Following the recent drone scare at Gatwick airport – which forced the airport to shut down resulting in considerable disruption and economic losses – widespread public criticism was expressed, and important questions were raised. How was it that such small, remotely controlled objects could create such mayhem, and nothing could be done to stop them? Why, indeed, could nothing be done to prevent drone interference in the first place? In the aftermath of the Gatwick incident, CASCADE team members were invited to consult on the danger of drone operations in close proximity of critical infrastructure, and Dr Kovac from the Imperial College London was invited to an advisory group meeting with the Rt Hon Ben Wallace, minister for security.
What emerged clearly from this incident is that while increased UAV use in everyday life is valuable, allowing for complex tasks to be carried out more efficiently and safely, it also implies a rise in drone misuse, ranging from accidental disregard of regulations by hobbyists to intentional disruption and illegal usage. In part, this problem is exacerbated by the always easier access to drones by the general public, which considerably increases the possibility of misuse. As UAV technology progresses rapidly, there is an equal need for effective drone counter-measures, as well as safety regulations, to prevent such vehicles from becoming hazardous. Moreover, it is evident that it is the engineers and operators working on or with UAVs who are best placed to develop effective counter measures. The CASCADE programme focuses on informing the community about UAV opportunities and challenges and as such it can support policy and technical development with relevant stakeholders.
Although significant work has been done on drone counter-measures, finding an effective solution remains challenging. UAVs can vary significantly in terms of configuration, weight, size, structure, flight behaviour, etc., as well as in communication systems and automation levels. Being unmanned, it may further be impossible to surmise exactly what a UAV is doing or planning to do. Similarly, it can be difficult to determine a UAV’s payload: a UAV may, for instance, be carrying explosives, which rules out shooting it down above a safety-critical area. Both the significant variation between UAVs and the element of unknown inherent in dealing with unmanned systems imply that developing a single strategy for all cases is difficult. Instead, different strategies may have to be employed in different situations. This, in turn, implies that the type of threat must be identified before it can be addressed, which can itself be difficult. A further important factor to consider is that technology is continually and rapidly evolving. To ensure that countermeasures can deal with the most recent UAV advances, anti-drone technology must move at least at the same pace – ideally faster, so as to anticipate UAV developments. Added to the technical difficulties of developing effective countermeasures, an overarching challenge in the UAV field is the regulatory aspect. Aviation has evolved over more than a century, centred around the human presence. Removing the human from the picture generates new problems. Furthermore, as the existing system is well-established, incorporating UAVs into it often requires some adaptation to current regulations, which may be unideal for UAVs. In UAV countering, as in UAV operation, a unified set of regulations is needed to establish what is permitted and how incidents and infractions should be handled. Even more so than in manned aviation, in unmanned aviation technology and regulations are tightly linked and need to be developed parallelly for full effectiveness.
A number of counter strategies are currently in use or under development. The most well-known are summarised briefly below, as well as visually in the figure below.
1. Communication interference involves disrupting the communication links of a suspect drone. A possible approach for this is radio jamming, i.e. emitting a powerful signal in the UAV’s radio communication frequency range, that overpowers the UAV’s own signals and thus interrupts its communication link with the operator. The company Drone Defence, for instance, developed the Sky Fence jamming system to prevent UAVs from flying into the airspace above a prison on Guernsey. A possible drawback of this solution is that once the signal is interrupted, what the drone will do is not certain. In many cases, the drone will be programmed to return to its base, which may help authorities identify the operator, however other behaviours are also possible and in some cases UAVs are highly autonomous in executing their mission. A drone’s specific communication links may also not be known or easily detected, hence this option is not always viable.
2. Capture nets. A very intuitive approach is to deploy a large net from the ground, capturing the suspicious drone and forcing it to the ground. The net entangles in the propellers, incapacitating the UAV. A parachute-type mechanism can then be deployed to prevent the disabled vehicle from crashing violently into the ground. While unsuitable for larger vehicles, this simple, intuitive approach can be highly effective for smaller platforms, and is one of the most widely known and routinely used solutions. Several companies have developed such nets, e.g. the Skywall system by the company Open Works Engineering.
3. Drone-capturing drones. Another option is to take down suspect drones using other drones. The hunting UAVs can, for instance, deploy a net: this is similar to approach #2 but launching the net directly from the air can allow for more targeted interventions and possibly for a wider range. The captured drone can be towed to a safe location and/or lowered to the ground using a form of parachute. While potentially more effective than ground-launched nets, this approach is also somewhat more complex to implement. This type of solution is for instance used by Dutch company Delft Dynamics.
4. Drone-fighting birds. More exotic solutions have also been considered. The Dutch police, supported by the company Guard From Above, has for instance trained bald eagles to catch drones in the air. This strategy was shown to be successful, however, it requires extensive training of the birds and is therefore inconvenient from a practical perspective.
5. Directed energy. Other approaches are still in an experimental phase or only newly developed. One such technique is using directed energy, e.g. powerful laser beams, to take down drones. This strategy was recently tested by the US Navy, which has developed the so-called Laser Weapons System (LaWS). The system can hit rapidly moving objects, such as UAVs, using invisible high-energy laser beams which instantly heat up the target to elevated temperatures, damaging it and taking it down. The emitted beams are invisible, silent and move at lightspeed, which allows them to achieve a remarkable accuracy and minimise the chances of collateral damage. An analogous system has also been investigated by Boeing. While highly effective, such approaches are currently only employed in a military context and constitute a significant safety risk, as the hit UAV will crash down.
6. Geo-fencing. Lastly, it is also possible to include protective measures on UAVs prior to distribution. Many manufacturers, for instance, include geo-fencing capabilities on drones, which makes these automatically identify and avoid no-fly zones. However, such systems can be circumvented or disabled and are thus insufficient insurance against intentional misuse.
While several anti-drone measures have been developed, all available approaches still present constraints and no single technology is effective in all cases. Ultimately, the choice of strategy is heavily dependent on both the type of vehicle and the specific scenario. While anti-drone measures exist, they are often used ad hoc, and a unified and effective framework is still lacking, as evident from the disruption at Gatwick last year. Bearing this in mind, it is essential to (i) consider the most likely occurrences and establish appropriate response strategies for these, (ii) be aware of and ideally anticipate the latest technological developments, (iii) develop clear regulations covering the most common scenarios, and maintaining sufficient flexibility for adaptation to new developments, (iv) continue seeking for better solutions that cover more cases, and are safer and more efficient.
The preceding discussion highlights the need for further work on anti-drone measures. To make such measures efficient, cost-effective, robust and safe, new technological advances are required. We need new capabilities, potentially new UAV platforms, and a more comprehensive regulatory framework. Initiatives such as CASCADE are well-placed to support not only the development of new UAV capabilities, but also the development of safer UAVs and a safer and more effective UAV ecosystem.