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