One of the United States’ little-noticed conventional arms control treaties turns 20 today, while its enforcement continues to rely on aircraft and surveillance gear stuck in the analog age.

The Open Skies Treaty enables countries to fly over each other’s territories, assuring signatories that their neighbors are not massing troops along a border, enabling them to monitor rail lines and generally building confidence between nations.

Russia is modernizing its equipment, purchasing and now testing a Tupolev Tu-214 outfitted with digital electro-optical/infrared sensors and synthetic aperture radar, says Diana Marvin, a senior adviser on the treaty for the State Department.

But so far, the U.S. has had difficulty finding funding to replace its Vietnam War-era imaging gear. It currently uses with panoramic framing cameras and vertical and oblique-mounted optical framing cameras on two OC-135Bs, which are modified aerial refueling tankers. “They all take film at this stage,” Marvin says, and film production is dwindling.

Funding to start the process of buying sensors and radar for the open-skies aircraft did not make it into the U.S. Air Force’s fiscal 2013 budget, but Marvin says senior leadership is aware of the requirement and could add it to the fiscal 2014 request. “They are working in the right direction,” she says. “I think there’s enough political will among the different agencies that it does have a much better chance this year than in previous years.”

The OC-135Bs also are aging, at 50 years and counting. The Air Force has certified the aircraft for use until 2040, and while there’s talk about potentially buying new aircraft, no formal proposals are being considered. In the meantime, maintenance requirements are growing. “We weren’t able to complete one of our missions this year,” because parts were not available quickly enough, Marvin says.

Open Skies is “an unsung, quiet treaty,” says Rose Gottemoeller, acting undersecretary of state for arms control, who hopes to build on its success as a foundation for other treaties, including one governing the conventional forces in Europe.