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