In another embodiment, {HEAD ~ the optimum source} of position data is chosen based on {TAIL ~ the distance} between the rover unit and the optical system 
is chosen based on
In another embodiment , { HEAD ~ the optimum source } of position data is chosen based on { TAIL ~ the distance } between the rover unit and the optical system
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH