Technology Supporting the Upcoming GRACE-C Mission and Other Mass Change Designated Observable Missions

Not only does BAE Systems Space and Mission Systems (formerly Ball Aerospace) work on large missions like building mirrors for the James Webb Space Telescope and science instruments for NOAA's Geostationary Extended Observations (GeoXO), we also develop component and subsystem technologies like the phased array antennas on Orion and laser stabilization cavities for the Gravity Recovery and Climage Experiment (GRACE) Follow-On and GRACE-C missions. BAE is currently exploring new accelerometer technologies with the Univeristy of Florida for better gravity measurements and other geodesy missions. The NASA-funded Simplified Gravitational Reference Sensor (S-GRS) IIP project, along with the upcoming InVEST program, GRATTIS, are developing an ultra-precise inertial sensor for future Earth geodesy missions. These sensors are used to measure or compensate for non-gravitational accelerations of the host spacecraft so that they can be removed in the data analysis to recover spacecraft motion due to Earth's gravity field, which is the main science observable. The S-GRS design follows that of the flight-proven LISA Pathfinder (LPF) Gravitational Reference Sensor (GRS) that represents the state of the art in precision inertial sensors [1], [2]. The S-GRS instrument presented in this paper is a scaled-down version of the LPF GRS, with reduced mass and complexity, and is optimized with respect to performance for future geodesy missions [3]. In the on-orbit operational state, the GRS functions by sensing the state of a precision cubic Test Mass (TM) with respect to the surrounding Electrode Housing via non-contacting electrostatic distance sensing and actuation electrodes. The S-GRS Instrument approach consists to two opposing Caging Mechanism (CM) assemblies that constrain the TM for launch and release into pure freefall on-orbit such that the residual velocity is within the non-contact actuation system control authority. This paper focuses on the novel mechanism design, initial functional testing, and recent results of the S-GRS CM.