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