Simplified Gravitational Reference Sensors for Future Earth Constellations

Our team led by the University of Florida, in collaboration with Caltech/JPL, Ball Aerospace, and Embry-Riddle Aeronautical University propose to elevate a Simplified Gravitational Reference Sensor (S-GRS), an ultra-precise inertial sensor for future Earth geodesy missions, from TRL 3 to TRL 5. 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. They consist of a dense metallic test mass that is free-falling inside an electrode housing. When operated as an accelerometer, small electrostatic forces are applied to the test mass to keep it centered in its housing. The applied force provides information about spacecraft acceleration. In a drag-compensated scheme, spacecraft propulsion is used to directly compensate for atmospheric drag, reducing the electrostatic force needed to keep the test mass centered and also the force noise on the test mass. 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, as well as by temporal aliasing associated with Earth's dynamic gravity field. The current accelerometers, used in GRACE and GRACE-FO have a limited sensitivity of ~1E–10 m/s^2 Hz^1/2 around 1 mHz. The S-GRS is estimated to be at least 40 times more sensitive than the GRACE accelerometers if operated on a GRACE-like spacecraft bus and more than 500 times more sensitive if operated on a drag-compensated platform. The improved performance is primarily enabled by (a) removing the small test mass grounding wire used in the GRACE accelerometers and replacing it with a non-contact UV photoemission-based charge management system, (b) increasing the mass of the sensor's test mass, and (c) increasing the gap between the test mass and its electrode housing. The S-GRS concept, as well as two candidate mission architectures, were developed in our current IIP Instrument Concept Demonstration (ICD) project. During our ICD effort we have shown that this level of improvement allows future missions to fully take advantage of the sensitivity of the GRACE-FO Laser Ranging Interferometer (LRI) 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 low Earth orbit spacecraft environment based on flight data from GRACE-FO. The relatively low volume (~5,000 cm^3), mass (<13 kg), and power consumption (<20 W) 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. Our approach to advancing the technology readiness will follow two primary paths. The first will be to develop a Metrology and Charge Management Testbed at the University of Florida that will be used to demonstrate S-GRS readout sensitivity, charge management performance, and test mass and drag-compensation control system performance in a hardware-in-the-loop configuration. These tests will first be done on a bench-top in air (TRL 4), then a higher fidelity unit will be tested in the UF thermal vacuum chamber (TRL 5). The second path will be to produce a Structural, Thermal, and Optical prototype and test it in the relevant environment through shock and vibration testing and thermal vacuum chamber testing. This TRL 5 prototype will be designed and fabricated by Ball Aerospace with oversight from UF. This programmatic choice expedites technology transfer to industry allowing earlier flight readiness. The success of this project will allow the S-GRS to be ready for a flight demonstration in the second half of this decade.

Enabling lower cost innovative remote sensing instrument development from concept through breadboard and demonstration

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