  We investigate the structure of the invariant measure of space-time chaos by
adopting an "open-system" point of view. We consider large but finite windows
of formally infinite one-dimensional lattices and quantify the effect of the
interaction with the outer region by mapping the problem on the dynamical
characterization of localized perturbations. This latter task is performed by
suitably generalizing the concept of Lyapunov spectrum to cope with
perturbations that propagate outside the region under investigation. As a
result, we are able to introduce a "volume"-propagation velocity, i.e. the
velocity with which ensembles of localized perturbations tend to fill volumes
in the neighbouring regions.
