  We consider the task of multiple parameter estimation in the presence of
strong correlated noise with a network of distributed sensors. We study how to
find and improve noise-insensitive strategies. We show that sequentially
probing GHZ states is optimal up to a factor of at most 4. This allows us to
connect the problem to single parameter estimation, and to use techniques such
as protection against correlated noise in a decoherence-free subspace, or
read-out by local measurements.
