The output of {TAIL ~ sensor} 80 is applied to summer 82 where it is compared to a signal representing {HEAD ~ a selected common reference frequency} from reference frequency source 81 
The output of { TAIL ~ sensor } 80 is applied to summer 82 where it is compared to a signal representing { HEAD ~ a selected common reference frequency } from reference frequency source 81
OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH