Comparison of GPS-based and Water Vapor Radiometer-based Earth troposphere calibration systems during Cassini’s radio science experiments in the Saturn system

The radio science experiments performed with Cassini on its way to Saturn required a very accurate troposphere calibration system, in order to reach a end-to-end Allan deviation of the radio link in the order of 3×10-15 at 1000 s integration time. The system developed in the 90’s has been used for the first time during the Cassini cruise radio science experiments between 2000 and 2004: a test of general relativity during the 2002 solar conjunction and the search of gravitational waves during the solar oppositions in 2001-2002 and 2003. Since late 2004, when Cassini reached the Saturnian system, such advanced media calibration (AMC) systems has been used, where available, during all tracking passes dedicated to gravity science (icy satellite and Titan fly-bys). This paper describes the behaviour of the AMC system since the first cruise experiment until the latest fly-bys dedicated to radio science gravity measurements. The focus is represented by the results obtained during the tour of the Saturnian system and in particular the four Titan fly-bys, which underline the utility of very accurate Earth troposphere calibration systems.