ALTIMETRY BY CASSINI RADAR: PROCESSING AND SIMULATION
Alberti, Giovanni (2009) ALTIMETRY BY CASSINI RADAR: PROCESSING AND SIMULATION. [Tesi di dottorato] (Inedito)
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The Cassini mission, that is a joint NASA/ESA/ASI effort, has recently offered the unique possibility of exploring Titan, the largest moon of Saturn, that is the only satellite in the solar system to host an appreciable atmosphere, which unfortunately made the surface below very difficult to be observed from the Earth with optical instruments. In fact, a smoggy haze, mostly composed of aerosols resulting from photochemistry between methane and hydrogen and other hydrocarbons, completely envelops the satellite. The Radar Altimeter of the Cassini Mission to Titan operates in a transition region between pulse and beam-limited condition. Due to the specific observation geometry, low values of mispointing angle have been found to significantly affect altimeter impulse response. This involves a non-conventional formulation of the system response which has been the main goal of this research doctorate. An analytical model of the average return power waveform, valid for near-nadir altimetry measurements, has been developed in order to cope with the particular operating conditions of Cassini Mission. The model used to approximate the altimeter waveform is based on the same general assumptions of the classical Brown’s model (1977), but exploits a flat surface response approximation by Prony’s methods. Both theoretical considerations and simulated data have been taken into account to support the accuracy of the proposed model. To infer the main geophysical parameters describing surface topography from altimetry data, a parametric estimation procedure has been used. The Maximum Likelihood Estimator (MLE) procedure has been chosen since in principle it can assure optimal performance as consequence of the analytical model we used to describe the system impulse response. Performances of the implemented method have been numerically evaluated through simulation of data received by CASSINI in high-resolution altimeter mode. The algorithms have been implemented in specific software tools for processing, managing, visualizing and archiving scientific output products containing all the retrieved information about the Titan surface topography, starting from the raw data as delivered by JPL/NASA. The developed processing system is currently in charge of producing standard altimetric Cassini products to be archived in the Planetary Data System (PDS) format. The retrieved topography of the fly-bys performed up to now are shown and shortly commented. Further activities have been dedicated to altimetry echo waveform simulation. The main reason for this effort is to better understand the Titan’s surface characteristics by analyzing the signals received by CASSINI radar in altimetric mode. The approach followed is based on a fractal characterization of Titan’s surface that enables a closed form for the scattering coefficient. A preliminary analysis has been performed on actual data (T30 fly-by) for estimating fractal parameters of Titan’s surface.
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