Astrometry of Saturn's Satellites from the Hubble Space TelescopeWFPC2

Highly accurate astrometric positions of 14 of Saturn's satellites have been obtained from 444 Hubble Space Telescopeimages taken with the Wide Field Planetary Camera 2 (WFPC2) between 1996 and 2005. In all, 1036 satellite positions were measured in Planetary Camera (PC) frames, with a typical uncertainty of ?PC= 0.?014 (80 km at Saturn), and 1403 positions from Wide Field (WF) frames, with ?WF= 0.?020 (120 km at Saturn). A key part of the reduction involved the application of an improved WFPC2 distortion-correction scheme (Anderson & King) and precise determination of the relative positions of the PC and WF chips, which varied substantially over the full course of the observation period. The time span covered by the observations is more than twice the nominal duration of the Cassinimission and thus provides an important baseline of measurements that is particularly important for studying time-variable phenomena such as the orbital exchange of Janus and Epimetheus and the chaotic interactions of Prometheus and Pandora. These results have been incorporated into ephemerides that are being used for planning and analysis of Cassinisatellite and ring observations.