  The precursor ESA mission LISA-Pathfinder, to be flown in 2013, aims at
demonstrating the feasibility of the free-fall, necessary for LISA, the
upcoming space-born gravitational wave observatory. LISA Technology Package
(LTP) is planned to carry out a number of experiments, whose main targets are
to identify and measure the disturbances on each test-mass, in order to reach
an unprecedented low-level residual force noise. To fulfill this plan, it is
then necessary to correctly design, set-up and optimize the experiments to be
performed on-flight and do a full system parameter estimation. Here we describe
the progress on the non-linear analysis using the methods developed in the
framework of the \textit{LTPDA Toolbox}, an object-oriented MATLAB Data
Analysis environment: the effort is to identify the critical parameters and
remove the degeneracy by properly combining the results of different
experiments coming from a closed-loop system like LTP.
