Of all the KPA's branches, the Ground Force is the largest, comprising approximately one million personnel divided into 80 infantry divisions, 30 artillery brigades, 25 special warfare brigades, 20 mechanized brigades, 10 tank brigades and seven tank regiments. It is equipped with 3,700 tanks, 2,100 armored personnel carriers and infantry fighting vehicles, 17,900 artillery pieces, 11,000 anti-aircraft guns and some 10,000 MANPADS and anti-tank guided missiles. The Air Force is estimated to possess around 1,600 aircraft (with between 545 - 810 serving combat roles), while the Navy operates approximately 800 vessels, including the largest submarine fleet in the world. The KPA's Special Operation Force is also the world's largest special forces unit.