INVITED PAPER A Hierarchical Flight Planning Framework for Air Traffic Management This paper aims to manage high-altitude air traffic flows by incorporating each flight operator’s preferences and aircraft type while considering weather risks and airspace capacity constraints.
By Wei Zhang, Member IEEE, Maryam Kamgarpour, Student Member IEEE, Dengfeng Sun, Member IEEE,andClaireJ.Tomlin,Fellow IEEE
ABSTRACT | The continuous growth of air traffic demand, tralization approach developed in this paper would also skyrocketing fuel price, and increasing concerns on safety and provide useful insights into the design of decision and infor- environmental impact of air transportation necessitate the mation hierarchies for other large-scale infrastructure systems. modernization of the air traffic management (ATM) system in the United States. The design of such a large-scale networked KEYWORDS | Air traffic control (ATC); cyber–physical systems system that involves complex interactions among automation (CPSs); decentralized optimization; hierarchical systems; path and human operators poses new challenges for many engi- planning neering fields. This paper investigates several important facets of the future ATM system from a systems-level point of view. In particular, we develop a hierarchical decentralized decision I. INTRODUCTION architecture that can design 4-D (spaceþtime) path plans for a The air traffic management (ATM) system of the United large number of flights while satisfying weather and capacity States is a large-scale, safety-critical, cyber–physical sys- constraints of the overall system. The proposed planning tem (CPS), which involves more than 50 000 flights daily, framework respects preferences of individual flights and en- being monitored and regulated by thousands of air traffic courages information sharing among different decision makers controllers based on weather and traffic measurements in the system, and thus has a great potential to reduce traffic obtained through numerous weather stations and radar delays and weather risks while maintaining safety standards. facilities located across the entire country. The physical The framework is validated through a large-scale simulation component in the system consists of a network of fast- based on real traffic data over the entire airspace of the conti- moving aircraft, whose coordination and control rely cru- guous United States. We envision that the hierarchical decen- cially on many cyber components such as weather and traffic prediction algorithms, decision-support software, and radio communications among pilots, dispatchers in Manuscript received April 15, 2011; revised June 17, 2011; accepted September 6, 2011. Airline Operation Centers (AOCs), and air traffic Date of publication September 6, 2011; date of current version December 21, 2011. This work was supported in part by the National Science Foundation (NSF) under controllers. Grant CNS-0931843, by the Air Force Office of Scientific Research (AFOSR) under Since its birth in 1920s, the ATM system has gradually Grant FA9550-06-1-0312, and by a National Science and Engineering Research Council of Canada (NSERC) fellowship. evolved from its primitive form that consisted of a set of W. Zhang and C. J. Tomlin are with the Department of Electrical Engineering and simple operation rules to its current version that is a com- Computer Sciences, University of California at Berkeley, Berkeley, CA 94720 USA (e-mail: [email protected]; [email protected]). plex network of sensing, communication, and control sub- M. Kamgarpour is with the Department of Mechanical Engineering, University of systems. Although various automation systems have been California at Berkeley, Berkeley, CA 94720 USA (e-mail: [email protected]). D. Sun is with the School of Aeronautics and Astronautics, Purdue University, West continuously introduced, the backbone of the current Lafayette, IN 47907 USA (e-mail: [email protected]). systemwasformedduringthe1950swhentheintro- Digital Object Identifier: 10.1109/JPROC.2011.2161243 duction of radar surveillance and radio communication
0018-9219/$26.00 2011 IEEE Vol. 100, No. 1, January 2012 | Proceedings of the IEEE 179 Zhang et al.: A Hierarchical Flight Planning Framework for Air Traffic Management
technologies revolutionized the way the system was B. Next Generation of ATM as a operating [1]. After more than half a century, another Cyber–Physical System major evolution of the ATM system to incorporate modern Clearly, the current air transportation system is sensing and information technologies is currently under- approaching its capacity and safety limits. To resolve its way [2] in order to address the growing concerns about its existing issues as well as to be able to support the rapid operational and energy efficiency, environmental impacts, growth of traffic demand, we need to automate the system and safety. at a faster pace. It is a common vision shared by the Federal Aviation Administration (FAA), air traffic controllers, and A. Notable Issues in Current ATM airline companies that the efficiency, environmental, and Air traffic demand in the past 20 years has grown by safety concerns of the current ATM system can be over 64%, while human traffic controllers and airspace considerably alleviated by properly incorporating modern resources such as airports and runways have not kept up sensing and information technologies to enable reliable with this growth rate [3]. It has been estimated that communications, real-time common situation awareness, domestic air traffic delays in 2007 cost the U.S. economy and prompt safety-guaranteed decision supports. Such a about $41 billion, including more than $19 billion in direct vision clearly coincides with the CPS viewpoint of modern operating costs [4]. The delays also contributed to about engineering systems, and is currently being implemented 740 million extra gallons of jet fuel, and an additional through the concept of Next Generation Air Transpor- emission of about 7.1 million tons of carbon dioxide. The tation System (NextGen) in the United States [2]. The situation will be further aggravated by the expected two- to NextGen concept advocates for an evolution from the cur- threefold increase in air traffic demand over the next two rent ground-based navigation system to a satellite-based decades [5]. Meanwhile, the need to constrain the rapid ATM system, where verbal communications and ground growth of aviation’s impact on the global climate is be- radar systems are replaced with more reliable and accurate coming increasingly clear. Global carbon dioxide emis- data-link communications and global positioning systems sions from aircraft grew about 45% between 1992 and (GPSs), so that many traffic control tasks can be handled 2005. It has also been forecasted that aviation emissions (semi)automatically. In addition, the increased automation will increase an additional 150% above the 2006 level by in conflict detection and resolution would facilitate 4-D 2036 [6], [7]. (space-time) trajectory-based operations (TBOs), in which Aside from being strained by the current level of individual flights would have freedom to adjust their tra- demand, air transportation safety has recently been jectories according to real-time traffic and weather condi- compromised to a level requiring immediate attention. It tions, rather than having to follow fixed nominal flight was reported that the number of certified air traffic plans as in the current clearance-based operations [9]. controllers in 2008 reached the lowest level in 15 years, The implementation of NextGen not only calls for an causing many major air traffic control (ATC) facilities to integrated engineering effort, but also poses new chal- declare staffing emergencies. The lack of enough air traffic lenges in many research disciplines as listed in Fig. 1, controllers has caused an upsurge of operational errors, including transportation engineering, human factors, many of which could have turned into major accidents. For communications, sensor networks, cyber security, opera- example, the plane crash at Lexington, KY, on August 27, tion research, and machine learning [2]. Information 2006, was partly due to the fatigue of the sole tower sharing has already played an important role in the current controller causing him to not respond promptly to the system. For example, many pilots report the actual weather incorrect runway use. As a more recent example, the sole conditions experienced in flight to air traffic controllers tower controller in the Ronald Reagan Washington through verbal communications. These reports are being National Airport (DCA), who was on his fourth straight updated continuously (see Fig. 2 for a snapshot of these overnight shift on March 23, 2011, fell asleep, forcing two reports) and used heavily for making aviation decisions commercial aircraft to land without assistance. [10]. With the vision of NextGen, one can imagine that the The efficiency and safety issues make the system even future air transportation system will soon evolve from this more vulnerable to weather disruptions. It was found that verbal information sharing platform to an automatic hazardous weather such as storms or high winds Bsensing and action web[ in the sky, in which onboard accounted for more than 70% of the air traffic delays in sensed data are disseminated through secure communica- 2007 [4] and about 30% of all aviation accidents [8]. tions, and are processed and analyzed in real time to Although various weather products with increasing provide reliable decision support for ATM. accuracy and resolution are continuously being developed, It is also important to notice that the modernization of their use in ATM is still in its infancy and depends heavily this kind of legacy and safety-critical system will have to on the experience of human controllers. A more efficient take many intermediate steps. Any proposed solution set and systematic way to utilize weather forecast to support for NextGen must respect the way the system is currently operation planning and traffic control requires substantial operating, and be able to work with both old and new research efforts. frameworks and technologies.
180 Proceedings of the IEEE |Vol.100,No.1,January2012 Zhang et al.: A Hierarchical Flight Planning Framework for Air Traffic Management
Fig. 1. Research areas that support the next generation air transportation system.
C. From Centralized to Hierarchical ential equation (PDE) models [12]–[14] that explicitly Decentralized Planning consider the spatial–temporal evolution of the traffic flow Among the various visions of NextGen, this work in- dynamics. On the other hand, numerous existing weather vestigates the en-route flight planning problem of the products can be used (mostly manually) to identify ha- future ATM system. The problem is concerned with mod- zardous regions or quantify capacity drops over the af- ifying,adjusting,orevenfullydesigningscheduledflight fected parts of the airspace [15], [16]. With these traffic plans (represented by high-altitude cruise waypoints) to and weather outlooks, the traffic management decisions meet en-route airspace capacity constraints and weather are often made with assistance from centralized optimi- restrictions. The planning decisions rely crucially upon zation algorithms subject to appropriate constraints. two classes of information: traffic prediction and weather Many centralized traffic management methods suffer forecast. The highly regulated nature of the air transpor- from complexity and scalability issues. They are often only tation system enables reliable predictions for the future capable to determine ground delays [17]–[19] rather than traffic distributions based on the approved flight path designing the entire flight path. As such, modifications to plans. The predictions can be facilitated by partial differ- flight plans to better utilize airspace resources are not explored. In addition, a centralized approach in general assumes a universal cost metric such as the total delay of all flights, which ignores the operation preferences of indi- vidual flights. For example, a large commercial flight with many passengers onboard may decide to take a relatively long detour to reduce departure delays as much as possible, while a small flight or a private plane would rather delay the departure until a shorter path becomes available. Moreover, it is difficult for a centralized traffic plan- ning approach to explicitly consider different aircraft limi- tations in handling hazardous weathers. A typical idea shared by most weather-aware flight planning strategies [20]–[23] is to extract hazardous weather regions from certain weather products and treat these regions as com- mon obstacles to be avoided by all flights during the plan- ning process. However, in reality, the effects of weather on
Fig. 2. Pilot reports of actual turbulence conditions. The various different flights could be substantially different [24]. For symbols indicate turbulence severity and frequency and are example, light or moderate icing conditions are important described in [11]. for intermediate-size aircraft with reciprocating engine
Vol. 100, No. 1, January 2012 | Proceedings of the IEEE 181 Zhang et al.: A Hierarchical Flight Planning Framework for Air Traffic Management
and straight wings, but will in general not affect large corresponding timestamps, instead of just computing the commercial aircraft. Moreover, even with the same type of ground delays. This is certainly in line with the TBO aircraft, experienced pilots may decide to fly through initiative in NextGen. regions with adverse weather conditions while inexperi- The rest of the paper proceeds as follows. The general enced ones would not. 4-D flight planning problem is formulated in Section II. A As the demand continuously increases, the need for a decentralized solution to this problem is developed in shift from the current centralized ATM system to a more Section III and its advantages and practical implications distributed traffic management architecture becomes more are carefully discussed in Section IV. Simulation results apparent. To enable such a shift, many research agendas based on real air traffic data of the United States is have been proposed. For example, the traffic flow man- presented in Section V and some concluding remarks are agement problem was formulated as a multiplayer game giveninSectionVI. played by airline companies in [25], and a market mecha- nism was then designed with a provable convergence to an equilibrium depending on the cost metrics of the airline II. 4-D FLIGHT PLANNING PROBLEM companies. Alternatively, the concept of credit points was introduced in [26] to specify flight priorities, which allows A. Background the ATM system to assign delays to flights that are rela- The en-route airspace of the United States is covered by tively less important to an airline. Although both ap- a network of airways as shown in Fig. 3, which were proaches incorporated airlines’ preferences on existing historically designed to connect ground-based navigational paths, optimizing individual path plans to further improve aids, such as radio beacons so that pilots could easily check the overall performance was not considered. where they are. With technological advances, the pilot may The increased information exchange in NextGen would request to fly directly over geographical waypoints, which provide numerous opportunities to improve efficiency and are points in airspace identified only by their latitude, address individual preferences. To design this future ATM longitude, and altitude, while reporting their locations system, a critical step is to determine how much responsi- periodically to the air traffic controllers. To simplify traffic bility should be distributed to the users1 and how to control tasks and divide responsibility, the en-route achieve such a transition in a reliable way. This paper airspace is divided geographically into 22 Air Route Traffic presents one set of preliminary results towards a better Control Centers (hereby referred to as Centers), and each understanding of these important questions. Our main Center is further divided into approximately 20 en-route contribution is the development of a hierarchical decision sectors. The maximum number of aircraft allowed in a and information architecture for large-scale en-route traf- given sector is referred to as sector capacity, which de- fic planning, which fully respects preferences of individual pendsontheweatherconditionsandthenumberofhuman flights and systematically considers both weather risks and controllers. For high-level path planning purpose, it is en-route capacity constraints. With the proposed architec- often assumed that once the sector capacity constraints are ture, the overall functionality of the ATM system is de- satisfied, local separation of aircraft can be accomplished composed into two interactive decision layers: traffic by the human controllers in the corresponding sector. regulation and performance optimization. The regulation layer is responsible for computing traffic and weather pre- B. Graph Model of En-Route Airspace dictions and setting up traffic regulation rules based on We consider a flight path planning problem over a these predictions, while the optimization layer optimizes bounded and connected subregion X R3 of the en-route the cost functions of individual flights subject to the regu- airspace, which represents either a Center, a collection of lation rules imposed by the regulation layer. Through this Centers, or the entire en-route airspace of the United hierarchical decomposition, the performance optimization States. The airway structure within X is described as a task can be accomplished in a fully decentralized way and directed graph G¼ðV; EÞ. Each node on the graph vi 2V can be distributed to individual users without violating represents a waypoint with a given latitude, longitude, and safety constraints. This gives each user full freedom to altitude, while each link linkðvi; vjÞ2E represents a make its preferred decisions subject to traffic regulations, directed airway, also referred to as jet route at high which may greatly improve its operational efficiency and altitudes, from waypoint vi to vj. The waypoints we consider passenger satisfaction. In addition, the proposed flight could represent both locations of navigational aids as well planning framework can design the entire 4-D flight path as geographical (virtual) waypoints with temporary posi- plans, represented by sequences of waypoints and the tions introduced to assist flight planning or monitoring. Suppose that the airspace region X consists of ns non- f g I ¼f; ...; g overlapping sectors Sm m ns .Denoteby S 1 ns 1In the rest of this paper, the term Buser[ is used interchangeably the index set for the sectors. Let : V!IS be the with Bflight,[ which refers to the planning decision maker for a flight, including the pilot in command as well as the flight dispatcher at the function that assigns each node on the graph to its corresponding airline operation center. corresponding sector, i.e., for any v 2V, ðvÞ¼m if and
182 Proceedings of the IEEE | Vol. 100, No. 1, January 2012 Zhang et al.: A Hierarchical Flight Planning Framework for Air Traffic Management
Fig. 3. Existing navigational aids, represented by solid circles, and a subset of the airways, represented by line segments connecting the solid circles, over the contiguous United States. Here the background polygons represent high-altitude ATC sectors. Data courtesy: Prof. D. Andrisani, School of Aeronautics and Astronautics, Purdue University.