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