ESA GNC Conference Papers Repository
Drag-free performance of the ST7 disturbance reduction system flight experiment on the LISA pathfinder
The Space Technology-7 Disturbance Reduction System (ST7-DRS) is an experiment package aboard the European Space Agency (ESA) LISA Pathfinder spacecraft. LISA Pathfinder launched from Kourou, French Guiana on December 3, 2015. The ST7-DRS is tasked to validate two specific technologies: colloidal micro-Newton thrusters (CMNT) to provide low-noise control capability of the spacecraft, and drag-free control/flight. This validation is performed using highly sensitive drag-free sensors (DFS), which are provided by the LISA Technology Package (LTP) of the European Space Agency. The ST7 Disturbance Reduction System is required to maintain the spacecraft's position with respect to two free-floating test masses to better than 10nm/vHz, along their sensitive axis (axis in optical metrology), with a goal of limiting the residual accelerations of those test masses to below 3x10-14 (1 + [f/3 mHz]²) m/s²/vHz, over the frequency range of 1 to 30 mHz. The ST7-DRS started its operations on August 14, 2016, after three periods of commissioning. All control modes and mode transitions were successfully tested and validated during commissioning. The dual drag-free and test mass acceleration requirements were evaluated in the Science mode (18-DOF) of the mission. This paper briefly describes the design and performance (based on the flight correlated models) of the control system for the Science mode. The control system uses 2 clusters of 4 micro-Newton thrusters for spacecraft attitude and position control, along with electrostatic actuation capability of the test masses, provided by the LTP. The sensing signals for the control loops are provided by the capacitive sensing of the LTP, with the option of using the Optical Measurement System (OMS) in six degrees of freedom for significantly increased accuracy and resolution. A star tracker provides attitude quaternion output for the spacecraft attitude control. Both, capacitive only sensing, as well as, capacitive and optical sensing options were exercised in flight. However, the drag-free and acceleration requirements were verified using the flight OMS data. The OMS flight data from multi-day noise runs, along with coherent thruster and electrostatic actuation commands, were used to construct spectra of the test mass position errors, as well as, test mass residual accelerations. These were compared to the requirements, as well as to the predicted spectra from flight-correlated simulation models. The paper provides a detailed assessment and discussion of the on-orbit performance.