Editor’s note: London’s Gatwick airport reopened for flights Friday while cautioning that it’s still on the hunt for illegal drones.

The scrapping of hundreds of flights at London’s Gatwick airport after it was buzzed by miniature drones shows just how easy it can be to disrupt advanced aviation networks with simple devices that can cost a few hundred dollars.

Airports have been raided by drones before. Dubai International was briefly closed in 2016, and Wellington, New Zealand’s main hub was shuttered for 30 minutes this year when a mystery craft was spotted close to the runway.

But as thousands of travelers at Britain’s second-busiest airport try desperately to salvage their holiday plans, the incident reveals how tough it is for authorities to combat the problem. Gatwick was still closed Thursday evening more than 20 hours after the drone sightings first shut down commercial flights.

The law already is clear in most jurisdictions and penalties for violation can be severe. In the U.K., drones aren’t allowed to fly within 1 kilometer (0.6 mile) of airports and can’t go above 400 feet to avoid conflict with flight paths. Aviation Minister Liz Sugg said the “pilots” of the Gatwick craft could face five-year jail terms if caught.

That’s a big “if.”

Modern airports rival the size of some cities, making their perimeters almost impossible to permanently police. And surrounding buildings offer almost limitless opportunities for drone operators to hide while maintaining line-of-site control of their troublesome craft. Such factors enabled the Provisional IRA to fire mortar shells into London’s Heathrow airport in 1994 without anyone being apprehended. Commercial airline pilots also have been distracted by laser pointers around dozens of airports.

British police say the Gatwick incursions were clearly deliberate, as the drones variously appeared, vanished and then emerged again from Wednesday night through most of Thursday. That could shift attention from how best to regulate drone flights to practical ways of neutralizing the threat and finding scofflaws.

Ken Quinn, a former chief counsel of the U.S. Federal Aviation Administration, said the disruption should come as a wake-up call to impose adequate tracking and identification of drones.

As the agency prepared to craft regulations allowing expanded drone flights two years ago, law-enforcement agencies halted the effort until they could devise separate security rules. The U.S. government is now drafting standards that would require all but the tiniest drones to broadcast their identity and position so authorities could identify operators who have crossed the line.

GPS, Falcons

One mechanism that’s being widely explored is “geofencing.” Drone makers including China’s DZ DJI Technology Inc. use built-in GPS to alert a pilot who is about to fly into restricted skies, such as around airports and prisons. Such technology can prevent accidental transgressions but won’t stop deliberate attempts to interfere with flights.

That’s where downing errant drones comes in — and proposals get pretty creative. One plan is a variation on using hawks to scare birds from runways, with Dutch police in 2016 proposing the use of trained raptors to take out rogue devices. A study funded by the U.S. Air Force and conducted last year by researchers at the University of Oxford suggested using peregrine falcons, which in their natural element kill their quarry by colliding with them.

Colorado-based Liteye Systems Inc. has developed a system that uses radio signals to halt a drone and force it to land, while Australia-based DroneShield Ltd. is developing a device resembling a traditional ballistic weapon. Police in Japan have experimented with snagging target objects out of the air using a net deployed from an even larger drone.

Meanwhile, people concerned that drones are invading their private space have developed countermeasures using open-source hardware like the Raspberry Pi computer and a strong Wi-Fi antenna to jam drone flight capabilities.

–With assistance from Justin Bachman and Alan Levin.

©2018 Bloomberg L.P.

This article was written by Christopher Jasper and Nate Lanxon from Bloomberg and was legally licensed through the NewsCred publisher network. Please direct all licensing questions to legal@newscred.com.

Photo Credit: Gatwick airport in London has been closed for a day as authorities figure out what to do about their drone problem.