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