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