dividing an airspace encompassing {HEAD ~ an initial point} and a destination point into {TAIL ~ a plurality} of grid cells 
dividing an airspace encompassing { HEAD ~ an initial point } and a destination point into { TAIL ~ a plurality } of grid cells
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH