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