Method for measuring PES noise of servo patterned media

A method of determining actuator position error in the measurement of servo position error signals (“PES”) from a magnetic disk having pre-patterned sector servo data pattern written into a plurality (“j”) of servo sectors defining substantially circular tracks whose centers are offset from the axis of rotation of a spindle onto which the disk is mounted, each servo sector generating a PES signal in a read-back magnetic transducer that is periodic in the radial, cross track dimension, comprising:

• a) stepping a magnetic transducer across a disc in i nominally equal radial increments, each step being offset from a nominal step by an actuator position error, where the number of increments is sufficient to span at least one PES period;
• b) at each step, measuring the PES at each of the servo sectors n times, where n is a predetermined number greater than zero;
• c) for each PES measurement, calculating an average of the n times measurements at each step to form an i ×j matrix of average PES measurements, each set of average measurements for each sector j having substantially identical cross track profiles, each cross track profile offset from the initial cross track profile by a phase difference, Φ[j];
• d) computing a radial phase offset Φ[j] of each cross track profile for each sector from the average PES measurements assuming, initially, that each step was of a nominal size;
• e) realigning the average PES measurements to have substantially the same phase;
• e) averaging the realigned PES measurements by step;
• f) decomposing, using a Fourier transformation, the averaged measurements to determine an average cross track profile;
• g) comparing the average cross track profile to the measurements to determine the radial distance between the two for each PES measurement;
• h) averaging the distances by step to determine an average radial distance for each step;
• i) updating a offset for each step by adding to the offset the average radial distance, the offset initially being set to zero;
• and
• j) repeating steps (d) through (i) by adding the respective offsets to the respective steps radial measurement locations until the largest average radial distance is less than a predetermined number;
• wherein the resultant offset is the actuator position error at each step.