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