## 3 System Architecture and Methodology
### System Architecture & Physical Layout  
To begin with, the digital twin-based platform was conceptualised and formulated via a local area network (LAN). Therefore, to present the system architecture effectively, a network diagram was deemed appropriate. Transmission Control Protocol (TCP) and Internet Protocol version 4 (IPv4) were applied in the transport layer to support the flow control in the LAN and the internet layer to formulate the LAN, respectively. Figure 1 shows the network diagram of the proposed system architecture for shared situational awareness and collaborative decision-making training and research. The platform was built with the hardware and software located in the Human Factors and Ergonomics Laboratory, _Department of Aeronautical and Aviation Engineering, The Hong  
Figure 1: Network diagram for demonstrating the system architecture. Kong Polytechnic University, Hong Kong SAR.The LAN framework consisted of two workstations and an integrated router with a wireless access point, as bounded by a purple rectangle in Figure 1; separate workstations were provided to the ATCOs and pilots. The workstation dedicated to the ATCOs (W-ATCOs) was a computer with Intel Core i7-10700 2.90 GHz CPU and 32 GB RAM in a Windows 10 Enterprise operating environment. It was equipped with a modified version of the _BlueSky ATC simulator_ (BlueSky) [31]. The _BlueSky ATC simulator_ is an open-sourced ATC simulator that allows modification to suit the user's need. Its open-sourced design permits the integration of pre-set scenarios, which are suitable for scenario training, as well as real-time communication with the pilots operating the flight simulator. Therefore, taking into account BlueSky's extensive scalability, we employed it to serve as the ATC simulator with a real-time ATC radar map. The modification of BlueSky for real-time flight status surveillance of the flight simulator is discussed in detail in Section 3.2. The workstation on the pilot's side consisted of two computers. Among the two computers, W-Visual was used for deploying the flight simulator software and W-Flightdeck was adopted to connect with the flight simulator hardware, as further explained in Section 3.3. The Prepar3D flight simulator software (version 3) was installed on W-Visual (specification: Intel Core i7-7700K 4.20 GHz CPU, 16 GB RAM, Windows 7 Home Premium), and a physical asset _CockpitSonic A320 FTD_ was connected to W-Flightdeck. The _CockpitSonic A320 FTD_ is full-scale simulator that can accommodate operations with two pilots, similar to that in real-life scenarios. Like W-ATCOs, a permanent private IP address at 192.168.0.130 was assigned to W-Visual to facilitate real-time data transmission. As W-Flightdeck is only responsible for coordinating the user inputs to manoeuvre the aircraft in the flight simulator, no permanent private IP address needs to be assigned. A well-developed application programming interface (API) _SimConnect_ provides a solid interface to request data from parameters in Prepar3D. Hence, it was adopted to support the linkage between W-Visual and W-Flightdeck, as well as between W-Visual and W-ATCOs. Lastly, the connection between the workstations was made wireless to facilitate the experiment's settings. In real situations, ATCOs would not be able to observe and hear the pilot's voice directly as they are physically separated. Therefore, to maintain the highest possible level of fidelity, the two workstations were not placed in the same room. Nonetheless, it is impossible to have a lengthy wired connection to formulate the LAN. Therefore, a router that served the role of a wireless access point was deployed to formulate the LAN and provide connections to the internet when necessary. Figure 2 shows the two-dimensional physical layout of the digital twin-based platform from the top view. Being placed in two separate rooms, it was infeasible to have wired connections between the workstations.