Different approaches to UAS Traffic flow control for mass scale Urban air transportation.
Estimating characteristics of the future urban air mobility and performance of airspace structures
The post here is a monologue for a system-centric approach depicting intensive analysis of airspace utilization for Air-taxi’s. The approach is based on numerical traffic simulations with a discernible model of Air-taxi’s for estimating characteristics of the future air traffic in urban areas and the consummation of airspace structures. A concept of Urban air mobility traffic flow control (UAMTFC) is proposed with various airspace structural designs of different levels of freedom in flight.
Advancement for unmanned aerial systems (UAS) in the last decade opened up the flood gates for operations beyond line-of-sight and integration into existing airspaces with augmented autonomy. This unfurled capability and wide-range applicability of UAS is drawing attention to become commercially feasible especially, the public and commercial needs of the Air-taxi operation in urban areas being increasingly raised. However, there exists a high degree of complexity: Air-taxi’s will operate at a low altitude above/below an uneven skyline in an area with high population density and vigorous ground activities. Ground assets, static obstacles, dynamic obstacles, unauthorized UASs, wind gust, and malicious activities (e.g. highjack, jamming, interception) are difficult factors for the Air-taxi operations in dense urban valleys.
In a broad view, two approaches will be presented: vehicle-centric and Air-taxi traffic management (UTM) system-centric approaches. The vehicle-centric approach includes developing on-board components, such as processors, power systems, and sensors, and enhancing the autonomy level of UASs (e.g. decision making, path/mission planning, etc.). System-level centric approach includes development for UTM problems: system designs, airspace designs, traffic control/management, navigation/surveillance infrastructures, and communication networks.
The post here presents a microscopic traffic model for numerical simulations in a lane-based airspace structure among the designs. The traffic model is devised with reasonable assumptions on an Air-taxi operating environment and vehicle motions and is comprised of a separation rule, a vehicle dynamics model, and a behavioral model of vehicles.
The UTFC framework has following businesses to take care of:
- Monitor the thickness of the traffic, throughput, and section-time;
- Supervise/control directional progressions of traffic;
- Broadcast traffic circumstances;
- Detect/track unapproved flights and caution other vehicles.
The criteria for the UTFC framework are as follows: confirmation of an adequate degree of safety; minimization of infra cost, system complexity and environment (noise, air pollution, sway on nearby eco-system), UAS execution prerequisites, UTFC scope/obligations and framework necessities, and load for taking care of traffic stream control problems.
The airspace in the urban area is divided into several layers by altitude and sectionalized according to a skyline of buildings. The use of airspaces below and above the skyline is determined by aircraft’s maneuverability, speed, and navigation capability. The flight above the skyline is allowed for high-speed Air-taxi’s equipped with navigation devices providing limited usability between high buildings. With greater speed, the vehicle must be required to be well-equipped with long-range sensors and have better detachment affirmation and collision evasion abilities that can slow down to stop and hover arbitrarily can fly below the skyline, as long as functioning navigation systems are provided. Because of restricted spaces between obstacles, more rules are required for the pattern and trajectory of the flight below the skyline than for the ones above the skyline.
This section presents three kinds of design concepts in airspace use:
Skylane, Sky-paths, and Sky-corridors frameworks.
An extension of a conventional road system into each layer to form a road system in the air with lanes. A lane is a rectangular bar-shaped space extruded from a square that confines the height and width of flight trajectories. Vehicles should follow reference lines placed along the center of lanes in nominal cases, except for changing lanes, turning or escaping in case of emergency, and strongly be requested not to deviate from the lane. Each vehicle in the lane is also responsible for separation assurance between the vehicles ahead/behind of it and collision avoidance of any kind of obstacle. In a layer, a one-/two-way group of parallel lanes is placed at a level height and constitutes a road strip. Vehicles can change their lanes laterally, and migrate to other layers or crossing strips via left-/right-turns, which are either controlled or free depending on variants of the design concept.
The design variants of sky-lanes are classified according to lane arrangements and configuration of intersections/interchanges. In the first variant (A1)- Figure 2, strips of intersecting roads share the same layer and thus the lanes of the strips are fused at the intersection. The traffic control service is provided via radio communication by UTFC system infrastructure built at the intersection.
The second (A2) and third (A3) design variants use exclusive stacking of crossed strips, which means no strips above a street intersection share the same layer (see Figures 3 and 4 ABOVE). Therefore, only the half number of strips can be placed than design A1 in the same amount of airspace. Although the utilization of airspace is worsened in the two latter design variants, straight flight along a lane is secured at an interchange, and thus the throughput of the system can be enhanced.
In both designs A2 and A3, right turns can be allowed by a ramp from the rightmost lane in each strip. The lane directions of different layers are coherent in design A2 (Figure 3), whereas the directions of lanes in neighboring layers alternate in design A3 (Figure 4). Since design A2 imitates a conventional road system into multiple layers in the air, there are reverse-way lanes before entering a crossed lane by using a left turn. Therefore, any non-stop left turns to merge with the traffic stream of the crossed strip are highly limited even though there exist vertical spaces above and below the lane. The left turns in design A2 are only allowed with carefully chosen routes or ordinarily controlled at leftmost lanes as in design A1. On the other hand, smooth paths of a left turn into a crossed lane in the lower/upper layer are not obstructed in design A3, since the lanes are placed in an alternating manner (Figure 4). Thus, no active traffic control system is required in design A3 and all-way traffic streams naturally flow down, which minimizes traffic delay.
The second airspace design concept is called sky-tubes. Unlike the sky-lanes, there is no reference line to follow and vehicles can more freely move inside each tube, but should correspond to the traffic direction of the tube. For example, a vehicle can have a lateral speed component but the velocity vector should not deviate greater than a certain angle from the tube’s direction. The size of the tubes can be adjusted for adaptive traffic regulation in case of an unbalance of two-way traffic.
As in the sky-lane system, right turns are allowed if ramps can be placed at the corners of an intersection. However, since a tube is a more bulky airspace structure than a lane and the traffic flow in the tube is not strictly organized, ramps for left turns are not utilized to avoid congestion at the exit to the ramps. Instead, tubes above a street intersection overlap at the same layer and fused at the intersection (see Figure 5). The sky-tube concept has two design variants in the configuration of intersections. The first one (B1) requires a traffic control system at every intersection, and the flight into the fused crossing is totally determined by a UTFC system (Figure 5 (left)). The other design variant (B2) controls the traffic by assigning transition waypoints and time windows of vehicles passing through the boundaries of a fused intersection (Figure 5 (right)). In both designs, separation assurance and collision avoidance among the traffic of more relaxed flights are obligatory for vehicles in either case of tubes and fused intersections.
The third design concept presented in this paper is sky-corridors. The airspace between buildings and above the lower altitude limit along a street is integrated into a bulk space called a corridor. A single corridor may be placed at each layer or it may collectively cover multiple layers below a skyline. The flight inside a corridor is totally managed by each vehicle and vehicles can freely move inside the corridor in any directions as long as safety is secured. Therefore, vertical separation is not guaranteed by layers anymore, rather it is ensured by vehicles themselves.
The corridors basically do not overlap above a street intersection: if the width of one street is larger than the other one and then the corridor above the wider street occupies the intersection area; or for similar-sized intersecting streets, one corridor goes down to the other one and they do not share the same space. The transition of vehicles between two corridors occurs at the contact surface of the corridors. Unlike the sky-tube system, a UTFC system does not provide waypoints and time windows, but each vehicle decides its transition plan. The role of the UTFC system for the corridor transition is to determine approval/dismissal of the vehicle’s plan and maybe to adjust time windows to regulate traffic.
Airspace structures have their own pros and cons. A suggestive measure to necessitate a universal solution would be dependent on the complexity of traffic flows in an urban environment. It is an arduous task to specify various structures to specific cities as mentioned above. Low-speed Air-taxis (<150KPH) would be preferable for intra-city transportation within 30-mile radius and hence would be an excellent option to utilize Sky-Lane concept in a bustling environment, since they fly close to the ground. High-speed Air-taxis (150 KPH>)(Tilt-wing, Ducted) would be preferable, to be argued for both, intra and inter-city operations, utilizing the Sky-tubes and sky-corridors concepts in addition to Sky-lanes, since they are capable of flying great heights above the ground level.