The Drag-free CubeSat

A drag-free spacecraft utilizes a Gravitational Reference Sensor (GRS) to shield an internal free-floating test mass (TM) from (a) external disturbances and (b) from disturbances caused by the spacecraft itself. The GRS measures the position of the spacecraft with respect to the TM and a feedback control system commands thrusters to maintain that position. In principle, the test mass is then completely freed from non-gravitational disturbances so that it and its “tender” spacecraft follow a pure geodesic. To date, three drag-free spacecraft have flown: TRIAD I in 1972, which provided the first navigation by satellite, Gravity Probe B in 2004, which tested predictions of Einstein’s general relativity theory, and the 2009 geodesy mission, GOCE (Steady-State Ocean Circulation Explorer). Next generation GRS technology for geodesy, fundamental physics and gravitational wave detection in space, has been under development at Stanford since 2004. Most recently a small scale instrument, called the 1U GRS has been proposed for a 3U CubeSat primarily for Earth aeronomy and geodesy applications. The 1U GRS consists of a 25 mm diameter spherical test mass housed inside a 50 mm cubic cavity. The sphere's position is sensed with a LED-based differential optical shadow sensor, its electric charge is controlled by photoemission using UV LEDs, and the spacecraft position is maintained with respect to the sphere using a cold gas micro-propulsion system. This paper highlights the history, applications, design, and laboratory technology development for this proposed CubeSat mission.