WASHINGTON — Even from nearly 6 kilometers up, the synthetic aperture radar on the Predator unmanned aerial vehicle (UAV) can detect tire tracks, footprints, even a small cut someone made trying to slice through the tall steel wall that separates the U.S. from Mexico along parts of the Southwest border.

Military technology devised for the wars in Iraq and Afghanistan is in increasing demand for homeland defense. The U.S. Customs and Border Protection Agency (CBP) has begun flying a Predator along the entire southern Texas border, and hopes to have two in service by early 2011. UAVs offer a key advantage over their manned counterparts — they can patrol nonstop for 20 hours. Helicopters run out of gas after about two hours; human pilots can fly about 20 hours a week.

“The emphasis behind this plane is endurance,” said David Gasho, head of air operations at the CBP’s Unmanned Systems Center in Arizona. “It can easily do 20 hours on one tank of gas.” For the time being, however, the Federal Aviation Administration (FAA) is limiting Predator flights to 16 hours a day when control towers along the border are manned, Gasho said.

Still, that adds substantially to surveillance along the border. In addition to their radars, Predators are armed with video and infrared cameras. From nearly 6 kilometers in the sky, they can zoom in to identify objects on the ground as small as 30 centimeters long, Gasho said.

The infrared cameras are sensitive enough, Gasho said, that at night it is possible to tell whether a hiker on the ground is carrying a drug-laden backpack — from nearly 13 kilometers away. Besides being sharp-eyed, the synthetic aperture radar also can see through clouds, dust and haze. A vehicle hidden under a tarp is plainly visible, he said.

One role for the radar is “change detection.” That is a process in which software compares what the radar is seeing now with what it saw earlier, pointing out new tire tracks, footprints and other changes that tip border agents off to the presence of smugglers and illegal immigrants.

The sensors on the Predator also are used to sort false alarms from real border incursions. Thousands of seismic sensors buried in the ground along the Southwest border alert U.S. Border Patrol agents whenever they detect movement.

But the sensors cannot tell the difference between drug smugglers and wildlife. So the Border Patrol would send a vehicle or a helicopter “full of guys in flak jackets” to investigate. And often they would find out that the disturbance was caused by a coyote, said a government researcher who has worked on UAV issues.

It is more cost-effective to send a UAV to investigate. A look with its cameras often can tell “whether you need guys with guns or medics with water,” or that no response at all is needed, he said.

Much of what Predators will do in Texas is collect intelligence so that border agents know where the most-used border crossing points are, whether illegal migrants are shifting from one area to another, and whether U.S. agents should be moved to different locations to deter crossings, make arrests and seize drugs, Gasho said.

The UAVs also are intended to be available for use in response to man­made and natural disasters. They can provide video feeds of hurricane and earthquake damage and serve as radio relay stations for emergency responders, he said.

Border agents have been flying Predators along the border in Arizona and New Mexico since 2005, and CBP credits the UAVs there with assisting in the arrests of more than 4,000 illegal aliens and the seizure of more than 6,800 kilograms of marijuana.

Such statistics prompted members of the Texas congressional delegation to pressure the FAA to allow UAV flights in Texas.

Predator flights that began in early September “mark a critical next step in securing the Texas-Mexico border,” said Rep. Henry Cuellar (D-Texas), whose congressional district includes an area along the border south of Laredo.

With Predator flights now approved for Texas, UAV surveillance along the U.S.-Mexico border now extends from El Centro, Calif., to the Gulf of Mexico, said Janet Napolitano, secretary of the Department of Homeland Security, CBP’s parent agency.


Collision Avoidance

But not everyone is convinced patrolling the border with Predators is a great idea.

The FAA, for example, worries that unmanned aircraft will create a hazard for manned planes flying in the same area. A key concern is collision avoidance.

In the past, the FAA has insisted unmanned aircraft had to have collision avoidance technology that is as good as a human operator to fly in U.S. airspace. So far, that technology does not exist.

But Gasho insists that along the Texas border, “there is no problem with collision avoidance.” The Predator’s Texas flight corridor is “sanitized airspace” that other aircraft must avoid, he said. And the Predators are continually watched by CBP controllers and by dedicated radars. In addition, they fly at “a unique altitude” that is above general aviation and below commercial airliners, Gasho said.

As for collision avoidance technology, it remains “a very difficult issue,” said Rocky Gmeiner, vice chairman of the Aerospace Industries Association’s unmanned systems subcommittee. “To be able to sense another airplane and avoid it” requires sophisticated sensors and advanced computer software. The algorithms that might steer UAVs away from other aircraft still have to be rigorously tested, he said.

Less-sophisticated ground-based “sense and avoid” systems are closer to reality. They use ground-based radars to keep ground-based UAV pilots apprised of air traffic in the vicinity of his UAV. It is up to the UAV operator to maintain a safe distance from other aircraft.

Cost has emerged as another concern; Predators are not cheap. The Congressional Research Service reports that Predators cost $4.5 million apiece. But other estimates say that when the sensors are added, the cost increases to about $10 million per aircraft.

Last spring Congress approved spending $32 million to buy two more Predators for border surveillance.

The Predator price tag compares with $8.6 million for a Black Hawk helicopter equipped for the border agency, according to the Congressional Research Service.

And despite being pilotless, Predators are encumbered by substantial personnel costs.

“There’s nothing unmanned about an unmanned airplane,” Gasho said. A ground-based pilot flies the UAV, a sensor operator controls the cameras and radar and a ground crew maintains the aircraft.

The satellite link through which the crew controls the UAV is another expense.

The CRS reports that Predator operating costs “are more than double” that of a manned aircraft.

But others say that because Predators can stay aloft for so long and cover so much more ground than helicopters or manned fixed-wing planes, their per-hour costs are actually lower.

“It depends on how you look at it,” Gasho said.

UAV industry officials hope that permission to fly Predators along the Texas-Mexico border marks another step toward more general use of UAVs in U.S. airspace.

“There is a pent-up commercial market” for UAVs for tasks from monitoring pipelines to analyzing irrigation and fertilizer needs for agriculture, Gmeiner said. “But we’re going to have to solve some of these flying issues.” The FAA already has granted special permission to use UAVs to survey forest fires and inspect hurricane damage, he said. And the agency is in the midst of writing standards for the use of small UAVs that weigh less than 25 kilograms.

But widespread UAV use may have to wait until the next-generation air traffic control system is in place. The FAA says that system “will fundamentally change the way aircraft fly above the United States by 2018.”