non- zero skewness coefficient. This has been also done applying an exponential non linearity to a bivariate Gaussian distribution: an update version of this model is also proposed. Moreover the backscattering model, recently developed for fractal Brownian surfaces, and applied to wind-roughened water surfaces, is also taken into account. A Software Simulator was implemented and the simulation results have validate this model.

The MARSIS observations are optimized during periods when the pericenter of the orbit is near or below zero degrees sun elevation (ldquonightsiderdquo) and the nightside phase, the last of the primary MEX mission, occurs on March-July 2005, in the northern latitude of MARS regions. This paper provides a description of the modeling approach and of the expected performance of the MARSIS radar in the northern hemisphere of Mars. Few models, suitable for a preliminary analysis of the MARSIS instruments are reported. The knowledge of these performance, evaluated according to the model used for the surface and subsurface of the Martian crust, are necessary in order to decide, during the planning activity of the mission, the radar operative mode. In addition the model utilized are an effective tool for the simulator that has to perform the radar equation inversion in order to evaluate, by the radar returns, the surface and subsurface dielectric characteristics. Few simulation results of the surface characteristics are reported and a radar gram is shown, as an example, in order to state the preliminary criteria for the radar equation inversion.

An approach to the inversion of the data available from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express is described. The data inversion gives an estimation of the materials composing the different detected interfaces, including the impurity (inclusion) of the first layer, if any, and its percentage, by the evaluation of the values of the permittivity that would generate the observed radio echoes. The data inversion method is based on the analysis of the surface to subsurface power ratio and the relative time delay as measured by MARSIS. The constraints, due to the known geological history of the surface, the local temperature and the thermal condition of the observed zones and the results of other instruments on Mars Express and other missions to Mars, have to be considered to improve the validity of the utilized models and the obtained results that are given in parametric way.

In this paper we describe an inversion approach in order to analyze data from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express. The inversion process allows the dielectric constant of the subsurface material to be estimated provided the dielectric constant of the surface is known. In addition, if impurity are present, it is possible to estimate the dielectric constant of any inclusions as well as the percentage amount of material in the inclusions relative to the host material provided knowledge of the host material up to the depth where the interface has been detected is available. The data inversion method is based on the analysis of the surface to subsurface power ratio and the relative time delay as measured by MARSIS. The data inversion has been performed at several frequencies in order to estimate the frequency dependent parameters affecting the behavior of the radar echoes. It is necessary that the surface and subsurface interfaces have the same roughness in order to estimate the Subsurface Fresnel reflectivity. As a preliminary approach, only flat surface have been selected. MOLA (Mars Orbiter Laser Altimeter) has already provided detailed data on the visible Martian surface and a simulator, with a facet model, has been utilized to use MOLA data in order to verify the correct selection of the frames that will be used for the data inversion (absence of clutter echoes).

SHARAD (SHAllow RADar) is a radar for the study of the Martian subsurface provided by the Italian Space Agency (ASI) as a facility instrument on board NASA’s Mars Reconnaissance Orbiter 2005 spacecraft. The scientific objective of SHARAD is the detection of water, either liquid or solid, and the profiling of subsurface ice layers in the first hundreds of meters of the Martian subsurface. Although the Martian surface is not uniformly amenable to subsurface sounding, it will be possible to find favourable conditions for the achievement of scientific objectives. SHARAD is complementary to the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) experiment on board ESA’s Mars Express spacecraft, as it is capable of a better resolution (because of the wider transmitted bandwidth) at the cost of a reduced penetration (higher operating frequency). SHARAD benefits from MARSIS experience both for the modelling of the expected surface clutter, and for the inversion of echo data. Preliminary data acquired during the instrument commissioning period around Mars have demonstrated the correct working of the instrument. Seu et al.: SHARAD, a radar sounder for MRO 27 Fig. 1. Plot of an individual processed radar echo (or “frame”) from the MARSIS experiment. The horizontal axis is the time delay of the echo in μs, while the vertical axis is the echo power in dB. The main peak in the plot is the reflection produced by the Martian surface, while smaller subsequent peaks are subsurface reflections. Distant peaks (i.e. peaks reaching the radar 15 μs or more after the surface reflection) are lateral surface echoes, the so-called “clutter”.

MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) is a subsurface sounding radar on board the European Space Agency mission Mars Express. The MARSIS primary scientific objective is to map the distribution of water, 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. Three secondary scientific objectives are subsurface geologic probing, surface characterization and ionosphere sounding. In this paper an inversion approach of MARSIS data is presented. The data inversion (estimation of the materials composing the surface and the subsurface by the estimation of the dielectric constants) is based on the analysis of the data available from the MARSIS observations, that is the surface to subsurface power ratio and the relative time delay. The data inversion has been performed with a multi frequency analysis in order to estimate the frequency dependent parameters affecting the behavior of the radar echoes. The data inversion needs a hypothesis on the surface composition to give an geological interpretation of the subsurface dielectric properties. To improve the validity 16 Picardi et al.: MARSIS, a radar for MEX of the obtained solutions it is necessary to introduce few constraints relevant to the geological history of the surface, to the local temperature and the thermal condition of the observed zones and the results of other instruments of Mars Express and of other missions to Mars. This approach, that is addressed to evidence the radar capabilities, is a first step for the interpretation of the results by the geologist.

The analysis of the surface return echoes in the subsurface data extraction in North Mars Polar region has shown the possibility to utilize simplified surface models, at least, for the purpose of the evaluation of the penetration depth capability. The surface simulation, obtained starting from MOLA data, has been utilized during the planning activity in order to select the MARSIS operative sequence in order to optimize the amount of scientific data taking into account the data rate available and the scientific target to be investigated during the next part of the mission. Moreover a simulator, still in progress, utilizing the surface characteristics will perform the analysis of the real data in order to make available the required information expected, by the mission, in terms of detection and identification of dielectric constant on the surface and subsurface.

According to the Mars Express mission , the MARSIS primary scientfIc objectives are to map the distribution ofwater, both liquid and solid, in the upperportions of the crust ofMars. Detection of such reservoirs of water wifl 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 seconda,y 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 and expected performances of the MARSIS, taking into account of Mars Orbital Laser Altimeter (MOLA) data. As matter of fact the analysis of MOLA data, from the current Mars Global Surveyor Mission, have shown indeed that Mars surface shows a fractal behavior in a range of scale going from 100m to 3÷ 10km. In this paper the results of the analysis of MOLA data by mean also a mapping and a statistical distribution of the fractal parameters performed all over the surface ofMars are shown. Then a new analytical model for the surface correlation function will be introduced taking into account the fractal behavior ofthe surface over a finite range of scales. Hence for the predicted range of variation of the surface parameters and accordingly to the instrument wavelengths the radar backscattering ofthe surface will be evaluated according to the kirchoffapproximation; in order also to predict the strength of the clutter signal and then of the penetration depth that is possible to reach with MARSIS instrument. Moreover the difference between results obtained with the fractal model and with the two scale model will be evaluated.

The MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument is a multi-spectral, low-frequency, nadir looking pulse limited radar sounder and altimeter with ground penetration capability. Moreover the detection of a subsurface interface will be possible only if the following conditions are met:the level of the subsurface reflection is higher than the noise floorthe surface/subsurface dynamic is included in the system dynamic rangethe subsurface reflection is higher than the corresponding surface clutter reflectionFor MARSIS the noise floor has been evaluated to be about 60 dB below the fully coherent surface echo, so that an overall 60 dB dynamic range will be allowed if sidelobes and non. linearities are controlled and reduced down to the noise level with a proper design. In this case we can assess that the penetration depth can be defined as that depth where the subsurface power is equal to the surface clutter power. In this paper the subsurface to surface clutter ratio will be evaluated, taking also into account the results related to the new fractal models of the structure of the planets surface and in particular the new MARS surface models obtained from the MGS/MOLA data.