The planning of optical navigation pictures for the Cassini extended mission

This paper describes the optical navigation image (opnav) planning for the Cassini extended mission, which includes nine low-altitude flybys of the icy satellites including seven of Enceladus, one of Dione and one of Rhea. Two of the Enceladus flybys have closest approach altitudes at or below 50 km. We present studies showing how much the uncertainties in the Enceladus ephemeris can be reduced by the inclusion of opnavs in the orbit determination. We show that the planned opnavs will maintain the precision of the satellite ephemerides to support the close flybys, cover periods of poor radiometric data during solar conjunctions, and provide a backup data type during the short periods between the two double flybys (where an icy satellite is flown by immediately preceding or following a Titan flyby, and Titan alone is targeted). The processes and software needed to compute the windows of observing opportunity for each satellite, making the initial selection of targets, and refining the selections in response to changes in the tour design during the process are dealt with. Opnavs often conflict with proposed science observations. The process for resolving these conflicts is described. The distribution and timing of opnavs in the extended mission navigation plan is presented to show that the plan meets the needs stated above. The need to periodically replan (mostly repoint) pictures during the extended mission in response to reference trajectory updates is explored and it is shown that the same techniques used to plan the opnavs can be used to replan them.