Data is selected and combined from the master data set to produce {TAIL ~ the data subset} required for {HEAD ~ position determination} 
Data is selected and combined from the master data set to produce { TAIL ~ the data subset } required for { HEAD ~ position determination }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD