once around and twice around components of runout tracking error these are stored in the microprocessor 562 as oncereal once around real components onceim once around imaginary components twicereal twice around real components and twiceim twice around imaginary components runout tables stored in ram for example are then generated by multiplying the dft by a complex number which takes into account the actuator characteristics and amplifier gains so as to correct for the measured repeatable runout error when applied to the actuator as an independent forcing function these forcing functions are stored as l 1 re l 1 im l 2 re and l 2 im which are dft representations of the once around and twice around runout correction functions fig3 d disk clamp centering disk tilt disk thickness and actuator geometries make the repeatable runout errors dependent on the position in the actuator stroke and on which surface of the disk is being used for this reason a different table is generated for each surface a correction function is also applied which is dependant on the track number and which compensates for variations in geometries between the disk and the actuator over the stroke this is the variable u201c target u201d in the flowchart where target is dependant on position in the stroke and represents the desired track specifically referring to the flow charts in fig3 a through 37 d a preferred embodiment of the invention is implemented as follows in fig3 a the overall structure of the process performed by the microprocessor 562 is set out in this.