SAE AS 5393:2009-10-21

Blade trackers measure: (a) rotor blade height and (b) lead-lag for use in a Rotor Track and Balance (RT&B) function in a Health and Usage Monitoring System (HUMS). HUMS is a generic term for a system used to measure, monitor, process, and store information relating to the functioning and usage of an aircraft's on-board primary systems, including the engine(s).