A solution may lie in overhead power lines. A program by Defense Research Associates (DRA) and the Air Force Research Laboratory (AFRL) seeks to develop a small UAV that can land on a power line and recharge its battery by drawing energy from the cable.

The PLUS (Power Line Urban Sentry) program seeks to create an autonomous, reusable platform with a long range that operates in confined areas, such as an urban battlefield. Surveillance and data collection would be improved because sensors recharge with the battery.

Dayton, Ohio-based DRA has received a Small Business Innovation Research (SBIR) grant from the Defense Dept. and is currently working on prototypes with AFRL in Phase II of the program.

The effort began three years ago when Patrick Marshall, a senior electronics engineer at AFRL, began investigating methods of charging small UAVs by flying them close to power lines. But the process was inefficient. "The best we could get was micro-watts of power," he says.

So Marshall built a toroidal test box that clamped around a power line to draw energy from the cable. The design was successful, though the latch mechanism needed work.

Marshall also worked with Mike McKinley, DRA's hardware engineering manager. They ran tests at Wright-Patterson AFB, Ohio, by putting a prototype on a power line and retrieving data through a radio-frequency link to see what happened as it charged, McKinley says. The battery charged in under three hours.

"From that point," McKinley recalls, "the thinking was, 'if we can get the charge, we can [find a way to] land on the line.'"

Then DRA hired a radio-controlled-aircraft operator to land model airplanes on power lines and attempt to latch on. In a span of nine months, they tested five or six models at a cost of about $100,000 each. "We landed two of them, and flew those planes many times," McKinley says.

The tests showed it was possible to connect to a power line, and that a UAV could withstand flying into one at 40 kt. A latching mechanism, however, still needs to be perfected.

Part of the Phase II SBIR also involves developing a power-line navigation capability. "There's a problem when a small UAV gets [electronically] jammed but keeps flying," notes McKinley.

If this happens, a UAV can be guided by power-line navigation. Operators would program coordinates for power lines into the UAV so that it follows a path that can be correlated back to any GPS start location.

If the GPS system becomes jammed, the stored power-grid map can be used to identify the UAV's position, McKinley says.

The UAV could be 100 mi. out and still transmit data. "Power lines are like highways in the sky," he says.

Even if a UAV is shot down, it will provide a data waypoint for troops. "You know someone there is armed," Marshall remarks. "They know about your UAV, but you know you want to look at that area."

An energy-harvesting UAV could be equipped also with multiple sensors. "The idea is that when you're recharging, you have all that power from the line to do whatever you need," McKinley observes.

One issue about recharging is detection. Can a small UAV be disguised? Rick Lind, an associate professor of mechanical and aerospace engineering at the University of Florida, thinks so.

Lind works with aircraft morphing technology and runs a facility called the Center for Morphing Control. One goal of the PLUS program is to create a small UAV that seems to disappear by morphing into a shape that doesn't look out of place on a power line. So Marshall and McKinley asked Lind to join their efforts in 2006.

"He made a small UAV look like a Coke can," Marshall recalls.

The first prototype flights are expected in 2008.