ESA GNC Conference Papers Repository

LISA Pathfinder is an ESA drag-free mission that is currently being developed to test a number of crucial technologies for the Laser Interferometer Space Antenna (LISA) mission and which scheduled to launch in 2010. The spacecraft includes two free flying test masses (TM) that are controlled with the Drag-Free and Attitude Control System (DFACS) which is one of the key technologies to be verified with the mission. The requirements for the different control loops in the DFACS have in the past been derived from the top level science requirement which requests a free fall performance of 3·10-14 m/s2/?Hz in the measurement bandwidth from 1 mHz to 30 mHz. However, this paper will focus on the impact of another top level requirement, namely the measurement accuracy of the Optical Metrology System (OMS) that is depending heavily on the misalignments between the optical bench and the test masses as they couple with the test mass jitter on cross-axes into the sensitive axis of the measurement. The paper will analyze the impact of the alignments in combination with test mass as well as spacecraft motion. Taking into account this analysis, the paper will show in a rigorous way, how to derive the additional requirements on the control loops from the alignment consideration. The current controller design will be analyzed with respect to these new requirements and it will be shown that a new set of drag-free controllers is necessary, in order to meet the additional requirements. The new set of controllers will be derived and it will be shown that the new set does indeed meet all requirements, the alignment driven ones as well as the original ones.