  We consider entanglement-assisted frequency estimation by Ramsey
interferometry, in the presence of dephasing noise from spatiotemporally
correlated environments.By working in the widely employed local estimation
regime, we show that even for infinite measurement statistics, noise renders
standard estimators biased or ill-defined. We introduce ratio estimators which,
at the cost of doubling the required resources, are insensitive to noise and
retain the asymptotic precision scaling of standard ones. While ratio
estimators are applicable also in the limit of Markovian noise, we focus on
non-Markovian dephasing from a bosonic bath and show how knowledge about the
noise spectrum may be used to maximize metrological advantage, by tailoring the
sensor's geometry. Notably, Heisenberg scaling is attained up to a logarithmic
prefactor by maximally entangled states.
