{HEAD ~ An airspace} is divide into {TAIL ~ a set} of grid cells, and a flight path is established between the initial point and the destination point 
{ HEAD ~ An airspace } is divide into { TAIL ~ a set } of grid cells , and a flight path is established between the initial point and the destination point
HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH