Modular Gravitational Reference Sensor (modular GRS) was proposed by the Stanford Team in 2004. In a modular GRS, the laser beam from the remote the sensor does not illuminate the proof mass directly. The internal measurement from the housing to proof mass is separated from the external interferometry. A double-sided grating further simplifies the structure and may better preserve the measurement precision. We review the recent progress in developing the modular GRS at Stanford. We are developing optical sensors with picometer resolution, capable of operating with a large gap for high precision readout. We have conducted an initial experiment incorporating RIF heterodyne detection and thus lowered the optical power compared with direct detection. We have demonstrated sub-nanoradian sensitivity of a grating angular sensor. We have successfully demonstrated fabrication of localized grating patterns on dielectric and gold surfaces. We have made critical progress in optical measurement of the mass center (MC) position of a spherical proof mass to a precision of a few micrometers. We are studying a method to experimentally determine the selfgravitational attraction via measurement of the moments of inertia. We have further demonstrated over 2700 hours of operation of a UV LED under typical AC charge management conditions. We are modeling the electrostatic field surrounding gapped housing wall, cubic and spherical proof masses. We have studied surface potential of metallic proof masses using a Kelvin probe and UV photoelectric current.
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