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The Pentagon appears undeterred by such concerns.

Air Force spokeswoman Capt. Erika Yepsen told The Times that a combination “of other capabilities and the Global Hawk” drone should meet the needs of the military. The Air Force, she added, looks “at what the mission requires and what combination of platforms and capabilities meet the requirements of that mission.”

But the Pentagon has not been entirely forthcoming about how it plans to spend the $1.9 billion that it wants to make the sensor switch over 10 years.

According to a March 3 Air Force talking points memo obtained by The Times, some $1.3 billion will go toward reliability and other improvements to the Global Hawk drones, with $500 million going specifically toward the sensor transfers.

It is not specifically clear, meanwhile, what precise sensor capability the Pentagon plans to transfer from the U-2 to the Global Hawk.

While the exact specifications of spy aircraft sensors is classified, published reports show the U-2 has multiple sensors on it, including the elite SYERS 2, which contains an optical infrared camera that adds to the aircraft’s long-held dominance over what defense officials describe as ISR — intelligence, surveillance and reconnaissance — missions.

One Defense Department source, who spoke with The Times on the condition of anonymity, said the Air Force is conducting a $10 million study on whether sensors from the U-2 can realistically be adapted to the Global Hawk.

The study pays particular attention to how differences in flight performance between the drones and the spy planes would affect sensor performance, the source said.

Last year, Air Force officials told lawmakers that strapping U-2 sensors onto the Global Hawk via a tool belt, known as a “universal payload adapter,” was feasible. But Air Force officials cautioned that it would take the military three years to develop and test the adapter and another two years to produce it in a final form.

There are also questions about the role of Northrop Grumman, the weapons company responsible for building Global Hawk drones for the Defense Department.

It was Northrop Grumman that originally pitched the sensor switch plan to the Pentagon after the company designed the “universal payload adapter” using in-house funds.

The Air Force initially was not responsive toward the plan, which arrived after a yearslong budget battle with Congress over the cheapest and best ways to advance overall capabilities of the nation’s aerial spying program.

The Pentagon initially pushed a proposal to simply pull the veteran U-2s out of the sky in favor of replacing them with an all-new and notably sexier fleet of Global Hawks. But Congress thwarted that plan by establishing a requirement that the Defense Department first must prove to lawmakers that the newer drones are more cost-efficient and effective.

In subsequent years, the Pentagon attempted to ditch some of the prized Global Hawks. Congress responded with the legislation that forced the Pentagon to continue flying both types of aircraft.