High-stability temperature control for ST-7/LISA Pathfinder gravitational reference sensor ground verification testing

This article demonstrates experimental results of a thermal control system developed for ST-7 gravitational reference sensor (GRS) ground verification testing which provides thermal stability δT < 1 mK/√Hz to f < 0.1 mHz, and which by extension is suitable for in-flight thermal control of the LISA spacecraft to compensate solar irradiate 1/f fluctuations. Although for ground testing these specifications can be met fairly readily with sufficient insulation and thermal mass, in contrast, for spacecraft the very limited thermal mass calls for an active control system which can simultaneously meet disturbance rejection and stability requirements in the presence of long time delay; a considerable design challenge. Simple control laws presently provide ∼ 1mK/√Hz for >24 hours. Continuing development of a model predictive feedforward control algorithm will extend performance to <1 mK/√Hz at f < 0.01 mHz and possibly lower, extending LISA coverage of super massive black hole mergers.