processing said echo signals to determine {HEAD ~ the distance} from said transducer to {TAIL ~ said surface}, 
to
processing said echo signals to determine { HEAD ~ the distance } from said transducer to { TAIL ~ said surface } ,
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH