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Jacco M. Hoekstra is a full professor at the faculty of Aerospace Engineering of the Delft University of Technology. After obtaining his M.Sc. and pilots license in 1990, and a brief excursion in telecommunications, he started working at the National Aerospace Laboratory (NLR). He then obtained his Ph.D. on Air Traffic Management and headed the NLR Air Transport Operations division, before joining the TU Delft faculty. He served two terms as a dean and now heads the CNS/ATM chair and holds a seat at the administrative board of SESAR.
Purpose of Review A lot of research into decentralised, state-based conflict detection and resolution, or detect and avoid algorithms has been executed. This paper explains the essential properties of state-based conflict detection and reviews the work in the context of applications for not only manned but also unmanned aerial vehicles, where this might be applied relatively soon. Recent Findings Lately, based on several reviews of a variety of published algorithms, a selection has been implemented and simulated in extremely high traffic densities for comparison. Summary The modified voltage potential has been surprisingly efficient, even compared with more complex algorithms or adaptations, as is apparent from looking at macroscopic metrics like domino effect, efficiency and safety. This indicates that to this date, it is so far the most suitable algorithm for the detect and avoid role for unmanned aerial vehicles in urban airspaces, or other areas where a high density is expected.
Jacco M. Hoekstra; Joost Ellerbroek. Aerial Robotics: State-based Conflict Detection and Resolution (Detect and Avoid) in High Traffic Densities and Complexities. Current Robotics Reports 2021, 1 -11.
AMA StyleJacco M. Hoekstra, Joost Ellerbroek. Aerial Robotics: State-based Conflict Detection and Resolution (Detect and Avoid) in High Traffic Densities and Complexities. Current Robotics Reports. 2021; ():1-11.
Chicago/Turabian StyleJacco M. Hoekstra; Joost Ellerbroek. 2021. "Aerial Robotics: State-based Conflict Detection and Resolution (Detect and Avoid) in High Traffic Densities and Complexities." Current Robotics Reports , no. : 1-11.
Current investigations into urban aerial mobility, as well as the continuing growth of global air transportation, have renewed interest in conflict detection and resolution (CD&R) methods. The use of drones for applications such as package delivery, would result in traffic densities that are orders of magnitude higher than those currently observed in manned aviation. Such densities do not only make automated conflict detection and resolution a necessity, but will also force a re-evaluation of aspects such as coordination vs. priority, or state vs. intent. This paper looks into enabling a safe introduction of drones into urban airspace by setting travelling rules in the operating airspace which benefit tactical conflict resolution. First, conflicts resulting from changes of direction are added to conflict resolution with intent trajectory propagation. Second, the likelihood of aircraft with opposing headings meeting in conflict is reduced by separating traffic into different layers per heading–altitude rules. Guidelines are set in place to make sure aircraft respect the heading ranges allowed at every crossed layer. Finally, we use a reinforcement learning agent to implement variable speed limits towards creating a more homogeneous traffic situation between cruising and climbing/descending aircraft. The effects of all of these variables were tested through fast-time simulations on an open source airspace simulation platform. Results showed that we were able to improve the operational safety of several scenarios.
Marta Ribeiro; Joost Ellerbroek; Jacco Hoekstra. Velocity Obstacle Based Conflict Avoidance in Urban Environment with Variable Speed Limit. Aerospace 2021, 8, 93 .
AMA StyleMarta Ribeiro, Joost Ellerbroek, Jacco Hoekstra. Velocity Obstacle Based Conflict Avoidance in Urban Environment with Variable Speed Limit. Aerospace. 2021; 8 (4):93.
Chicago/Turabian StyleMarta Ribeiro; Joost Ellerbroek; Jacco Hoekstra. 2021. "Velocity Obstacle Based Conflict Avoidance in Urban Environment with Variable Speed Limit." Aerospace 8, no. 4: 93.
Large-scale adoption of drone-based delivery in urban areas promise societal benefits with respect to emissions and on-ground traffic congestion, as well as potential cost savings for drone-based logistic companies. However, for this to materialise, the ability of accommodating high volumes of drone traffic in an urban airspace is one of the biggest challenges. For unconstrained airspace, it has been shown that traffic alignment and segmentation can be used to mitigate conflict probability. The current study investigates the application of these principles to a highly constrained airspace. We propose two urban airspace concepts, applying road-based analogies of two-way and one-way streets by imposing horizontal structure. Both of the airspace concepts employ heading-altitude rules to vertically segment cruising traffic according to their travel direction. These airspace configurations also feature transition altitudes to accommodate turning flights that need to decrease the flight speed in order to make safe turns at intersections. While using fast-time simulation experiments, the performance of these airspace concepts is compared and evaluated for multiple traffic demand densities in terms of safety, stability, and efficiency. The results reveal that an effective way to structure drone traffic in a constrained urban area is to have vertically segmented altitude layers with respect to travel direction as well as horizontal constraints imposed to the flow of traffic. The study also makes recommendations for areas of future research, which are aimed at supporting dynamic traffic demand patterns.
Malik Doole; Joost Ellerbroek; Victor L. Knoop; Jacco M. Hoekstra. Constrained Urban Airspace Design for Large-Scale Drone-Based Delivery Traffic. Aerospace 2021, 8, 38 .
AMA StyleMalik Doole, Joost Ellerbroek, Victor L. Knoop, Jacco M. Hoekstra. Constrained Urban Airspace Design for Large-Scale Drone-Based Delivery Traffic. Aerospace. 2021; 8 (2):38.
Chicago/Turabian StyleMalik Doole; Joost Ellerbroek; Victor L. Knoop; Jacco M. Hoekstra. 2021. "Constrained Urban Airspace Design for Large-Scale Drone-Based Delivery Traffic." Aerospace 8, no. 2: 38.
Bird strike prevention in civil aviation has traditionally focused on the airport perimeter. Since the risk of especially damaging bird strikes outside the airport boundaries is rising, this paper investigates the safety potential of operational bird strike prevention involving pilots and controllers. In such a concept, controllers would be equipped with a bird strike advisory system, allowing them to delay departures which are most vulnerable to the consequences of bird strikes in case of high bird strike risk. An initial study has shown the strong potential of the concept to prevent bird strikes in case of perfect bird movement prediction. This paper takes the research to the next level by taking into account the limited predictability of bird tracks. As such, the collision avoidance algorithm is extended to a bird strike risk algorithm. The risk of bird strikes is calculated for birds expected to cross the extended runway center line and to cause aircraft damage upon impact. By specifically targeting these birds and excluding birds lingering on the runway which are taken care of by the local wildlife control, capacity reductions should be limited, and the implementation remain feasible. The extrapolation of bird tracks is performed by simple linear regression based on the bird positions known at the intended take-off times. To calculate the probability of collision, uncertainties resulting from variability in bird velocity and track are included. The study demonstrates the necessity to limit alerts to potentially damaging strikes with birds crossing the extended runway center line to keep the imposed delays tolerable for airports operating at their capacity limits. It is shown that predicting bird movements based on simple linear regression without considering individual bird behavior is insufficient to achieve a safety-effect. Hence, in-depth studies of multi-year bird data to develop bird behavior models and reliable predictions are recommended for future research. This is expected to facilitate the implementation of a bird strike advisory system satisfying both safety and capacity aspects.
Isabel C. Metz; Joost Ellerbroek; Thorsten Mühlhausen; Dirk Kügler; Jacco M. Hoekstra. Analysis of Risk-Based Operational Bird Strike Prevention. Aerospace 2021, 8, 32 .
AMA StyleIsabel C. Metz, Joost Ellerbroek, Thorsten Mühlhausen, Dirk Kügler, Jacco M. Hoekstra. Analysis of Risk-Based Operational Bird Strike Prevention. Aerospace. 2021; 8 (2):32.
Chicago/Turabian StyleIsabel C. Metz; Joost Ellerbroek; Thorsten Mühlhausen; Dirk Kügler; Jacco M. Hoekstra. 2021. "Analysis of Risk-Based Operational Bird Strike Prevention." Aerospace 8, no. 2: 32.
Involving air traffic controllers and pilots into the bird strike prevention process is considered an essential step to increase aviation and avian safety. Prior to implementing operational measures such as real-time warning systems, it is vital to evaluate their feasibility. This paper studies the efficacy of a bird strike advisory system for air traffic control. In addition to the potential safety benefit, the possible impact on airport operations is analyzed. To this end, a previously developed collision avoidance algorithm underlying the system was tested in fast-time Monte Carlo simulations involving various air traffic and bird densities to obtain representative conclusions for different operational conditions. The results demonstrate the strong safety potential of operational bird strike prevention in case of precise bird movement prediction. Unless airports operate close to their capacity limits while bird abundance is high, the induced delays remain tolerable. Prioritization of hazardous strikes involving large individuals as well as flocks of birds are expected to support operational feasibility in all conditions.
Isabel C. Metz; Joost Ellerbroek; Thorsten Mühlhausen; Dirk Kügler; Stefan Kern; Jacco M. Hoekstra. The Efficacy of Operational Bird Strike Prevention. Aerospace 2021, 8, 17 .
AMA StyleIsabel C. Metz, Joost Ellerbroek, Thorsten Mühlhausen, Dirk Kügler, Stefan Kern, Jacco M. Hoekstra. The Efficacy of Operational Bird Strike Prevention. Aerospace. 2021; 8 (1):17.
Chicago/Turabian StyleIsabel C. Metz; Joost Ellerbroek; Thorsten Mühlhausen; Dirk Kügler; Stefan Kern; Jacco M. Hoekstra. 2021. "The Efficacy of Operational Bird Strike Prevention." Aerospace 8, no. 1: 17.
Air traffic simulations serve as common practice to evaluate different concepts and methods for air transportation studies. The aircraft performance model is a key element that supports these simulation-based studies. It is also an important component for simulation-independent studies, such as air traffic optimization and prediction studies. Commonly, contemporary studies have to rely on proprietary aircraft performance models that restrict the redistribution of the data and code. To promote openness and research comparability, an alternative open performance model would be beneficial for the air transportation research community. In this paper, we introduce an open aircraft performance model (OpenAP). It is an open-source model that is based on a number of our previous studies, which were focused on different components of the aircraft performance. The unique characteristic of OpenAP is that it was built upon open aircraft surveillance data and open literature models. The model is composed of four main components, including aircraft and engine properties, kinematic performances, dynamic performances, and utility libraries. Alongside the performance model, we are publishing an open-source toolkit to facilitate the use of this model. The main objective of this paper is to describe each main component, their connections, and how they can be used for simulation and research in practice. Finally, we analyzed the performance of OpenAP by comparing it with an existing performance model and sample flight data.
Junzi Sun; Jacco M. Hoekstra; Joost Ellerbroek. OpenAP: An Open-Source Aircraft Performance Model for Air Transportation Studies and Simulations. Aerospace 2020, 7, 104 .
AMA StyleJunzi Sun, Jacco M. Hoekstra, Joost Ellerbroek. OpenAP: An Open-Source Aircraft Performance Model for Air Transportation Studies and Simulations. Aerospace. 2020; 7 (8):104.
Chicago/Turabian StyleJunzi Sun; Jacco M. Hoekstra; Joost Ellerbroek. 2020. "OpenAP: An Open-Source Aircraft Performance Model for Air Transportation Studies and Simulations." Aerospace 7, no. 8: 104.
Current investigations into urban aerial mobility, as well as the continuing growth of global air transportation, have renewed interest in Conflict Detection and Resolution (CD&R) methods. With the new applications of drones, and the implications of a profoundly different urban airspace, new demands are placed on such algorithms, further spurring new research. This paper presents a review of current CR methods for both manned and unmanned aviation. It presents a taxonomy that categorises algorithms in terms of their approach to avoidance planning, surveillance, control, trajectory propagation, predictability assumption, resolution manoeuvre, multi-actor conflict resolution, considered obstacle types, optimization, and method category. More than a hundred CR methods were considered, showing how most work on a tactical, distributed framework. To enable a reliable comparison between methods, this paper argues that an open and ideally common simulation platform, common test scenarios, and common metrics are required. This paper presents an overview of four CR algorithms, each representing a commonly used CR algorithm category. Both manned and unmanned scenarios were tested, through fast-time simulations on an open-source airspace simulation platform.
Marta Ribeiro; Joost Ellerbroek; Jacco Hoekstra. Review of Conflict Resolution Methods for Manned and Unmanned Aviation. Aerospace 2020, 7, 79 .
AMA StyleMarta Ribeiro, Joost Ellerbroek, Jacco Hoekstra. Review of Conflict Resolution Methods for Manned and Unmanned Aviation. Aerospace. 2020; 7 (6):79.
Chicago/Turabian StyleMarta Ribeiro; Joost Ellerbroek; Jacco Hoekstra. 2020. "Review of Conflict Resolution Methods for Manned and Unmanned Aviation." Aerospace 7, no. 6: 79.
Collisions between birds and aircraft pose a severe threat to aviation and avian safety. To understand and prevent these bird strikes, knowledge about the factors leading to these bird strikes is vital. However, even though it is a global issue, data availability strongly varies and is difficult to put into a global picture. This paper aims to close this gap by providing an in-depth review of studies and statistics to obtain a concise overview of the bird strike problem in commercial aviation on an international level. The paper illustrates the factors contributing to the occurrence and the potential consequences in terms of effect on flight and damage. This is followed by a presentation of the risk-reducing measures currently in place as well as their limitations. The paper closes with an insight into current research investigating novel methods to prevent bird strikes.
Isabel C. Metz; Joost Ellerbroek; Thorsten Mühlhausen; Dirk Kügler; Jacco M. Hoekstra. The Bird Strike Challenge. Aerospace 2020, 7, 26 .
AMA StyleIsabel C. Metz, Joost Ellerbroek, Thorsten Mühlhausen, Dirk Kügler, Jacco M. Hoekstra. The Bird Strike Challenge. Aerospace. 2020; 7 (3):26.
Chicago/Turabian StyleIsabel C. Metz; Joost Ellerbroek; Thorsten Mühlhausen; Dirk Kügler; Jacco M. Hoekstra. 2020. "The Bird Strike Challenge." Aerospace 7, no. 3: 26.
Mode S surveillance allows air traffic controllers to interrogate certain information from aircraft, such as airspeeds, turn parameters, target altitudes, and meteorological conditions. However, not all aircraft have enabled the same capabilities. Before performing any specific interrogation, the surveillance radar must acquire the transponder capabilities of an aircraft. This is obtained via the common usage Ground-initiated Comm-B (GICB) capabilities report (BDS 1,7). With this report, third-party researchers can further improve the identification accuracy of different Mode S Comm-B message types, as well as study the compliance of surveillance standards. Thanks to the OpenSky network’s large-scale global coverage, a full picture of current Mode S capabilities over the world can be constructed. In this paper, using the OpenSky Impala data interface, we first sample over one month of raw BDS 1,7 messages from around the world. Around 40 million messages are obtained. We then decode and analyze the GICB capability messages. The resulting data contain Comm-B capabilities for all aircraft available to OpenSky during this month. The analyses in this paper focus on exploring statistics of GICB capabilities among all aircraft and within each aircraft type. The resulting GICB capability database is shared as an open dataset.
Junzi Sun; Huy Vû; Xavier Olive; Jacco M. Hoekstra. Mode S Transponder Comm-B Capabilities in Current Operational Aircraft. Proceedings 2020, 59, 4 .
AMA StyleJunzi Sun, Huy Vû, Xavier Olive, Jacco M. Hoekstra. Mode S Transponder Comm-B Capabilities in Current Operational Aircraft. Proceedings. 2020; 59 (1):4.
Chicago/Turabian StyleJunzi Sun; Huy Vû; Xavier Olive; Jacco M. Hoekstra. 2020. "Mode S Transponder Comm-B Capabilities in Current Operational Aircraft." Proceedings 59, no. 1: 4.
Annually, thousands of birds collide with aircraft. The impact usually has lethal consequences for the bird, the involved aircraft can experience severe damage. The highest bird strike risk occurs at low altitudes. Therefore, aircraft within the airport area as well as the adjacent approach and departure corridors are especially vulnerable to collisions with birds. To analyse risk-reducing measures in these areas, a fast-time bird strike simulation environment was developed. An open-source Air Traffic Management simulator was enhanced with a model to represent bird movements and to recognize bird strikes. To confirm the reproducibility of the outcome, Monte Carlo simulations were performed. They included bird movement data from one year and air traffic flight plans for various air traffic volumes. The number of strikes and near misses showed an expected variance within the individual replications. The results indicate that the predictability of the number of strikes and near misses increases with rising number of birds, and rising air traffic intensity. Thus, by considering simulation scenarios including bird movement information from all seasons and a sufficient air traffic volume, the described set-up leads to stable results.
Isabel C. Metz; Thorsten Mühlhausen; Joost Ellerbroek; Dirk Kügler; Hans Van Gasteren; Jan Kraemer; Jacco M. Hoekstra. Simulation Model to Calculate Bird-Aircraft Collisions and Near Misses in the Airport Vicinity. Aerospace 2018, 5, 112 .
AMA StyleIsabel C. Metz, Thorsten Mühlhausen, Joost Ellerbroek, Dirk Kügler, Hans Van Gasteren, Jan Kraemer, Jacco M. Hoekstra. Simulation Model to Calculate Bird-Aircraft Collisions and Near Misses in the Airport Vicinity. Aerospace. 2018; 5 (4):112.
Chicago/Turabian StyleIsabel C. Metz; Thorsten Mühlhausen; Joost Ellerbroek; Dirk Kügler; Hans Van Gasteren; Jan Kraemer; Jacco M. Hoekstra. 2018. "Simulation Model to Calculate Bird-Aircraft Collisions and Near Misses in the Airport Vicinity." Aerospace 5, no. 4: 112.
This paper presents analytical models that describe the safety of unstructured and layered en route airspace designs. Here, ‘unstructured airspace’ refers to airspace designs that offer operators complete freedom in path planning, whereas ‘layered airspace’ refers to airspace concepts that utilize heading-altitude rules to vertically separate cruising aircraft based on their travel directions. With a focus on the intrinsic safety provided by an airspace design, the models compute instantaneous conflict counts as a function of traffic demand and airspace design parameters, such as traffic separation requirements and the permitted heading range per flight level. While previous studies have focused primarily on conflicts between cruising aircraft, the models presented here also take into account conflicts involving climbing and descending traffic. Fast-time simulation experiments used to validate the modeling approach indicate that the models estimate instantaneous conflict counts with high accuracy for both airspace designs. The simulation results also show that climbing and descending traffic caused the majority of conflicts for layered airspaces with a narrow heading range per flight level, highlighting the importance of including all aircraft flight phases for a comprehensive safety analysis. Because such trends could be accurately predicted by the three-dimensional models derived here, these analytical models can be used as tools for airspace design applications as they provide a detailed understanding of the relationships between the parameters that influence the safety of unstructured and layered airspace designs.
Emmanuel Sunil; Joost Ellerbroek; Jacco M. Hoekstra; Jerom Maas. Three-dimensional conflict count models for unstructured and layered airspace designs. Transportation Research Part C: Emerging Technologies 2018, 95, 295 -319.
AMA StyleEmmanuel Sunil, Joost Ellerbroek, Jacco M. Hoekstra, Jerom Maas. Three-dimensional conflict count models for unstructured and layered airspace designs. Transportation Research Part C: Emerging Technologies. 2018; 95 ():295-319.
Chicago/Turabian StyleEmmanuel Sunil; Joost Ellerbroek; Jacco M. Hoekstra; Jerom Maas. 2018. "Three-dimensional conflict count models for unstructured and layered airspace designs." Transportation Research Part C: Emerging Technologies 95, no. : 295-319.
Aircraft mass is a crucial piece of information for studies on aircraft performance, trajectory prediction, and many other topics of aircraft traffic management. However, It is a common challenge for researchers, as well as air traffic control, to access this proprietary information. Previously, several studies have proposed methods to estimate aircraft weight based on specific parts of the flight. Due to inaccurate input data or biased assumptions, this often leads to less confident or inaccurate estimations. In this paper, combined with a fuel-flow model, different aircraft initial masses are computed independently using the total energy model and reference model at first. It then adopts a Bayesian approach that uses a prior probability of aircraft mass based on empirical knowledge and computed aircraft initial masses to produce the maximum a posteriori estimation. Variation in results caused by dependent factors such as prior, thrust and wind are also studied. The method is validated using 50 test flights of a Cessna Citation II aircraft, for which measurements of the true mass were available. The validation results show a mean absolute error of 4.3% of the actual aircraft mass.
Junzi Sun; Joost Ellerbroek; Jacco M. Hoekstra. Aircraft initial mass estimation using Bayesian inference method. Transportation Research Part C: Emerging Technologies 2018, 90, 59 -73.
AMA StyleJunzi Sun, Joost Ellerbroek, Jacco M. Hoekstra. Aircraft initial mass estimation using Bayesian inference method. Transportation Research Part C: Emerging Technologies. 2018; 90 ():59-73.
Chicago/Turabian StyleJunzi Sun; Joost Ellerbroek; Jacco M. Hoekstra. 2018. "Aircraft initial mass estimation using Bayesian inference method." Transportation Research Part C: Emerging Technologies 90, no. : 59-73.
This paper presents a safety assessment method for unmanned aerial vehicle (UAV) operations, including the effect of a distributed conflict detection and resolution (CD&R) system, in a high density airspace environment. Here, the expected conflicts occurrence and chances for each CD&R system to perform are sufficiently high to extract two parameters of safety, i.e., the frequency of near mid-air collision (NMAC) and the frequency of mid-air collision (MAC), by series of Monte Carlo simulations. The results are then used to derive the safety parameters in a more realistic, less dense airspace. Two cases of distributed CD&R protocols are assessed and compared, i.e., 1) uncoordinated protocol, where each vehicle has its own avoidance preferences, and 2) implicitly coordinated protocol where each vehicle, while still independent from each other, applies simple common rules. Using those CD&R protocols, the result shows a reduction of more than 94% of possible NMAC. Moreover, while maintaining the target level of safety in the airspace, the maximum number of UAVs under an implicitly coordinated CD&R protocol can be at least ten times more than cases when no CD&R is applied.
Yazdi I. Jenie; Erik-Jan Van Kampen; Joost Ellerbroek; Jacco M. Hoekstra. Safety Assessment of a UAV CD&R System in High Density Airspace Using Monte Carlo Simulations. IEEE Transactions on Intelligent Transportation Systems 2017, 19, 2686 -2695.
AMA StyleYazdi I. Jenie, Erik-Jan Van Kampen, Joost Ellerbroek, Jacco M. Hoekstra. Safety Assessment of a UAV CD&R System in High Density Airspace Using Monte Carlo Simulations. IEEE Transactions on Intelligent Transportation Systems. 2017; 19 (8):2686-2695.
Chicago/Turabian StyleYazdi I. Jenie; Erik-Jan Van Kampen; Joost Ellerbroek; Jacco M. Hoekstra. 2017. "Safety Assessment of a UAV CD&R System in High Density Airspace Using Monte Carlo Simulations." IEEE Transactions on Intelligent Transportation Systems 19, no. 8: 2686-2695.
This paper proposes a novel avoidance method called the three-dimensional velocity obstacle method. The method is designed for unmanned aerial vehicle applications, in particular to autonomously handle uncoordinated multiple encounters in an integrated airspace, by exploiting the limited space in a three-dimensional manner. The method is a three-dimensional extension of the velocity obstacle method that can reactively generate an avoidance maneuver by changing the vehicle velocity vector based on the encounter geometry. Adverse maneuvers of the obstacle are anticipated by introducing the concept of a buffer velocity set, which ensures that the ownship will diverge with sufficient space in case of sudden imminence. A three-dimensional resolution is generated by choosing the right plane for avoidance, in which the unmanned aerial vehicle conducts a pure turning maneuver. Implementation of the three-dimensional velocity obstacle method is tested in several simulations that demonstrate its capability to resolve various three-dimensional conflicts. A validation using Monte Carlo simulations is also conducted in stressful superconflict scenarios, which results in zero collisions occurrences for the entire 25,000 samples.
Yazdi I. Jenie; Erik-Jan van Kampen; Cornelis C. de Visser; Joost Ellerbroek; Jacco M. Hoekstra. Three-Dimensional Velocity Obstacle Method for Uncoordinated Avoidance Maneuvers of Unmanned Aerial Vehicles. Journal of Guidance, Control, and Dynamics 2016, 39, 2312 -2323.
AMA StyleYazdi I. Jenie, Erik-Jan van Kampen, Cornelis C. de Visser, Joost Ellerbroek, Jacco M. Hoekstra. Three-Dimensional Velocity Obstacle Method for Uncoordinated Avoidance Maneuvers of Unmanned Aerial Vehicles. Journal of Guidance, Control, and Dynamics. 2016; 39 (10):2312-2323.
Chicago/Turabian StyleYazdi I. Jenie; Erik-Jan van Kampen; Cornelis C. de Visser; Joost Ellerbroek; Jacco M. Hoekstra. 2016. "Three-Dimensional Velocity Obstacle Method for Uncoordinated Avoidance Maneuvers of Unmanned Aerial Vehicles." Journal of Guidance, Control, and Dynamics 39, no. 10: 2312-2323.