A modified Russian military passenger jet will overfly the United States next week, taking photographs of the ground below and monitoring U.S. government activities. It’s basically spying but it’s all completely legal, thanks to the Open Skies Treaty of 1992 that allows the Russians to overfly the United States and Canada—and vice versa.

The Russian aircraft will overfly the United States on April 10 to 15, and overfly Canada (part of the North American Air Defense Command) April 18 to 22. The aircraft will be looking for signs the United States is violating nuclear arms agreements, such as the New START Treaty and Intermediate Nuclear Forces Treaty. At the same time, U.S. and Canadian specialists will overfly Russia in their own aircraft.

The Open Skies Treaty was signed in 1992 by then-president George H.W. Bush. Designed to boost confidence in arms control agreements, the treaty allows participants to fly over any part of any territory of other participants to make sure everyone is adhering to treaties limiting the numbers and types of weapons.

Member states are allowed to fly unarmed aircraft equipped with a variety of sensors to identify objects on the ground. The United States, for example, flies a U.S. Air Force OC-135B transport, while the Russian Aerospace Forces use a similar-sized Tupolev Tu-154ON transport and a smaller Antonov An-30. Other countries use smaller, shorter range aircraft to get the job done. Technicians and other specialists from the country being overflown are allowed onboard to ensure that the surveillance equipment is being used properly.

Open Skies aircraft carry a variety of electro-optical sensors to do their job. Permissible surveillance systems include “video cameras and panoramic and framing cameras for daylight photography; infra-red line scanning systems, which can operate by day and night; and synthetic aperture radar, which can operate day and night in any weather”. All equipment is inspected to ensure that it is allowed under the treaty. For example, installing gear that records radio and radar transmissions as the aircraft flies over a military base is a no-no.

The treaty stipulates that aircraft are allowed to carry surveillance equipment to carry out their task. Not the most sophisticated spy equipment, but enough to tell a truck from a tank. That satisfied the requirements of most participants from Europe, who had a conventional arms control agreement limiting tanks and other weaponry with Russia—before Russia pulled out of the treaty in 2015. Any data gathered by any treaty member is accessible to any other treaty member, so long as they pay for the cost of copying it.

In the United States, Russian Aerospace Force aircraft participating in Open Skies must enter the country through Dulles International Airport on the East Coast and Travis Air Force Base on the West Coast. They may operate from Travis AFB, Dulles, Elemendorf Air Force Base in Alaska, and Lincoln Municipal Airport in Nebraska. They may refuel at air bases and airports in Hawaii, Wisconsin, Montana, Arizona, and Tennessee. Likewise, American planes operating over Belarus and Russia operate under similar restrictions.

Is Open Skies invasive? Sure, it’s a little disconcerting that Russian Aerospace Forces aircraft can fly over our most sensitive military installations—indeed, any point over America. But at the same time, we can do the same to them. And without these flights, all parties would be a lot more suspicious that others have something to hide.