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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 24 hours. Continuing development of a model predictive feedforward control algorithm will extend performance to

We describe the calibration and operation of the ST7 GRS capacitive sensor system. A novel optical interferometer, featuring both fiber optic delivery and readout, is used for calibration of the capacitive sensor system and as a witness sensor during testing. The capacitive sensor showed 1 nm/√Hz performance from 1 mHz to 10 Hz. Above 10 mHz the optical system showed significantly better performance than the capacitive sensor, with a noise floor at 2 × 10−11 m/√Hz and a longer measurement standoff distance. The optical system was limited to 5 nm/√Hz at 1 mHz due to thermal disturbances of the laboratory environment, and could be reduced with a revised design and/or improved thermal stability.