By David Hambling, technology expert
Just before Christmas, sightings of small drones around Gatwick forced the airport to suspend operations for 36 hours, causing major disruption which rumbled on for three days.
And on Wednesday Heathrow Airport also temporarily halted flights after a drone was spotted with the military being deployed.
Following the Gatwick incident, many people asked why the military could not simply shoot down the intruders as soon as they were spotted... but whilst military personnel and equipment were deployed to Gatwick and now Heathrow after the drones were spotted, it is not quite as simple as just shooting them down.
Drones, unlike other air vehicles, are very small and fragile it doesn't take much to knock one down.
One video online shows a football fan in Argentina take out a quadcopter with a roll of toilet paper, while another shows that even a chimpanzee can down a drone with a stick.
This all gives the impression that drones are easy to shoot down but is not that simple.
Operational experience has shown that in practice, consumer drones can be very difficult targets indeed. There is a huge difference between a drone hanging around taking pictures at low altitude, hovering or moving slowly, and one which is avoiding fire.
This was shown during operations in Mosul, Syria, in 2017, where so-called Islamic State used modified consumer drones to drop 40mm grenades on allied forces. They attacked from an altitude of a few hundred metres, so the drone was just a dot in the sky.
The drones were a serious problem, at one point almost bringing operations to a halt because their attacks were so disruptive.
The US Army has explored the new threat from “low, slow and small” drones in some detail.
In April 2017, new guidance was issued on Counter-Unmanned Aircraft System Techniques. This advised that the best way to bring down a drone is for a unit to fire all available rifles and machine-machines a fixed aim point in the sky ahead of the drone’s flight path.
The idea is that the drone flies into this wall of fire, because it is too difficult to hit with aimed shots – but it is much harder if the drone is not flying in a straight path.
Defending a large site like Gatwick would require a huge number of firing positions to be maintained.
Even if this could be achieved, and sufficient fire could be brought to bear to shoot down a drone, there remains the question of overshoot and accidental damage.
A bullet fired at a target in the air will land somewhere, and while it is much less lethal than at shorter ranges, even a 5.56mm bullet retains enough energy to cause injury a mile away.
In crowded south-east England, there is a significant chance that a bullet falling at random will hit a building or a vehicle – or even a person.
Firing hundreds or thousands of bullets, the sort of number that might be needed to hit a drone, would almost guarantee an accident.
Buckshot from shotguns has a much smaller danger zone, but also a much shorter effective range. Any drone posing a hazard to aircraft would be well out of shotgun reach.
This was why shooting down the drones was not considered a viable option, and alternate means were deployed.
While there is no official comment on the system used, according to some reports, it is thought the Drone Dome system produced by Israeli company Rafael was brought in and set up on a rooftop at Gatwick.
Drone Dome consists of three elements, a radar to detect and track drones, an electro-optical/infra-red system (which is essentially an advanced camera for fine tracking and identification), and a radio-frequency jammer.
The jammer is able to interfere with the radio link between the drone and the operator or with the drone's GPS satellite navigation system.
When this happens, drones are generally forced to land on the spot, unable to receive instructions or find their way back.
A system like Drone Dome may not work for long, though.
However, some military drones can already be operated in ‘jam-proof’ mode, flying a pre-programmed flight path and using navigation systems which do not rely on a satellite signal. This type of drone is immune to jamming and can only be brought down by more direct ‘kinetic’ means.
Some solutions that have been used range from net-guns to trained eagles, all of which have their drawbacks -nets have a short range, while eagles only fly in daylight.
Anyone who can find a simple, effective way of dealing with drones will soon find plenty of customers – not least at Britain’s airports.