Orbit Determination Adaptations for the Cassini Grand Finale

satellite encounters. Over this period, there have been several papers describing the orbit determination process and
performance up through 2016 [1-5]. In April of 2017, Cassini will enter its Grand Finale mission phase when it will traverse
the gap between the D-ring and the Saturn atmosphere twenty-two times before plunging deep into the atmosphere to end the
mission. The lack of targeted satellite encounters during this period necessitates updates to the nominal Cassini Orbit
Determination (OD) process. This paper describes these planned adaptations for the operation of the Grand Finale. During
the Equinox and Solstice Mission Phase (2008-2016), navigation analysis has been divided into segments focused on two
particular targeted satellite encounters, called an “arc”. Maneuvers in an arc were usually targeted to encounter B-plane
position and time, so the OD state and covariance were mapped forward to the B-plane of the encounter within the arc.
Trajectory dispersions during the Grand Finale need instead to be mapped to equator crossings and targeted Cartesian
positions. In addition, trajectory arcs have typically covered a few orbital revolutions (~2-8 weeks), in order to span the
time between two encounters. However, the Grand Finale will encompass five months of time without an encounter which
necessitates an adjusted arc strategy. A modified arc strategy was developed based on OD behavior during long multi-rev
periods between encounters in the year leading up to the Grand Finale. The OD covariance study conducted for the Grand
Finale mission phase will also be examined.