The Nancy Grace Roman Space Telescope's Attitude Control System

The Nancy Grace Roman Space Telescope ("Roman") mission has completed its design phase and is currently in integration and testing, with a launch readiness date of October 2026. The mission is designed to investigate essential questions in the areas of dark energy, exoplanets, and infrared astrophysics. Roman uses a 2.4-meter primary telescope and two instruments: the Wide Field Instrument (WFI) and the Coronagraph Instrument (CGI). In order to address the critical science requirements, the Roman mission will conduct large-scale surveys of the infrared sky, requiring both agility and precision pointing (8 milli-arcsec stability, 14 milli-arcsec jitter). This paper describes some of the challenges this mission profile presents to the attitude control system (ACS), and some of the design elements chosen to accommodate those challenges. The Fine Guidance System (FGS), which is integral to the WFI instrument, is capable of tracking up to 18 guide stars, enabling robust FGS acquisition and precision pointing. Mission requirements drive the FGS acquisition process to be rapid, with a requirement of ten seconds on average. Control and estimator designs that support this capability are described. The wheel nullspace is used to minimize zero-crossing disturbances and jitter excitation. Aside from pointing precision, one of the most challenging ACS requirements was thruster maneuver accuracy. The design for meeting this requirement is discussed. Also, algorithms for pointing the high-gain antenna (HGA) are described. Finally, the ACS hardware suite is presented.