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