Cassini Altimetric Data Processing

Abstract

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. Funded by the Italian Space Agency (ASI), consortium CO.RI.S.T.A. and Thales Alenia Space Italia designed and realized a hardware and software system devoted to process altimetric data for the nominal four-year mission lifetime (i.e. 45 envisaged flybys of Titan). This facility offers all the specific 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. This chapter provides an overview of the processing chain architecture, in terms of implemented functionalities, component applications, and system design. The authors developed an original closed form for altimeter impulse response valid for near-nadir measurements and suitable for special Cassini condition and geometry. This allowed to use statistical fitting method (maximum likelihood) for retrieving Titan's surface height with high degree of accuracy, also in the presence of spacecraft uncompensated attitude errors. 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 flybys performed up to now are shown and shortly commented.