Both Republican and Democratic lawmakers have expressed alarm about reports that the Trump administration may abandon the Open Skies Treaty. Put into effect in 2002, it allows the United States, Russia, and 32 other countries to conduct short-notice flights over one another’s territories to monitor military deployments. The pact’s defenders point out that helps NATO allies monitor Russian moves — even as technical limitations prevent Russia from making much use of the imagery for purposes of espionage. That’s part of why the treaty is worth keeping.

Rep. Eliot Engel, D-New York, chair of the House Foreign Affairs Committee, quickly voiced his concerns. “American withdrawal would only benefit Russia and be harmful to our allies’ and partners’ national security interests,” Engel said in a Monday letter to National Security Advisor Robert C. O’Brien.

Rep. Don Bacon, R-Nebraska, a retired United States Air Force brigadier general, said, “The Open Skies Treaty promotes understanding, trust and stability among the 34 member nations. As a signatory to the treaty, we get valuable access to Russian airspace and military airfields on short notice... I believe the US was justified in terminating our participation in the INF treaty, but I’ve yet to see a compelling reason to withdraw from Open Skies.”

As with the Intermediate-Range Nuclear Forces Treaty, which the administration recently backed out of, Russia hasn’t always been a perfect party to the agreement. For years, for instance, they have restricted flights over Kaliningrad, a Russian exclave on Poland’s border; as well as over Abkhazia and South Ossetia in Georgia. Specifically, in 2014, they announced a 500-kilometer sublimit on Open Skies flights over Kaliningrad and restricted several flights to above 500 km, according to the State Department. They also denied flight access to a ten-kilometer corridor over the disputed region of Georgia, which Russia claims is part of Russia but which the international community recognizes as Georgia. The United States responded by restricting Russian flights over its territory. Engel supports the U.S. restrictions. The State Department has concluded that Russia's moves don't actually present an obstacle to U.S. intelligence collection over the areas. But they did determine that the Russian restrictions violated Article VI of the treaty.

The bigger concern is that Russia gets more out of the treaty than does the United States since U.S. intelligence satellites are better than their Russian counterparts.

In 2016, Adm. Cecil Haney, then-commander of U.S. STRATCOM, testified to lawmakers, “I don’t have the Russian intelligence guidebook available to me, but I will say that given the lack of overhead capability that the Russians have, Open Skies gives them a capability to be able to reconnoiter parts of our country and other nations as part of that.”

Another concern is that technology advances in sensors and cameras could allow Russia to see more than treaty participants realize (without exactly cheating per se.)

“As we look at how technology has developed, it is not surprising to me that there would be a desire to use more advanced capabilities in order to conduct that Open Skies Treaty,” said Haney, discussing technology trends generally.

U.S. and Russian officials have disagreed about what new sensors can be used under the treaty, as Joseph Trevithick wrote in this 2018 report for The Drive.

The treaty doesn’t ban any particular type of sensor but it does limit their resolution to about 30 centimeters of ground sample distance. A United Nations explainer says,

“Vertical and oblique optical framing cameras at 30 centimeters resolution (ground sample distance);

Panorama cameras at 30 centimeters ground resolution;

Thermal infrared line scanners at 50 centimeters resolution;

Sideward looking synthetic aperture radar (SAR) at 3 meters resolution.”

That resolution number is the treaty’s “most vital limitation,” says Arthur Holland Michel, author of Eyes in the Sky. “It’s the big equation that describes the number of pixels, field of view and altitude, those factors together give you ground sample distance.” It’s a bit better than the photos you can buy today from the Maxar WorldView-3 high-resolution satellite.

Nathan Crawford of Consolidated Resource Imaging, a company that’s helping the U.S. Air Force convert the photographic intelligence it collects under Open Skies into digital format, says that the treaty doesn’t allow technological innovations to give one country more advantage than another. “We can’t get clever. It has to be provable that its to specifications,” he said. “This is a verification sensor, not an undiscovered intelligence sensor. It’s meant to verify what [the signatory nations] say [about their forces and deployments] not discover things we don’t know.”

All of the parties to the treaty can monitor the Open Skies flights, and even shut down the image collection if they want to. If an aircraft deviates off course by 5 degrees, Crawford said, a monitor needs only to push a button. “It kills the capture systems and blocks the shutters.”

It’s one reason why Haney remained a fan of the treaty. “While I am concerned in terms of overflights of any ability of another nation to learn more about our overall critical infrastructure, I do have respect for said treaty in terms of the 32-some nations that are also part of that treaty, in which it allows for transparency and the ability of sharing immediately that information that that treaty is associated with.”

Perhaps the most important thing that the treaty provides is the ability of the other countries—without fancy high-resolution intelligence satellites—to monitor Russian moves. That, perhaps, is the main reason it's worth keeping.

Said Michael Carpenter, senior director at the Biden Center for Diplomacy and Global Engagement at the University of Pennsylvania and a former Deputy Assistant Secretary of Defense for Russia, Ukraine, Eurasia: “such concerns call for a discussion of the treaty's pros and cons; they don't justify a unilateral move to exit the treaty without proper consultation and deliberation with our European allies, who cannot match our ISR capabilities and very much rely on the Open Skies treaty for military transparency.”