PRECISE AND EFFICIENT FORMATION KEEPING AT EARTH-SUN L2 FOR STARSHADE MISSIONS

Current starshade concepts for imaging exo-Earths would operate at the Earth-Sun L2 point and consist of a starshade flying in formation tens to hundreds of thousands of kilometers from its telescope. The starshade would need to be aligned to the meter level across the line of sight from telescope to target star. This paper reports work aimed at maturing the technology readiness level of starshade formation sensing and control for the starshade science phase, during which precision alignment is required. First, the enabling formation fine lateral sensor using pupil plane images and a formation control algorithm that provides high efficiency for science observations are presented. Then, other elements of an end-to-end starshade GNC system for the precision alignment phase are outlined: the longitudinal formation control algorithm, an RF communication link and ranging sensor, the formation estimator, and a thrust allocation algorithm. A high-fidelity simulation environment, which includes dynamics, actuator, and sensor models, is also summarized. Finally, for a representative observation scenario with WFIRST, this GNC system for precision alignment is demonstrated in a Monte Carlo simulation, robustly achieving observationally efficient meter-level control.