The GRATTIS mission will demonstrate the end-to-end functionality and sensitivity of the Simplified Gravitational Reference Sensor (S-GRS), an ultra-precise inertial sensor for future Earth geodesy missions. These sensors are used to measure or compensate for all 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, the main science observable.
The S-GRS concept is a simplified version of the flight-proven LISA Pathfinder GRS. It consists of a free-falling cubic test mass (TM) inside an electrode housing (EH) that senses the position and orientation of the test mass and electrostatically applies forces and torques to it to keep it centered at the nanometer-level. The applied forces and torques required to do so are also used to determine the non-gravitational forces acting on the host spacecraft, as well as the spacecraft’s angular acceleration. Low-low satellite-to-satellite tracking missions like GRACE-FO and GRACE-C that utilize laser ranging interferometers are technologically limited by the acceleration noise performance of the electrostatic accelerometers currently in use, as well as by temporal aliasing associated with Earth’s dynamic gravity field. The S-GRS offers performance enhancements by removing the small test mass grounding wire used in the GRACE accelerometers, which limits its performance, and replacing it with a UV LED-based charge management system. The UV discharge system in turn enables (a) a more massive TM and (b) a larger gap between the TM and its electrode housing. These two parameters are the most important for acceleration noise performance.
GRATTIS will fly two identical S-GRS mounted next to one another near the center of mass of a 235 kg ESPA-class commercial microsatellite. The six-axis acceleration measurement capability of the S-GRS allows precision measurement of the spacecraft drag-induced translational acceleration, as well as the residual angular acceleration of the nominally inertially-pointed bus. By combining the outputs of each sensor and with the known relative position of the two TMs, we can recover the acceleration sensitivity (noise floor) of the S-GRS.
Fabrication and integration of the GRATTIS payload and spacecraft will be complete by the time of the 2026 NASA ESTIM. Launch is expected No Earlier Than February 1, 2027. This presentation will describe the status of the mission, the planned mission operations, the expected results, as well as implications for future use of this technology.