Airspace immediately above the land and water areas are vested with title to the underlying soils (see United States v. Causby 328 U.S. 256, 266 (1946)). Altitudes above 500 ft 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).
The upper limit of national airspace is not defined by law.
NATO Air Policing
As part of the principal of collective defence in peacetime NATO members carry out Air Policing missions to protect the integrity of Allied airspace. As part of the mission aircraft are used in a Quick Reaction Air role to respond to both civilian and military aircraft in distress and any aircraft that approach allied airspace and fail to identify themselves, fail to communicate with Air Traffic Control or fail to file a flight plan.
A number of NATO countries cant sustain a force of quick reaction aircraft and aircraft of other member states provide cover in the Baltic Area. In the Benelux area (Belgium, Luxembourg and the Netherlands) the Belgian and Dutch air forces provide the quick reaction aircraft for four-months each.
The Swiss Air Force provides aircraft and systems to protect the sovereignty of Swiss airspace, it also covers Leichenstein airspace.
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.