Navigation of the GRAIL Spacecraft Pair Through the Extended Mission at the Moon

The GRAIL extended mission (XM) dramatically expands the scope of GRAIL's gravity science investigation by flying the pair of spacecraft at the lowest orbit the flight team can safely support. From the perspective of the Navigation team, the low orbit altitude introduces new challenges. At this lower altitude, navigation is more sensitive to higher-order terms of the gravity field so that orbit determination solutions are more difficult and there is less certainty of achieving maneuver targets. This paper reports on the strategy and performance of the Navigation system for GRAIL's XM. On a weekly basis, the Navigation team provided reference trajectory updates, designed three maneuvers, and reconstructed the execution of those maneuvers. In all, the XM involved 55 planned maneuvers; five were canceled. The results of the Navigation team's efforts, in terms of maintaining the reference-trajectory targets, satisfying requirements, and achieving desired separation distances, are assessed.