Determining a skew error signal (SES) offset used to determine an SES to adjust heads in a drive unit

1. A computer program product for controlling heads in a drive to access data on a recordable storage media, the computer program product comprising a computer readable storage medium having computer readable program code embodied therein that executes to perform operations, the operations comprising:

reading, by a first servo read element and second servo read element on a first head, first and second position information from a servo pattern on a recordable storage media;

determining a first difference in a first orientation with respect to a direction of movement of the recordable storage media based on the read first and second position information;

reading, by the first servo read element and a third servo read element on a second head, third and fourth position information from the servo pattern;

determining a second difference in a second orientation with respect to the direction of movement of the recordable storage media based on the read third and fourth position information;

calculating an offset based on the determined first and second differences; and

using the calculated offset to generate an error signal to adjust the first and second heads during read and write operations with respect to the recordable storage media.

2. The computer program product of claim 1 , wherein the first head includes a write element and the second head includes a read element positioned to read data written by the write element on the first head, wherein the read element on the second head performs a read following a write written by the write element on the first head.

3. The computer program product of claim 1 , wherein the first orientation is horizontal with respect to the direction of movement of the recordable storage media and wherein the second orientation is vertical with respect tot he direction of movement.

4. The computer program product of claim 3 , wherein the offset comprises a skew error signal (SES) offset, and wherein calculating the SES offset comprises:

calculating a first SES from the first difference;

calculating a second SES from the second difference;

calculating the SES offset from the first SES and the second SES.

5. The computer program product of claim 4 , wherein calculating the first SES considers a first distance between the first and second servo read elements, and wherein calculating the second SES considers a second distance between the first and third servo read elements in the first orientation.

6. The computer program product of claim 5 , wherein calculating the first SES comprises calculating an arctangent of the first difference divided by the first distance, and wherein calculating the second SES comprises calculating an arctangent of the second difference divided by the second distance, and wherein the SES offset comprises the second SES minus the first SES.

7. The computer program product of claim 5 , wherein the servo pattern has a first and second servo bands, wherein the first servo band is read by the first servo read element and the third servo read element and wherein the second servo band is read by the second servo read element, wherein the first and second position information comprises temporal information indicating when the first and second servo read elements read the first and second servo bands, respectively, and wherein the first distance comprises a distance in physical space between the first and second servo bands and wherein the second distance comprises a distance in physical space between the first and third servo read elements along.

8. The computer program product of claim 1 , wherein the recordable storage media comprises a magnetic tape storage media, and wherein the drive comprises a tape drive.

9. The computer program product of claim 4 , wherein the generated error signal comprises a SES used to skew the first and second heads, wherein using the calculated SES offset comprises:

reading, with the first servo read element and the second servo read element on the first head, new first position information and new second position information from the servo pattern;

determining a new first difference in the first orientation based on the read new first and second positions;

calculating a new first SES from the new first difference;

calculating the SES from the new first SES and the SES offset; and

using the calculated SES to adjust the skew of the first and second heads.

10. The computer program product of claim 9 , wherein calculating the SES comprises adding the new first SES to the SES offset.

11. A system to access data on a recordable storage media having a servo pattern, comprising:

a first head having a first servo read element and second servo read element;

a second head having a third servo read element;

a computer readable storage media having logic executed to perform operations, the operations comprising:

receiving first and second position information read by the first servo read element and the second servo read element from the servo pattern on the recordable storage media;

determining a first difference in a first orientation with respect to a direction of movement of the recordable storage media based on the read first and second position information;

receiving third and fourth position information read by the first servo read element and the third servo read element from the servo pattern;

determining a second difference in a second orientation with respect to the direction of movement of the recordable storage media based on the read third and fourth position information;

calculating an offset based on the determined first and second differences; and

using the calculated offset to generate an error signal to adjust the first and second heads during read and write operations with respect to the recordable storage media.

12. The system of claim 11 , wherein the first head includes a write element and the second head includes a read element positioned to read data written by the write element on the first head, wherein the read element on the second head performs a read following a write written by the write element on the first head.

13. The system of claim 11 , wherein the first orientation is horizontal with respect to the direction of movement of the recordable storage media and wherein the second orientation is vertical with respect tot he direction of movement.

14. The system of claim 13 , wherein the offset comprises a skew error signal (SES) offset, and wherein calculating the SES offset comprises:

calculating a first SES from the first difference;

calculating a second SES from the second difference;

calculating the SES offset from the first SES and the second SES.

15. The system of claim 14 , wherein calculating the first SES considers a first distance between the first and second servo read elements, and wherein calculating the second SES considers a second distance between the first and third servo read elements in the first orientation.

16. The system of claim 15 , wherein calculating the first SES comprises calculating an arctangent of the first difference divided by the first distance, and wherein calculating the second SES comprises calculating an arctangent of the second difference divided by the second distance, and wherein the SES offset comprises the second SES minus the first SES.

17. The system of claim 15 , wherein the servo pattern has a first and second servo bands, wherein the first servo band is read by the first servo read element and the third servo read element and wherein the second servo band is read by the second servo read element, wherein the first and second position information comprises temporal information indicating when the first and second servo read elements read the first and second servo bands, respectively, and wherein the first distance comprises a distance in physical space between the first and second servo bands and wherein the second distance comprises a distance in physical space between the first and third servo read elements along.

18. The system of claim 11 , wherein the recordable storage media comprises a magnetic tape storage media, and wherein the system comprises a tape drive.

19. The system of claim 14 , wherein the generated error signal comprises a SES used to skew the first and second heads, wherein using the calculated SES offset comprises:

receiving new first position information and new second position information read by the first servo read element and the second servo read element from the servo pattern;

determining a new first difference in the first orientation based on the read new first and second positions;

calculating a new first SES from the new first difference;

calculating the SES from the new first SES and the SES offset; and

using the calculated SES to adjust the skew of the first and second heads.

20. The system of claim 19 , wherein calculating the SES comprises adding the new first SES to the SES offset.

21. A method for controlling heads in a drive to access data on a recordable storage media, comprising:

reading, by a first servo read element and second servo read element on a first head, first and second position information from a servo pattern on a recordable storage media;

determining a first difference in a first orientation with respect to a direction of movement of the recordable storage media based on the read first and second position information;

reading, by the first servo read element and a third servo read element on a second head, third and fourth position information from the servo pattern;

determining a second difference in a second orientation with respect to the direction of movement of the recordable storage media based on the read third and fourth position information;

calculating an offset based on the determined first and second differences; and

using the calculated offset to generate an error signal to adjust the first and second heads during read and write operations with respect to the recordable storage media.

22. The method of claim 21 , wherein the first head includes a write element and the second head includes a read element positioned to read data written by the write element on the first head, wherein the read element on the second head performs a read following a write written by the write element on the first head.

23. The method of claim 21 , wherein the first orientation is horizontal with respect to the direction of movement of the recordable storage media and wherein the second orientation is vertical with respect tot he direction of movement.

24. The method of claim 23 , wherein the offset comprises a skew error signal (SES) offset, and wherein calculating the SES offset comprises:

calculating a first SES from the first difference;

calculating a second SES from the second difference;

calculating the SES offset from the first SES and the second SES.

25. The method of claim 24 , wherein calculating the first SES considers a first distance between the first and second servo read elements, and wherein calculating the second SES considers a second distance between the first and third servo read elements in the first orientation.

26. The method of claim 25 , wherein calculating the first SES comprises calculating an arctangent of the first difference divided by the first distance, and wherein calculating the second SES comprises calculating an arctangent of the second difference divided by the second distance, and wherein the SES offset comprises the second SES minus the first SES.

27. The method of claim 25 , wherein the servo pattern has a first and second servo bands, wherein the first servo band is read by the first servo read element and the third servo read element and wherein the second servo band is read by the second servo read element, wherein the first and second position information comprises temporal information indicating when the first and second servo read elements read the first and second servo bands, respectively, and wherein the first distance comprises a distance in physical space between the first and second servo bands and wherein the second distance comprises a distance in physical space between the first and third servo read elements along.

28. The method of claim 21 , wherein the recordable storage media comprises a magnetic tape storage media, and wherein the drive comprises a tape drive.

29. The method of claim 24 , wherein the generated error signal comprises a SES used to skew the first and second heads, wherein using the calculated SES offset comprises:

reading, with the first servo read element and the second servo read element on the first head, new first position information and new second position information from the servo pattern;

determining a new first difference in the first orientation based on the read new first and second positions;

calculating a new first SES from the new first difference;

calculating the SES from the new first SES and the SES offset; and

using the calculated SES to adjust the skew of the first and second heads.

30. The method of claim 29 , wherein calculating the SES comprises adding the new first SES to the SES offset.