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