Optical Navigation During Cassini's Solstice Mission

After nearly twenty years in flight, Cassini’s mission at Saturn will conclude as
it purposely dives into Saturn’s atmosphere on September 15, 2017. Primarily
to avoid moons potentially harboring conditions for life and with propellant very
low, the intentional plunge into the atmosphere was set in motion years ago. We
take this opportunity to give an overview of the optical navigation and its roles
throughout the mission. The paper describes the navigation process and the evolution
of optical navigation over the past thirteen years. The last equatorial phase
of the Cassini mission was particularly challenging for the OD team as the Saturn
system was not being estimated anymore, and it had been a few years since the last
icy moon flybys. Science pictures of Enceladus one month prior to the Enceladus
encounters confirmed the moon’s position to be in good agreement with the Saturn
system dynamical modeling used. This reduced Enceladus’s absolute uncertainty
by a factor of three, less than 1 km, and gave confidence the navigation team could
achieve acceptable flybys and meet science objectives.