How do you measure movement as small as 1/100th of a human hair? Yet track widths and track movement of that size will need to be measured if tape cartridge capacities are to increase into the 1 to 10 TB areas. Track widths will have to be less than 1 micron. But before tape drives can achieve this, one must be able to accurately measure movements in the order of 0.1 micron in the track position, Figure 1. If you can’t measure it, you don’t know if you’ve achieved it. Up to this time, such precision was not needed. Track pitches were wide, as much as 500 microns in early 3490s. The tape could wander laterally half the track width and the signal could be read without servos because the track was so wide compared to the reader head. Guiding was simple. However, today tracks are typically 20 microns wide in LTO drives. Researchers are presently working on drives with track widths in the 5 to 1 micron region.