"The United States, Canada, and 22 European nations signed the Treaty on Open Skies on March 24, 1992. The treaty entered into force on January 1, 2002, and now has 34 members. Each participant must permit unarmed observation aircraft to fly over its entire territory to observe military forces and activities. The treaty is designed to increase transparency, build confidence, reduce the chances of military confrontation, and encourage cooperation among European nations. According to the U.S. State Department, the parties have conducted more than 1,200 observation flights since 2002. While the United States has conducted more flights over Russia than Russia has conducted over the United States, Russia recently conducted flights over both the Washington DC area and Bedminster, New Jersey. Some Pentagon officials have questioned Russia's plan to upgrade cameras and sensors on the aircraft it employs for U.S. overflights, and some in Congress have questioned U.S. continued participation in the treaty."
CRS Insight, IN10502
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html