ACCORD: A Formal Model for the Digitalization and Automation of Drone Coordination Processes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors,

the paper is very relevant and provides processes and implementation details for easing the coordination requirements in the course of drone operations within aeronautical infrastructures and geographical zones. The neccessity for these kinds of automated coordination processes in clearly argued and not only eases the overall coordination requirements between different actors, but also provided a traceable and secure basis for implementing this process required by regulation.

Please find below some review comments:

1. Introduction: The second paragraph (line 30) starts with a rather complex and long sentence. Please revise.

2. State of the Art: The state of the Art could be enriched by giving an overview of current research projects and results that provide solutions for the problem stated. One of the solutions is the Dynamic Airspace Reconfiguration (DAR) conceptualized and specified in several EU-projects (e.g. ENSURE, PJ34 AURA). In these research projects detailed coordination sequence diagrams have been elaborated coordinating the process between ANSP and drone operator and U-space. 

The tables could be reduced/condensed, because the data on drone categories (table 1) isn't needed in that detail and also provided in the tables following (table 3).

3. Structure of a drone operation request: In chapter 3.2 (line 294) it could be argued that missions details wouldn't be sufficient. For coordinating an operation with the infrastructure or airspace responsible operators, some details about the drone missions objective and especially defined operational/technical requirements from the authorities following a SORA process could be neccessary. 

In 3.4 (line 349) the authors mention airspace CTR, CTZ and FIZ. In Germany UAS Test Centers located at regional airports often use ATZ (Aerodrome Traffic Zone). 

In Chapter 4 the Figure 4 is hard to read (bad quality and text in Spanish) and should be replaced.

Overall, a good paper that provides good argumentation for the implementation of automated, national processes for the coordination of drone missions. It is important that similar processes are implemented throughout Europe in order to facilitate the transferability and cross-border operation of drones.

Author Response

Dear reviewer, thanks for taking the time to review our manuscript. We have addressed your comments in detail and the additional comments provided by other reviewers. As a result, we have generated a new document version which includes (in addition to minor improvements and editorial corrections) the following high-level improvements:

• Additional references to existing initiatives in other parts of the world have been introduced.
• The quality of the images has been greatly improved by employing specialised techniques that allow web interface screenshots to be captured at a higher resolution than the one offered by standard screen captures.
• We have emphasised the benefits and underlying contributions of our proposal in a better way.
• A limited performance evaluation has been carried out to provide a high-level perception of the benefits achieved by digitalising the coordination process.

[Comment 1]. Introduction: The second paragraph (line 30) starts with a rather complex and long sentence. Please revise.

[Response 1] Completely right. We failed to introduce that specific comment from our English checker properly.

[Comment 2]. State of the Art: The state of the art could be enriched by giving an overview of current research projects and results that provide solutions for the problem stated. One of the solutions is the Dynamic Airspace Reconfiguration (DAR) conceptualised and specified in several EU-projects (e.g. ENSURE, PJ34 AURA). In these research projects detailed coordination sequence diagrams have been elaborated coordinating the process between ANSP and drone operator and U-space.

[Response 2] The reviewer correctly indicates that other airspace design/management strategies exist for drones. The DAR concept originates as a strategy to manage parts of U-space dynamically, allowing controlled air traffic to cross a portion of airspace designated as U-space airspace, ensuring segregation is maintained between UAS in U-space and manned aircraft. In that sense, a DAR is a dynamic piece of airspace which does not require explicit coordination. Nevertheless, we believe it is relevant to emphasise the existence of DAR's and their differences regarding geographical areas.

[Comment 3]. The tables could be reduced/condensed, because the data on drone categories (table 1) isn't needed in that detail and also provided in the tables following (table 3).

[Response 3] The authors will try to condense those tables, but we indicate that the information provided would be later employed when describing the geographical area inner classification employed by SEM.

[Comment 4]. Structure of a drone operation request: In chapter 3.2 (line 294) it could be argued that missions details wouldn't be sufficient. For coordinating an operation with the infrastructure or airspace responsible operators, some details about the drone mission’s objective and especially defined operational/technical requirements from the authorities following a SORA process could be necessary.

[Response 4] Geographical areas are defined to restrict access to parts of the airspace near critical infrastructures. From our experience, SORA mitigations do not reduce the risk the coordination entities perceive when a drone enters that airspace. However, in some situations, an existing SORA could be considered a relevant mitigation to enable a positive response for coordination. As a result, we have implemented a mechanism to attach relevant documents to the operation definition, including an existing SORA. The concept is also extended to other applicable documents, like operator's documentation, insurance, etc.

[Comment 5]. In 3.4 (line 349) the authors mention airspace CTR, CTZ and FIZ. In Germany UAS Test Centers located at regional airports often use ATZ (Aerodrome Traffic Zone).

[Response 5] This is an omission on our side. Thanks for noting the error.

[Comment 6]. In Chapter 4 the Figure 4 is hard to read (bad quality and text in Spanish) and should be replaced.

[Response 6] A significant effort has been carried out to improve the quality of images, which, in some cases, are screenshots from the actual application.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper shows that ACCORD is a solution for drone coordination, but it does not bring new ideas. It explains that ACCORD uses U-space and LAANC, but it does not introduce new methods. It states that the study follows current regulations but does not suggest better ways to manage risks. Please follow my concerns and queries:

1. The paper does not prove that ACCORD is effective. It does not compare ACCORD with manual methods, and it does not have real-world pilot data. It is unclear how much ACCORD improves efficiency because there is no data.
2. The paper says that LAANC exists, but it does not compare response times or automation. It does not explain how ACCORD is different from other digital systems. It shows that the ACCORD model is like current air traffic models, which makes originality a concern.
3. The study does not test ACCORD in busy areas. It does not check how ACCORD handles many requests at the same time. It does not provide security data.
4. The paper explains that ACCORD organizes existing processes, but it does not create new ideas. It shows that digitizing drone coordination is useful, but it does not improve automation.
5. The analysis explains that ACCORD helps organize drone coordination in Catalonia. It shows that drone operations need many steps, but it does not measure time savings. It raises questions about how well the system works during busy times.
6. The paper shows that ACCORD sorts drone operations by risk and sets restrictions. It does not explain if the system works well in emergencies. It does not answer how often requests get approved. It is unclear if ACCORD can adapt to new regulations.
7. The study mentions that regulations affect drone coordination, but it warns about delays. It does not say how many operations fail compliance checks. It does not explain if ACCORD is better than manual methods. It is unclear if ACCORD will be required or optional.

Comments on the Quality of English Language

Needs improvement.

Major revisions required.

 

Author Response

Dear reviewer, thanks for taking the time to review our manuscript. We have addressed your comments in detail and the additional comments provided by other reviewers. As a result, we have generated a new document version which includes (in addition to minor improvements and editorial corrections) the following high-level improvements:

• Additional references to existing initiatives in other parts of the world have been introduced.
• The quality of the images has been greatly improved by employing specialised techniques that allow web interface screenshots to be captured at a higher resolution than the one offered by standard screen captures.
• We have emphasised the benefits and underlying contributions of our proposal in a better way.
• A limited performance evaluation has been carried out to provide a high-level perception of the benefits achieved by digitalising the coordination process.

 

[Comment 1]. The paper does not prove that ACCORD is effective. It does not compare ACCORD with manual methods, and it does not have real-world pilot data. It is unclear how much ACCORD improves efficiency because there is no data.

[Response 2] ACCORD has been designed according to the functional requests requested by infrastructure operators and the suggestions made by several drone operators on how to input operational data efficiently. In response to the reviewer's comments, we have introduced a high-level evaluation effort that should help provide a better justification for the efficiency of the proposed process versus a manual document-based process.

It is true, however, that the actual efficiency of the proposed methodology can only be captured once the system reaches a minimum level of deployment and users are actively questioned about potential inefficiencies or improvements to the system. Such a level of understanding, especially when studying human-machine interfaces and human perception, can only be achieved through an extensive evaluation campaign that, in our opinion, is beyond the scope of this initial system deployment.

[Comment 2]. The paper says that LAANC exists, but it does not compare response times or automation. It does not explain how ACCORD is different from other digital systems. It shows that the ACCORD model is like current air traffic models, which makes originality a concern.

[Response 2] As described in the introduction, LAANC is the platform US drone operators should use to coordinate their requests with FAA-managed aeronautical infrastructures. LAANC does not extend to other aeronautical infrastructures not managed by the FAA or other types of geographical areas. Unfortunately, LAANC access is unavailable for test or non-US drone operators. Even more critical, the interfaces available to FAA agents are inaccessible or documented.

Beyond LAANC, existing digital interfaces are either limited to querying the structure of the airspace (similar to the AIP information portals) or are limited to collecting coordination requests to specific infrastructures. Overall, to the author's best knowledge, no toolset exists in the aeronautical or drone domain which enables a drone operator to manage the coordination and the operation execution phase with multiple independent infrastructures and entities simultaneously and through a single entry point. To the best of our understanding, such type of innovation is not described anywhere in the existing bibliography or known aeronautical practices. 

[Comment 3]. The study does not test ACCORD in busy areas. It does not check how ACCORD handles many requests at the same time. It does not provide security data.

[Response 3] The Barcelona metropolitan area is extremely busy, with many aeronautical infrastructures of all types. The level of complexity will be further described in an additional evaluation section introduced in the document. The computational requirements of the system, according to the volume of demand, are yet another factor that needs to be evaluated once deployment is initiated. Finally, the data encryption mechanism employed for the system communications and the ACCORD server is beyond the scope of this publication, which focuses on the operational concept, automation mechanism and data management.

[Comment 4]. The paper explains that ACCORD organises existing processes, but it does not create new ideas. It shows that digitising drone coordination is useful, but it does not improve automation.

[Response 4] Dear reviewer, we do not fully understand this comment. On the one hand, existing processes in most countries, including Spain, are managed manually. No existing tool beyond LAANC incorporates any level of automation but with the restrictions discussed in the previous paragraphs. On the other hand, we have introduced a platform that formalises the coordination process and implements several automated workflows that can be parameterised and configured by the users, ranging from its direct participation in each coordination authorisation to automatic authorisation under certain premises. However, your comment indicates that our proposal does not improve automation.

As a result of these doubts, we have addressed your comment by emphasising [EP1]  the existence of those capabilities in the document and increased the numeric evaluation process efficiency.

[Comment 5]. The analysis explains that ACCORD helps organise drone coordination in Catalonia. It shows that drone operations need many steps, but it does not measure time savings. It raises questions about how well the system works during busy times.

[Response 5] As discussed before, an introductory analysis concerning the efficiency of the proposed process has been introduced in the reviewed document, 

[Comment 6]. The paper shows that ACCORD sorts drone operations by risk and sets restrictions. It does not explain if the system works well in emergencies. It does not answer how often requests get approved. It is unclear if ACCORD can adapt to new regulations.

[Response 6] The authors do not fully understand the extent of the emergency concept introduced by the reviewer. If it refers to a drone experiencing an emergency and that emergency being notified to the various coordinated infrastructures, this is an interesting concept that was not initially included but will be incorporated into the system as it is a valuable functionality. Conversely, suppose an infrastructure perceives an increased risk (like an increase in operations in and out of a medical heliport). In that case, ACCORD allows the infrastructure managers to activate an "emergency" mode that requires all drone operators to cease their operation. 

[Comment 7]. The study mentions that regulations affect drone coordination, but it warns about delays. It does not say how many operations fail compliance checks. It does not explain if ACCORD is better than manual methods. It is unclear if ACCORD will be required or optional.

[Response 7] Drone operations would comply with the coordination restrictions according to the rules defined by the infrastructure managers. The number of rejections would be directly proportional to the operation's risk and the protected infrastructure's proximity. Statistics would be collected as the platform is deployed and data collected.

Again, we note that an initial evaluation has been introduced in a dedicated section. However, a full quantification of the benefits could only be quantified once an extensive deployment is achieved and analysed.

Finally, the deployment conditions would depend on each country's legal situation. In the Spanish case, the recipients of the coordination are entitled to determine the coordination mechanism, so in that sense, ACCORD would become mandatory if embraced by a specific infrastructure manager.

Reviewer 3 Report

Comments and Suggestions for Authors

The work presents relevant issues in the field of Digitalization and Automation of Drone Coordination Processes. The work is written in an understandable manner. The following elements of the work need improvement:

- literature review - Note 1. The authors conducted a literature review of restrictions on the implementation of UAV flight operations in the territory of America and the European Union. The review should be supplemented (perhaps as another subsection) with legal restrictions in other countries of the world.

- literature review - Note 2. There is a lack of scientific approach to the literature review. Only two items in the literature review are scientific publications. The literature review should be expanded to include numerous literature (scientific) items on issues related to the research area.

- case study - note 1. The description of the implementation of the UAV mission in Figures 8 through 12 should be made more detailed. The quality of the drawings is poor which makes it impossible to read the information contained in them. It is necessary to indicate the essence that the authors wanted to convey by indicating all the information contained in Figures 8 through 12.

Author Response

Dear reviewer, thanks for taking the time to review our manuscript. We have addressed your comments in detail and the additional comments provided by other reviewers. As a result, we have generated a new document version which includes (in addition to minor improvements and editorial corrections) the following high-level improvements:

• Additional references to existing initiatives in other parts of the world have been introduced.
• The quality of the images has been greatly improved by employing specialised techniques that allow web interface screenshots to be captured at a higher resolution than the one offered by standard screen captures.
• We have emphasised the benefits and underlying contributions of our proposal in a better way.
• A limited performance evaluation has been carried out to provide a high-level perception of the benefits achieved by digitalising the coordination process.

[Comment 1]. Literature review - Note 1. The authors conducted a literature review of restrictions on the implementation of UAV flight operations in the territory of America and the European Union. The review should be supplemented (perhaps as another subsection) with legal restrictions in other countries of the world.

[Response 1] The authors have extended the regulatory review to several countries in other parts of the world and added that information to the introduction section. 

[Comment 2]. Literature review - Note 2. There is a lack of scientific approach to the literature review. Only two items in the literature review are scientific publications. The literature review should be expanded to include numerous literature (scientific) items on issues related to the research area.

[Response 2] The authors extensively searched the existing academic literature. Such a search was performed at the time of the submission and repeated as a response to the reviewer's request. Unfortunately, no relevant publication directly linked to the proposed subject has been identified. The authors believe that most existing initiatives are directly connected to the countries' aeronautical authorities, who, in turn, are not prone to publishing in research journals. Given that ACCORD is a research group initiative, it can be considered an outlier to the norm.

[Comment 3]. Case study - note 1. The description of the implementation of the UAV mission in Figures 8 through 12 should be made more detailed. The quality of the drawings is poor which makes it impossible to read the information contained in them. It is necessary to indicate the essence that the authors wanted to convey by indicating all the information contained in Figures 8 through 12.

[Response 3] The authors have increased the resolution of the images. Given that they are screenshots of the ACCORD web interface, special mechanisms have been required to achieve a reasonable resolution.

Reviewer 4 Report

Comments and Suggestions for Authors

The paper introduces ACCORD, a support platform designed to digitalize and automate the  coordination processes required by the drone regulatory framework. The paper includes a good description of the state of the art in terms of the de facto procedures used in different countries to access controlled airspace. The rest of the paper is a detailed description of the ACCORD platform. At the same time, from the paper, it is unclear what novel solutions are used in the ACCORD platform. The whole solution is new, but is it actually built of scientifically new methods and approaches? Authors should directly list the full set of contributions to the state of the art at the end of the introduction.

The second important point is experimental results. Section 8 states that multiple entities already signed preliminary agreements with end-users. However, it is not clear if any of them actually use it in daily matters. Moreover, it is unclear whether using the ACCORD platform has any positive effect. Authors must add some numerical results and critically analyze the pros and cons of deploying their solution. If this experimental data is not ready, they should suspend publishing until they get these results. In another case, their paper cannot be published in a scientific journal.

Minor issues:

Figure 4,8 has small text, which is difficult to read

Figures 4-8 have very low quality. Authors must raise it up to 300 dpi.

Author Response

Dear reviewer, thanks for taking the time to review our manuscript. We have addressed your comments in detail and the additional comments provided by other reviewers. As a result, we have generated a new document version which includes (in addition to minor improvements and editorial corrections) the following high-level improvements:

• Additional references to existing initiatives in other parts of the world have been introduced.
• The quality of the images has been greatly improved by employing specialised techniques that allow web interface screenshots to be captured at a higher resolution than the one offered by standard screen captures.
• We have emphasised the benefits and underlying contributions of our proposal in a better way.
• A limited performance evaluation has been carried out to provide a high-level perception of the benefits achieved by digitalising the coordination process.

[Comment 1]. The paper includes a good description of the state of the art in terms of the de facto procedures used in different countries to access controlled airspace. The rest of the paper is a detailed description of the ACCORD platform. At the same time, from the paper, it is unclear what novel solutions are used in the ACCORD platform. The whole solution is new, but is it actually built of scientifically new methods and approaches? Authors should directly list the full set of contributions to the state of the art at the end of the introduction.

[Response 1]. The ACCORD proposal is built on top of the formalisation of the various restrictions that may apply to a coordination process. Such a formalisation enables the natural extension of its scope to new requirements, following a concept similar to inheritance in the CS domain. Furthermore, state-transition formalisation describes the various actions required along with coordination. At each transition, it is properly defined which stakeholders have the responsibility to act and under which circumstances that transition can be carried out automatically, eliminating the need for human intervention, thus significantly improving the efficiency of the process.

The authors have modified some parts of the text to emphasise the proposed strategies better. In particular, the most relevant contributions are stressed at the end of the introduction section.

[Comment 2]. The second important point is experimental results. Section 8 states that multiple entities already signed preliminary agreements with end-users. However, it is not clear if any of them actually use it in daily matters. Moreover, it is unclear whether using the ACCORD platform has any positive effect. Authors must add some numerical results and critically analyse the pros and cons of deploying their solution. If this experimental data is not ready, they should suspend publishing until they get these results. In another case, their paper cannot be published in a scientific journal.

[Response 2] ACCORD has been designed according to the functional requests requested by infrastructure operators and the suggestions made by several drone operators on how to input operational data efficiently. The initial evaluation was carried out in a limited way through several operations commonly agreed upon between the drone operators and the infrastructure managers collaborating on the project. ACCORD is not yet operationally deployed, as the Catalan authorities evaluate it for its actual adoption.

In response to the reviewer's comments, we have introduced a high-level evaluation effort that should help provide a better justification for the efficiency of the proposed process versus a manual document-based process. The analysis highlights those areas in which efficiency is greatly improved.

It is true, however, that the actual efficiency of the proposed methodology can only be captured once the system reaches a minimum level of deployment and users are actively questioned about potential inefficiencies or improvements to the system. Such a level of understanding, especially when studying human-machine interfaces and human perception, can only be achieved through an extensive evaluation campaign that, in our opinion, is beyond the scope of this initial system deployment.

[Comment 3]. Figure 4,8 has small text, which is difficult to read

Figures 4-8 have very low quality. Authors must raise it up to 300 dpi.

[Response 3]. Both elements have been corrected, improving the readability of the text and images.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for updating the manuscript in response to the reviewers' previous comments. While some improvements have been made, several key concerns remain inadequately addressed. The modifications and new changes made in the revised manuscript are not clearly explained or easily identifiable. I kindly request the authors to highlight all the new changes in color (e.g., red or blue) within the manuscript for clarity. Additionally, the technical contribution remains weak and lacks substantial improvement. A more rigorous enhancement in both modeling and validation is necessary to meet the journal's standards. Below is a detailed critique organized by topic.

1. The modifications introduced in the revised manuscript are not clearly explained or easily identifiable. Please highlight all changes in color (e.g., red or blue) within the manuscript to facilitate review.
2. The technical contribution remains weak and lacks substantial improvement. The modeling approach is not rigorous, and validation efforts are insufficient to meet the standards of a scientific journal. There is no formal description of the system architecture, algorithms, rule engines, or digital logic that enable automation. Key terms such as "formal model", "traceability", and "automation" are mentioned repeatedly but are not clearly defined or technically elaborated. The manuscript lacks a structured formalism (e.g., Petri Nets, FSM, BPMN) or a clear software stack to explain the system’s internal operation.
3. No computational validation is presented (e.g., performance benchmarks, system latency, scalability under concurrent requests). The study does not test the system (ACCORD) in high-load or congested environments, nor does it evaluate how ACCORD handles multiple simultaneous requests. There is no data related to security, failure rates, or system robustness. The manuscript does not compare ACCORD with existing manual coordination methods. There is no pilot study or deployment in real-world scenarios to validate the effectiveness of the platform. It is unclear how ACCORD improves efficiency, as no quantitative metrics or comparative results are provided.
4. The discussion section lacks depth and does not sufficiently connect the results to practical implications. A stronger link between system performance, use-case outcomes, and real-world impact is needed. The core contribution appears to be limited to integrating existing coordination workflows into a digital platform. The work is more aligned with an engineering implementation responding to regulatory requirements than an academic advancement with generalizable contributions. The manuscript reads more like a technical report or policy document than a peer-reviewed research article.

Author Response

Dear reviewer, we kindly respond to your comments in the attached file.

We are submitting the review document and the document with changes for your evaluation.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

All of my comments have been addressed and incorporated. The article can be published in its current form.

Author Response

Thank you for your time and suggestions to help improve the paper.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors modified manuscripts following my suggestions.

Author Response

Thank you for your time and suggestions to help improve the paper.

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

Thus, the revision is good for me. I have no further comment.