Position sensors for flight testing of LISA drag-free control

Space-based detection of gravitational radiation is limited at low frequencies by stray force contamination of the orbits of ideally "free-falling" test masses. Shielding a test mass from external forces with a satellite requires precision position sensing that does not itself introduce excessive stray force or springlike coupling to satellite motion. The LISA (Laser Interferometer Space Antenna) mission to observe gravitational waves between 0.1 and 1000 mHz must limit stray acceleration to the fm/s2/√Hz level, with satellite position control at the nm/√Hz level. We present here a design, and a discussion of electrostatic disturbances, for a capacitive position sensor for LISA "drag-free" control, which will be flight-tested with the LISA Technology Package (LTP).