  We build a model of radius-to-frequency mapping in magnetospheres of neutron
stars and apply it to frequency drifts observed in Fast Radio Bursts. We assume
that an emission patch propagates along the dipolar magnetic field lines
producing coherent emission with frequency, direction and polarization defined
by the local magnetic field. The observed temporal evolution of the frequency
depends on relativistic effects of time contraction and the curvature of the
magnetic field lines. The model generically produces linear scaling of the
drift rate, $\dot{\omega} \propto - \omega$, matching both numerically and
parametrically the rates observed in FBRs; a more complicated behavior of
$\dot{\omega} $ is also possible. Fast rotating magnetospheres produce higher
drifts rates for similar viewing parameters than the slowly rotating ones. In
the case of repeaters same source may show variable drift pattens depending on
the observing phase. We expect rotational of polarization position angle
through a burst, though by smaller amount than in radio pulsars. All these
findings compare favorably with properties of FBRs, strengthening their
possible loci in the magnetospheres of neutron stars.
