Gliding Algorithm Lets Drones Surf The Winds For Hours

No fuel needed

Launch an unmanned SBXC sailplane into the air with a catapult and it glides lazily down again in about three minutes. But when the sailplane is equipped with software to find plumes of warm air rising from the ground, known as thermals, it can ride them upward and extend its flight to a record-breaking five hours.

The software, known as ALOFT, for Autonomous Locator of Thermals, was developed by Dr. Dan Edwards and colleagues at the Naval Research Laboratory. The team tested and launched more than 20 flights with the software at the Phillips Army Airfield in Maryland last October. In total, those flights lasted more than 30 hours, proving that using algorithms to update data on thermals can help unmanned sailplanes fly much longer than previously possible.

Many birds and even insects take advantage of thermals. Eagles and vultures use them reach high altitudes without having to flap their wings, then glide around looking for prey until they find the next thermal. Sailplane pilots use them to extend their flying time, and now ALOFT gives the same capability to drones.

“The biggest challenge for autonomous soaring is teasing out the skills that a soaring pilot uses into an algorithm that an autopilot can follow,” says Edwards. “We have spent many hours watching the auto-soaring algorithm work, but behave not quite the way that makes the most sense, only then to return to the computer and tweak the programming.”

ALOFT uses the sensors already built into the aircraft for airspeed and altitude pressures, along with GPS and inertial navigation, to ‘feel’ when it is getting lift from a thermal. ALOFT then steers the drone around in a tight loop to keep the aircraft inside the rising column of air.

solar arrays
UAV images for Rob Walters; SunPower arrays and the SBXC airframe; courtesy U.S. Naval Research Laboratory

Five hours may seem like a long time to stay up using nothing but warm air, but far more is possible. A recent study found that frigate birds fly continuously for several months at a stretch, riding thermals continuously without coming down to settle on the water. If drone are to do the same they need to be better at locating thermals.

The original system relied on running into thermals by chance. Thermals tend to form in the same place repeatedly, often over tarmac or bare rock which soaks up the sun better than the surrounding terrain. So the researchers gave ALOFT a memory of where it had found a thermal before.

“At the beginning of the week, the aircraft wandered around aimlessly looking for thermals,” says Edwards. “By the end of the week, the aircraft would move between frequent hot-spots and almost immediately find good lift. That was super exciting.”

Flight path
Researchers monitored how the software improved flight paths at Phillips Army Airfield in Maryland. courtesy U.S. Naval Research Laboratory

The team is now working with the Laboratory’s Monterey Marine Meteorology Division to develop route-optimization algorithms so a drone can plot a route which takes advantage of likely thermals.

Another approach, which the team worked on with help from Pennsylvania State University’s Air Vehicle Intelligence and Autonomy Laboratory, was to have two soaring drones working co-operatively. As soon as one found a thermal it signaled the other. This matches the behavior of soaring birds, which often fly over to share a thermal. Edwards found that birds would join in when ALOFT was soaring.

“It’s a patriotic feeling watching a bald eagle come over to soar in the same thermal as your robots,” says Edwards.

The cooperative approach will be even more effective with more drones.

A third approach is to equip the drone with sensors such as LIDAR allowing it to detect a thermal in the distance. This would require additional hardware, whereas the basic ALOFT can be easily added to almost any drone with an autopilot.

Basic sensors may help too: researchers found that frigate birds look for cumulus clouds, a sure sign of an updraft carrying moist air upward, and glider pilots may look for grass or insects carried up on a thermal.

The basic ALOFT software can be easily added to almost any drone with an autopilot. courtesy U.S. Naval Research Laboratory

Soaring algorithms could be retrofitted to existing small drones carrying out tasks like mapping or surveying. Pausing to gain altitude on a thermal might mean the drone takes longer to reach a given destination, but greatly extends the mission time.

“ALOFT could be programmed into nearly any autopilot,” says Edwards. “We’ve adapted ALOFT for three different aircraft and have been successful in soaring with all three.”

Edwards says that the next project is to integrate the soaring software with solar power and a hydrogen fuel cell to create a hybrid craft with extreme endurance.

“The Naval Research Laboratory’s Ion Tiger aircraft flew for forty-eight hours on hydrogen before,” says Edwards. “Adding the auto-soaring and solar systems is projected to double the endurance with only minor modifications to the aircraft.”

ALOFT is being re-engineered to fly on new hardware in an NRL program called Solar Soaring, and the combination of solar and soaring should fly in October.

Soaring could provide an easy boost to the long-endurance solar unmanned aircraft like those being developed by Google and Facebook as an alternative to satellites, providing communications to remote or under-served areas. And solar-plus-soaring drones might travel the world.

“One day in the future, I can imagine a UAV flying across the ocean on just the power from the sun and the wind,” says Edwards.