ESA GNC Conference Papers Repository

The Lisa Pathfinder (LPF) mission is a technology demonstrator for a gravity-wave observatory carrying the most critical technologies needed to measure gravity waves in space. These include an optical interferometer to measure very precisely the distance between two free-floating test masses, one in drag-free motion and one electrostatically controlled. In order to demonstrate that the noise levels that can be achieved are sufficiently low to measure gravity waves, the spacecraft had to be placed far from Earth's gravity at the Earth-Sun Lagrangian point L1. Lisa Pathfinder was launched on 3rd December 2015 on Vega. 50 days after launch LPF reached its operational orbit at L1. At this point the propulsion module (PRM) separated from the science module (SCM), leaving the spacecraft to rely only on micro-propulsion for attitude and orbit control. The effect of tip-off rates imparted by the separation system to the SCM, combined with the low thrust authority of the micro-propulsion, would have potentially exposed the spacecraft to loss of a Sun pointed attitude, which would have been disastrous for the mission (no power generation from the solar array and Sun not in the field of view of the Sun Sensors). This risk was overcome by spinning up LPF to 5 deg/s before SCM-PRM separation in order to achieve gyroscopic stability on the SCM, leaving then MPACS in charge of a long (8 hours) SCM de-nutation and de-spin performed with the Cold Gas thrusters. The next MPACS operation was then to achieve a steady state Sun pointing attitude. The SCM then entered the Micro-propulsion Normal Mode (MNM), where the attitude was very finely controlled with a gyroless AOCS architecture, only relying on the STR as attitude sensor. A series of orbit control manoeuvres was executed in order to correct the disturbances generated on the orbit by the spin-up, separation and de-spin phases: while the orbit disturbance was significantly less than 1m/s, some of the orbit control manoeuvres lasted more than 24 hours due to the limited thrust authority of the Cold Gas micropropulsion. A peculiarity of the MPACS is that it has a unidirectional thrusters configuration (all thrusters are producing acceleration towards the Sun); the full 6 DOF control is however restored, when the spacecraft is in steady state, by using the Sun as a virtual thruster. This paper will present an overview of the MPACS performance, focusing on the De-nutation/De-spin phase, Orbit Control manoeuvres, gyroless MNM and 6 DOF control with virtual thruster. LPF MPACS has exhibited flawless in orbit performance, with flight TM showing a very good match with the predicted performance, as this paper will show.