Throughout history, humans have employed falcons as lethal hunters of other animals. Now those raptors are being sent after drones.
It turns out that many of the skills feathered predators use to find a tasty lunch can be applied to the developing field of drone defence.
A US Air Force-funded study by zoology researchers at Oxford University suggests that the means by which a peregrine falcon tracks its quarry could be effective in defending against drones that threaten troops, police or airports.
The researchers fitted the falcons with miniature video cameras and GPS receivers to track their angle and method of attack on other birds, or on bait being towed through the air by a drone.
In a paper published Monday in the Proceedings of the National Academy of Sciences, the falcons' approach to intercepting its target aligned closely with the rules of proportional navigation, a guidance system used by visually-directed missiles.
The principle is such that a missile - or a falcon on the hunt - will reach a target as long as its line-of-sight remains unobstructed while it closes in. The earliest AIM-9 Sidewinder heat-seeking missiles, dating to the 1950s, used this technique with a rotating mirror to "see" the target.
A key difference, however, is that falcons adjust their angle of attack to compensate for their slower speeds - which is where drones come in.
The work, the researchers suggested, could be applied to the development of small, visually-guided drones that can disable other drones.
"We think that the finer details of how peregrines operate could certainly find application in small drones designed to remove other drones from protected airspace," Professor Graham Taylor, the principal investigator, wrote in an email.
The research involved data from 55 attack flights in Wales with falconers and a certified drone operator.
For soldiers on the battlefield and even law enforcement at home, the threat of drone attack has grown as every day passes - as has a desire for a working defence.
In July, the Pentagon authorised 133 US military installations to shoot down private or commercial drones that threaten their airspace. That move followed a decision earlier this year to ban aerial drones near these facilities.
Cheap, small drones have become a handy weapon for militants, with the Defense Department working to field new technology and techniques to protect troops and equipment.
To date, the Pentagon has explored a variety of methods to deter hostile drones. These range from the most basic - machine gun fire - to more sophisticated approaches including lasers, frequency jamming to render them inoperable and more advanced techniques to actually gain control of the drone.
The application to drone defence "emerged naturally through the course of the study" given research by several police forces to eliminate drones using trained raptors, the authors said.
Police in the Netherlands, for example, have studied whether eagles can be an effective means to capture and disable small drones.
"The problem with this approach is that raptors are only motivated to chase targets if they are hungry or defending a territory, and spinning rotor blades pose as much of a threat to a birds' talons as they do to our own fingers," Taylor wrote in an email.
"Keeping a defence team of hungry raptors on call isn't practical, and flying them at large multi-rotors isn't ethical." (The Air Force didn't immediately return a request for comment.)
Taylor called the study's findings "an elegant convergence" of raptor behaviour and missile guidance law, "which reflects how natural selection and engineering design are constrained similarly by maths and physics.
It's also quite beautiful how well the model fits the data, and thrilling for me as a mathematically-minded biologist to see how the flight trajectories of real birds engaged in real attacks emerge from the equations that ultimately govern them."
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This article was originally published by Bloomberg.