The Army is planning to launch a pair of blimps over Maryland this fall to watch the Eastern Seaboard for incoming cruise missiles.

It's what else they might be able to see from up there that worries privacy advocates.

The Army says the aerostats — blimps that will be tethered to the ground in Harford and Baltimore counties — will carry technology capable of detecting, tracking and targeting cruise missiles and rockets up to 340 miles away. That means they can cover an area from North Carolina to the Canadian border.

The helium-filled aircraft, which have been deployed in Afghanistan and Iraq, are on their way to Aberdeen Proving Ground. Officials are planning a three-year exercise to test the system's effectiveness in the National Capital Region.

"People have been using balloons for military purposes since the Civil War," said Maj. Beth Smith, a spokeswoman for U.S. Northern Command and North American Aerospace Defense Command. "There's a couple that are flying right now; we use them all the time, and they work great."

But the American Civil Liberties Union of Maryland, which has pushed the General Assembly for legislation to limit the deployment of drones in the state, is raising concerns about the surveillance technology to be mounted on the aerostats and whether it might be trained on private citizens.

The Electronic Privacy Information Center, meanwhile, has sued the Army for details about the equipment and its capabilities.

"It's designed to be a system that can surveil a large area," said Ginger McCall, an attorney for the Washington-based center. "That would have profound implications for the way that people would choose to live their lives."

The Joint Land Attack Cruise Missile Defense Elevated Netted Sensor System, or JLENS, comes to Maryland at a time of rising concern over government eavesdropping. The past year has brought a succession of revelations about the collection of telephone and email data by the National Security Agency, a Defense Department organization headquartered at Fort Meade.

The Army says the aerostats to be tested over Aberdeen Proving Ground will be outfitted only with radar, not cameras. Smith says the system will be "looking outward," for external threats.

"There's no intent to spy on any Americans with this," she said. "It's an exercise."

Privacy advocates note a demonstration last year by defense contractor Raytheon, which boasted in a news release that it had successfully equipped a JLENS aerostat with electro-optical infrared sensors, "enabling operators to watch [a] live feed of trucks, trains and cars from dozens of miles away."

As part of the demonstration, Raytheon reported, operators were able to observe actors placing a mock roadside bomb in the Utah desert.

It's that kind of technology that concerns David Rocha, an attorney with the ACLU of Maryland.

"We don't really care about radar being aimed into the Atlantic Ocean to detect cruise missiles," Rocha said. "There's no privacy implications in cruise missiles. But they have said that this same technology can also detect vehicles on the ground, and that they're not ruling out mounting other surveillance technology on this platform. And that does raise huge concerns."

McCall warns of "mission creep."

"Oftentimes a new technology that's very invasive will at first be proposed as a military technology," she said. "It will be rolled out oftentimes in Middle East conflicts. So, for instance, the drones. Or some of the facial recognition technology. These things are first used in conflict zones.

"And then they begin to be used domestically. At first, they're usually couched as an antiterrorism tool. And then there's mission creep from there, and it becomes a tool that's used for garden-variety law enforcement."

McCall says Americans expect not to be watched or tracked by the government in their everyday lives.

"They go to doctor's appointments. They might go to an abortion clinic, or a psychiatrist's office. They might go to a political protest," she said. "Your religious activity or associations — all of these things, people expect privacy in."