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 aircrafts on-board primary systems, including the engine(s).
SAE AS5393-2002 Referenced Document
SAE AS5391-2002 Health and Usage Monitoring System Accelerometer Interface Specification
SAE AS5392-2002 Health and Usage Monitoring System, Rotational System Indexing Sensor Specification