  LISA PathFinder (LPF) will be flown with the objective to test in space key
technologies for LISA. However its sensitivity goals are, for good reason, one
order of magnitude less than those which LISA will have to meet, both in
drag-free and optical metrology requirements, and in the observation frequency
band. While the expected success of LPF will of course be of itself a major
step forward to LISA, one might not forget that a further improvement by an
order of magnitude in performance will still be needed. Clues for the last leap
are to be derived from proper disentanglement of the various sources of noise
which contribute to the total noise, as measured in flight during the
PathFinder mission. This paper describes the principles, workings and
requirements of one of the key tools to serve the above objective: the
diagnostics subsystem. This consists in sets of temperature, magnetic field,
and particle counter sensors, together with generators of controlled thermal
and magnetic perturbations. At least during the commissioning phase, the latter
will be used to identify feed-through coefficients between diagnostics sensor
readings and associated actual noise contributions. A brief progress report of
the current state of development of the diagnostics subsystem will be given as
well.
