A method is disclosed for writing servo patterns for tracks on a rotating magnetic
disk medium of a disk drive. Servo patterns are written on a reference track of
the disk medium. Track following of the written servo patterns on the reference
track is performed using a servo loop having a closed-loop response. A position
error signal is generated for the reference track based on the track following.
A correction signal is generated based on the track following using an observer
of a one-dimensional state model that is equivalent to a two-dimensional state
model of the servo loop. Servo patterns are written on a target track during track
following of the reference track. The position error signal of the servo loop is
adjusted based on the correction signal to reduce radial error propagation from
the reference track to the target track.
