Cassini’s Determination of the Gravity Fields of the Saturnian Satellites

The determination of the gravity fields of the Saturnian satellites is one of the main scientific goals and responsibilities of the Cassini Radio Science team. Radiometric tracking data were acquired during the flybys of Phoebe, Enceladus, Dione, Rhea, Hyperion, and Iapetus, allowing an accurate determination of the masses of these satellites as well as the gravity quadrupole field of Rhea. Our technique consists of using X-band and Ka-band coherent, two-way Doppler data and fit them in JPL’s Orbit Determination Program in short data arcs. Doppler data are fitted using a spacecraft dynamical model that includes the gravitational accelerations from all Saturn system bodies, as well as non-gravitational accelerations from the spacecraft Radioisotope Thermoelectric Generators and from solar radiation pressure. Calibrations of the noise introduced by the Earth troposphere, using an Advanced Media Calibration system, and charged particles in the solar corona and the Earth ionosphere are applied. The paper will describe the technique used for the short-arc orbit determination process and the data calibration and analysis procedure, emphasizing the advantages offered by Cassini’s advanced deep-space tracking system.