Cassini Radar: data analysis of the Earth flyby and simulation of Titan flybys' data

摘要

Saturn and its largest satellite, Titan, are the principal objectives of the NASA-ESA-ASI Cassini-Huygens mission. Launched in 1997, the spacecraft will reach its destination in July 2004 and the mission will end in 2008, after 44 Titan flybys. A Ku-band radar, in particular, will investigate the nature of Titan surface, using four operative modes: imaging radar, scatterometer, altimeter and radiometer. During the Earth flyby, in August 1999, Cassini Radar acquired radiometric and scatterometric data over Pacific Ocean and South America for calibration purposes. These data have been compared with the co-located X-SAR, Seasat, other sensors' and reference database, retrieving information for Cassini Radar calibration. Furthermore, an e.m. simulation is applied in order to assess the possibility of radar discrimination among three different surface scenarios expected for Titan: oceans or lakes of ethane, water ice or ammonia-rich ice. The surfaces have been simulated using fractals, described as 3-D Fractional Brownian Motion processes, and their e.m. response has been calculated using the Kirchhoff approximation. The results indicate a good possibility of discrimination because of the higher sensitivity of backscattering coefficient to dielectric constant variations than to surface roughness. For very smooth surfaces, liquid methane in absence of wind, signals at the low limit of the radar detectivity are expected.