摘要：We report results from multi-epoch radio astrometry of the Cassini spacecraft with the Very Long Baseline Array (VLBA).These observations are part of a program to determine a series of accurate positions for the Saturn system barycenter in the inertial International Celestial Reference Frame (ICRF) and to use these position measurements to improve our knowledge of Saturn's orbit in the planetary ephemeris.Our VLBA observations cover the full duration of the orbital phase of the Cassini mission, from Saturn orbit insertion in 2004 to the end of mission in 2017.This period covers more than one-third of Saturn's orbital period, allowing us to obtain good orbit constraints for Saturn, particularly on the inclination and ascending node longitude.During the early years of Cassini's orbital mission our VLBA data dominated the determination of orbit orientation, while later in the mission range measurements become more significant.The orientation of Saturn's orbit is now known to approximately 0.25 milli-arcseconds (1.25 nrad), an order of magnitude improvement since the start of Cassini observations.Continuing improvements in the ICRF position accuracy for our phase reference sources, and possible improvements in the final orbit solutions for Cassini, may lead to a still better Saturn orbit over the coming years.
