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