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Airspace immediately above the land and water areas are vested with title to the underlying soils (see United States v. Cuasby 328 U.S. 256, 266 (1946)). Altitudes above 500ft over private lands and all the airspace above public lands forms part of the sovereign state area. Flights by some civil aircraft into the airspace of a UN member state does not need prior permission (Convention on International Civil Aviation). In the United States, the air sovereignty mission had been renamed "Air Sovereignty Alert", but in 2011 it was renamed "Aerospace Control Alert." The lion's share of the aerospace control alert missions in the U.S. are carried out by the Air National Guard by units flying, at present time, the F-15 Eagle and F-16 Falcon jets.
The upper limit of national airspace is not defined by law.