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