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