A Lockheed Martin "unattended ground sensor," or UGS, disguised as a rock. Photo: Lockheed Martin A soldier attaches an unattended sensor to the side of a mock building. Photo: U.S. Army A Textron "MicroObserver" UGS, buried in the ground. Photo: Textron A smaller model of the "MicroObserver" UGS. Photo: Textron A soldier emplaces an older UGS. Photo: BAE Systems Northrop Grumman's "Scorpion" suite of unattended sensors. Photo: Northrop Grumman A Lockheed promotion video for its unattended sensor network. Members of the 22nd Marine Expeditionary Unit clandestinely monitor signals in a 2010 field test. Photo: USMC

America is supposed to wind down its war in Afghanistan by 2014. But U.S. forces may continue to track Afghans for years after the conflict is officially done. Palm-sized sensors, developed for the American military, will remain littered across the Afghan countryside – detecting anyone who moves nearby and reporting their locations back to a remote headquarters. Some of these surveillance tools could be buried in the ground, all-but-unnoticeable by passersby. Others might be disguised as rocks, with wafer-sized, solar-rechargeable batteries that could enable the sensors' operation for perhaps as long as two decades, if their makers are to be believed.

Traditionally, when armies clash, they leave behind a horrific legacy: leftover mines which can blow civilians apart long after the shooting war is over. These "unattended ground sensors," or UGSs, won't do that kind of damage. But they could give the Pentagon an enduring ability to monitor a one-time battlefield long, long after regular American forces are supposed to have returned home.

"Were going to leave behind a lot of special operators in Afghanistan. And they need the kind of capability that's easy to put out so they can monitor a village without a lot of overt U.S.-made material on pathways and roadways," says Matt Plyburn, an executive at Lockheed Martin, the world's largest defense contractor.

The U.S. military has used unattended ground sensors in one form or another since 1966, when American forces dropped acoustic monitors on the Ho Chi Minh trail. Tens of thousands of UGSs have been emplaced around Afghanistan and Iraq, forming electronic perimeters around combat outposts and keeping tabs on remote locations. It's a way to monitor the largest possible area with the smallest number of troops.

"You use them to cover up your dead space – the areas you're concerned about but can't cover with other ISR [intelligence surveillance and reconnaissance] assets," says Lt. Col. Matt Russell, an Army program manager overseeing the deployment of unattended sensors.

But earlier UGSs – even ones of the recent past – were relatively large and clunky, prone to false alarms, and had lifespans measurable in days or weeks. "What we found in the field was significant under-usage," Russell tells Danger Room. Plans to incorporate them into every combat brigade fizzled as the Army's proposed $200 billion revamp, Future Combat Systems, went south.

The new models are dramatically smaller and consume far less power, enabling them to operate for months – maybe even years – at a time with only the slimmest chance of being detected. Lockheed calls them "field and forget" systems for "persistent surveillance."

And they won't just be used overseas. U.S. Customs and Border Patrol today employs more than 7,500 UGSs on the Mexican border to spot illegal migrants. Defense contractors believe one of the biggest markets for the next generation of the sensors will be here at home.

"They could be used for border security or even around corporate headquarters," Plyburn tells Danger Room.

In early 2011, commanders in Afghanistan issued an "urgent operational needs statement" for better sensors. In response, the Army shipped a new line of about 1,500 "expendable" UGSs to the warzone. The size of a few stacked hockey pucks with a four-inch antenna, these sensors are easily hidden, and can "pick up wheels or footprints" for up to three months at a time, Russell says. It's a perfect surveillance tool for the remote valleys of eastern Afghanistan.

Soon, when one of the sensors picks up a signal, it'll queue a spy blimp to focus in on the spot. "That's a capability coming to a theater near you soon," he adds.

Even more sophisticated are the UGSs being tested northeast of Norfolk, Virginia, at a Lockheed proving ground. Arrays of up to 50 palm-sized acoustic and seismic sensors form a mesh network. When one sensor detects a person or a vehicle passing by, it uses unlicensed radio frequency bands to pass an alert from one node to the next. The alert finally hits a communications gateway, which can send the signal via satellite, tactical radio network, or Wi-Fi to a command and control center. That signal can tip off additional sensors – or it can send a Twitter-like message to an intelligence officer's phone or tablet.

When they're not picking up signals or passing along messages, the sensors are all-but-shut-down, barely consuming any power. That allows them to last for weeks, buried underground. Or the sensors can be encased in hollow "rocks" equipped with miniature solar panels. A quick recharge from the sun will allow the sensor to "get through the night anywhere on Earth that U.S. forces operate," says Plyburn.

Plyburn claims that the sensor's battery, about the size of a postage stamp, has been able to go through 80,000 recharges, compared to a few hundred cycles for a typical lithium-ion battery. Even if he's off by a factor of 10, the sensor's battery could keep the machine operational for nearly twenty-two years.

Russell is skeptical of these assertions of longevity. "I'm sure there are a lot of claims by contractors," he says. "My experience is: the longer the lifespan, the bigger the battery."

Nor does Lockheed currently have a contract with Defense Department to mass-produce the sensors. But Plyburn says there has been interest around the armed forces, especially since the system is relatively cheap. Plyburn says each sensor could cost as little as $1,000 each – practically expendable for a military paying $80,000 for a single guided artillery round.

Lockheed isn't the only company claiming that its sensors can operate for years on end. U.S. Special Operations Command has handed out at least $12 million in UGS contracts to tiny Camgian Microsystems, based out of Starksville, Mississippi. Company CEO Gary Butler, who spent years developing ultra-low power integrated circuits for Darpa, was awarded in March a patent for such a next-gen unattended sensor suite.

Rather than relaying alerts from node to node, each of Butler's sensors is designed to send signals directly to a satellite – speeding up notifications, and cutting down on power consumed. Rather than a simple acoustic or seismic detector, the sensor relies a steerable, phased-array radar and moving-target indicator algorithms. That could give it a much greater ability to detect people and vehicles on the run. High-powered solar cells provide will enable up to "500,000 recharge cycles" could give the sensor a "10-20 year life," according to the patent.

Butler won't say how U.S. special operators are using his research, if at all. But when I ask him about the possibility of leaving UGS networks behind after American troops have officially left, Butler calls that "plausible. Very Plausible."

Camgian's patent claims that the sensor's ease-of-use and small size means it "is easily emplaced in difficult areas, using airborne assets such as Unmanned Aerial Vehicles." Edward Carapezza, who has been overseeing UGS research for more than two decades, says drones are already dropping unattended sensors into hostile locations.

"In certain areas, we certainly are using unmanned vehicles and unattended sensors together," says Carapezza, who now works at the defense contractor General Atomics. He declined to name where these operations were being conducted. He simply gave the rationale for the missions. "Instead of sending patrols of our guys in, we send in drones and unattended sensors – dropping arrays, locating bad guys, and then putting weapons on target."

The "MicroObserver" UGS from defense contractor Textron has been in the field since 2008. The U.S. Army is currently using the sensors in Afghanistan. "Another customer – we're not allowed to say who or where – used it as part of a comprehensive border security program in a Middle Eastern country," says Patty Shafer, a Textron executive.

Textron's seismic sensors come in two varieties. The smaller, three inch-long model, weighing 1.4 pounds, will last about a month. The bigger system, a 4.4 pound spike, can be buried in the ground and gather intelligence for more than two years. It can detect and characterize people from 100 meters away, and vehicles from three times that distance, Shafer says. A conformal antenna allows it to communicate with a gateway five kilometers away.

Northrop Grumman employs a family of sensors for its Scorpion surveillance network.

"Seismic sensors work well detecting vehicles on bumpy roads, but lose range as the road becomes smoother, or the vehicle lighter. Typically, magnetic sensors sense only large vehicles at fairly short distances. The range of acoustic sensors depends upon environmental conditions such as humidity and surroundings. Most sense engine exhaust noise or other periodic pulse trains and measure the period to determine numbers of cylinders and classify the source," explains a Northrop presentation to an academic conference on unattended sensors.

The Army has purchased over a thousand of the original versions, with an average of four sensors, each. The vast majority have been sent to Iraq and Afghanistan. Another 20 Scorpion II systems were recently bought by the Army Research Lab. The sensors can today spot people from 800 meters away, and vehicles from 2,100 meters. The sensors' batteries wear out after a month.

These might have been eye-popping results, not long ago. But the U.S. military now has plans to keep its network of tiny, hidden spies going for much longer than that.