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