In-flight calibration of the Cassini-Radio and Plasma Wave Science (RPWS) antenna system for direction-finding and polarization measurements

International audienceOne major objective of the Cassini mission is the analysis of Saturnian radio emissions of magnetospheric (auroral) as well as atmospheric (lightning) origin. The Radio and Plasma Wave Science (RPWS) experiment is designed to measure the full polarization and the wave vector of the incoming radio waves, allowing us to retrieve information on source locations and emission modes. For that purpose, RPWS uses a two-channel receiver, connected to two electric monopoles (selected among three), which measures the voltages induced by the electric field of the incident waves and their various correlations. The accuracy of retrieved source locations depends directly on the precise knowledge of the orientation of the three effective monopole axes and lengths, which do not coincide with the physical ones owing to interaction with the spacecraft body. Antenna calibration aims at determining the so-called effective length vector of each antenna (combining orientation and length information). For that purpose, roll maneuvers of the Cassini spacecraft were performed before and after the Jupiter flyby, at distances such that Jovian radio sources can be identified with the planet's center but still provide a high signal-to-noise ratio. The resulting modulations of the measured signals allow us to derive the orientation and length of the effective antennas. The analysis is performed in two steps: first, the Stokes parameters (wave polarization) are determined using approximate antenna orientations derived from laboratory measurements on a scale model of the spacecraft. Second, measurements with high signal-to-noise ratio and pure circular polarization are selected and used for the determination of the effective length vectors of the RPWS antennas. Two methods have been developed for inverting the system of equations relating antenna parameters, wave parameters, and measurements (least squares fit and analytical inversion), both of which provide consistent results and present different advantages and limitations which are discussed. A final set of antenna parameters to be used for direction finding studies with the RPWS experiment is obtained