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The classical space borne radar altimeters have been flown on most of the remote sensing missions, for the primary task of determining the distance from the observed surface to the radar with a very high degree of accuracy: typical altimeters utilize wide bandwidth pulses to obtain a high spatial resolution. In this kind of instrument, known as pulse-limited (PL) altimeters the spatial resolution is limited by the pulse width and the orbital height to values of some kilometers: the same concept is applicable to bistatic altimeter. A bistatic remote sensing system consists of a constellation of satellites flying at the same altitude with an operating geometry such that the incidence and scattering angles are equal. Usually opportunity sources could be used as GPS and Glonass. Moreover, when encountering rougher surfaces, with prominent features and non zero slopes, such as land surfaces, coastal regions, ice sheets, etc. the poor spatial resolution inherent in the pulse limited approach becomes inadequate to the observed surface and the altimeter performance degrades severely. In order to overcome these problems the spatial resolution can be improved at least in the along track direction, adapting well-known processing techniques developed for Synthetic Aperture Radar (SAR) (Doppler Beam Sharpened concept) by low-pass filtering the received echoes in the Doppler frequency domain. The pulse limited radar altimeter concept could be applied also to obtain penetration capability in radar sounder. 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 floor • the surface/subsurface dynamic is included in the system dynamic range • the subsurface reflection is higher than the corresponding surface clutter reflection In many cases the limits of the penetration depth will be imposed by the surface clutter level (the last of the list above), which can be found through the evaluation of the depth at which the subsurface power is equal to the surface clutter power. This will be detailed in this paper, taking into account some typical surface and subsurface model in planetary application. Obviously in presence of rough surfaces the dynamic range is strongly reduced by the surface clutter. To increase the detection performance against surface clutter, the SAR-DBS concept could be also applied: as it will be demonstrated the Doppler azimuth processing significantly reduces the surface echoes coming from along track off nadir reflections. In this paper the SAR-DBS concept will be discussed and its performance and the optimum radar parameters will be evaluated.
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