The current system of Air Traffic Control (ATC) relies on a centralized control architecture. At its core, this system is heavily dependent on manual intervention by human Air Traffic Controllers (ATCos) to ensure safe operations. The capacity of this system is, therefore, closely tied to the maximum workload that can be tolerated by ATCos. Although this system has served the needs of the air transportation industry thus far, the increasing delays and congestion reported in many areas indicates that the current centralized operational model is rapidly approaching saturation levels. To cope with the expected future increases of traffic demand, many researchers have proposed a transition to a decentralized traffic separation paradigm in en route airspaces. Although there are several variants of decentralized ATC, this thesis focuses on a variant known as self-separation. In self-separated airspace, each individual aircraft is responsible for its own separation with all surrounding traffic. To facilitate self-separation, significant research effort has been devoted towards the development of new algorithms for automated airborne Conflict Detection and Resolution (CD&R). However, in spite of over two decades of active research highlighting its theorized benefits, decentralization/self-separation is yet to be deployed in the field. From a technical point of view a lack of understanding on three open issues namely airspace design, airspace safety modeling, and airspace capacity modeling, have impeded its further development and implementation. The goal of this research is to address these three open problems in order to bring self-separated ATC closer to reality. Consequently, the main body of this thesis is divided into three parts, with each part tackling one of the three aforementioned open problems...@en

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.

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Decentralized en-route airspace concepts have been proposed by many studies to increase airspace safety and capacity. Most of these studies, including our own forays into this domain, have used fast-time simulation experiments to explore the benefits offered by decentralization. While simulations are indispensable during the initial design phase of any new airspace concept, the understanding gained using this approach can be difficult to generalize beyond the tested conditions. To address this issue, some researchers have presented analytical conflict count models to quantitatively analyze the effect of physical factors, such as traffic separation requirements, on the intrinsic safety of decentralized airspace concepts. However, the derivation of these models often make use of idealized assumptions regarding the behavior of traffic that do not always reflect realistic operations. To this end, this paper investigates the effect of these assumptions on the accuracy of the analytical conflict count models using targeted fast-time simulations of a direct-routing unstructured en-route airspace concept for a number of more realistic traffic patterns. The data collected from these simulations is also used to test so called ‘model adjustments’ that aim to relax the dependency of the models on the idealized traffic scenario assumptions. The results show that the assumptions do affect the accuracy of the analytical models, with some assumptions leading to a substantial under-estimation of conflicts. The results also show that the model adjustments increased accuracy for the more realistic scenarios to the levels previously found for the ideal traffic settings for all cases. Therefore, in addition to providing a physical understanding of the factors that affect airspace safety, the adjusted models can also be used as tools for practical airspace design applications. Keywords—Airspace safety; airspace design; conflict rate; conflict @en

This paper presents a semi-empirical method to determine the maximum theoretical capacity of decentralized airspace concepts. The method considered here, named Capacity Assessment Method for Decentralized ATC (CAMDA), formalizes an earlier approach described in literature, extends it for threedimensional airspace, and also improves the accuracy of the underlying models. CAMDA defines capacity as the traffic density at which conflict chain reactions propagate uncontrollably throughout the entire airspace. CAMDA identifies this critical density using a semi-empirical approach whereby models describing the actions of decentralized conflict detection and resolution algorithms are combined with empirically obtained conflict count data. The CAMDA method is demonstrated in this work for a decentralized direct routing en-route airspace concept that utilizes a state-based conflict detection algorithm, and a voltage potentialbased conflict resolution algorithm. Three fast-time simulation experiments were performed to study how the capacity of this particular airspace design is affected by: a) conflict detection parameters; b) conflict resolution dimension; and c) the speed distribution of aircraft. The results showed that CAMDA estimated the occurrence of conflict chain reactions with high accuracy for all cases, enabling capacity estimations using relatively non-intensive low density traffic simulations. Therefore, CAMDA can be used to speed up the airspace design process by reducing the number of time consuming high-density traffic simulations that are required when performing a trade-off between different airspace designs, or when fine-tuning the parameters of the selected airspace design. @en

Both U-space in Europe, as well as UTM in the USA, develop concepts and tools for UAV airspace. Enabling highdensity operations is one of the goals of these studies. Past and recent studies have analysed which factors affect the capacity of a UAV airspace. An improved understanding of this can lead to control methods for capacity management. Two general principles for capacity management can be distinguished: controlling the traffic density, and controlling the traffic complexity. The first approach can be achieved using geofencing or geocaging, which is foreseen for UAV airspace. The second approach is hardly addressed in the planned concepts. In this paper a new, general concept, called geovectoring, is proposed which could increase the capacity by reducing the traffic complexity for U-Space and UTM. This paper therefore proposes to add geovectoring as a third service to the already planned concepts of geofencing and geocaging. @en

Previous research relating airspace structure and ca-pacity has shown that a decentralized layered airspace concept, in which each altitude band limited horizontal travel to within a prede?ned heading range, improved safety when compared to unstructured airspace. However, the extent of the safety bene?ts of such layered airspace designs were not quanti?ed. To this end, in this paper, con?ict rate models are developed to determine the intrinsic safety of unstructured and layered airspace designs. In comparison to previous work, the present models consider con-?icts between aircraft in different ?ight phases. Thus, con?icts for climbing and descending traf?c, as well as for cruising aircraft, are taken into account when computing the total con?ict rate. To validate the models, fast-time simulations were performed for several different layered airspace concepts, and for unstructured airspace. The results indicate that the models are able to estimate the con?ict rate for high traf?c densities using a model ?t for low densities. When comparing the different layered airspace concepts tested, the model predicted, and the simulation results con?rmed, a clear safety improvement when the permitted heading range per altitude band is reduced. Thus the models can be used to study the effect of airspace design parameters on the safety of unstructured and layered airspace concepts. @en

The work that is presented in this paper is part of an ongoing study on the relationship between airspace structure and capacity. The present paper investigates the degree of structuring needed to maximize capacity for decentralized en-routeairspace. To this end, four decentralized en-route airspace concepts, which vary in terms of the number of constrained degrees of freedom, were comparedusing fast-time simulations, for both nominal and non-nominal conditions. The airspace structure-capacity relationship was studied from the effect of multiple traffic demand densities on airspace metrics. The results indicated that structuring methods that over-constrained the horizontal path of aircraft reduced capacity, as traffic demand displays no predominant patterns in the horizontal dimension for decentralization. The results also showed that capacity was maximized when a vertical segmentation of airspace was used to separate traffic with different travel directions at different flight levels.This mode of structuring improved performance over completely unstructured airspace by reducing relative velocities between aircraft cruising at the same altitude,while allowing direct horizontal routes.

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In the context of decentralized separation, airspace stability pertains to the propagation of con?ict chain reactions as a result of tactical con?ict resolution maneuvers. This notion of airspace stability has been used in previous literature to develop a semi-empirical method for determining the capacity of a decentralized direct-routing airspace concept in the horizontal plane. The present paper extends this method by explicitly mod-eling: a) the effect of a given Con?ict Detection and Resolution (CD&R) strategy on the stability of the airspace; b) the in?uence of direct-routing on instantaneous con?ict probability; and c) the impact of ?nite-time measurements on the determination of airspace states. To validate the resulting analytical capacity model, fast-time simulations were performed. The results indicate that the predictions of the analytical model are close to that of the previous semi-empirical approach. Thus, the analytical model can be used to obtain a ?rst-order estimate of the maximum theoretical capacity, as along as simulation settings do not cause the ‘local’, or per aircraft, con?ict rate to deviate signi?cantly from assumptions made during the model derivation. Future work will focus on relaxing model assumptions, and extending the modeling approach to three-dimensional airspace.@en

Haptic guidance is a promising way to support unmanned aerial vehicle (UAV) operators, but the design of haptic guidance forces is often heuristic. This paper describes the design and experimental validation of a systematic neuromuscular analysis-based tuning procedure for haptic guidance, here applied to haptic collision avoidance system for UAV teleoperation. This tuning procedure is hypothesized to reduce operator workload as compared with current heuristic tuning methods. The proposed procedure takes into consideration the estimated mechanical response of the neuromuscular system (NMS) to haptic cues. A “relax-task” setting of the NMS, for which reflexive and muscular activation is minimal, is chosen as the design point for tuning the haptic support, as this setting is expected to yield minimal physical workload. The paper first presents a neuromuscular identification experiment, performed to estimate the “relax task” admittance of an operator's arm. The averaged admittance of a group of subjects (n=10) was then used for tuning the haptic shared controller, which was subsequently evaluated in its ability to support different operators (n=12) in a simulated unmanned aerial vehicle surveillance task. Results show that our novel tuning procedure indeed reduces operator workload and also improves situation awareness compared with haptic settings that ignore the NMS. In fact, it is shown that overtuning, which frequently occurs for these heuristically tuned systems, leads to even lower user acceptance scores than interfaces without any haptic support.@en

Several conflict resolution algorithms for airborne self-separation rely on principles derived from the repulsive forces that exist between similarly charged particles. This research investigates whether the performance of the Modified Voltage Potential algorithm, which is based on this algorithm, can be improved using bio-inspired swarming behavior. To this end, the collision avoidance function of the algorithm is augmented with the velocity alignment and flock centering swarming traits displayed by animals such as birds and fish. The basic and swarm augmented versions of the algorithm were compared using large-scale fast time simulations, for multiple traffic demand scenarios. For ideal conditions, the results show that the process of aligning with neighboring traffic triggered a large number of conflicts. However, when noise was added to scenarios, swarming led to a lower increase in the number of intrusions, which could indicate that it can be used to improve the robustness of the Modified Voltage Potential algorithm. Furthermore, the stability results suggest that both versions of the algorithm could reduce the number of conflict chain reactions with respect to simulations without resolution. Future research will further explore the effect of conflict resolution on airspace stability, as well as whether varying the relative weights of swarming elements can improve the safety of swarm augmentations.@en

Most Free-Flight concepts rely on self-separation by means of airborne Conflict Detection and Resolution (CD&R) algorithms. A key enabling technology for airborne CD&R is the Automatic Dependent Surveillance-Broadcast (ADS-B) system, which is used for direct state information exchange between aircraft. Similar to other communication systems, ADS-B is affected by a number of limitations which can be broadly classified as system and situation related deficiencies. This re- search investigates the impact of these limitations on the viability of using ADS-B for airborne CD&R within the Free-Flight context. Here, ‘state-based’ conflict detection and the modified voltage potential conflict resolution algorithm are used as a case- study. An ADS-B model is developed, and its effect on the aforementioned CD&R method is measured using three fast- time simulation experiments. The experiments studied overall safety with ADS-B, as well as the specific effect of situation related characteristics, i.e., transmission range and interference, on safety. The results indicated that the overall safety with ADS- B was comparable to the case where perfect state information was assumed. Additionally, it was found that increasing ADS- B transmission range also increased signal interference, which in turn lowered safety. This suggests that the degrading effect of ADS-B signal interference should be considered in future airborne CD&R research, particularly for high traffic densities.@en

In many ATM studies experiments are performed to determine the capacity. This paper looks at the effect of airspace design on the capacity. Using an algebraic approach a relation is derived between the design parameters of a layered airspace design and the capacity of the airspace. The validity of the assumptions which are used in this derivation are tested experimentally. This airspace lay-out proved to be the airspace design which had the highest capacity for the unstructured, extremely high traffic demand used in an earlier experimental study. The result is both a method to relate an airspace design to the capacity as well as a relation which shows the effect on the airspace capacity for an airspace design where different levels or layers are defined each with their own segment of heading angles. @en

Airspace structure can be used as a procedural mechanism for a priori separation and organization of en-route air traffic. Although many studies have explored novel structuring methods to increase en-route airspace capacity, the relationship between the level of structuring of traffic and airspace capacity is not well established. To better understand the influence of traffic structure on airspace capacity, in this research, four airspace concepts, representing discrete points along the dimension of structure, were compared using large-scale simulation experiments. By subjecting the concepts to multiple traffic demand scenarios, the structure-capacity relationship was inferred from the effect of traffic demand variations on safety, efficiency and stability metrics. These simulations were performed within the context of a future personal aerial transportation system, and considered both nominal and non-nominal conditions. Simulation results suggest that the structuring of traffic must take into account the expected traffic demand pattern to be beneficial in terms of capacity. Furthermore, for the heterogeneous, or uniformly distributed, traffic demand patterns considered in this work, a decentralized layered airspace concept, in which each altitude band limited horizontal travel to within a predefined heading range, led to the best balance of all the metrics considered.@en

The work that is presented in this paper is part of an ongoing study on the relationship between structure and capacity of decentralized airspace concepts. In this paper, the effect of traffic stability, which considers the occurrence of conflict chain reactions as a result of conflict resolution maneuvers, on capacity is examined closely. Using the domino effect parameter as a measure of traffic stability, a model relating stability and capacity is derived. Although the derivation of this model is not complete, its current form shows that traffic stability, and therefore capacity, is also affected by the safety and efficiency characteristics of decentralized concepts. This suggests that the capacity measurement of decentralized concepts must consider the variation of intrinsic system-wide properties with density, using a minimum of safety, efficiency and stability metrics. Future work will continue the development of the model, and its validation using large-scale simulation experiments.@en

This research investigates a neuromuscular analysis based tuning algorithm for haptic cues that has been hypothesized to simultaneously improve safety and workload when compared to heuristic tuning, applied to a haptic collision avoidance system for unmanned aircraft teleoperation. This novel tuning method considers the combined stiffness of the human arm and the control inceptor when computing ideal haptic cues. The `relaxed' setting of the neuromuscular system, for which neural reflexes are suppressed, is chosen as the design point for tuning haptic cues as it is expected to lead to the lowest workload, contrary to the `force' and `position' settings. Theoretical investigations using offline simulations verified the novel approach and the selection of the `relaxed' setting. Subsequently, a teleoperation experiment (n=12) in an obstacle laden urban environment was conducted with six different tuning profiles, including a manual control condition. Results showed that safety, workload and situational awareness was substantially improved over conditions that ignored the neuromuscular system. Additionally, over-tuning haptic cues was found to be worse than manual control for user acceptance of the system. No significant differences were found between the `relaxed' and `force' settings, suggesting that selection between these two options depends on the specific application of haptic cues. The admittance-trajectory relationship during teleoperation was studied, without haptic cues, to further improve the tuning method. Here, no statistical differences in admittance were observed between different obstacles. However, a significant effect was found for admittance variations within obstacles, and an inverse relationship was established between admittance and UAV velocity/yaw rate.