  We investigate phase and frequency estimation with different measurement
strategies under the effect of collective phase noise. First, we consider the
standard linear estimation scheme and present an experimentally realisable
optimization of the initial probe states by collective rotations. We identify
the optimal rotation angle for different measurement times. Second, we show
that sub-shot noise sensitivity - up to the Heisenberg limit - can be reached
in presence of collective phase noise by using differential interferometry,
where one part of the system is used to monitor the noise. For this, not only
GHZ states but also symmetric Dicke states are suitable. We investigate the
optimal splitting for a general symmetric Dicke state at both inputs and
discuss possible experimental realisations of differential interferometry.
