The Swiss military, in turn, attacked Allied aircraft overflying Switzerland with fighters and anti-aircraft cannons. There are some areas that aircraft are barred from overflying. They also forbade overflying by British bombers to and from Germany. On 9 February 1941, Canadian forces engaged a single German aircraft overflying the island. Highly accurate data can be collected by aircraft overflying the terrain in question. The primary mission of the U-2s was overflying the Soviet Union. This required special State Department approval, as it meant overflying Cuba. Therefore, there is an important overflying problem and noise pollution. It was found in Oklahoma, identified simultaneously by ground searchers and the overflying ARCHER system.