The displacements are selected to reduce {HEAD ~ the overall cyclic error contribution} to {TAIL ~ an average position measurement} 
The displacements are selected to reduce { HEAD ~ the overall cyclic error contribution } to { TAIL ~ an average position measurement }
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL