Dynamic Separation Standards for Multi-Category UAV Operations

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dynamic separation minima represent an important concept in air traffic management and have been explored by various research groups. However, classical fixed separation minimums are historically derived from the total system error, which includes both flight technical error and navigation system error. Due to approximately the same positioning performance for all airspace users limited within a particular volume of airspace, it makes sense to use hard fixed separation minimums for horizontal and vertical planes. From a regulatory point of view, safety compliance can be readily verified by comparing a UAV’s deviations from its pre-planned trajectory, making fixed minima particularly effective for oversight and enforcement.

Consequently, the large-scale implementation of dynamic separation minima in real-world operations does not appear feasible in the near future. Nonetheless, the approach presented in the article offers valuable insight and may contribute to future U-space development roadmaps as technology frameworks evolve.

Methodological comment:

Do you consider environmental factors in the development of separation minima? UAV operations at low altitudes are strongly influenced by terrain, artificial structures, and other obstacles. In addition, weather conditions can have a significant impact on UAV performance and safety. Could you clarify whether these factors were accounted for in your model?

Comments:
1. Dots in lines 142-160 are missing

2 Fig. 1 is low information quality

3. Add reference to the source of UAV Categories in 2.1.1.
4. Fig. 2 should be placed after appearing in the text
5. Add mode description of Route Type Factor in 2.2.2. How could this parameter be calculated based on coordinates?
6. Line 288, what does it mean “sin?(”?
7. Better would be to provide a structural scheme of AHDS algorithm in 2.4.2

Author Response

Comment 1: Lines 142-160 need punctuation (dots/periods) to be added.

Response 1: Thank you for pointing this out. We agree with this comment. We have carefully reviewed the entire manuscript and added appropriate punctuation throughout. The corrected text can be found in the revised manuscript at lines 148-166 (Page 4), where all sentences now end with proper periods.

Comment 2: Figure 1 has low information quality and should be improved.

Response 2: Thank you for this valuable suggestion. We agree that Figure 1 needed improvement. We have completely redesigned Figure 1 to provide a more informative and clearer visualization of the three-tier framework architecture. The new figure now clearly depicts:

• The hierarchical relationship between Tier 1 (Strategic), Tier 2 (Pre-tactical), and Tier 3 (Tactical) layers
• Information flow between tiers with labeled arrows
• Update frequencies at each level
• Input/output components

The redesigned Figure 1 can be found on Page 5, lines 185-186.

Comment 3: Section 2.1.1 UAV categories need a source reference for the classification system used.

Response 3: Thank you for highlighting this important point. We agree that the UAV categorization required proper sourcing. We have added Reference [32] (U.S. Department of Defense Joint Publication 3-30: Joint Air Operations) to support our categorization system. The revised text now reads (Page 6, lines 213-216):

"This categorization aligns with U.S. Department of Defense UAS Groups 2-3 classification [32], where Group 2 encompasses 9.5-25 kg platforms and Group 3 covers 25-600 kg systems, and is consistent with operational classifications observed in urban air mobility deployments [22]."

Comment 4: Figure 2 placement should appear after its first mention in the text.

Response 4: Thank you for this observation. We agree with this formatting requirement. We have repositioned Figure 2 to appear immediately after its first citation in the text. In the revised manuscript, Figure 2 now appears at lines 247-248 (Page 7), following its first mention at line 238.

Comment 5: Section 2.2.2 Route Type Factor needs detailed calculation procedure from coordinates.

Response 5: Thank you for this important methodological clarification request. We agree that the Route Type Factor calculation needed more detailed explanation. We have expanded Section 2.2.2 with a comprehensive step-by-step procedure (Page 8, lines 285-295):

"Step 1 Route Vector Extraction: Compute dominant route directions over a 5 km planning horizon:

Step 2 Intersection Angle Computation:

Step 3 Factor Assignment..."

Comment 6: Line 288 contains unclear notation "sin?(" which needs correction.

Response 6: Thank you for identifying this typographical error. We have corrected the equation to properly display the trigonometric function. The corrected equation (9) at line 337 (Page 9) now reads:

""

Comment 7: Section 2.4.2 needs an AHDS algorithm structural diagram.

Response 7: Thank you for this suggestion to improve algorithm clarity. We agree that a visual representation would enhance understanding. We have added a new Figure 5 presenting the AHDS Algorithm Architecture, which illustrates the sequential decision-making process through four phases:

• Step 1: Conflict Detection
• Step 2: Priority Assignment
• Step 3: Maneuver Planning
• Step 4: Vertical Prioritization

The new Figure 5 can be found at lines 378-379 (Page 11).

Comment 8: How does the framework consider environmental factors such as terrain, buildings, and weather?

Response 8: Thank you for raising this important practical consideration. We have addressed this limitation in Section 4.4 (Limitations and Challenges) on Page 20, lines 673-678. The revised text acknowledges:

"Additionally, the current framework does not explicitly model terrain, artificial structures, or obstacle interactions. UAV operations at low altitudes are strongly influenced by buildings, towers, and topographical features that create both physical hazards and complex wind patterns. For low-altitude operations in complex urban environments, integration with digital elevation models (DEM) and obstacle databases represents an important future enhancement to ensure comprehensive safety coverage."

We acknowledge that terrain and structure modeling is beyond the current scope and recommend DEM integration as future work.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The article presents new methods and techniques for applying dynamic separation between drones in low-altitude airspace. I enjoyed reviewing this article and my congratulations to the researchers as it is a complete work with high scientific rigor. The comments are in the attached document.

Comments for author File: Comments.pdf

Author Response

Comment 1: Line 27 - "dramatically" is too informal for academic writing.

Response 1: Thank you for this language suggestion. We agree that more formal academic language is appropriate. We have changed "dramatically" to "accelerated substantially" at line 26 (Page 1):

"The proliferation of unmanned aerial vehicles (UAVs) in civilian airspace has accelerated substantially..."

Comment 2: Line 28 - "environments" cannot simultaneously be urban and precision agriculture; this creates logical inconsistency.

Response 2: Thank you for identifying this logical error. We agree with this observation. We have revised the text to "urban and suburban environments" at lines 27-28 (Page 1):

"...with projections indicating over one million concurrent UAV operations in urban and suburban environments by 2030 [22]."

Comment 3: Line 32 - "platforms" is repeated; consider alternative wording.

Response 3: Thank you for noting this repetition. We have replaced the second occurrence with "airspace" at line 32 (Page 1):

"...within shared low-altitude airspace."

Comment 4: Line 36 - The vertical separation values stated (1,000-2,000 feet) may be incorrect for manned aviation in the context described.

Response 4: Thank you for this technical clarification. We have revised the text to provide more accurate context (Page 1, lines 35-37):

"For en-route operations, lateral separations of 5 nautical miles and vertical separations of 1,000-2,000 feet are standard."

This now correctly specifies the context as en-route operations where these values apply.

Comment 5: Line 38 - "studies" should be singular.

Response 5: Thank you for catching this grammatical error. We have corrected "studies" to "study" at line 39 (Page 1):

"Recent study estimate that fixed separation approaches can reduce effective airspace capacity by 40-60%..."

Comment 6: Line 46 - "thresholds" refers to manned aviation standards, not unmanned; clarification needed.

Response 6: Thank you for highlighting this important distinction. We have added clarifying text explaining the adoption rationale (Page 2, lines 46-50):

"While ICAO specifies TLS ≤10⁻⁷ per flight hour for manned aviation [12], this benchmark is increasingly adopted as a reference standard for high-density UAV operations pending development of UAV-specific regulatory guidance [13], [14]."

Comment 7: Line 93 - The sentence about "management" is unclear and needs rewriting.

Response 7: Thank you for this observation. We have rewritten the sentence for clarity at lines 92-93 (Page 3):

"However, RL methods face challenges in interpretability, training requirements, and certification for safety-critical aviation systems."

Comment 8: Line 121 - "establishment" needs clarification regarding what is being established.

Response 8: Thank you for requesting this clarification. We have expanded the explanation at lines 124-127 (Page 3):

"Current fixed separation standards apply uniform distances regardless of performance variations, while dynamic methods lack systematic frameworks for category-specific baseline establishment that mirror the structured approach used in conventional manned aviation separation standards."

Comment 9: Lines 142-165 - The Introduction contains content that should be in Results/Conclusions sections.

Response 9: Thank you for this structural observation. Rather than moving the content entirely (which provides important context for readers), we have added a forward reference to indicate that detailed results appear later. At lines 166-167 (Page 4):

"Comprehensive Monte Carlo simulations validate the framework's effectiveness, with detailed results presented in Section 3."

Comment 10: Line 183 - Figure text is too small to read.

Response 10: Thank you for identifying this readability issue. We have completely redesigned Figure 1 with larger, more legible text and clearer visual elements. The improved Figure 1 appears on Page 5, lines 185-186.

Comment 11: Line 186 - "Tier" descriptions need bullet point separation for readability.

Response 11: Thank you for this formatting suggestion. We have reformatted the Tier descriptions using bullet points (⚫) at lines 189-203 (Pages 5-6):

"

• Tier 1 Strategic Baseline (hours to pre-flight): Establishes category-specific separation minima (30-80m)...
• Tier 2 Pre-Tactical Dynamic Adjustment (minutes to seconds ahead): Refines strategic baselines...
• Tier 3 Tactical Real-Time Resolution (seconds to sub-seconds): Decomposes adjusted separations..."

Comment 12: Line 208 - Lists throughout the section need consistent bullet formatting.

Response 12: Thank you for noting this formatting inconsistency. We have applied consistent bullet formatting throughout Section 2.1.1, as shown at lines 219-229 (Page 6) for the UAV category descriptions.

Comment 13: Line 222 - Why use 3 encounter types instead of the traditional 5 angular sectors?

Response 13: Thank you for this methodological question. We have added an explanation with reference to sensitivity analysis validation at lines 234-238 (Page 6):

"While conventional aviation encounter classification employs five angular sectors (0°-20°, 20°-70°, 70°-110°, 110°-160°, 160°-180°) [12], this framework consolidates them into three operationally distinct categories for computational efficiency. This simplification reduces Tier 2 computation time by approximately 40% while preserving >95% of collision risk discrimination capability, as validated through sensitivity analysis in Section 3.3."

Comment 14: Line 227 - The source of encounter type distribution information is unclear.

Response 14: Thank you for requesting source clarification. We have added explanatory text at lines 243-246 (Page 6):

"These proportions are derived from analysis of N=10,000 simulated encounters across all experimental scenarios described in Section 3.1, and are consistent with operational patterns reported in urban UAV traffic studies [22]."

Comment 15: Line 250 - The source of uncertainty values (2-5m) needs clarification.

Response 15: Thank you for this request for parameter sourcing. We have added manufacturer specifications at lines 270-272 (Page 7):

"...where is detection uncertainty (2-5 m, representing typical consumer-grade GNSS horizontal accuracy based on manufacturer specifications from DJI, senseFly, and Parrot [15], with sensitivity analyzed in Section 3.3)..."

Comment 16: Line 253 - "margin" requires clarification regarding its derivation and purpose.

Response 16: Thank you for requesting this clarification. We have provided detailed explanation at lines 276-278 (Page 8):

"where is a safety margin factor (derived from aviation industry practice where 20% buffers are standard for separation assurance [12], and validated through Monte Carlo analysis in Section 3.3)..."

Comment 17: Line 257 - "routes" needs explanation of how route types are determined.

Response 17: Thank you for this observation. The explanation has been integrated into the expanded Route Type Factor section (Section 2.2.2) with the detailed Step 1-3 calculation procedure at lines 282-292 (Page 8).

Comment 18: Line 272 - The source of "buffer" value needs clarification.

Response 18: Thank you for this request. We have explained the buffer derivation at lines 276-278 (Page 8), citing aviation industry practice where 20% buffers are standard for separation assurance.

Comment 19: Line 274 - The rationale for tier-based scaling needs more detailed explanation.

Response 19: Thank you for this request for additional justification. We have expanded the principle explanation at lines 312-321 (Page 9):

"The pre-tactical tier addresses a fundamental observation in collision avoidance: UAV encounters present varying levels of risk. Head-on encounters with high closure rates (combined velocities up to 60+ m/s for MF-MF pairs) demand larger separation buffers than slow overtaking scenarios (relative velocities as low as 5 m/s). By dynamically scaling Tier 1 strategic baselines according to predicted encounter geometry, Tier 2 enables efficient airspace utilization in low-risk situations while preserving and even enhancing safety margins where collision probability is elevated."

Comment 20: Line 318 - "Detection" section needs conceptual description before presenting pseudocode.

Response 20: Thank you for this structural suggestion. We have added assumptions (A1-A5) and a conceptual description before the algorithm details at lines 362-377 (Pages 10-11):

"The AHDS algorithm operates under the following assumptions:

- (A1) All UAVs broadcast position and velocity states at ≥10 Hz frequency via Remote ID or V2V communication protocols.

- (A2) Position measurement accuracy is ≤5 m (95% confidence interval).

- (A3) End-to-end communication latency is ≤300 ms.

- (A4) UAVs can execute commanded velocity changes within 1 second response time.

- (A5) No malicious or non-cooperative UAVs are present in the operational airspace.

The algorithm addresses the tactical separation problem through a four-phase sequential process..."

Comment 21: Line 399 - "Efficiency" metrics need clarification to distinguish from airspace utilization.

Response 21: Thank you for this clarification request. We have distinguished between traffic management efficiency and individual aircraft efficiency at lines 462-465 (Page 14):

"This section evaluates system-level performance metrics including airspace utilization (measured as UAVs/km³) and mission efficiency indicators. Note that 'operational efficiency' in this context refers to traffic management system performance rather than individual aircraft fuel or energy efficiency."

Comment 22: Line 409 - "Performance" figures are illegible and need regeneration.

Response 22: Thank you for identifying this quality issue. We have regenerated all performance figures with improved resolution and clarity. The updated figures (Figures 6-12) are now clearly legible throughout Section 3.

Comment 23: Line 440 - "modest" values need supporting sources.

Response 23: Thank you for this request. The modest values are now supported by the comprehensive sensitivity analysis in Section 3.3 (Pages 16-17), which provides quantitative regression analysis with trend slopes and R² values.

Comment 24: Line 565 - The unexplained frequencies need clarification.

Response 24: Thank you for noting this. We have added a reference to Section 2.1 where update frequencies are defined (Page 19, lines 632-633):

"The three tiers operate at the update frequencies specified in Section 2.1 (0.1 Hz for Tier 1, 1 Hz for Tier 2, 10 Hz for Tier 3)..."

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for article improvement according to my comments.

Reviewer 2 Report

Comments and Suggestions for Authors

Thx to complete the review