Sweden's policy of neutrality and nonalignment requires a strong, modern, and independent defense establishment. The budget allocated $4.4 billion for defense in 2001, or 2.1% of GDP. Active armed forces in 2002 totaled 33,900 with reservists numbering 262,000. The army numbers 19,100 personnel with 160 main battle tanks. The navy has 7,100 personnel including the 1,100-member coastal defense force, and a naval aviation wing. There are naval stations at Stockholm, Karlskrona, and Göteborg. Equipment includes seven submarines, 45 patrol and coastal combatants, and 22 mine warfare vessels. The air force of 7,700 personnel operates 203 combat aircraft. There is a civil defense service, and all persons between the ages of 16 and 25 can be called to serve in it. As part of the civil defense program, nuclear-resistant shelters were built over a 10-year period in the large urban areas. The shelters, completed in 1970, are used as garages in peacetime and can hold 6.3 million people in a national emergency. Sweden participates in UN and peacekeeping missions in 12 regions.