What's the best way to detect and destroy drones?

In the northern Danish city of Aalborg, the firm MyDefence makes equipment that jams and repels drones.

"We've had a big surge of interest," says chief executive, Dan Hermansen.

He says that up until early October his company was mainly dealing with defence firms, but now it has "completely shifted".

The small, box-like kit made by MyDefence is mostly used by the military of Nato countries and Ukraine.

However lately demand has grown from civilian customers.

"It's coming from critical infrastructure," he adds, "from big companies, looking to protect their own assets".

The device detects communication between the drone and its pilot, then breaks that connection, explains Mr Hermansen, by emitting a powerful radio signal on the same frequency.

Rather than falling out of the sky, the drone is pushed away and has a controlled landing. If it tries to reconnect to a GPS signal, that can be blocked too, he adds.

Mr Hermansen reckons that radio frequency jamming works against 80 to 90% of the drones that are flown.

While forcing an unwanted drone to crash land is a good result, it's essential to be able to detect it first.

"The first part is really about identification. And the second part is an interceptor system," explains Kasper Hallenborg, director of The Maersk Mc-Kinney Moller Institute at University of Southern Denmark.

Identifying a drone is not so easy, points out Andreas Graae, the head of research at the Institute of Military Technology at the Danish Defence Academy.

"[Drones] can be very small or really big, and are often produced from materials like plastic or fabrics that are very hard to detect on a traditional radar," he says.

A suite of technologies are under constant development, to help find drones.

That includes acoustic sensors that listen for the drone's buzzing; advanced optical cameras, with very high resolution; and increasingly sophisticated tactical radars, which work over longer ranges and can even differentiate between a drone or a bird.

Once detected, a drone needs to be disabled. Electronic jamming, similar to that used by MyDefence has leapt forward, thanks in large part to the war in Ukraine.

"[Ukraine's] frontlines are totally jammed," Mr Graae says, which means that drone controllers lose control of their machines.

So, Russia and Urkaine have adapted by using drones controlled by fibre optic cables, or using drones that can navigate autonomously, or fly along pre-programmed routes.

Such drones need to be intercepted or shot down and plenty of firms are working on novel ways to do that.

Among them is Swedish start-up, Nordic Air Defence. It is developing a low-cost interceptor designed to strike the targeted drone, forcing it to crash.

"It's missile shaped, so travels incredibly fast," he adds. "It's incredibly easy to manufacture. It is basically 3D printed," says Jens Holzapfel, the company's business director.

Cost is a criticial factor in countering drones.

Last month, Nato Secretary-General Mark Rutte said: "It's unacceptable to shoot down drones costing one or two thousand dollars with missiles that may cost half a million or even a million dollars."

That's been a big lesson from Ukraine, says Mr Graae. "It's become a competition of how cheap you can actually make a drone attack, and how expensive it is to defend against."

"As hostile drones become cheaper, it puts pressure on the defender to manufacture low cost products," agrees Mr Holzapfel.

Low-cost drones are increasingly a security issue away from the frontlines of Ukraine.

Poland and Romania had their airspace breached by Russian drones; while separate drone incidents were reported, in Norway, Sweden, Lithuania, Romania and most recently at Germany's Munich airport.

In Denmark tensions have also run high after a string of mysterious sightings at airports and military installations around the country.

That spurred the defence ministry to deploy "several capacities" that can detect, track and jam drones; and last week Sweden announced plans to invest more than $365m (£275m) in anti-drone systems, including measures to jam and shoot them down, as well as the deployment of hunter drones.

Mr Holzapfel at Nordic Air Defence currently works with Sweden and its European allies. As well as the military, clients are from law enforcement agencies and security companies.

But he also sees civilian sectors like shipping and the oil and offshore industries as potential markets.

In a civilian setting. simply shooting down a drone might be too risky.

"It could be rather dangerous," says Kasper Hallenborg, pointing to the falling parts and potentially flammable fuel.

"We saw the impact in Poland," he continues. "That was just drone fragments, which more or less removed the roof of a house."

Early detection would help, says Mr Hallenborg: "Then you can probably take it down somewhere it's more safe to do so."

At short ranges, shooting out nets to tangle up the drone is another method and cheap lasers are also being developed.

There are also safer, so-called soft-kill options, including hacking. "That's a more secure way to neutralize the drone, because then you can actually control the landing," says Mr Graae.

Crucially, a traffic management system is urgently needed, suggests Mr Hallenborg, involving electronic license plates for each drone device and way for users to register the flight in advance.

"Then we can quickly identify which drones are allowed to be there and those that aren't," says Mr Hallenborg.

"The [Danish] police have been overloaded with people telling them about what they've seen in the sky. A lot of these drones are probably there with a [legitimate] purpose," he says.