One of the amazing things about the recent drone incident at London Gatwick is that the two unmanned aerial vehicles flying into operational runway space prompted the closure of Britain's second-busiest airport for more than a day.

With further sightings of drones, Gatwick reopened to limited service only after a 36-hour interruption.

With more than 110,000 passengers on 760 flights due to depart Gatwick on just one of the affected days, these drone incursions have left a trail of disruption.

This is by no means the first incident of drones causing problems at airports - there have been similar incidents in Canada, Dubai, Poland and China. But the event at Gatwick is unusual in the length of its duration and the presence and repeated use of multiple drones.

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The growing availability and affordability of consumer drones means risks to airports and other secure spaces will rise - and the counter-measures used against them leave room for improvement and need to be more widely adopted.

A study by the Remote Control Project estimates about 200,000 drones are being sold for civilian use around the world every month.

Readily available from a range of online and high-street outlets, drones are becoming more commonplace and more affordable for the hobbyist.

As they move more mainstream they have also caught the eye of more hostile groups, and state militaries as well as terrorists and other non-state entities are using drones in battle.

Islamic State (Isis), for example, has used drones to drop explosives, to observe and direct fire for others, and to capture footage for propaganda.

Elsewhere drones have been used to cause disruption at home, such as the drone "assassination attempt" on the Venezuelan President, Nicolas Maduro, in August.

The incident at Gatwick has not been labelled a "terrorist event", but whether "criminal, careless, or clueless" it shows even unarmed consumer drones can cause risk to life and economic activity.

Sussex police have called the drone pilot's actions "deliberate disruption". At a recent Countering Drones conference I spoke about how both consumer and DIY drones may be flown and modified to do this. In seeking to future proof how we think about drones and their risks, it is worth considering how drone technology and software is developing.

There are now intelligent flight modes that allow drones to track and follow designated individuals, basic swarming functionalities that allow multiple drones to act in unison, and the livestreaming of images to social media, meaning drones can potentially be used for live propaganda.

In the Gatwick case, a question often asked is, why don't the police shoot down the drone? Answer: It is dangerous due to the risks of falling objects and stray bullets, plus they're so small they're hard to detect before they are near enough to be a problem.

But there has been a boom in developing measures designed to stop drones. A recent report by Arthur Holland Michel of the Centre for the Study of the Drone profiled more than 230 products produced by 155 manufacturers.

Counter drone equipment deployed on a rooftop at Gatwick airport. Photo / AP
Counter drone equipment deployed on a rooftop at Gatwick airport. Photo / AP

Among them are those which seek to detect and alert users of approaching drones, to impede and stall drones through GPS and radio jamming or the embedding of electronic tagging and geo-fencing software, which prevent drones from being used near sensitive locations such as airports, prisons or power stations. There are also ways to to intercept and capture the drones using net-equipped drones and guns. Dutch police have even trained eagles to intercept them.

But cost and laws limit many such responses. And many reports have shown how defences built into drones such as geo-fencing or altitude restrictions can be tampered with, over-ridden, or even just switched off.

It's not the first time Gatwick has had to contend with a drone, but this should be a wake-up call to the need for reliable counter-measures.

Eagles could intercept

Reckless or criminal use of drones is on the rise, mirroring the growth in the technology's popularity.

This ranges from hobbyist operators fined for "flying dangerously" and a rise in close-calls with manned aircraft, to incidents of drones ferrying drugs into prison and even unspecified "sexual offences". Authorities around the world have also warned of the growing use of drones or unmanned aerial vehicles (UAVs) by the likes of insurgents and terrorists.

The mounting threat of users ignoring regulations or committing crimes means police need ways to stop and capture rogue devices. One novel idea that was explored by the Dutch National Police was training bald eagles to down drones. While this "low-tech solution for a hi-tech problem" has advantages, the dangers it poses to the birds suggests we shouldn't write off alternative counter-measures.

The small size of most drones makes them hard to detect and target. They can use thermal cameras to operate day and night. The bald eagle may seem well suited to downing them because of its ability to spot and rapidly intercept a target. By seizing the drone out of the sky, the bird disables it without it falling on people below, and instinctively finds a safe area to land.

But critics say bald eagles are not falconry predators who typically grab prey on the wing but eat mostly fish and carrion. Other issues include the cost of training and keeping eagles for occasional use against drones, and the time it could take to deploy them.

Anna Jackman, Lecturer in Political Geography, Royal Holloway

- The Conversation