# Solutions

In this section, we consider the requirements of solutions for the problem proposed in the specific problem description, all possible solutions, and both the advantages and disadvantages of each solution.

## Categories

When considering the state of the art research presented in the relevant Section, we can distinguish multiple categories in which the presented solutions might fall. In this Section, we further elaborate on these different categories, and as such provide a better overview and allow for more a more specific formulation of requirements. Firstly, different anti-UAV systems serve different purposes. For our study, we differentiate between the following purposes:

### Purposes

UAV Detection
These systems serve to detect the presence of UAVs in unwanted airspaces. They often also locate the UAV in question and sometimes include the possibility of continuous location tracking to assist systems categorised under the other purposes.
UAV Identification
Systems from this category serve to identify UAVs, obtaining more information about the UAV than simply its location. This information might include simple statistics, such as the average size of the drone which can often be observed by a human, given that the UAV is present in their field of view. More complicated statistics might also be obtained, such as a serial tracking number to identify commercial UAVs.
UAV Neutralisation
Drone neutralisation systems serve to neutralise a drone. This is the main topic of our study since UAV presence in the airspace above an airport introduces various risks, discussed in other Sections, that have to be neutralised in order to maintain public and societal security.

Now that the scope of the purpose of the anti UAV systems for airport security that we consider has become clear, we might further distinguish the main purpose considered in this study. As such, we differentiate between 3 different subcategories, all part of the drone neutralisation purpose. These categories are as follows:

### Categories

Preventative solutions
This category encompasses all solutions that serve to prevent the problem from occurring. More specifically, entries of this category focus on keeping UAVs away from airspace belonging to airports. An example might include the geofencing system that was described previously and will be elaborated on further in the following sections.
Corrective solutions
Solutions from this category focus on solving the problem of UAV presence in the airspace over airports, especially when said UAV is already present in that airspace. These solutions attempt to do so with minimal damage to the parties involved, an example might consist of a procedure where the control of the drone is overridden, either automatically or by a human, before the drone is removed from the airspace by landing or flight and after which control could be passed back to the pilot.
Destructive solutions
These solutions have the same area of focus as the previous category of corrective solutions, namely the minimising of further risk to air traffic above airports after a UAV has entered the airspace. The main difference is that, while corrective solutions attempt to do so in a non-destructive way, this limitation does not apply to destructive solutions. Sub-systems of a UAV or the UAV as a whole may be destroyed or permanently disabled. A coarse example consists of taking down unwanted UAVs with firearms, causing damage to the UAV and rendering it unable to continue operations.

This division into categories is not entirely black on white, however. Consider an abstract example system that temporarily incapacitates a UAV in flight, causing it to cease operation and enter a free fall towards the ground. This might result in the destruction of the drone, given the collision with the ground. We have found a grey area in our division into subcategories, and as such, we further define destructive solutions as those solutions, where the incapacitation of the drone follows from the destruction, and not the other way around. We also require the destruction to be an integral part of the solution, if we want it to count as a destructive solution. In this example, the destruction is not guaranteed nor does the incapacitation follow from the destruction. Instead, the destruction might follow from the incapacitation, dependent on other circumstances. Therefore, this specific example counts as a preventative or corrective solution, based on where the UAV in question is located. Note, however, that this is based on the keywords temporarily incapacitate'. If the incapacitation of the UAV or one of its subsystems were permanent, the destruction would be guaranteed since it does not depend on how hard the UAV hits the ground anymore. In this case, it would count as a destructive solution.

## Requirements

A solution to the specific problem described will have to adhere to requirements. These requirements are not simply capabilities the solution has to provide in the form of functional requirements, but they should also cover constraints posed on the solution. The constraints can be on the design of the solution in order to meet specified levels of quality, on the environment and technology of the system, and the project plan and development methods. Note that there can exist multiple different types of solutions and that we, therefore, have to keep the requirements of a solution as abstract as possible. We should not limit the solution space with these requirements. Instead, we should provide a general outline of what capabilities (functional requirements) the solution should provide and under what constraints (non-functional requirements).

Furthermore, these requirements might serve as a basic framework for further development of solutions to similar problems, thereby widening the scope to other problem spaces involving UAVs as well.

In this case, it is rather simple to provide some basic requirements and let the input of the airports' decide on further requirements.

For detection, the solution should be able to detect UAVs. For identification, the solution should be able to identify a detected UAV. For Neutralisation, the solution should be able to neutralise the detected and identified UAV. Of course, there are many other requirements, but we let the airport place these requirements as certain airports might argue that the safety of bystanders is more critical than other airports, such as recreational airfields.

## Possible solutions

As we have already elaborated on, a possible solution can be categorised into the purpose it fulfils with respect to anti-UAV systems at and around airports. Since a full anti-UAV system should be able to do three things: detect flying objects, identify that this object is an (unwanted) UAV, and lastly neutralisation of the UAV. However, the identification of the object might be something that is up for discussion, since it might be safer to neutralise every flying object, we will discuss this later on. As most possible (partial) solutions only cover one or two of the three things it should be able to do, before it can be considered at a full anti-UAV system, for each of the (partial) solutions listed below, they are divided up into categories of its purposes it fulfils. Such that, later on, we can compare and afterwards combine multiple of these partial solutions into one system that meets the needs of the users.

#### UAV Detection

• Radar system for detecting the location and height of an object in the air. The radar makes use of a transmitter which produces an electromagnetic signal which is radiated into airspace with an antenna. If this signal hits an areal object, it will get reflected in many directions. This reflected signal is received by the radar antenna then it is processed to determine the geographical data of the object.[1]
• A Wi-Fi receiver can be used to detect a UAV based on the signature of the signal reflected from the propellers of a UAV. Similar to radar, a transmitter broadcasts signals and a receiver captures reflected signals that bounce of a UAV. [2]
• Detect a UAV by listening to the communication channel between the UAV and its controller using a wireless receiver. Usually, UAVs communicate with their controllers a few times per second to update their status and to receive commands from the controller. A system could collect wireless samples and observes the signal, analyse them and can then detect a UAV's presence. [2]
• Detection of UAVs with the use of other UAVs that fly around the airports, carrying lightweight radar systems or cameras to scan their environment.
• Echodyne's 3D Security radar that offers superior sensor performance in a compact, solid-state, all-weather product. A recent winner in the SOFWERX Game of Drones competition.
• Human detection, for example by using watchtowers or pilots in the aeroplanes to spot UAVs. (Currently what Eindhoven Airport uses to detect UAVs)
• 3D Radio frequency antenna (https://drone-detection-system.com/the-system/)

#### UAV Identification

• Identification of any specific aircraft can be made by broadcasting a coded signal, which is decoded by air traffic control towers. Such that allies and enemies can be identified and to avoid targeting a friendly aircraft. As a result, all aircraft where radar service is provided should require systems that are able to broadcast coded signals for identification, for this solution to work. [1]
• For identification of UAVs, employing a 2D antenna and appropriate signal processing to create a multibeam, 3D, wide area overcomes the weakness of scanning radars and achieves high detection sensitivity. A decision tree based classifier can be used to identify the difference between UAVs and other moving objects. Where it rejects non-UAV targets, decreasing the number of false positives and increases true positives. Such that when neutralising such a moving object in the air, with high probability, it will be a drone instead of for example a flying bird. [3]
• A lightweight, X-Band (10.5GHz) radar system for use on a small-scale (less than 25 kg) rotorcraft. The prototype implementation of the radar is small enough to be carried by a drone and is able to differentiate other 'miniature rotorcrafts' (drones) by their doppler signature. The prototype uses a radar system which utilises electromagnetic energy to gain information on objects by analysing the reflected energy. [4]

#### UAV Neutralization

• Taking out UAVs by using air to air missiles, where these air missiles could be launched from other UAVs used by the airport or possibly any other aerial vehicle.[1]
• Taking out UAVs or disabling specific subsystems might be achievable by using lasers. Different kinds of lasers can be used for different purposes, either permanently or temporarily disabling a UAV. [1]
• Electromagnetic attacks to interfere with the GPS signals of the UAV, that the UAV uses to position itself. Jamming the GPS signals causes the UAV not to be able to follow the pilot's navigation commands accurately.[1]
• Taking control of a UAV by spoofing the GPS signals of the UAV, such that the UAV thinks that it is still talking to the original pilot when it is actually being taken over. This way the drone can easily and safely be landed somewhere out of danger.[1]
• Capturing a UAV using another UAV carrying a net, which drops the net over the unwanted UAV. Thereby taking control of the UAV as the net makes sure the UAVs rotors get tangled in the net making sure it is unusable for the pilot. Then with a parachute on the net, it can be made sure that the UAV lands safely on the ground[5]
• A bazooka with an intelligent locking system to aid the controller to hit the UAV successfully, that shoots a net to capture a UAV. The rotors of the UAV will then get tangled in the net, making sure it cannot cause any harm anymore. Then a parachute that is attached to the net will make sure that the UAV will land safely on the ground. [6]
• Transmitting geo-fence coordinates, avoidance commands or disruption of radio communication in order to avoid UAV's entering no-fly zone. [7]
• Using trained eagles to neutralise UAVs. These eagles would be trained into considering UAVs as preys so that they could catch these drones and place them in a safe area. [8]
• Geo-fencing software built into the UAVs restricts consumer UAVs even to be able to fly within a certain range of unwanted areas such as airports. [9]
• Using high powered radio waves to disable drones, it blocks their communication with the controller and switches them off mid-air. [10]

### UAV Detection

When it comes to UAV detection, radar systems provide a sufficient solution. There already exists much research on these type of systems. This partly helps with financing the solution as it is already existing technology, which should be cheaper than technology that is not fully developed yet. The disadvantage is that many airports already make use of radar systems, but that they do not seem to suffice. What one should ask themselves is whether or not these radar systems can be made in such a way that they would be reliable right now. Furthermore, how reliable would these radar systems be in the future if they are apparently' already not reliable enough right now? It seems to be the case that UAVs can be designed with certain materials such that they will not reflect the reflections of a radar system such that a radar system will not notice/detect any flying object while there might be a hostile UAV flying over an airport. All in all, radar systems offer an inexpensive way to detect UAVs, but not each type of UAV is detected.

Echodyne designs and manufactures radars with unparalleled price-performance. MESA technology is used, which is a fundamental breakthrough in high-performance radar with game-changing benefits in many markets. Acuity is an intelligent radar control software suite to enable user configurability. At an order of magnitude lower cost, Echodyne radar radically outperforms all other radar sensors in its class[11]. Their 3D Security radar offers superior sensor performance in a compact, solid-state, all-weather product. A recent winner in the SOFWERX Game of Drones competition, EchoGuard is the perfect' radar for a multilayered perimeter defence solution. Furthermore, Acuity API integrates seamlessly with existing security ecosystems to provide situational awareness.

Their radar can be seen in usage in the following video. Echodyne provides another video that depicts a visualisation of the working of their radar.

The specifications of the 3D Security radar are as follows:

• Size: 8.0in x 6.4in x 1.57in (20.3cm x 16.3cm x 4cm)
• Weight: 1.25kg
• Power: DC +15V to +28V
• Operating: <50W
• Hot standby: <15W
• Hibernate TBR: <100mW
• Field of View: 120° Azimuth x 80° Elevation

This radar reliably detects and tracks aircraft and cars at 3km, people walking at 2km, and sUAS at 1km.

The Guardian reported that one such system could cast around 150 000 dollars[12].

WiFi receivers can accurately determine the position of drones. They are, however, very susceptible to interference. For example, WiFi signals can be blocked by obstacles. While it might be sufficient for right now, it is possible for malicious attackers to attach WiFi interfering tools to their UAVs. Then, WiFi receivers might not be as reliable in the future. When it comes to solutions, reliability is one of the main concerns as the solutions has to work in all cases.

Listening on communication between drone and ground

Listening to communication between a drone and its operator can be an easy way to detect the presence of both the drone and the operator. Often, this type of communication is not encrypted. The U.S. government displayed how easy it is to hack drones made by Parrot, DBPower, and Cheerson[13]. One significant disadvantage, however, is that custom built drones might use significantly different communication standards which do make use of encryption. So this solution is only useful when the communication is not encrypted, which still happens quite often as of now. The number of communications that do make use of encryption is suspected of increasing with the years as the technology gets more established. Then, this solution does not provide a way of detecting drones.

Detecting drones with other drones

When we use drones to detect other drones, we do not depend on insecure channels. With this solution, we are not just limited to drones in the line of sight as the drone-detecting drone can fly around. A disadvantage, however, is that flying around with a drone at a busy airport can be quite dangerous. Furthermore, these drones can only stay in the air for a limited time due to battery-related constraints. This can be mitigated by simply using larger batteries, but this increases the weight of the drone, which leads to some negatives again. This might, however, still provide to be a sound solution as it makes dealing with the illegal drone activity easier as the drone itself can, for example, be weaponised.

### UAV Identification

Identification by coded signal

UAV identification through coded signals can quickly identify activity. A disadvantage, however, is that other areal entities, such as bird, might also be targeted. This is due to birds being able to be roughly similarly sized as drones. Thus, this method can identify the aerial activity, but there is no guarantee that only illegal drone activity is identified. Then, this could lead to negative results when we consider, for example, birds. It is possible they are targeted by the drone interception system.

3D radar system with machine learning

UAV identification through a 3D radar system that uses machine learning can eventually lead to a precise system. The issue with it is that it first needs data to learn from. The gathering of this data can provide to be difficult. Furthermore, even if it learns from this data, it does not always have to lead to correct results as there are, often, biases in data.

UAV identification through an X-band radar system can perform accurate shape analysis of flying objects using doppler and high-frequency radar signals. Attaching a radar system to a drone, however, can be an issue around airports as this might result in interference with already existing systems.

### UAV Neutralisation

Missiles

The use of missiles might, on the one hand, be a rapid method and affordable method to take out hostile drones. However, there are a few downsides to this method. First of all, missiles are very dangerous, especially in an area where, apart from hostile UAVs, many aeroplanes with innocent passengers fly. The chance exists that a missile might miss an unwanted UAV and, instead, hit an aeroplane. Which would be disastrous and would only make the situation worse, especially if the unwanted drone was just a hindrance to the airport. UAVs are often quite small and can move/switch directions pretty quickly it is actually quite hard for a missile to correctly take out a UAV. Furthermore, might the unwanted UAV actually be taken out by a missile, then it will most definitely be destroyed meaning that police investigation will be more difficult. All in all, the use of missiles at and around airports is most likely a bad idea.

Lasers

Lasers are very precise and can be used in multiple ways to deal with drones. On the one hand, a very narrow laser beam can be pointed at an unwanted UAV to melt the body of the UAV causing structural failure and crashing of the UAV. On the other hand, a wide laser beam can be used to target multiple unwanted UAVs at the same time, taking out their control systems causing them to crash. Both methods require pretty close range to a target, the exact range depends on the type of laser that is used, and clear sight to the targetted UAV(s). However such systems could be attached to moving vehicles making such systems very mobile. Another advantage is, compared to the use of missiles, is that there is no need to reload as it uses the energy of a generator or the vehicle it is attached to. However, this means that the energy could deplete might there be too many targets, or might a target take too long to take out. Also, one might not always have clear sight to a target, or the range might be too long, making this method ineffective.

Interfering with GPS

Interfering with the GPS of a UAV, will not cause any harm to the drone. Then the drone can be inspected, once landed, to find out who is responsible for the UAV. However, when interfering with the GPS of a UAV, the pilot will be unable to send commands to the UAV making the UAV uncontrollable. This might cause the UAV to crash into aeroplanes, buildings or crowds of people. Hence this method can only be used when a UAV is in a so-called 'safe space' where it cannot harm anyone/anything. Furthermore, GPS interference might also affect the GPS systems of the aeroplanes at the airport. It might be the case that this method of neutralisation will not work against every UAV, as some UAVs might not use GPS to communicate with its pilot. Also, if a UAV is autonomous, it does not even need communication with a pilot necessarily. Lastly, it might also be difficult to get regulatory approval for the use of jamming devices due to jurisdictions.

GPS spoofing

This method is similar to the method discussed above (Interfering with GPS) and thus also shares the most advantages and disadvantages. However, an advantage of this method compared to GPS interference is that the targetted UAV will not be uncontrollable, but instead, the UAV can just be safely landed on the desired location. Apart from that, it shares the same disadvantages as GPS interference.

Capturing UAVs using nets underneath other UAVs

This method is a method that is not harmful to UAVs/the surroundings and does use interference which might cause problems for aeroplanes at the airport; those are two significant advantages. Another advantage is that it is a very affordable method to deploy to counter unwanted UAVs and allows for safe retrieval of UAVs. However, UAVs carrying a net to capture hostile UAVs, mostly just have room for carrying a single net to capture a hostile UAV. Meaning that if the UAV misses the net, it needs to return and completely be reset. Another thing is that these UAVs need to be able to follow small and very fast hostile UAVs to be able to place a net over them. This might be quite an issue, since carrying a net might be pretty heavy for a UAV causing it to move more slowly. Lastly, using this method, there will be even more UAVs in the air space, meaning even more interference for the airport.

Bazooka net system

This method is quite similar to previously mentioned method (Capturing UAVs using nets underneath other UAVs), however, in this case instead of using a UAV carrying a net, it uses a bazooka to capture a hostile UAV. It will also be quite affordable and straightforward to implement. Another advantage of this method is that there will not be any extra UAVs in the air space which might cause more interference. However, this method might be more inaccurate since a net needs to travel in the air for a more extended period before reaching an unwanted UAV, giving the UAV more time to evade the net. Furthermore, a bazooka firing a net will have more limitations in its range compared to UAVs carrying a net as it might not be able to reach unwanted UAVs which are high up in the air space.

Geo-fence coordinates

This method is the most straightforward solutions for the airports, as they will not need to implement a system to neutralise UAVs since UAVs simply will not be able to enter the air space of airports. However, this method is dependent on drones being programmed not to enter certain areas and will therefore not always avoid unwanted UAVs at airports. This might help against unwanted consumer UAVs that accidentally enter the air space above airports due to ignorant pilots. However, if someone really has terrible intentions with an airport, it will be straightforward to either turn off the geo-fencing software on a UAV or simply design their drone which will not have any geo-fencing software either. Furthermore, the software of the UAVs must continuously be updated according to new areas that might not be entered by UAVs, which might not be possible. Lastly, this solution also does not help against a large number of UAVs that already exist and can still be used for interference at airports. So in summary, this method might be useful for taking on a large part of newly bought UAVs, but can easily be avoided and should not be relied on by airports as the only method against UAVs.

Eagles

Using eagles to intercept drones is an economically friendly solution. Furthermore, the chances of a technical malfunction are non-existing. It is, however, still possible for the birds to deviate from the standard procedure when intercepting a drone even after extensive training. Moreover, flying birds near and around airports can be dangerous as they can get damaged by aeroplanes and other obstacles.

The Dutch police started using eagles to intercept drones back in 2016 already[14]. This, initially, seemed like a successful approach to seize drones mid-air. Not long after their initial usage, the Dutch political part Partij voor de dieren' expressed their concerns regarding the safety and the wellbeing of the eagles.

After a year of training the birds, the police have concluded that the Eagles were barely used. Furthermore, the NOS reports, the training of the eagles is more complex and more expensive than the police expected. Additionally, there was little to no return in training these birds. Moreover, the birds did not always follow the procedures they were instructed to follow and therefore, the police was not convinced the birds would follow these procedures in real use[14].

Anti-Drone Eagles - title text: It's cool, it's totally ethical--they're all programmed to hunt whichever bird of prey is most numerous at the moment, so they leave the endangered ones alone until near the end.

Let us consider the following comic created by Randall Munroe on May 26, 2017. This comic raises a crucial ethical argument against the use of eagles in anti-drone mechanisms. While eagles, the predators they are, have natural inclinations to attack central parts of drones while evading sharp bits, their lives are still put at risk.

Cueball (person in the middle) argues that using eagles as anti-drone mechanisms is unethical as it forces a rare animal to put their lives at risk. Cueball compares it to using police dogs for traffic control, which is something that most people would frown upon after giving it some thought.

The effectiveness of eagles depends a lot on the conditions of how they are used. Naturally, eagles cannot be used everywhere, but they are often effectively used where some form of ground security is present that can be used to identify and arrest those illegally flying their drones. This is partly due to these people not being able to replenish their hardware indefinitely.

Not only would the use of eagles be unethical, but also ineffective. That is partly due to the supply of eagles being somewhat limited. Furthermore, there are natural boundaries to how fast they can be replenished, whereas more drones can easily be created to replace those that have been destroyed. Of course, this will involve more money, but we should be prepared for the worst. As brought up in the third part of the comic, traffic control dogs would be similarly ineffective, as dogs would struggle to run equally fast as racing motorcycles. Moreover, they would, in most cases, be too powerless to stop the motorcycle even if they could keep up them.

Megan (the girl on the right) states that both ideas, the usage of eagles and dogs, sound cool'. She does, however, understand the ethical argument that Cueball raises against their use for traffic control. On the other hand, Black hat (the man on the left) goes a step further and states that he has created a drone that hunts eagles. This flips the premise from anti-drone eagles' to anti-eagle drones'. In the title text, which represents a statement from Black hat, he continues that it is ethical because they - the `anti-eagle drones' - only target the most populous species first, although they will eventually eradicate the endangered ones once they bring down the number of all birds of prey. Here, Black hat seems to miss the point that it is not merely the relative number of birds that creates an ethical problem, but the fact that animals' lives are being put at direct risk by humans. This is especially negative when other mechanisms can be used that are similarly effective for a bit more money, if not equal or less. The construction of his anti-eagle drone may simply be for the point of making the eagles' goals not only dangerous but also entirely ineffective.

All things considered, the comic above raises a critical ethical argument against the use of eagles in anti-drone mechanisms.

This method is actually really similar to the previously discussed GPS interference method and shares the same advantages and disadvantages as that method.

## Discussion

After finishing the collection of solutions (for now), it quickly became clear that the split into the three subcategories, namely detection, identification, and neutralisation made sense in quite a few cases. There were, however, solutions where identification and detection were merged. Furthermore, in order for identification to be something tangible, there needs to be detection. Many solutions already provided forms of identification, which is (by the previously mentioned prerequisite) also included detection. Therefore, we have decided to merge detection and identification. This would make the execution of the decision model a lot cleaner and also makes sense from a logical point of view. We want to group things together as much as we can as long as this does not degrade the quality of the result. From this point onwards, we only consider the detection (and identification) and neutralisation subcategories for all solutions.

## Comparison chart

In order to summarise the differences between the above solutions, we made a comparison chart. In this chart, we quantify the various attributes of each solution to allow us to see what solution is better for which type of airport.

We have chosen to look at the following attributes:

• Range, differently sized airports need different ranges of effect
• Speed of operation, for some airports a threat needs to be detected and neutralised in seconds, other airports do not have that much time pressure
• Disturbance to the environment, often airports closed to urban areas have noise restrictions, while others may not
• Effect on different types of drones, some solutions may only work on a targeted subgroup of drones, which can differ per airport
• Scalability, some airports may expand in the future and do not want to be limited by the investment in this technology
• Number of drones it can concurrently handle
• Emission, sustainability is essential for some airports. Other airports find this to be less of a concern
• Size, some solutions are way larger than others
• Identification, some solutions have a way also to identify
• Level of autonomy, the autonomy of the solution can play an important role as well
• Power Outage risks, it is possible to argue that the solution should always work, even when there is a power outage
• Weather, certain solutions might not work when it is really bad weather
• Uptime, some solutions have to be active 24/7, whereas others do not
• Portability, some solutions should be portable
• Danger to humans, at some airports many civilians may be present, for other airports this is not that much of a concern
• Destructivity, it can be possible that there should not be any destructivity be involved
• Level of training needed, sometimes it is possible that individuals operating the solution need a form of training

Rendering the attribute - solution table does not result in a clear overview. Therefore, we choose to only redirect the user to the tables rather than posting the tables here. These tables can be found here and here.

Back to the root page.

## References

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6. Burns, M., https://techcrunch.com/2016/03/04/the-skywall-100-bazooka-captures-drones-with-a-giant-net/?guccounter=1 "The SkyWall 100 bazooka captures drones with a giant net"], TechCrunch, 2016, Retrieved on 2019-02-07.
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8. Thuy Ong. "Dutch police will stop using drone-hunting eagles since they weren't doing what they're told", 12 December 2017, Retrieved on 14-02-2019
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11. Snapshot: DHS Silicon Valley Innovation Program Successfully Transitions Three Technologies to CBP https://www.dhs.gov/science-and-technology/news/2019/02/05/snapshot-svip-successfully-transitions-three-technologies-cbp
12. Super Bowl: experimental radar aims to stop drone drama at game https://www.theguardian.com/technology/2019/jan/28/super-bowl-drones-radar-start-up-experiment
13. The U.S. government showed just how easy it is to hack drones made by Parrot, DBPower and Cheerson. (2017). Recode. Retrieved 15 February 2019, from [1]
14. Haye Kesteloo, Dutch police halts use of eagles to intercept drones (2017) [2]