1. College of Civil Aviation, Nanjing University of Aeronautics and Astronautics (NUAA), Nanjing, People’s Republic of China
2. Geography & Earth Science Department, Shippensburg University, Shippensburg, PA, USA
3. National Key Laboratory of Air Traffic Flow Management at NUAA, Nanjing, People’s Republic of China
4. Department of Geography, Ghent University, Gent, Belgium