Hard disk drives access data stored on their disk surfaces by positioning a slider containing a read-write head over a track on one of the disk surfaces.  They require careful control of the clearance of the slider over the accessed disk surface to insure the reliability of reading and writing the data.  Since the incorporation of Flying height On Demand (FOD) capabilities, tables of FOD control values for various zones of tracks on the disk surface are often used.  These tables may list an extreme control value, after which the slider experiences Head to Disk Contact, which is referred herein as Touch Down (TD). 
The tracks of the disk surface are often organized into servo wedges radiating from the disk drive's spindle hub.  Each servo wedge contains a servo region separating a user data region from the user data region of a neighboring servo wedge.  The servo region often includes an Amplitude Gain Control (AGC) signal region.  The AGC signal region is written with an analog signal pattern that may be read to create an AGC calibration signal, which is then used to create a Servo AGC (SAGC) value.  The SAGC value controls the amplification gain the read-write head in accessing the servo data region. 
The need to know when touch down occurs has been a long term concern, but the problems of detecting touch down have worsened as clearance tolerances have narrowed.  What works well at a clearance of ten nanometers (nm) may be too erratic for clearances below three nm.  For instance, in the past touchdown could be determined by considering the average SAGC for different power levels of FOD control, but this has been found unreliable at low clearances.  Also methods that estimate slider tangential (off-track) disturbance due to contact fail near a Mid-Disk track, because the disturbance is parallel to the track.