The US Air Force is funding work to let drones tag suspects or cars with a spray that gives them a distinct spectral signature, making them easy to track

Better mist than missiles (Image: Bryan Denton/Corbis)

ON A dusty road in northern Pakistan, a nondescript vehicle rounds a corner. Fifty metres overhead, a tiny drone buzzes unseen, spraying a fine mist across the vehicle’s roof as it passes below. The vehicle is now tagged, and can be tracked from many kilometres away by an infrared scanner on a larger drone.

This scenario may soon be played out now that Voxtel, a firm in Beaverton, Oregon, has won a US Air Force contract to develop a drone-based tagging system. Voxtel makes tagging materials – taggants – that can be used to discreetly label vehicles carrying smuggled goods, or people who are involved in civil disobedience or attempting to cross international borders illegally.

Interest in tagging technology has been driven in part by growing pressure on the White House over civilian deaths in US drone attacks. During a recent visit to Pakistan, US Secretary of State John Kerry stated that drone strikes there will end “very, very soon”. Tagging by drones would allow people to be tracked for subsequent arrest.


Voxtel’s taggants are based on quantum dots – semiconductor nanocrystals less than 50 atoms across. Because of quantum effects, they absorb and emit light at specific wavelengths. The company has demonstrated a taggant powder that, when illuminated with an invisible ultraviolet laser, can be detected by infrared cameras 2 kilometres away. The powder is delivered as an aerosol that clings to metal, glass and cloth, and batches can be engineered to have distinct spectral signatures.

The nanocrystals would be sprayed by a hand-launched drone such as the Raven (pictured). With a wingspan of less than 1.5 metres, it is quiet and has a range of several kilometres. A larger Predator drone could then illuminate the target with an ultraviolet laser and track its progress.

Nanocrystals can be sprayed by a hand-launched drone and illuminated with a laser

But spraying the taggant accurately can be tricky, as Kevin Jones and colleagues at the Naval Postgraduate School in Monterey, California, discovered. They experimented with small drones that delivered a simulated taggant made from coloured sugar beads used in cake decoration. They wanted to coat a road with the stuff so that it would stick to the wheels of any vehicle that drove through.

But the wind blew the beads around as soon as they were sprayed. So Jones’s team developed software to model the effects of wind so they could allow for it when spraying. When they fed in estimates of wind speed and direction based on readings from the drone’s sensors, the drone could hit a target from an altitude of 45 metres.

A more advanced system would allow accurate tagging from greater distances, which would be more effective as small drones can be inaudible when flying further than 60 metres away.

The US Department of Homeland Security has expressed interest in giving non-lethal offensive capabilities to drones used by its Customs and Border Protection service. Any such move is bound to be contentious, and tagging might prove more acceptable to US public opinion. Drones could also use smart tagging during riots so that the people involved can be identified and later arrested.

Chris Cole of anti-drone pressure group Drone Wars UK is not reassured. He is concerned that any form of tagging could affect bystanders.

“If a drone pilot sees someone whose clothes have the tagging mark, how will they know the clothes have not been simply borrowed?” he says.

Many ways to make your mark TAGGING technology has moved on since the days of using water cannon with indelible dye to mark rioters. Selectamark Security Systems, a company based in London, produces a range of products containing unique synthetic DNA sequences. These include automatic sprays for marking intruders, a personal defence spray and a device similar to a paintball pistol that can tag an individual from 30 metres away. Defence giant Lockheed Martin has developed a grenade which disperses nanoparticle taggants (see main story). Standard grenade launchers can fire it at targets hundreds of metres away, marking vehicles and people over a wide radius from the point of impact.

This article appeared in print under the headline “A new game of tag”