GRATTIS: The Gravitational Reference Advanced Technology Test In Space

The Gravitational Reference Advanced Technology Test In Space (GRATTIS) mission will demonstrate the functionality and sensitivity performance 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. Low-low satellite-to-satellite tracking missions like GRACE-FO that utilize laser ranging are technologically limited by the acceleration noise performance of their accelerometers, as well as temporal aliasing associated with Earth’s dynamic gravity field. The S-GRS is estimated to be 10 times more sensitive than the GRACE accelerometer requirement and more than 200 times more sensitive if operated drag-free.

The S-GRS concept is a simplified version of the LISA Pathfinder GRS. It consists of a free-falling cubic test mass (TM) inside an electrode housing that senses the position and orientation of the TM and electrostatically applies forces and torques to keep it centered at a nanometer-level. The applied forces and torques required are also used to determine the non-gravitational forces acting on the host spacecraft, as well as the spacecraft’s angular acceleration. The improved performance of the S-GRS relative to the accelerometers used on the GRACE missions is enabled by removing the small grounding wire used in the GRACE accelerometers, which limits its performance, and replacing it with a UV LED-based charge management system, increasing the mass of the sensor’s TM, and increasing the gap between the TM and its electrode housing.

GRATTIS will fly two identical S-GRS mounted next to one another near the center of mass of a 180 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. Our mission goal is to demonstrate acceleration noise performance of ≤10^–11 m/s²Hz^1/2.

The PI and science team is led by the University of Florida (UF) and includes relevant experts from Texas A&M University (TAMU) and CrossTrac Engineering. The S-GRS mechanical sensor heads are provided by BAE Space & Mission Systems (formerly Ball Aerospace), while the S-GRS electronics units are provided by Fibertek, Inc. CrossTrac Engineering provides the S-GRS software and program management. The UF-led team will integrate the flight payload into a single thermal/mechanical enclosure and perform ground testing to the extent possible. Apex Space will provide the Aries microsatellite bus and launch services via a SpaceX F9 Transporter mission planned for Q1 2027. This presentation will describe the S-GRS technology development and planned GRATTIS demonstration mission.