1. Introduction
Worldwide Air Traffic Management (ATM) system is undergoing the process of upgrading and transformation, in order to cope with increasing air traffic demand and congestion, as well as diverse expectations of stakeholders for safety, efficiency, economy, and pro-environment, especially in high-density airports and terminal airspace [
1]. Trajectory-Based Operation (TBO) and Performance-Based Operation (PBO) have been identified as the core concept of the future ATM system [
2].
Continuous Descent Operation (CDO), one of the key elements in TBO and PBO, has been recognized as an effective procedure that may improve operation efficiency and environmental benefits in terminal airspace, which was initially designed to abate noise [
3]. A series of CDO flight trials validated that CDO could significantly reduce fuel burn and noise impact during arrival phase by keeping arriving aircraft at their cruising altitude for longer and then executing continuous descent with no level-flight segments [
4]. In 2010, the International Civil Aviation Organization (ICAO) published
Doc 9931 Continuous Descent Operation Manual, which provides guidelines for CDO procedure design [
5]. At present, CDO has become one of the building blocks for Global Air Navigation Plan (GANP), Single European Sky ATM Research (SESAR), and Next Generation Air Transportation System (NextGen).
Previous studies demonstrated that some factors, like vertical profile [
6], speed profile [
6], flight path angle [
7], meteorology [
8], sequencing and flight scheduling [
9], and capacity [
10] have great effects on the performance of CDO. In view of such factors, pre-tactical CDO trajectory optimization with various constraints and objectives in single-aircraft or multi-aircraft scenarios attracted great attention [
11,
12]. In most of the literature, the objectives of trajectory optimization include fuel consumption [
13], flight time [
14], emission [
15] and noise impact [
16], etc. A few scholars also assessed trajectories based on optimization in terms of safety, efficiency, airspace capacity, and ecological compatibility [
14,
17]. Samà et al. optimized CDO trajectories in a busy Terminal Maneuvering Area (TMA) in terms of flight time and fuel consumption using lexicographic method to make a decision among alternative approaches, by presetting the primary and secondary performance indicator [
14]. It was essentially a greedy strategy with subjectively prioritizing the importance of objectives. In addition, real-time CDO trajectory control strategies have been investigated in the presence of abnormal operation deviating from the planned path [
18], as well as adverse meteorological factors, such as storm [
19] and gust [
20].
With the deepening of studies, it suggested that traditional Standard Terminal Arrival Route (STAR) is too rigid to exploit the potential of the CDO benefits. Alam et al. established a new type of transition airspace to generate dynamic CDO trajectories in order to achieve lateral optimization [
21]. However, it is difficult to execute path recovery once the approach aborted. Furthermore, in Reference [
21], the aircraft has to adjust headings continuously in low-altitude area, which would increase the operational complexity for both controllers and pilots. From the practical perspective, it is suggested that, without advanced Air Traffic Control (ATC) automations or flexible flight procedures, 4D CDO trajectory operation that mostly relies on Air Traffic Controller’s mental cognition could only be applied in some less busy airports or off-peak hours [
12].
The Point Merge System (PMS) is a promising procedure to improve the performance of CDO, which was proposed by the EUROCONTROL Experimental Center (EEC) [
22]. It comprises sequencing legs and a merge point, as shown in
Figure 1. The PMS enables flights maintain altitude and fly along the sequencing legs, and then descend and fly to the merge point once a “direct-to” instruction is issued. The landing sequence and the longitudinal inter-aircraft separations are controlled during operation. Research has validated that the Point Merge procedure possesses higher flexibility and lateral predictability, which brings benefits to operator’s workload, arrival efficiency [
22], and environmental impact [
23]. Nevertheless, the delay absorption ability of the sequencing legs in the PMS is constrained by their limited length. Once the sequencing legs became saturated or in some emergent situations, inbound flow should be vectored off from the PMS. Such abnormal maneuver would significantly increase the traffic complexity, as well as degrade operation safety and efficiency [
24]. Therefore, designing a new CDO airspace system and automated 4D conflict-free trajectory planning, would be a practical breakthrough to improve the flexibility, controllability, and predictability of CDO operation.
In this paper, inspired by PMS, we propose a novel terminal airspace structure named Inverted Crown-Shaped Arrival Airspace (ICSAA) that supports Omni-directional arrival. Based on such an airspace system, the conflict-free, economical, and efficient CDO trajectories are generated and assigned to each aircraft automatically using Multi-Objective Decision-Making(MODM) methods. Simulation experiments validate overall performance of optimized trajectories within the ICSAA in terms of collision probability, fuel consumption, and trip time. The main work of this paper are illustrated as follows:
The design of ICSAA with intuitional operation modes: In order to increase the elasticity of traditional STAR, and the capacity of sequencing legs in the PMS, a flexible terminal airspace called ICSAA was designed to merge traffic streams from all the directions and accommodate more aircraft using circular design. Based on ICSAA, two concise and intuitive operation modes: Mode H and Mode V, were designed to ensure the predictability, safety, and transparency of automated CDO operations.
Pareto optimal front generation for Conflict-free CDO trajectories in the ICSAA: Inside the ICSAA, a multi-objective optimization model that aims at generating conflict-free CDO trajectories with minimal fuel consumption and trip time is proposed by simulating flight dynamics. By comparing with some state-of-the-art multi-objective algorithms, the Non-dominated Sorting Genetic Algorithm with Elitist Strategy (NSGA-II) shows its best performance in convergence near the true Pareto optimal front efficiently, thus is selected to solve the proposed optimization problem.
Multi-attribute decision-making for optimal CDO trajectories selection: In order to select the optimal trajectories among the Pareto optimal front, from the perspective of both pilots’ and controllers’ preferences, we additionally take collision risk as an attribute besides fuel consumption and trip time. The entropy-based TOPSIS method is adopted to select an unique solution by multi-attribute decision-making strategies. To validate the proposed model and algorithm, the overall performance in single-aircraft, low-density and high-density scenarios are investigated. The results verified that proposed automated CDO trajectory planning in the ICSAA are of high efficiency and strike a better trade-off in terms of economy, efficiency, and safety.
The remainder of this paper is organized as follows: We first illustrate the structure and operational procedures of the ICSAA in
Section 2; then, the descriptions are made to explain the detailed trajectory planning problems to be solved in the novel airspace in
Section 3. In order to derive optimization objectives, the aircraft performance model and fuel consumption model are established in
Section 4 and
Section 5, respectively, followed by a multi-objective optimization model in
Section 6 and multi-attribute decision-making strategies in
Section 7.
Section 8 shows the numerical experiments of verifying the effectiveness of proposed methods in different scenarios. Conclusion and the future work are discussed in
Section 9.
2. The Novel CDO Airspace Design
2.1. Basic Structure of the Inverted Crown-Shaped Arrival Airspace
According to Doc 9931 Continuous Descent Operation Manual, CDO is an aircraft operating technique aided by appropriate airspace and procedure design and reliable ATC clearances enabling the execution of an optimized trajectories with low engine thrust settings and, where possible, a low drag configuration, thereby reducing fuel burn and emissions during descent. Based on that ideology and enlightened by PMS, we proposed a novel flexible terminal airspace structure and operation procedure, named the ICSAA.
In the ICSAA, we still instruct flights with “direct to” instructions instead of radar vectoring, which could reduce the pilot-ATC communication. Given that the length and number of sequencing legs impose constraints on the performance of the PMS, we extend the arc-shaped sequencing leg to a circle. As shown in
Figure 2, we define the artificial terminal area as a series of concentric sequencing rings with the merge point in the center. Each ring has preset waypoints spaced 10° apart from each other along the circumference to ensure safety separation between arriving aircraft. The artificial ICSAA has five levels of rings at different altitude. The altitude of the outermost ring is 6000 m, on which waypoints represent the entry points, while the innermost ring is at 4800 m, on which each waypoint represents the beginning of the CDO trajectory. The vertical separation between adjacent rings is 300 m. It is worth noting that, for the fifth-level ring, there are three layers at the same altitude. Departing from different waypoints on different layers, aircraft could fly to the merge point with different flight path angles ranged from 3° to 4°. In addition, to assure a smooth and efficient interception to the glide slope, the height of the merge point and the central angle of the fan-shaped envelope is set as 900 m and 120°, respectively. For the sake of simplicity, we numbered the waypoints along each ring, as shown in
Figure 2b. Please note that, at the fifth level, the waypoints on the outermost layer, the second layer, and the third layer are numbered from 1 to 13, 14–26, and 27–39, respectively.
2.2. The Operational Procedure
Based on flight dynamics of aircraft in the air, two switchable operational modes are defined in the ICSAA: Mode H and Mode V, a multiphase mixed-integer optimal control approach.
Definition 1 (Mode H). The aircraft is considered as flying into a horizontal plane in a clockwise direction (or anti-clockwise direction). Thus, flight path angle γ, the derivative of flight path angle and the derivative of height are set to 0. In this mode, the equation also holds: , where L is lift; ϕ is bank angle; m is the mass of aircraft, and g is the gravitational acceleration.
Definition 2 (Mode V). The aircraft is considered as flying into the vertical plane, performing a leveled-wing descent. Therefore, flight path angle γ, the derivative of flight path angle and the derivative of height are not equal to 0, while the bank angle ϕ is set to 0.
Furthermore, to assure trajectory predictability and operation order, aircraft should descend level by level and cannot go through the interior of ICSAA. In other words, the aircraft must always fly on the wall of the ICSAA. Therefore, we define Separation Z. If and only if the separation Z is less than a certain value, the aircraft could descend with Mode V; otherwise, the aircraft had to fly along the circumference of rings in a clockwise direction (or anti-clockwise direction) with Mode H.
Definition 3 (Separation Z). Suppose that the present position of aircraft is at the waypoint , for any waypoint on the adjacent inner ring, which the aircraft is going to fly to, theSeparation Zis presented in Figure 2b and defined by . In the multi-aircraft scenario, the situation would be more complex. Not only should Separation Z be less than the certain value but also no potential conflicts exist during descending, when the aircraft could start to fly to the next inner ring. Otherwise, it would fly with Mode H until meeting descent conditions. Obviously, when flying along the circumference of rings, aircraft should keep the safety separation with preceding aircraft by adjusting speed.
Compared with traditional PMS, thanks to the circular design, the ICSAA could integrate inbound flow from all directions, and the sequencing legs (i.e., the rings of ICSAA) would accommodate more aircraft. The proposed intuitive flight modes also possess positive potentials in reducing operational complexity while simplifying automated conflict-free trajectory planning. To some extent, this design also renders more flexible segregation of the inbound and outbound flow. Furthermore, the fifth-level rings provide more continuous descending options for aircraft with different flight path angles, which would significantly affect the fuel consumption and trip time. Therefore, the proposed ICSAA has higher structural and procedural flexibility that would provide larger solution space for CDO trajectory optimization.
3. Problem Definition
In order to search optimal CDO trajectories, the airspace is modeled as a network , and each vertex represents waypoint of ICSAA. The adjacency matrix is constructed, of which the element, , represents connectivity of the directed network, in accordance with the operation rules of ICSAA mentioned above. If the edge, , is connected, ; otherwise, . Therefore, the path of the aircraft is expressed as a set of connected vertices in network G. The problem we studied can be stated as follows: based on the expected entry points and expected arrival time of aircraft, we would generate optimal CDO trajectories automatically in the ICSAA, aiming at achieving best performance in economy, efficiency, and safety on the promises of acceptable computation time, to support the pre-tactic operation of air traffic management.
4. Aircraft Performance Model
Aircraft performance model is used to control the flight dynamics in accordance with the flight modes in the ICSAA. Glover and Lygeros [
25] came up with the Point Mass Model (PMM), derived from the Newtonian dynamics. EUROCONTROL [
26] proposed the Total-Energy Model (TEM) in Base of Aircraft Data (BADA), derived from the work-energy theorem for continuous descent. In this research, in order to simplify the trajectory optimization problem, a Point-Mass performance model, adapted from the work mentioned above, is adopted to simulate the CDO flight while measuring the fuel consumption and trip time. The model can be stated as follows:
where the dotted terms are derivatives with respect to time;
x,
y, and
h represent the position of aircraft in the three-dimension coordinate system;
v is true airspeed;
is bank angle;
is attack angle;
is flight path angle;
is course angle;
is wind speed;
T is engine thrust;
D and
L is the aerodynamic drag and lift, which are modeled, respectively, as:
where
is air density;
S is the wing reference area; and
and
are the drag coefficient and the lift coefficient, respectively, modeled as:
where
and
are BADA coefficients. The drag coefficient
is expressed as a function of the lift coefficient
.
When simulating the aircraft descent, we use the standard speed profile recommended by BADA and perform curve fitting for those data, as shown in
Figure 3. The goodness of curve fitting
= 0.9820. The result is stated as follows:
In the above expressions, velocity is the function of height. In order to obtain the derivative of velocity versus time, we made the following deformations.
It is worth noting that, in order to improve the accuracy of the flight descent path modeling, some atmospheric properties (temperature, pressure, density) are expressed as a function of altitude, based on the BADA reversion 3.11 issued in 2013, rather than fixed values [
27].
where
R is real gas constant for air, the value of which is 287.05287 m
2/(K · s
2);
is the standard atmospheric temperature at mean sea level, of which the value is 288.15 K;
is temperature differential at mean sea level, which is the difference in atmospheric temperature at mean sea level between a given non-standard atmosphere and International Standard Atmosphere (ISA), and the value is set to 0;
is ISA temperature gradient with altitude below the tropopause, the value of which is −0.0065 K/m;
is standard atmospheric pressure at mean sea level; and
g is gravitational acceleration, the value of which is 9.80665 m/s
2.
Based on the aircraft performance model (those equations are denoted as
M), once the starting point
, ending point
, flight path
, and initial speed
are determined, the real-time position of aircraft
at time
t could be obtained following speed-altitude profile recommended by BADA, which can be expressed by the Equation (7). Such an equation enables the conflict detection of CDO trajectories.
9. Conclusions and Future Work
Improving flexibility of approach airspace is one of cornerstone for implementing CDO especially in high-density traffic scenarios. Inspired by traditional PMS, this paper broke through the traditional rigid terminal airspace structure by designing ICSAA and its operational procedures to deliver Omni-directional CDO. A multi-objective trajectory optimization model aiming at minimizing fuel consumption and trip time was solved by NSGA-II. In order to provide decision-making support to air traffic controller, the best trade-off was determined using entropy-based TOPSIS from the perspective of safety, efficiency and economy.
The proposed models and algorithms were validated in the single aircraft scenario, the multi-aircraft scenarios with simultaneous and continuous arrivals to gain insights into the interplay of traffic demand and given objectives. The results verified the effectiveness and efficiency of optimal CDO operation in ICSAA dealing with complex traffic patterns, which demonstrates the potential of CDO operation in high-density terminal area and enables the application of TBO.
This paper is a preliminary exploration of CDO trajectory planning in the novel airspace. Further research shall be carried out to solve the limitations and to facilitate the transition from theoretical studies to real-world practice.
The ICSAA was designed based on an artificial airspace irrespective to terrain, obstacles, noise abatement, airspace restriction and runway configurations, etc. The very essential work is to adapt the ICSAA to a real airport and thoroughly validate the effectiveness by both fast-time simulations and Human-In-The-Loop (HITL) experiments.
Aircraft positioning error was considered in trajectory selection in this paper. However, more realistic uncertain factors, like flight control error, meteorological conditions, stochastic runway scheduling, etc., shall also be included in robust CDO trajectory planning and implementation.
Human-machine system is another emerging topic in future autonomous ATM. The decision-making by machine shall be transparent and understandable, which allows air traffic controllers and pilots to take over automated equipment at any time safely. It means the cognition of human and machine needs to be strongly synchronized [
42]. For automated CDO trajectory planning and negotiation in flexible airspace, designing transparent Decision Support System (DSS) based on explainable Artificial Intelligence(AI) would be an indispensable approach to enhance human cognition, operational efficiency, and safety, especially for a high-density traffic scenario.