In a disk storage system, the disk may include a full null servo position error signal (“PES”) wedge, followed by a data wedge, and then a short null servo PES wedge. To improve the accuracy with which information read from the short null servo PES wedge can be used to help keep the read head centered over the information track being read, the short null servo PES wedge may include a calibration field. Information read from the calibration field can be used to compensate the subsequently read PES information in the short wedge for possible sampling phase error that may have accumulated since the full null servo PES wedge was read. This type of short null servo PES wedge also has other possible uses.