  LISA Pathfinder is the technological demonstrator space mission for the
future gravitational waves observatory in space eLISA, with the aim of measure
the differential acceleration between free-falling test masses orbiting in the
same apparatus at a level of 30 fm/s-2Hz-1/2 at 1 mHz. Because the satellite
can't follow the two masses at the same time, the second mass must be forced to
follow either the other one or the spacecraft. The actuation force applied to
compensate this effect introduces a dominant source of force noise in the
mission noise budget at frequency near and below the mHz. The free-fall mode
actuation control scheme has been designed to suppress this noise source and
avoid actuation instabilities: actuation is limited to brief periodic impulses,
with test masses in free fall in between two kicks. This actuation-free motion
is then analyzed for the remaining sources of acceleration ultra noise. A
free-fall mode parallel testing has been successfully implemented on torsion
pendulum facility at the University of Trento and the results of the
measurement campaign are ready to be shown and discussed. The implementation of
the data analysis algorithms to estimate the spectrum of the acceleration of
the controlled test mass, and the estimation of system parameters such as the
force gradients, stiffness terms, are also part of this work. The free-fall
mode 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 tests
have nevertheless allowed to develop and test quantitatively this innovative
method of noise reduction due to actuation noise, contributing to its
development for the LISA Pathfinder mission.
