At a campaign rally in Dresden on September 15, a small quadrocopter flew within feet of German Chancellor Angela Merkel and Defense Minister Thomas de Maiziere, hovering briefly in front of them before crashing into the stage practically at Merkel's feet. Merkel appeared to be amused by the "drone attack," but de Maiziere and others on the stage seemed a bit more unsettled by the robo-kamikaze.

The quadrocopter, a Parrot AR drone, was operated by a member of the German Pirate Party as a protest against government surveillance and the ongoing scandal over the Euro Hawk drone program—which failed because it could not get certified to fly in European airspace. In a statement, the deputy head of the Pirate Party, Markus Barenhoff, said, "The goal of the effort was to make Chancellor Merkel and Defense Minister de Maiziere realize what it's like to be subjugated to drone observation." The drone was harmless, aside from potential political collateral damage to Merkel's Christian Democratic Party, and the pilot of the drone was released after being briefly held by police.

While Merkel smirked off the drone in Dresden, even a small explosive charge or grenade aboard a similar drone would have been catastrophic—and defending against such attacks is difficult at best. Unmanned Aerial Vehicle (UAV) researchers from TNO Defense Research, an organization in The Netherlands, recently showed the real risk of that sort of attack, demonstrating that terrorists and insurgents could effectively use current commercial and do-it-yourself drones as weapons in a number of scenarios, including one much like the Dresden event.

It’s a bird! It’s a plane! It’s a flying hand grenade!

In a paper published during the Unmanned Systems 2013 conference last month, Klaas Jan de Kraker and Rob van de Wiel of TNO Defence Research analyzed the threat posed by "mini-UAVs"—small remote-controlled and autonomous drones weighing less than 20 kilograms (44 pounds).

The research was in part prompted by two incidents in 2010—the crashing of a radio-controlled plane into The Netherlands' House of Parliament as a prank and the foiling of a plot to use explosives-packed, radio-controlled model airplanes to attack the Capitol and the Pentagon by the FBI.

TNO researchers found that small drones, especially those using autonomous navigation, could be stealthy, accurate, and potentially deadly weapons, and the probability of their use is rapidly increasing. The paper presented the following potential scenarios:

During a large public event in a stadium, a terrorist launches a Mini UAV, which is equipped with a machine gun, from a building at some distance. He directs the Mini UAV into the stadium and remotely fires the machine gun. In the panic that occurs in the stadium numerous people are overrun and die.

During a public speech by a VIP, the VIP is shielded from the audience by bulletproof glass. However, a terrorist deploys a Mini UAV equipped with an explosive, which flies over the shielding glass. The explosive detonates close to the VIP wounding him fatally.

During an expeditionary mission, opposing forces launch a Mini UAV toward a compound. When the Mini UAV has reached the center of the compound, it releases a chemical agent. Luckily this only causes some minor physical effects on people that were present and unprotected. However this has caused significant fear among the compound inhabitants.

During an expeditionary mission, an opposing militant group launches a small swarm of Mini UAVs, each equipped with an explosive, toward an airbase. The Mini UAVs fly toward the fighter jets that are parked on the airbase, and their explosives detonate just above the fighter jets. This significantly damages a number of jets and even destroys one of them.

Because of their size, their low flying altitude, and their relatively low speed, mini-UAVs are particularly hard to detect—especially in an urban environment, the researchers found. Even if they are detected, identifying whether they're a threat or not is still an issue, because it's difficult to determine whether they're armed or just carrying a camera. And because of the short range they're detectable at, security forces would have only seconds of warning to decide whether to attack the drone or not.

"Detection and classification are very difficult," de Kraker and van de Wiel wrote. "This is not only due to their small size but also to their very low flying altitude and speed, and to clutter that occurs from trees and buildings." Tests of a number of commercial and do-it-yourself mini-UAVs in TNO's anechoic radar room revealed that they had a "bird-size" radar cross-section, and their relatively low speed makes them hard to distinguish from birds under even more ideal conditions.

The TNO researchers looked at a number of other ways to detect micro-UAVs, including audio sensors, radio detection of control signals, continuous-wave radar, and infrared. The best results came from mixing radar and infrared—using radar for initial detection and infrared sensors for classification.

Burn them with lasers

Taking down potentially hostile drones once they're detected comes with another set of problems. While radio jamming can be used to interrupt remote-control signals for drones, it might not keep them from reaching their target and would be ineffective against autonomous drones using GPS or GLONASS satellite navigation. Jamming commercial navigation signals could cause autonomous drones to fail to find their target, but could cause other security and safety problems at the same time.

And just shooting down drones in a crowded urban environment could cause more damage than the drones themselves. "Missile systems with small missiles and a suitable guidance mechanism, (rapid fire) guns with suitable ammunition, and machine guns are considered as very effective means for neutralizing Mini UAVs," the researchers wrote, but "downsides may be that missile systems are relatively expensive and that these hard kill systems could generate collateral damage."

The best answer, de Kraker and van de Weil suggested, might be laser and high-power microwave "directed energy solutions," which could be used to heat the drones up until their batteries or electronics are destroyed. These weapons could be deployed in a truck to provide protection for events at public places with lower risk to people and property on the ground than a chain gun or small missiles.