  We describe a 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, which is the main science
observable. Low-low satellite-to-satellite tracking missions like GRACE-FO that
utilize laser ranging interferometers are technologically limited by the
acceleration noise performance of their electrostatic accelerometers, in
addition to temporal aliasing associated with Earth's dynamic gravity field.
The S-GRS is estimated to be at least 40 times more sensitive than the GRACE
accelerometers and more than 500 times more sensitive if operated on a
drag-compensated platform. The improved performance is enabled by increasing
the mass of the sensor's test mass, increasing the gap between the test mass
and its electrode housing, removing the small grounding wire used in the GRACE
accelerometers and replacing them with a UV LED-based charge management system.
This level of improvement allows future missions to fully take advantage of the
sensitivity of the GRACE-FO laser Ranging Interferometer in the gravity
recovery analysis. The S-GRS concept is a simplified version of the
flight-proven LISA Pathfinder GRS. Our performance estimates are based on
models vetted during the LISA Pathfinder flight and the expected Earth orbiting
spacecraft environment based on flight data from GRACE-FO. The relatively low
volume, mass, and a power consumption enables use of the S-GRS on ESPA-class
microsatellites, reducing launch costs or enabling larger numbers of satellite
pairs to be utilized to improve the temporal resolution of Earth gravity field
maps.

Instrumentation and Detectors