From the Document: "The United States, Canada, and 22 European nations signed the Treaty on Open Skies on March 24, 1992. It 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, and encourage cooperation among European nations. The parties to the Open Skies Treaty have conducted 1,500 flights through early October 2019. Some parties provide their own aircraft, but the parties can also join on overflights on aircraft provided by other nations. Both the observing nation and observed nation have access to the data from each flight; other parties can purchase copies of the data, so all can share information collected during all flights. The United States and Russia have both conducted numerous observation flights over the other's territory, although, according to the State Department, the United States conducted nearly three times as many flights over Russia as Russia did over the United States. Further, the parties can invite flights over their territories in special circumstances, as Ukraine did in 2014, when Open Skies flights helped monitor activities along the Ukraine-Russian border."
CRS Insight, IN10502
Congressional Research Service: https://crsreports.congress.gov/