- Chaos overhead -

Drones have overflown several airports across Denmark this week in what the prime minister called "hybrid attacks", forcing closures and fuelling security fears.

Danish military intelligence said it had not been able to identify who was behind the drones, but officials suggested Russia could be the culprit -- which Moscow denied.

Authorities in Norway meanwhile said they had seized a drone piloted by a foreign operator in Oslo airport's no-fly zone Wednesday night.

Commercial drones emerged in the 2010s and fast became widespread.

But the whirring bird's-eye-view devices can pose a threat.

Drones have become key weapons in Russia's war with Ukraine, used by both sides to strike behind enemy lines.

Even small recreational drones can be made into deadly weapons by fitting them with explosives.

And airports face a challenge protecting aircraft.

In 2018, London's Gatwick airport had to cancel more than 1,000 flights after scores of drones flew within its perimeter in the space of three days. Their origin has not been definitively established.

- Detect, neutralise -

Airports generally use several tools to spot low-altitude drones, including radar, acoustic detection and visual surveillance.

If a drone comes too close, they seek to neutralise it.

One option is to destroy it mid-air with a laser beam. But the falling debris risks causing injuries or damage on the ground.

Another solution: rapid security drones equipped with nets to intercept the intruder.

The most common approach is jamming the drone's signal with a shoulder-mounted scrambling device.

But that method is not 100-percent effective.

Russia and Ukraine have both developed jamming workarounds since their war began in 2022.

- Evolving technology -

"The concept of the signal jammer is that if you point it at the drone, it will hover in place" until its battery dies and it lands, said Xavier Tytelman, an aviation security consultant and editor of the aerospace industry magazine Air et Cosmos.

"The problem is the Russians and Ukrainians have reworked the internal software: when it's jammed, now the drone flies upwards until it's out of the jammer's range."

Some drones can now be programmed to reach a target and continue advancing until impact, even if their GPS signal is jammed.

Even when airports try to adapt to the threat, their operations are typically restricted to handling commercial and private drones. More-advanced drones, including military drones, are the responsibility of the armed forces.

In France, for example, the civil aviation authority, the DGAC, said it was responsible for "detecting and neutralising light drones", along with air transportation authorities.

But for more-advanced drones, "protecting France's airspace is the responsibility of the armed forces ministry," it said.

The armed forces can use surface-to-air missiles, helicopters or fighter jets to counter a drone threat. But unless the drone is carrying explosives, the risk of collateral damage on the ground is considered too high.