Rocha warned of the "movement of battlefield technology to the domestic sphere."

"What's appropriate in a battlefield is not appropriate here," he said. "Maryland is not a battlefield, and we are not the enemy."

Smith says she understands the advocates' questions.

"We recognize that people take these concerns seriously," she said. But the system "is designed to link in with existing infrastructures to better protect the nation. And it's an exercise to see how well it does that. … There's no hidden purpose."

Army officials have been trying to head off public concern about the exercise. Once launched, scheduled to start in October, the aerostats — 243 feet long, bright white and nearly 2 miles up — will be obvious to motorists on Interstate 95. On a clear day, they'll be visible from downtown Baltimore.

Representatives of Aberdeen Proving Ground have been visiting the surrounding community to let neighbors know what's coming.

"We are certain this will generate phone calls and Internet and social media buzz," Kelly Luster, a spokesman for the Army installation, told the Harford County Council during a presentation this month. "I say this because the aerostats are quite large and they're visible for about 50 miles."

So far, however, the public has shown little interest. About 25 people attended a hearing in January on wetlands permits for the project; most were members of the military or its contractors. During the portion of the hearing set aside for public comments, no one spoke.

Spokespersons for Harford County, where Aberdeen Proving Ground is based, and for Rep. C.A. Dutch Ruppersberger, in whose district the aerostats are to be deployed, say they have received no questions or complaints from the public.

"It is our understanding that the mission is to test new and much-needed surveillance equipment to protect us from air attacks as countries like Iran, China and Russia work to develop their own cruise missiles," said Ruppersberger, the top Democrat on the House Intelligence Committee. "And we'll be in regular contact with the Army to make sure that remains the case."

Ruppersberger and other officials have promoted the economic benefits of the exercise for the region. Up to 150 workers, a mix of service members and civilians, are to begin arriving in August.

Officials plan to construct pads, buildings, utilities and parking for each of the aerostats, which are to be tethered 4 miles apart — one within the Edgewood area of Aberdeen Proving Ground, the other on Graces Quarters Road, southeast of the Hammerman Area of Gunpowder Falls State Park in Baltimore County.

Army officials say the aircraft can stay aloft for up to 30 days at a time, many times longer than helicopters or airplanes, and they're much cheaper to operate.

They are to be tethered to the ground with 10,000-foot Kevlar cables that contain power lines and fiber optics. The elevation extends the range of the radar.

"JLENS works for the same reason that you can see further when you are in a 10-story building then you can from the ground," said Smith, the NORAD spokeswoman.

The aerostats are making their way to Maryland from Utah, where soldiers and civilians from Dugway Proving Ground have spent the last three years testing them in the Snake Valley.

The Citizens Education Project in Salt Lake City raised concerns about the use of drones as targets during the testing in Utah, the possibility of a crash or fire, and whether the project was the best use of public money.

It is not clear whether drones are to be used during the exercise at Aberdeen Proving Ground.

Steve Erickson, the director of the Citizens Education Project, said there was no public outcry in Utah about privacy.

"This is quite a remote area where they were operating in," Erickson said. "It was out on military land in the Utah Test and Training Range. That's inaccessible to the general public.

"While you could see these two blimps from the freeway, there's no residences out there."