  Tidal interactions between Saturn and its satellites play a crucial role in
both the orbital migration of the satellites and the heating of their
interiors. Therefore constraining the tidal dissipation of Saturn (here the
ratio k2/Q) opens the door to the past evolution of the whole system. If
Saturn's tidal ratio can be determined at different frequencies, it may also be
possible to constrain the giant planet's interior structure, which is still
uncertain. Here, we try to determine Saturn's tidal ratio through its current
effect on the orbits of the main moons, using astrometric data spanning more
than a century. We find an intense tidal dissipation (k2/Q= (2.3 \pm 0.7)
\times 10-4), which is about ten times higher than the usual value estimated
from theoretical arguments. As a consequence, eccentricity equilibrium for
Enceladus can now account for the huge heat emitted from Enceladus' south pole.
Moreover, the measured k2/Q is found to be poorly sensitive to the tidal
frequency, on the short frequency interval considered. This suggests that
Saturn's dissipation may not be controlled by turbulent friction in the fluid
envelope as commonly believed. If correct, the large tidal expansion of the
moon orbits due to this strong Saturnian dissipation would be inconsistent with
the moon formations 4.5 Byr ago above the synchronous orbit in the Saturnian
subnebulae. But it would be compatible with a new model of satellite formation
in which the Saturnian satellites formed possibly over longer time scale at the
outer edge of the main rings. In an attempt to take into account for possible
significant torques exerted by the rings on Mimas, we fitted a constant rate
da/dt on Mimas semi-major axis, also. We obtained an unexpected large
acceleration related to a negative value of da/dt= -(15.7 \pm 4.4) \times 10-15
au/day.
