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