LISA pathfinder drag-free control and system implications

The top-level requirement of the LISA Pathfinder mission is the verification of pure relative free fall between two test masses with an accuracy of about 3×10^−14 m s^−2 Hz^−1/2 in a measurement bandwidth between 1 mHz and 30 mHz. The drag-free control system is one of the key technology elements that shall be verified. Its design is strongly connected to the overall system and experimental design, in particular, via the following issues: the differential test mass motion and thus the science measurements depend on the control system; design constraints, such as negative stiffness of test masses and electrostatic actuation cross-talk, have an impact on science and control system performance; derived requirements for control system components, in particular, the micro- propulsion system, must be within reasonable and feasible limits. In this paper, the control design approach is outlined and the system-related issues are addressed.