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