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LISA Pathfinder is the technological demonstrator space mission for thefuture gravitational waves observatory in space eLISA, with the aim of measurethe differential acceleration between free-falling test masses orbiting in thesame apparatus at a level of 30 fm/s-2Hz-1/2 at 1 mHz. Because the satellitecan't follow the two masses at the same time, the second mass must be forced tofollow either the other one or the spacecraft. The actuation force applied tocompensate this effect introduces a dominant source of force noise in themission noise budget at frequency near and below the mHz. The free-fall modeactuation control scheme has been designed to suppress this noise source andavoid actuation instabilities: actuation is limited to brief periodic impulses,with test masses in free fall in between two kicks. This actuation-free motionis then analyzed for the remaining sources of acceleration ultra noise. Afree-fall mode parallel testing has been successfully implemented on torsionpendulum facility at the University of Trento and the results of themeasurement campaign are ready to be shown and discussed. The implementation ofthe data analysis algorithms to estimate the spectrum of the acceleration ofthe controlled test mass, and the estimation of system parameters such as theforce gradients, stiffness terms, are also part of this work. The free-fallmode has been tested at the level of 2 fNm/Hz-1/2, at the frequencies of 1 mHz,corresponding to an acceleration of about 100 fm /s-2 Hz-1/2. The ground testshave nevertheless allowed to develop and test quantitatively this innovativemethod of noise reduction due to actuation noise, contributing to itsdevelopment for the LISA Pathfinder mission.

A torsion pendulum ground test of the LISA Pathfinder Free-fall mode