Drag-free Small Satellite Platforms for Future Geodesy Missions

Continuous satellite geodesy measurements lasting into the foreseeable future are critical for the understanding of our changing planet. It is therefore imperative that we explore ways to reduce costs, while maintaining science return. Small satellite platforms represent a promising path forward if ways can be found to reduce the size, weight, and power of the necessary instrumentation. One key enabling technology is a precision small-scale drag-free system under development at the University of Florida and Stanford University. A drag-free satellite (a) contains and shields a free-floating test mass from all non-gravitational forces, and (b) precisely measures the position of the test mass inside the satellite. A feedback control system commands thrusters to fly the 'tender' spacecraft with respect to the test mass. Thus, both test mass and spacecraft follow a pure geodesic in spacetime. By tracking the relative positions of low Earth orbiting drag-free satellites, using laser interferometry for example, the detailed shape of geodesics, and through analysis, the higher order harmonics of the Earth's geopotential can be determined. Drag-free systems can be orders of magnitude more accurate that accelerometer-based systems because they fundamentally operate at extremely low acceleration levels, and are therefore not limited by dynamic range like accelerometers. Since no test mass suspension force is required, larger gaps between the test mass and satellite are possible, which reduces the level of unwanted disturbing forces produced by the satellite itself. The small satellite platform also enables cost-effective constellations, which can increase the temporal resolution of gravity field maps by more-frequently observing given locations on the Earth. Mixed-orbit constellations can also markedly enhance observational strength, decorrelate gravity coefficient estimates, and help address the fundamental aliasing problem that exists with previous missions. The constellation approach is also scalable. The drag-free system under development is based on the design of the first drag-free satellite, TRIAD I, flown in 1972 by the U.S. Navy and borrows some technologies from NASA's 2004 Gravity Probe B satellite. It consists of a 25 cm diameter sphere housed inside a 50 cm cubic cavity. The sphere's position is sensed with an LED-based differential optical shadow sensor, which provides feedback information to a microfabricated cold gas or electric propulsion system. The propulsion system, in conjunction with an attitude control system, maintains the satellite's position with respect to the test mass to < 100 micrometers. The test mass is secured during launch by a mechanical caging mechanism and its electric charge is controlled by photoemission using UV LEDs. The entire system, including thruster, is less than 10 kg and consumes less than 10 W. This talk will present the design of this small-scale drag-free system, the simulated performance of its drag-free control design, and the estimated sensitivity with respect to Earth' geopotential of pairs of drag-free small satellites equipped with a laser ranging system. The talk will conclude with a description of the ongoing technology demonstration campaign using small satellites and zero-g atmospheric flights, including a description the upcoming UV LED-based charge control demonstration mission, scheduled launch in November 2013.