Guido Manfredi
email: guido.manfredi@enac.fr

Elgiz Baskaya

Jim Sharples

Yannick Jestin

Unmanned Aerial System Operations for Retail

Keywords: UAS, RPAS, drones, retail, mass distribution, integration, drone operations

The number of Unmanned Aerial System (UAS) applications is quickly increasing as technology, standards and regulation allow them. With each new application, more industrial sectors get affected, and the retail sector is already being impacted. This paper presents five UAS applications that will impact the retail sector: freight, monitoring, guiding, delivery, and advertisement. For each application, concepts of operation are provided along with the associated technological, standard and regulatory locks. These operations are then organized along time, from earliest to latest accessible, with accompanying explanation as to why and when. It is shown that the applications currently most publicized are not the ones that will come first. Finally, a discussion regarding the accuracy of our forecast is proposed and leads to support the enabling of drones, in the retail sector, are provided.

I. INTRODUCTION

The retail sector has been quickly evolving in the last decades, as it aptly integrated emerging technologies. The advent of the Internet enabled big data and machine learning approaches, e.g., recommender systems based on customers browsing history to target advertisement [START_REF] Smith | Two decades of recommender systems at amazon.com[END_REF]; automation changed the way general merchandise stores worked with self-checkout machines and automated drive supermarkets [START_REF] Reuters | French retailers in Drive to win market share[END_REF]. Smart sensors changed the way online retail stores present their goods, with the development of virtual dressing rooms [START_REF]Fashion ecommerce: are virtual fitting rooms the silver bullet[END_REF]. Even the most complex technologies have been integrated in the retail sector, with warehouse robots allowing fast and efficient content management [START_REF] Wurman | Coordinating hundreds of cooperative, autonomous vehicles in warehouses[END_REF] and robotic guides improving the customers' experience [START_REF]Softbankrobotics[END_REF]. The successive technologies transformed the industry in many ways by changing how goods are stored and managed, reducing the number of intermediates, automating dull and dirty tasks, and changing the customers' habits and experience. Now, a new technology is about to become fully available: Unmanned Aerial Systems (UAS). A well-known example is the Amazon prototype [START_REF]Amazon Prime Air[END_REF] for autonomous package delivery. However, there are lesser known applications, which could have more impact on the retail industry and sooner than one might think. This paper studies five applications in which UAS operations will impact the retail sector. For each application, different concepts of operation are considered. For each operation type, the key enablers are listed. Exemples of such operations, and ongoing research on the topic, are also provided. As the reader may not be familiar with UASs, Section II introduces their architectures, their environment, and their regulation. With this information in mind, Section III presents the five applications along with the technologies and regulation required to enable them. Tentative dates at which these applications will become possible are provided in Section IV. Finally, the conclusion provides future leads to explore in order to enable UAS operations for retail.

II. THE UAS WORLD

There are numerous words to designate drones, each with its own meaningful nuance. For the sake of clarity, in the rest of this paper, we use indifferently the terms UAS and drones, with the definitions provided hereafter.

A. UAS Architecture

The S in UAS stands for "System", a UAS is composed of three elements: an Unmanned Aerial Vehicle (UAV), a Command and Control link (C2 link), and a Ground Control Station (GCS). We provide some general information regarding each one of these elements, as it will be useful to understand the limitations in different applications.

1) UAV: The UAV is the aerial part of the UAS. There are numerous ways to categorize UAVs. For clarity we will consider that they can belong to one of three families: rotorcraft, fixed wind, and airship. The most represented type of UAV is the rotorcraft. It does not have wings and entirely relies on the thrust from its propellers to fly. Easy to pilot, especially for take-off and landing, it allows the pilot precise positioning. Its drawback is the energy needed to remain aloft, as well as the noise and wind created by the propulsion. With low endurance, it remains the most popular form of UAV. The fixed wing UAV flies thanks to the lift produced by the flow of air above its wings, like most manned airplanes. For this reason, it must remain inside a given flight envelope and is thus less maneuverable than rotorcrafts. As the lift comes from the air reaction and not purely from the propellers' thrust, it has high endurance and can fly for long periods of time. Plus, it can carry high payload relative to its own weight. It needs an initial speed for takeoff, and some distance for landing, though it is possible to design Vertical Take Off and Landing (VTOL) fixed wing aircraft, greatly simplifying the beginning and end of flight. Its aerodynamic shape allows it to reach high speeds. For a airship type UAV, the majority of its lift comes from a lighter-than-air gas inside its structure allowing it to stay aloft with energy spent only for motion. Consequently, it has the highest endurance of the three types. However, having to store gas onboard gives it a bulky shape which reduces the maximum speed/maneuverability and makes it sensitive to external perturbations (e.g., wind). The UAV also carries the equipage, i.e., the onboard equipment needed for the UAS operation (e.g., flight, rules compliance) but not necessary to perform its assigned task. The equipage comprises the sensors used to navigate (e.g., GPS, cameras), the communication systems, and the equipment required by regulation (e.g., lights, sound). A brief summary of the differences between these types of aircraft is provided in Table I.

2) C2 link: The C2 link is the medium used to uplink/downlink Command and Control (C2) data to/from the UAV. From an operation point of view, C2 links can belong to one of three categories: direct link, terrestrial network, satellite network. The direct link allows short range (few kilometers) control over a UAV. It has the benefit of not relying on a service provider. For long range operations, the C2 link can be carried over a terrestrial network. Due to the cost of such installations, a service provider is required. The delay in such type of communication is low enough to be comparable to direct link. Similarly, satellite networks allow very long range operations and operations where no ground network is available (e.g., over mountains, water). But it has longer delay than direct or terrestrial networks. This type of C2 link may not be useable for certain types of operations, e.g., indoor operations. Again, due to the cost of such structures, it requires a service provider.

3) Ground Control Station: The GCS can be a simple remote control, a cockpit sized control room, or even bigger. Its size and complexity is related to the type of UAV and the type of operation. However, as the GCS is not considered a limiting factor, we will not take into account the different types of GCS in the rest of this paper.

The capabilities of a UAS change greatly depending on its type and equipage. These need to be adapted to the type of operation to ensure a safe flight while allowing the UAS to perform its mission. Note that, for a same UAS, the regulatory constraints vary depending on the environment and type of operation.

B. UAS Environment

The environment of an aircraft can be classified based on three elements: the available services, the systems required on the aircraft and the applicable procedures. Services encompass all external support to the operation, like aeronautical meteorology or conflicts management. System requirements designate onboard equipment needed to interact with services, other aircraft, and to ensure proper following of procedures. Procedures are pre-defined behaviors that an aircraft needs to follow in certain airspaces or situations. When considering manned aviation, these three elements form the Air Traffic Management (ATM) system; these same elements, when specific to drone operation, form the U-Space system.

1) ATM: The ATM has a strong safety record due to its numerous services, high system requirements and numerous procedures. As a consequence, integrating in such an environment is complex. Plus, the fact that manned aviation is currently flying in these environments asks for drones to be integrated seamlessly, i.e., a drone should be able to behave like a manned aircraft in every aspect. Drones capable of integrating in the ATM system are likely to be large ones, with performances close to these of existing manned aviation.

2) U-Space: The notion of U-Space encompasses the services and procedures offered to drones to allow their integration. The U-Space is planned to be rolled out in four phases: U1, U2, U3, and U4 [START_REF]U-Space blueprint[END_REF]. Phase U1will bring e-registration, eidentification, geofencing (don't go in this area) and geocaging (don't leave this area). Phase U2 will set up management of drone operations through flight planning, flight approval, tracking, airspace dynamic information, and procedural interfaces with ATM. In Phase U3, operations in dense areas, capacity management, assistance for conflict detection will be rolled out. Finally Phase U4 will provide integrated services with manned aviation, high levels of automation for all services. When reaching U4, all types of operations, even the most complex ones, will be supported by the U-Space services and procedures. U-Space will provide traffic management for small UAS but also drone specific services for large UAS.

Depending on the type of operation, the drones will have to integrate with: neither, one, or both of these environments. However, the complexity of integrating a drone not only comes from the environment but also from the risk of the operation, as will be explained in the next section.

C. UAS Regulation

In order to provide an unified answer to the problem of regulating UASs, national regulators and industries gathered to create the Joint Authority for Rulemaking on Unmanned Systems (JARUS). This entity, building on the European Aviation Safety Agency (EASA) rules, proposed to split UAS operations based on risk levels. The risk is mainly defined in terms of probabilities to hit a human on the ground, another airspace user or a critical infrastructure. This led to the definition of three operations categories (A, B and C) described below.

1) Category A: Category A encompasses low risk operations, like most of leisure flights and some professional activities. Operations falling in this category do not require an explicit authorization from civil aviation authorities. But, they are subject to strict operational limitations (e.g., no proximity to people, traffic, infrastructures; no dangerous items; one pilot per UAS; no item dropping). These operational limitations are sufficient to mitigate the low risk. Though some professional activities fit in this category, the main goal is to regulate leisure types of operations.

2) Category B: Category B regroups medium risk operations, such as operation beyond visual line of sight (i.e., no visual contact between pilot and UAV during flight). To facilitate the regulation process, a set of scenarios with specific operational limitations are designed. To operate within a scenario, the UAS needs to comply with a list of requirements. For operations outside the scope of the scenarios, a risk analysis must be carried out to show that the existing risks are properly mitigated. To facilitate this risk analysis process, JARUS developed a framework called the Specific Operations Risk Assessment (SORA). The SORA considers threats, which contribute to the risk, and barriers, which mitigate the risk, to evaluate the actual risk of an operation and decide if the resulting mitigated risk is low enough to allow the operation. The risk analysis needs to be validated by the authorities in order to authorize an operation not included in the standard scenarios.

3) Category C: Operations with risk that cannot be mitigated in Category B are evaluated in Category C. These represent high risk operations, for example large cargo delivery in urban areas. These are likely to be operations with a risk close to current manned aviation's one. As such, the aircraft, avionics, pilot/crew and operator will need to be certified in order to fly these operations. The fact that a certification process is involved increases the complexity of the introduction of UAS in these types of operations. Indeed, before being able to certify a piece of equipment, a standard must be developed for this equipment. Standardization can be understood as the process of defining details and minimum requirements for safe and uniform operation across a diverse range of implementations. However, as of today, not all the parts required to fly a UAS have corresponding standards, e.g., Detect And Avoid (DAA) systems, C2 links, pilot training. So, enabling this category of operation requires extra effort and time for the industry to agree on standards. For Category C, harmonization at the International Civil Aviation Organization (ICAO) is planned to allow international operations.

4) The special case of indoors operations: Indoor operations are not regulated by civil aviation authorities, so the above categories do not apply. In fact, indoor operations fall under the responsibility of the operator and usually no risk analysis or certification is required by the authorities.

With a knowledge of the existing technologies, environments, and regulations, next section introduces UAS operations that will impact the retail sector in the near to long term.

III. UAS APPLICATIONS FOR RETAIL

In the following, we consider five domains where the UASs will impact the retail industry and examine the prerequisites for integration, as well as the obstacles faced on the technological, standardization and regulatory levels.

A. Freight

Freight operations involve the transportation of large quantities of goods, which represents the input of most retail stores. Depending on the way suppliers operate, opportunities for the retail sector can change. Thus, an evolution of the freight can have an impact on the retail sectors. In the following we analyze two freight operations: cargo flights transporting goods on long distances and cargo delivery to urban areas.

1) LUCA: Large Unmanned Cargo Aircraft (LUCA) offer economic opportunities, as demonstrated by Collins [START_REF] Collins | The future market for large unmanned cargo aircraft in the national airspace system[END_REF], especially when it comes to long haul point to point, delivery to/from remote places and high value cargo over distances requiring multiple piloting crews. Removing the cockpit will cut costs related to having a crew onboard and the associated maintenance. Plus, the UAV will be freed of dynamical constraints related to the presence of humans onboard (e.g., limited acceleration, turning angles, braking force). This kind of operation will ask for integration in the ATM system. Aircraft performing these missions will fly along pre-determined routes, will have to access airport environments, and to follow specific ATM procedures. Because it will fly in the middle of the traffic, this type of operation is classified as high risk, Category C, and will ask for certification. From a technical point of view, transforming large manned aircraft into UAVs is possible with existing technology; the UAV having the same capabilities as existing aircraft, it will integrate seamlessly with the ATM environment. But it will require to agree on some standards. Notably for the C2 link, DAA and Automated Take Off and Landing (ATOL) equipment. Indeed, long range operations ask for a terrestrial or satellite communication network, collision avoidance is a requirement to fly with the rest of the traffic, and operating in airport environments remotely requires ATOL and auto-taxi (from gate to runway) capabilities. The rest of the equipment remains the same so no additional standardization effort is required. Once these hurdles are removed, this type of operation will be at hand. Using LUCA could even open new opportunities. Indeed, in the context of the SESAR2020 projects [START_REF]PJ10-05[END_REF], experiments have been carried out to determine drone specific trajectories which could facilitate their landing on airports without disturbing the existing traffic. It has been determined that UAVs can use steeper descent angles and sharper turning angles. Added to the fact that there is no human onboard, so less airport services are required, this could open access to more options in terms of airports and available supply routes.

2) Urban freight: Urban is currently defined in some civil aviation instances as an area with a density of population higher than 1295 people per square kilometer (plus a 0.5NM buffer around it). Nowadays, delivery of goods to urban outlets is mainly done with trucks. This brings problems related to traffic, and increases the cities' complexity. Using air transport can simplify the way deliveries are carried out. It would still require some urban planning for the landing sites and air routes, but could remove the impact and dependency on road traffic. Moreover, little infrastructure would be required for operations contrary to trucks, which require at least roads, facilitating delivery to peculiar places (e.g., islands, old cities).

Comparatively to what can be done today with helicopters, using a UAV would allow saving space and weight by removing the cockpit. Plus, today's helicopters operations over urban areas are mostly exceptional. Developing an appropriate regulation for UAS will allow them to operate routinely. The urban freight operation is complex due to the fact that the UAV is likely to have to go both into integrated airspace (cruise) and low level (take-off and landing), so these UAVs must both integrate with ATM and U-Space. In both cases, flying above urban areas will be classified as a category C operation. A benefit of flying above urban areas is that manned aviation is not allowed near buildings, simplifying operations in terms of DAA for traffic. However, the density of infrastructures requires a DAA for fixed obstacles. Plus, these operations are likely to use stable flight routes, with deconfliction services required considering the operational risk. This asks for U-Space services from phase U3. Due to the size of these UAVs, and to reduce nuisance to the population, it is likely that some areas will be forbidden. Because of the large payloads, delivery from the air or dropping seems unlikely, so the freight UAV will have to land on some properly defined spaces. The limited amount of space will ask for rotorcrafts and/or possibly VTOL UAVs. Urban areas being well covered in terms of communication networks, ground communication will be the preferred solution.

Enabling these operations will take time, as there is no equivalent in manned aviation, so there is little experience.

Having a UAV transition from integrated airspace to low level will ask for equipage to deal with both environments. In integrated airspace, a DAA for traffic will be required, while in urban environment (take-off and landing) a DAA for obstacles will be needed. Developing both of these pieces of equipment for medium sized aircraft is a significant challenge and will ask for research to be done, especially on sensor miniaturization. The sensors will also be crucial for the localization part, as some urban areas can be GPS denied, asking for an independent localization mean. From a procedure point of view, insertion of a relatively slow UAV in the traffic will be challenging. At lower levels, dynamical flight planning will be needed to adapt to a future busy airspace around cities. Procedure for helipad use will also need to be defined, though this could benefit from regulations of cities with high helicopter traffic. A solution to the miniaturization problem can come from the automotive industry. Indeed, hardware and software developed for autonomous cars could answer the requirements for low level flight. The envisioned UAV being on a scale similar to cars, the Size Weight and Power (SWaP) requirements could match. From a procedure perspective, this type of operation will benefit from the advances brought by the current exploratory research projects on U-Space (see current SESAR2020 ERC projects). Alternatively, in the future, a drone could stay aloft and be used as a warehouse while being supplied by ground-to-air freight: Amazons floating warehouse [START_REF]Amazon's flying warehouses to dispatch drone deliveries from the sky[END_REF].The idea is to have a medium altitude floating warehouse to which a swarm of drones have access. This might serve as a fast delivery platform, assuming the regulatory hurdles behind would be solved in the future. The company patented the design, with details about its fuel efficiency solutions for the route from airship to the delivery destination.

Freight related flights are likely to be among the first high impact operations to be enabled since they already exist and only require modifying the transport medium (from aircraft to UAV). Plus, they don't involve the transportation of humans, greatly reducing the risk. However, operations in urban areas will still ask for complex procedures and systems to be set up before they can take place.

B. Warehouses, Distribution Centers and Outlets Monitoring

Monitoring in those large buildings offers some challenges which can be eased with the help of drones. A wide range of applications can be envisioned like security protocol improvement, customer habits studies, process improvement, and inventorying [START_REF] Overflo | How Drones Are Changing the Face of Warehousing[END_REF]. Todays solutions can be expensive and lack efficiency. For example, inventory management of distribution centers is most of the time done by human workers via handheld devices scanning through stocked pallets of goods in labyrinth like aisles. UASs abilities to move quickly and in 3-D will facilitate these tasks with an automated, flexible and cost-efficient solution. Fleets of UAVs going around a building with appropriate sensors can reduce the number of required sensors as well as their cost (closer range, so less costly). The autonomy level required for such tasks is to be able to navigate and avoid collisions with static obstacles and humans (assuming there are no other flying object). Automation will allow the routes to be changed dynamically by the operator providing flexibility and usage of a same fleet for different tasks. The fleet will operate for long periods of times with automated UAV replacement and charging. In this section, we limit ourselves to indoor operations. These applications don't ask for high speed, large payload or maneuverability, the main requirement will be endurance, plus it is indoors operations in a known environment subject to little perturbations. For such tasks, airships are the best pick as they provide high endurance and stability for onboard sensors. The two main tasks of navigation and collision avoidance ask for a precise localization system, which will most likely be based on Simultaneous Localization and Mapping (SLAM) methods which have been extensively studied. Existing sensors, even lowcost ones, like monocular cameras, would allow to perform this task. As there is no interaction with traffic, ATM, or U-Space, regulatory constraints will be low. The two main obstacles for this application are the high level of automation and the operation above the public, which asks for Category B regulatory requirements. Though it will not be the case for all applications, if the UASs operate above the public, they will require mitigation means to prevent any incident developing into injuries on ground. Again, airship provide some de-facto mitigation means because they are lighter than air. For the level of automation, operations with a pilot per UAV, or even one pilot for multiple UAV, does not seem like an economically viable solution. These applications will most likely ask for supervision only from one human who could perform other tasks.

Though not existing yet on the market, all the pieces for these applications are on the shelves. There are no strong regulatory or technological barriers to the introduction of UAS in this context.

C. Improving Customer Experience

According to 2018 retail predictions, the organizations have to review their structure to set the experience of the customers as their priority [START_REF]Retail predictions. From drones to the changing customer experience[END_REF]. With the data based methods, such as machine learning, offering powerful solutions for customer data, it is easy to have a direct link to the habits of individuals which would help to offer preferable solutions to customers. This customer-centric approach would be likely to involve interaction between the customer and the organization both online and in stores, where a drone can be a part of this link. While the concerns of the public about drones is already on the rise before they have started to populate the sky, maybe indoors applications can serve as a warming environment to start with human-machine interactions. A scenario will be to have drone guides that will show you the path to the item of preference, saving time during the supermarket item searches. Their ability to move in 3-D already will allow them to point items in higher shelves where it might not be easy with other solutions. They can even calculate the best route to follow if a list of items is provided. They can offer alternatives for the missing items, shuffling through the database. Another possible application will be drones for assisting with car parking in malls. For big cities, finding a spot in a mall parking might be quite challenging. Multiple drones can work in harmony to guide the customers to the closest parking lot, and win the hearts of the public in time. Yet another application of drones might be to merge with virtual goggles, displaying the stream through a camera on the drone to wander in shops to see the items or environment without the need to actually be there. For example, this would help customers choose the less crowded time to go shopping, naturally improving the customer flow. In this type of application, two operations have to be distinguished: indoors and outdoors. As mentioned earlier, indoors operations are not concerned with civil aviation rules. However, contact with the public asks for risk mitigation measures. To reduce safety concerns, one solution could be to use very small drones (<800g). A rotorcraft or airship of this size would have sufficient autonomy to guide a customer while being small enough not to be a hazard. Since they will be flying in large aisles, avoiding obstacles is not a concern and avoiding the other drones will be the main challenge. Now, for the outdoor case, things get complicated. Mitigation measures to ensure that drones won't crash onto cars will be required. Indeed, a failure to the drone could lead to a car crash. The resulting risk is likely to classify this application in Category C. Moreover, at least U1 U-Space services will be required to ensure tracking and geocaging of the drones in the mall area. In both cases, the fleet management and autonomy aspects will be the main concerns, as they will determine the overall efficiency of the system. Human-machine interaction will also be crucial to allow easy communication between drones and customers. As far as guiding humans with drones is concerned, feasibility studies showed that drones can be used to guide blind runners [START_REF] Al Zayer | Exploring the use of a drone to guide blind runners[END_REF]. Similarly, prototypes of flying street lights have been designed to guide people through cities [START_REF] Designboom | Fleetlights: a prototype team of flying torch drones to guide the way at night[END_REF]. Still, guiding simultaneously a group of persons is a challenge yet to be undertaken. For guiding cars, manufacturers like ford are currently exploring the solution for a wider application [START_REF]Ford Studies Drone Use to Guide Self-Driving Cars[END_REF]; guiding cars through a parking could be a first step, yet a challenging one.

When it comes to customer experience, customers are used to have what they need, as they need it, at the time they want, most probably as fast as possible thanks to interactions with computers, internet and mobile networks. Thus, the companies are in a rush to keep up with the race of expectations, otherwise they face to lose the customers quite fast as well. Faster deliveries is of importance and how it might be achieved is described in the following section.

D. Delivery

As mentioned earlier, the advent of internet shopping has profoundly altered the logistics of retail. According to the BBC [START_REF] Bbc | Crowded streets: Cities face a surge in online deliveries[END_REF], parcel volumes surged almost 50% globally between 2014-2016 and they are on track to increase at rates of 17-28% annually up to 2021. This has bolstered the need to rethink logistics, particularly in the delivery part. Currently, delivery is mostly done with delivery vans and trucks. Because of the sheer volume of parcels, this is causing problems in big cities. According to [START_REF]Transport, la logistique du dernier kilometre (Transports, the logistics of the last mile)[END_REF], in Paris, one moving vehicle in five is a delivery vehicle, generating 25% of urban CO2 emissions. Last mile delivery is also costly: it represents 20% of the cost of the whole delivery chain [START_REF] Technologyreview | Urban Drone Deliveries Are Finally Taking Flight[END_REF]. This is where drones may come in handy. With their low cost, automation, high speed and flexibility, they could become the future of delivery. This section focuses on the last leg of the journey, more specifically on the last mile delivery, between the warehouse and the customer or a pick-up point. This application can be declined in three operational scenarios: delivery to a remote location, to an urban pickup point, to a house or person in an urban area. Each of these scenarios has its own constraints that will be detailed in the following. For all applications, the main technological capacities needed are endurance, automation, maneuverability, payload carrying ability and DAA.

The most important aspect is ensuring the integrity of the data used to automate the flight. If this data is corrupted then the flight might be jeopardized. This means several onboard sensors must be used and fused. Global Navigation Satellite System (GNSS) is to be avoided in the cities (too imprecise). And malicious corruption of the information source must be mitigated. Parcel delivery operations will likely fall within Category B. If parcel delivery falls outside of the set of preestablished specific scenarios, a risk analysis must be carried out and an authorization must be requested. Since delivery drones might be numerous, and certainly will not be the only drones in the urban sky, services from U-Space will need to be at least in phase U2.

1) To a remote location: This is the simplest scenario, flying to a remote location avoiding urban areas greatly reduces the risk to humans. This type of long range mission is best suited to fixed wing or VTOL UAV, with a connection through terrestrial or satellite network. Depending on the remote place to reach, flight can be performed at low levels, taking advantage of U-Space services, or at medium altitude which would require interactions with ATM, thus adding requirements on the operation. Assuming that the flight goes through low density air traffic, because of the remote location, constraints on DAA and deconfliction services are low. However, such long-range operations are likely to ask for a stable flight plan. Thus, a U-Space in phase U2 would be sufficient for this type of operation.

2) To a pickup point: Flying to a pickup point has various benefits. Firstly, the objective is fixed so a stable flight plan can be used. Secondly, a landing facility will be available at the pickup point. However, due to these operations being in urban areas, the landing facility is likely to be small, like a helipad, thus VTOL and rotorcraft will be the preferred UAVs. This type of operation will have to be integrated in the middle of a complex airspace above cities, asking for a U-Space in phase U2 to U3. Protection areas around cities will prevent interactions with manned aviation, so no interaction with ATM would be required in normal operations.

3) To a house/person: This is the most complex case, as each delivery has a different destination which can potentially move (moving person as target), asking for maneuverable aircraft like rotorcraft. From a procedural perspective, this asks for services capable of dynamic flight planning, dynamic geofencing and deconfliction to ensure that the drone is able to reach the person/house. Such a high level of dynamic airspace management asks for a U-Space in phase 4. On top of the equipage required to be integrated in the U-Space, the UAV will need to carry identification means to ensure that the parcel is delivered to the correct customer, increasing the payload. It is still unclear how delivery could be done: dropping the parcel would be dangerous for the customer, and the parcel; attaching it to a rope would jeopardize the flight and be dangerous to the population below; landing for the delivery would expose the drone to hazards like animals, or malicious humans.

As a summary, delivery drones must be agile, enduring, and loaded with sensors; this means that the maximum parcel weight they can carry might be reduced. This also means that the number of drones delivering parcels will be high; and since other drones will be flying for other missions (police, etc.) a high level of airspace management will be required, thus delaying the integration of these operations.

E. Advertisement

Advertisement has evolved recently with the appearance of dynamic ads and recommender systems. Instead of advertising goods to an as large as possible audience, tailoring advertisement to the customer's needs is now possible. The integration of UASs is an opportunity to develop both approaches. For mass advertisement, UASs could be equipped with banners or signs, depending on their size. For targeted advertisement, displays on a UAV could allow displaying the relevant information to the right customer. For example, one could envision a delivery drone asking for the customer to watch an advertisement (just like on some web videos) before releasing the parcel. This would imply some design challenges. Indeed, UASs are systems optimized for a given operation, with all design factors: Cost, Size, Weight And Power (C-SWAP), being pushed to the limit. Any additional element not directly related to the mission or the UAS might jeopardize the optimality of the design. For example, adding advertisement structures would add significant aerodynamic drag asking for more power or a different structural design. Similarly, adding displays would increase the weight and require more power. This effect can be lessened by new technologies, e.g., by providing more powerful motors, more efficient batteries, better control laws. From a regulatory perspective, privacy concerns might arise from the fact that targeted advertisement in a public place can reveal personal information that a customer would prefer to keep private. This could be solved by technological means (e.g., displaying the advertisement on the customer's cellphone) but it could lead to more customer acceptability issues, on top of the ones linked to the presence of the advertisement itself. This type of operation could be started as soon as drone operations will be allowed in close proximity to humans.

This section presented the operational concepts related to each of the five considered applications, as well as the obstacles preventing their integration and existing projects. The next section compares the different obstacles with the timelines of large European efforts in order to forecast dates at which the required services and procedures will make the operation available.

IV. ENABLING TIMELINE IN EUROPE

Since the beginning, three types of obstacles have been considered for the integration of drones in various types of operations: technological, standardization and regulatory. In the European integration effort, each of these locks is currently tackled by an international effort. Technological locks are being identified and dealt with by SESAR2020, the coordination body for ATM research and development. Lack of standardization is addressed by EUROCAE, the main aeronautical standardisation body in Europe, especially the working group WG-105 dedicated to UAS. In terms of regulation, the EASA is developing a regulation for drone operations.

In terms of obstacles, it appears from the analysis of the previous operations that the biggest obstacles when introducing UAS are likely to be:

• Safety aspects, especially related to the presence of humans near the operation, • The need to integrate with ATM, or U-Space services, or both, • High automation levels, • Definition of standards for UAS sub-systems.

The obstacles and ongoing efforts deployed in Europe are presented side-by-side for each type of operation in Table II.

When the humans are involved in the operation, the risk is lower, and in fact such a scenario is on its way to being regulated in some countries. For this reason, drones for monitoring in non-public places are already at hand. Now when the humans are not part of the operations, like in the customers monitoring and customer assistance operations, mitigation factors are required to ensure a low overall risk level. For both applications, the UAV design and size can be chosen to provide safe operation (e.g., UAV in a ball, micro UAV). For flights far from humans and traffic, the complexity of the operation is related to the amount of interactions with the rest of the traffic. Operating in the middle of a dense traffic is a lot more complex than in less occupied airspaces. For this reason, delivery to remote places will likely come fairly soon. The second obstacle is interactions with the ATM system and manned traffic, though this is a highly complex problem, the fact that it is being actively investigated will enable these operations, like cargo drones, relatively soon. Now, another factor that may delay some operations is the automation level. Indeed, the regulation being developed envisions a pilot for each UAV in Category C operations and possibly one pilot for multiple UAVs for Category B operations. However, to be feasible, some operations will require full autonomy with little or no supervision. We believe this level of acceptance and trust will come only after years of successful drone operations, at which point offering drone services for customer experience outdoors will become a realistic application. Finally, the integration of UAS in high traffic density airspaces, like the future airspace above cities, asks for numerous mitigation measures that large UAV (e.g., freight UAV) will be able to carry, allowing them to operate earlier than smaller ones. Small UAV operations, like parcel delivery, will come with the miniaturization of equipment and setup of UAS traffic management, but that will ask for time.

Based on the SESAR-JU roadmap for integration of drones in ATM and EASA's U-Space roll-out (see Figure 1), considering that EASA regulation is planned for 2019, and in view of EUROCAE's ongoing work, we propose tentative dates for the beginning of each operation. For the EUROCAE timeline, currently studied topics (DAA traffic, C2 link, ATOL) are expected to yield standards in three years (around 2021). Topics not studied yet, if started this year, would still take four to five years to complete. Note that these dates represent the times at which integration will be possible, however it does not ensure that the industry will develop the applications and that the public will be ready to accept them.

V. CONCLUSION This paper presented five envisioned applications, with associated operations for UAS, that will impact the retail sector. Each type of operation has been described, and enabling technologies, standards, rules and services have been put forward. This allowed to provide an estimation as to when the different operations will become possible. One must remain careful regarding these estimations as the retail habits are very different from one country to another, and some countries may want to push some applications regardless of the difficulties. The same applies for regulation, with some countries going faster and being more liberal.

Figure 1 .

 1 Figure 1. The European Union timelines for integration of drones with ATM (green), and with U-Space (orange).

TABLE I .

 I PERFORMANCES OF DIFFERENT UAV TYPES.

	UAV type	Rotorcraft	Fixed-wing	Airship
	Endurance	Low	High	Very high
	Speed	Medium	High	Low
	Manoeuvrability	Very high	Medium	Low
	Payload	Medium	High	Low
	Cost	Medium	Very low	Low

TABLE II .

 II OPERATION TYPES, MAIN OBSTACLES TO INTEGRATION, CURRENT EFFORTS TO SOLVE THEM, AND TENTATIVE DATES FOR INTEGRATION IN THE EUROPEAN CIVIL AVIATION CONTEXT.

	Application	Main obstacles	Main ongoing efforts from civil aviation communities	Prospective begin-
				ning date
	Freight
	LUCA	ATM integration, C2 link,	SESAR2020 projects for RPAS, EUROCAE WG-105 developing	2022
		DAA traffic, ATOL	standards (DAA, C2link, ATOL).
	Urban	ATM integration, U-Space U2	SESAR2020 projects for RPAS, SESAR2020 projects for U-SPACE,	2027
		deployment, DAA obstacles,	automotive sector developing sensors, existing helicopter procedures.
		safety (Cat. C)
	Monitoring
	Public	High level of automation,	Not regulated by civil aviation authorities.	2018
		public safety
	No public	High level of automation	Not regulated by civil aviation authorities.	2018
	Guide
	Indoors	High level of automation,	Not regulated by civil aviation authorities.	2018
		public acceptance
	Outdoors	High level of automation,	SESAR2020 projects for U-SPACE, EASA's regulatory framework	2022-2023
		public acceptance, U-Space
		U1 deployment, safety (Cat
		C.)
	Delivery
	to remote location	ATM integration, U-Space U2	SESAR2020 projects for RPAS, SESAR2020 projects for U-SPACE,	2022
		deployment, C2 link, DAA	EUROCAE WG-105 developing standards (DAA, C2link).
		traffic, safety (Cat B.)
	to pickup point	U-Space U3 deployment,	SESAR2020 projects for U-SPACE, EASA's regulatory framework	2027
		safety (Cat B.)
	to house/person	U-Space U4 deployment,	SESAR2020 projects for U-SPACE, EASA's regulatory framework	2035
		safety (Cat B.)
	Advertisement	Public acceptance	Not regulated by civil aviation authorities.	2018


Guido Manfredi
email: guido.manfredi@enac.fr

Yannick Jestin
email: yannick.jestin@enac.fr

Are You Clear About "Well Clear" ?

Keywords: Well Clear, Remain Well Clear, Self-Separation, Traffic Avoidance, Detect And Avoid, Sense And Avoid, RPAS, UAS

Regulations from the ICAO use the term Well Clear without defining it. Now, this definition is needed to design air traffic Detect And Avoid systems. A definition is currently discussed at the ICAO level, with work on the associated Remain Well Clear (RWC) function underway at standardisation bodies level (RTCA, EUROCAE). But many members of the communities impacted by these works are not well aware of their state. To adress this lack of awareness, this paper provides three contributions. First, it derives from ICAO texts the components of a RWC function: boundaries, alerts and guidances. These are linked to essential elements required to define the Well Clear term: a start and end, the actors involved, and the expected actions. Second, it summarizes the current regulatory efforts in RTCA, EUROCAE and ICAO regarding the Well Clear and Remain Well Clear notions. Third, it proposes discussion topics to move forward. From a DAA perspective, the notion of Well Clear is key to unlock RPAS full integration, i.e. operation in all classes of airspaces. Though existing works make good progress, the ressources engaged on this topic seem insufficient when compared with the complexity and importance of the task at hand.

I. INTRODUCTION

Though mentioned ten times in ICAO's Rules of the Air, the term "Well Clear" (WC) is never defined in the current ICAO regulation [START_REF] Jestin | Rules of the Air[END_REF]. Until recently, this was not a problem since the interpretation of this term was left to the appreciation of highly trained Air Traffic Control Operators (ATCOs) and pilots. However, with the ongoing effort to integrate Remotely Piloted Aircraft Systems (RPASs) in non-segregated airspaces, providing an objective definition of WC has become urgent.

In many types of operations, the integration of an RPAS in the existing traffic is likely to require the RPA to be equipped with an air traffic Detect And Avoid system (DAA). Depending on the operational and regulatory environment, the DAA system should provide one or both of the following functions: Collision Avoidance (CA) and Remain Well Clear (RWC). The former is a last minute manoeuver to avoid imminent collision with air traffic and reach a safe state. The latter consists in smooth manoeuvers considering multiple factors (e.g. safety, operational, mission) to avoid conflicting traffic. The main differences between CA and RWC are highlighted in Table I.

The notion of CA is built around the precise definition of Collision Volume as defined in [START_REF]Manual On Remotely Piloted Aircaft Systems (RPAS)[END_REF]. However, there is no such objective definition of a WC volume, thus preventing the definition of a RWC function. Hence the current efforts, reported in this paper, to aggree on a definition for WC and then RWC. The notion of RWC originates from regulatory requirements of the ICAO Annex 2 Rules of the Air [START_REF] Jestin | Rules of the Air[END_REF].

It is strongly linked with the notions of collision risk and right-of-way rules. Hence, coming up with a definition is no easy task. On top of an effort from the ICAO's Standard and Recommended Practice (SARP) panels, two standardization bodies have taken up the challenge: EUROCAE and the RTCA, along with helper projects. The goal of this paper is threefold. First, based on existing definitions form ICAO documents, an operational decomposition of the RWC function is proposed and linked to the definition of WC. Second, an overview of current efforts on the definition of the RWC function is provided. Third, topics are provided to fuel discussions about the possible evolutions around the WC and RWC notions. This paper first describes the RWC function in more details. Then, the current efforts deployed to tackle the WC and RWC definition problems in the ICAO, EUROCAE and RTCA are described, with a stress on the particular contributions and hypothesis of each entity. A discussion is proposed about the paths not being currently explored. This work concludes on the short term objective of current works and ways to go.

II. THE NOTION OF REMAIN WELL CLEAR

As mentioned in the previous section, the notion of Remain Well Clear is directly linked to ICAO's Rules of the Air [START_REF] Jestin | Rules of the Air[END_REF], "An aircraft shall not be operated in such proximity to other aircraft as to create a collision hazard.". So the primary objective of a RWC function is to prevent collision hazards. Moreover, according to ICAO's Manual on RPAS, RWC is "the ability to detect, analyse and manoeuvre to avoid a potential conflict by applying adjustments to the current flight path in order to prevent the conflict from developing into a collision hazard" [START_REF]Manual On Remotely Piloted Aircaft Systems (RPAS)[END_REF]. Meaning that the RWC function should start when a conflict is detected, and finish when the conflict is solved (RWC succeeded); or when it developed into a collision hazard (RWC failed), in which case the CA function shall engage, if available. Finally, though this is not clear from the ICAO Annex 2 [START_REF] Jestin | Rules of the Air[END_REF], it seems reasonable to assume that the right-of-way rules apply as soon as a conflict exists.

The previous RWC definition relies on the term "conflict" as a starting point and in its relationship with the right-ofway rules. But the term "conflict" is not defined in these documents. The rest of this paper uses the definition of conflict from ICAO's Air Traffic Services Planning Manual [3] and formulated as a "Predicted converging of aircraft in space and time which constitutes a violation of a given set of separation minima". By relying on the term "conflict", the definition of RWC asks for the definition of separation minima.

These minima are materialised by boundaries which separate the airspace in volumes where different rules apply. Considering there is a Remote Pilot (RP) in or on the loop, such boundaries need to be associated with alerts and guidances. An important thing to consider when reading this section is that WC and RWC are different concepts. WC is an aircraft state influencing the application of the right-of-way rules, while RWC should be understood as a separation minima and the RWC functions is a function aimed at ensuring that the RPAS stays out of the RWC minima. DEBUG

A. Boundaries

According to the Right of Way rules, three states are relevant to describe the level of constraint during an encounter: in conflict, well-clear and has right-of-way. In presence of an intruder, i.e. when in conflict, the RPAS needs to follow the RoW rules. If the ownship has the right-of-way, it needs to maintain heading and speed. If not, it is free to maneuver. When an intruder becomes a threat, i.e. when WC is lost, the aircraft giving way is restricted in its possible maneuvers. For the ACAS community, note that the intruder and threat notions used for RWC are different from the ones in ACAS documents where intruder means under surveillance, and threats require RAs.

The different combinations of these states yield two boundaries which split the space in three volumes (see Figure 2). According to the ICAO's RPAS Manual [START_REF]Manual On Remotely Piloted Aircaft Systems (RPAS)[END_REF], the two first volumes are called the RWC volume and RWC threshold (see Figure 1). The third one encompasses the space beyond the RWC threshold, we call it the conflict-free volume. Recent discussions led to a proposition to rename the "Remain WC volume" into "Regain WC volume" to stress the necessity to be WC at any time. This proposition comes from communities which envision DAA systems with no CA capabilities. In this case the RWC function is the only safety layer and should continue performing as long as possible. This is similar to a CA function integrated in the RWC function. For simplicity, in the rest of this paper we consider that a separate CA function always exists and will not use the term "Regain WC". The boundaries of these volumes can be defined using time and/or distance measures (e.g. slant range, time to NMAC). These values should be chosen carefully as the volumes they define must ideally remain larger than the CA volumes so as to infringe them before any CA volume and ensure that the RP and/or conflicting traffic pilot are not upset by a sudden high priority alert. DEBUG

The previous boundaries separate the different steps of an evolving encounter depending on the constraints on the RPA motions. With these boundaries defined, the system needs to communicate to the RP the state of the conflict as well as the level of maneuver available. This is done through appropriately timed alerts.

B. Alerts

Through the boundaries, the RP can monitor the evolution of the encounter. However, the RP is already concerned with numerous tasks (aviate, navigate, communicate) and the navigation is not his/her first priority. That is why well located alerts are required to draw the RP attention to the right information at the right time. As proposed by Veitenburger [START_REF] Veitengruber | Design criteria for aircraft warning, caution, and advisory alerting systems[END_REF], four types of alerts can be considered for a RWC function: information, advisory, caution, and warning. The information alert requests awareness, though no action is needed. The advisory alert requests awareness and possible action. The caution alert requires immediate awareness and possible corrective or compensatory action. The warning alert requires immediate awareness and immediate corrective or compensatory action. Special care should be taken to have coherent alerts between the RWC and CA functions with CA alerts having a higher level of priority than RWC alerts. Their evolution should be progressive along time and contradictory alerts of the same level at the same time should be avoided. The minimum number of alerts required and their timing is still open to discussions.

To be most useful, alerts should come at optimal times to ensure boundaries are not violated. They should be accompanied with relevant information on the situation and, in the case of a RWC function, with avoidance information, it is to say a guidance.

C. Guidances

The type of information associated to an alert level belongs to one of four categories: informative, suggestive, directive, automatic. Informative guidance provides situational awareness; suggestive guidance limits the set of possible actions, e.g. headings, altitudes and speeds to avoid. Directive guidance provides a limited set of actions to execute, e.g. a 3-D trajectory, a vertical direction and/or a heading along with a manoeuver strength. Finally, with automatic guidance, the system informs the RP of its intent and executes a manoeuver, while the RP monitors it and can inhibit the manoeuver at any time. The guidance type should be chosen depending on the alert level and the efficiency of the system to solve the task at hand. In the particular case of the RWC function, directive only guidance should be avoided. Indeed, a RWC maneuver should take into consideration numerous factors (traffic, weather, clearance, etc.) when the system might only consider part of them and propose sub-optimal solutions. With a general idea of how the fundamental elements of a RWC function are defined, the next section introduces the state of current efforts to provide a definition for the RWC function. The scope is on the contributions of ICAO to provide a WC definition, and EUROCAE's and RTCA's in defining the minimum requirements for a RWC function.

III. CURRENT WORK

The greatest effort to define objectively and precisely the notion of RWC is currently carried out by two standardization bodies: the RTCA and the EUROCAE. Each has different hypothesis and, though coordination is strong, their conclusions may differ. In the following, the current state of their efforts regarding the definition of the RWC function is presented, a summary of the main hypothesis and methodologies is available in Table ??.

A. RTCA

The topic of DAA is tackled by two Special Committees (SCs) of the RTCA, the SC-228 and SC-147. The SC-228 is tasked with developing Minimum Operational Performance Standards (MOPSs) for RPAS, including for the DAA part. Their work considers the DAA problem as a whole and in all operational aspects (airspace classes, equipage, etc.). The SC-147 is tasked with defining and updating the ACAS performance standard. The work on RPAS started with ACAS Xu, the new generation of CA avionics for UAS, and was then extended to the RWC function thus describing a full DAA system. Both RTCA groups rely on a bottom up approach with their work being based on existing implementations, NASA's DAIDALUS + TCAS for SC-228 [START_REF] Muñoz | Daidalus: detect and avoid alerting logic for unmanned systems[END_REF] and Honeywell's ACAS Xu for SC-147 [START_REF] Kochenderfer | Next generation airborne collision avoidance system[END_REF]. The MOPSs are written based on the capabilities of these existing systems. In the following, we provide an overview of the state of the work in both groups, and the harmonisation and reconciliation efforts currently taking place.

1) SC-228: The work of the SC-228 is organised in successive phases with a scope increasing at each phase. Phase I considered "en route" IFR flights in airspaces of class D, E and G, with an altitude comprised between 1000ft AGL and FL180, though the lower limit is likely to be higher due to sensor limitations (e.g. around 3000ft for air-to-air radar). Both cooperative (i.e. transponder or ADS-B equipped) and non-cooperative intruders were considered, equipped with any type of CA system or DAA, and with the following maximum performances: speed 600knts, vertical rate 5000ft/mn, horizontal acceleration 1.5g. One of the crucial hypothesis of the SC-228 work is that an RPA might fly without a CA function, and rely solely on a RWC function. In this context, the DAA system was required to have a RWC function and optionally a ACAS II system for CA. But the focus of this work was on the RWC part. Based on models and experiment results provided by the NASA, AFRL and MIT, the SC-228 selected a fixed definition for the RWC volume based on time and distance values (see Figure 4). The experiments leading to this choice are described in detail in [START_REF] Cook | Defining Well Clear for Unmanned Aircraft Systems[END_REF]. A RWC threshold around this volume is considered to provide an alert when LoWC is predicted.

Because CA is optional, three alert, with two different levels, have been used for the RWC function alone: preventive (=caution), corrective (=caution) and warning; additionally, ACAS II, if equipped, provides a second warning alert in the form of a Resolution Advisory (RA), note that Traffic Advisories (TA) are not issued. Guidances include showing headings leading to a loss of well clear (suggestive) and providing instructions to regain well clear (directive) (cf. Figure 3). Automatic execution of RWC manoeuvers are not considered in Phase 1, though the CA maneuver can be automatic. These manoeuvers can be performed in the vertical or horizontal dimensions. With the start of Phase 2, efforts aim at looking into operations in more airspaces and in cases which require new definitions of RWC boundaries. Specifically, low cost, size, weight, and power sensors will have limited range so appropriate RWC boundaries should be chosen; and terminal area operations will require reduced separation distances asking for suitable RWC boundaries.

Because of harmonisation constraints, choices made by the SC-228 affect the ACAS Xu definition work lead by the SC-147. Indeed, the ACAS Xu MOPS is expected to comply with the Phase 1 DAA MOPS, as is explained below.

2) SC-147:

The SC-147 focuses on the whole ACAS family, especially on most recent one: ACAS X. In the RPAS case, they are in charge of writing MOPSs for the ACAS Xu system. Though ACAS Xu was initially planned to perform only the CA function, it evolved into a full DAA system integrating both RWC and CA functions. Thus, the work of SC-147 on ACAS Xu has been extended to RWC. The work from SC-147 is different from the one developed in SC-228, but the ACAS Xu RWC definition strives to support SC-228 DAA MOPS requirements. The evolution of ACAS Xu is organised around successive validation Runs (spirales) followed by tuning phases. In parallel, the SC-147 MOPS redaction is organised in Phases, just like for SC-228. The hypothesis of this group's work are similar to the ones of SC-228 except that the minimum considered altitude is 3800ft. Though the RWC and CA functions are mixed into a single function, we only consider here the information relative to the RWC part of this function. The ACAS Xu RWC function considers two boundaries: a look-ahead boundary, whose role is similar to the RWC threshold; and a RWC boundary, the limit of the RWC volume. Unlike in SC-228 definition, these two boundaries are not defined by a geometrical description but rather described in the ACAS Xu tables and optimized with each successive Runs. These boundaries are associated with three levels of alerts: preventive (=advisory), corrective(=caution) and warning (cf. Figure 3). The advisory and caution alerts are accompanied by a suggestive guidance while the warning alert gives a directive guidance. It is interesting to note that the warning alert can be given both as a part of RWC or CA sub-functions, but there is only one warning alert. In Run 3, guidance is only in the horizontal direction. Vertical manoeuvers are planned to be added in Run 4. In the meantime, Phase 2 will see timeline adjustments to improve the integration of the RWC and CA functionalities. Writing of the ACAS Xu ConUse is planned to be finished by the end of 2017 while writing of the ACAS Xu MOPS is planned to end by 2020 with harmonisation with SC-228 Phase 2 work. Resulting SC-147 ACAS Xu MOPS will try to comply as much as possible with the more general SC-228 DAA MOPS.

3) Harmonisation: With the extension of ACAS Xu to provide a RWC functionality, overlapping between the SC-228 and SC-147 scopes became a risk and led to the creation of three joint harmonisation groups, so called Tiger Teams, for surveillance, threat, and metrics and modelling. These three groups include members from SC-228 and SC-147 as well as members from EUROCAE's WG75 and WG105 (see below) for international harmonisation. The TTs have three goals: organise the work on the ACAS Xu MOPS, deal with scoping and design decisions thrown by the SCs, and deal with scoping overlap between the DAA and ACAS Xu MOPSs.

B. EUROCAE

In EUROCAE's history, three work groups have been involved in the standardisation effort for DAA systems, or parts of it. First, the WG-73 has been tasked with developing support documents for a CA function in airspaces A-C. The output of the group has been a CA function OSED. The WG-73 has since been reshaped to create the WG-105, described in the following. Second, the WG-75 focuses on ACAS systems, and lately on the ACAS X family. Their work is supported by external projects like Eurocontrol's CAFE simulator and the development of an encounter model for CA systems evaluation. This model could later be extended to evaluate RWC systems as well, though this is a difficult task that will ask for time and effort. The last, and largest, group dealing with DAA in EUROCAE is the WG-105, a counterpart to RTCA's SC-228. It tackles RPAS related standardisation efforts (C3, DAA, design and airworthiness, SORA, ERA; and more recently UTM and RPS). The DAA focus team is separated in three sub-groups: DAA airspaces A-C (CA only); DAA airspaces D-G (RWC + CA); and DAA airspaces Very Low Levels (VLL). The RWC definition is in the scope of the DAA D-G sub group, as there is no mandatory ATC separation services in these airspaces, thus requiring a self-separation function: the RWC function. As opposed to the RTCA, work done by EUROCAE adopts a top-down approach and actively seeks to avoid limiting future implementation choices.

Though starting their work on the topic somewhat later than the SC-228, current work from EUROCAE WG-105 tackles a broader scope than RTCAs phase 1. Indeed, it considers "en route" phases of the flight in airspaces A-G + VLL. Intruders hypothesis are the same as for SC-228. For ownship, minimum equipage includes a CA system, plus a RWC system in airspaces D-G. Even if the CA and RWC functions belong to two different systems, it is considered that the CA system engages as soon as WC is lost. Pilots acceptability studies have been carried out to propose RWC boundaries [START_REF]Trawa pilot project[END_REF]. Such boundaries would allow to provide advisory and caution alerts with continuous guidance being displayed and updated as long as the CA does not provide a warning, as in [START_REF] Pellebergs | The midcas project[END_REF]. The hypothesis that all RPAS will have a CA function led to the absence of warning alert in the RWC function (cf. Figure 3). This represents a difference with the RTCA approach, as there is no alert when the Right of Way rules start constraining the RPA manoeuvres (LoWC). Current work focuses on the choice of guidance type at the different levels of alert with the possibility to have semi-automatic execution of the RWC guidance. More generally, an Operational Service and Environment Description (OSED) is being developed to bring forward requirements and assumptions for the DAA system, including the RWC function.

C. ICAO

The ICAO is currently pressured into proposing a useful definition for WC. Discussions in the DAA SARP panel are centered around the formulation of such definition. It should be precise enough to be useful but have a large enough scope to allow a diversity of solutions answering the problem. Their output is crucial as this will provide a definition on which EUROCAE and the RTCA will continue to build their work. According to the previous decomposition of a RWC definition, a formal definition from their part can be expected to define the events which start and end the RWC function (related to boundaries) as well as the actors involved (related to alerting) and possibly the actions expected (related to guidance). Care need to be taken not to enter into too much details so as to not limit possible solutions. On all sides, the work on defining WC and RWC is advancing and coordination between the entities will ensure a coherent result. The main situations are being addressed, still some topics are not part of the discussions. The next section mentions some of them and proposes some discussions.

IV. DISCUSSIONS

This section aims at fueling discussions on some topics which are not yet covered by existing efforts: non-binary loss of well clear, CA vs Regain Well Clear, RWC for low performance RPAS.

To begin with, the loss of WC (LoWC) is currently being considered as a binary value: an aircraft is well clear or not. Though, metrics to measure the severity of a LoWC have been proposed [START_REF] Mueller | Piloted well clear performance evaluation of detect and avoid systems with suggestive guidance[END_REF] and are already used in Verification and Validation simulations [START_REF] Cook | Defining Well Clear for Unmanned Aircraft Systems[END_REF]. Though, including them in the current discussions would increase the difficulty of an already difficult task, it could be beneficial for the safety of a RWC function. Indeed, allowing low severity LoWC could allow a RPA to lose WC momentarily in order to avoid a potentially dangerous situation later. Some approaches do not consider a CA function, but they do consider a Regain Well Clear (RegWC) sub-function. The difference between RegWC and CA is not clear, so we propose three differentiating elements. First, some regulatory bodies ask for the RWC and CA functions to be independent, which would not be the case in a RWC/RegWC system. Second, the RWC/RegWC system implies that a LoWC would immediately trigger the RegWC, which would not be necessarily the case for a CA function which could trigger some time after LoWC. Third, the end of a CA event is indicated by a Clear of Conflict (CoC) related to a collision volume, the end of a RegWC event would be linked to a WC volume. From a coordination point of view, it is not clear if a RegWC manoeuver would be required to be interoperable with existing ACAS systems. As the RegWC solutions is being pushed, a clear definition of RegWC will become necessary.

Current definitions associated to the notion of RWC are being developed for integrated airspace operations with RPAS performances at least close to those of General Aviation (GA). However, as lower performances RPAS will ask to enter integrated airspace, there will be a need to adapt existing definitions for their performances in terms of RPA dynamics and sensors capabilities. To allow low performance RPAS into airspace will ask for a delicate balance between safety and performance requirements.

V. CONCLUSION

Many RPAS integration efforts are currently focused on quick wins, mainly integration in controlled airspace. However, complete integration will only be accomplished when RPAS will be able to fly in any class of airspace. For this purpose, the definition of WC and its associated DAA functionality, the RWC, are crucial elements. Current efforts from RTCA and EUROCA, with helper projects from NASA and SESAR-JU, are progressing in this aspect, but the available resources does not seem to match the complexity of the task at hand. In the meantime, work from the ICAO's SARPS, generic as it may be, will provide strong foundations on which regulators will be able to lean when defining country specific rules. Even if the basis for nominal operation are not established yet, one can see that there is still more work to do e.g. for closely spaced operations and terminal manoeuvering areas. From the point of view of our work, future leads include gathering elements to propose a formal definition of the Regain Well Clear function and assessing low power sensors capabilities for RWC.

Figure 1 :

 1 Figure 1: The Remain Well Clear threshold and Remain Well Clear volume as defined in the ICAO RPAS Manual. The collision avoidance threshold and collision volume are not considered here as they are not related to the RWC function but rather to the CA function. (image from the ICAO's RPAS Manual)

Figure 2 :

 2 Figure 2: Two boundaries (yellow, orange) separate the different states. Top, the ownship does not have the right of way. Bottom, the ownship does have the right of way. As an encounter evolves, the ownship possible manoeuvers are increasingly constrained: none (clear), maintain heading and speed (darker), constrained according to Right of Way rules (darkest).

Figure 3 :

 3 Figure 3: The alerting timeline has three main events: earliest time to make a Remain Well Clear (RWC) manoeuver, latest time to make a RWC manoeuver, and earliest time to make a Collision Avoidance manoeuver. The alerts for each of the systems envisioned by RTCA SC-228, SC-147 and EUROCAE WG-105 are distributed with respect with these milestones.

Figure 4 :

 4 Figure 4: The Regain Well Clear volume defined by the RTCA SC-228.

TABLE I :

 I Main differences between Collision Avoidance (CA) and Remain Well Clear (RWC) functions. CoC is for Clear of Conflict; NMAC is for Near Mid Air Collision.

		CA	RWC
	Decision factors	Safety	Safety, acceptability, strategic
	Responsibility	Pilot	Depends on airspace
			(can be shared with pilot's)
	Contact ATC?	If time allows	Yes, notably if under clearance
	Start/	Collision hazard/	Conflict/
	End	NMAC or CoC	Collision hazard or CoC
	Time horizon	Tens of seconds	Few minutes
	Manoeuver	Strong	Smooth
	Manoeuver	None	Right of Way
	constraints		rules, clearance

TABLE II :

 II

Comparison of RTCA SC-228, SC-147, and EUROCAE WG-105 working hypothesis, support projects and methodologies. CAS is for Collision Avoidance (CA) System.

ACKNOWLEDGMENT

This work was funded by the Engine Ineo -Groupe ADP -Safran RPAS Chair led by ENAC. The authors would like to thank Catherine Ronfle-Nadaud, Alain Vallee, Eric Thomas, Bengt-Goran Sundqvist and Ted Lester for the precious insights.