An initial interferometric position measurement of the stage is averaged with at least one additional measurement corresponding to {HEAD ~ a displacement} of {TAIL ~ the stage} from its initial position 
of
An initial interferometric position measurement of the stage is averaged with at least one additional measurement corresponding to { HEAD ~ a displacement } of { TAIL ~ the stage } from its initial position
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH