## 2 Related Work
### Academic Community  
The U.S. Government Accountability Office (GAO) concluded that the U.S. ATM system has significant security control vulnerabilities that threaten the safe and uninterrupted operation of the National Airspace System (NAS) [11]. The GAO recommended measures such as system boundary protection, encryption of sensitive data, and implementation of access controls to reduce security risks due to security flaws and system vulnerabilities. Boeing presented its views on a framework towards improving ATM critical infrastructure cybersecurity for the future of networked ATM [12]. Boeing believes that various U.S. aviation companies and departments should collaborate to share cyber threat information and best practices to develop and deploy a more secure information infrastructure. Furthermore, it can prevent, detect, and defend cyber attacks, and recover quickly from disasters caused by attacks. Professor [PERSON] et al. [13] from Embry-Riddle Aeronautical University studied the High-Level Safety Risk Assessment (HSRA) methodology for the NextGen program. Their study showed that there is a high security risk in data exchange due to the lack of strong information security measures in the ground-air data link system and aircraft access System-Wide Information Management (SWIM). A study by [PERSON] [14], a U.S. aviation strategist, showed not only the security risks of airborne networks but also the access and frequent data exchange between airborne networks and the separate resource system devices in ATM, creating inherent security vulnerabilities in ATM. He emphasized that the increasingly tight interconnectivity between air vehiclesand ATM makes them vulnerable to unauthorized remote access attacks. He concluded with recommendations for protecting civil aviation networks against cyber attacks. [PERSON] [15] from the Flight Instruction Institute of the German Aerospace Center considered the ATM as a whole and divided the CNS and automation systems into separate subsystems for security assessment. He described a possible methodology for security prototype validation and aimed to enhance the security of ATC voice communication system by adapting to security prototypes. In addition, [PERSON] and [PERSON] [16] from this institution evaluated several new systems involving several security prototypes developed in the European Aviation Security Research Program. The specific validation efforts of the selected methods were detailed for complex ATM with wide area distribution. Their research was funded by the EU Seventh Framework FP7 program project. [PERSON] et al. [17] from RMIT University studied the security risks faced by aviation assets and analyzed the security vulnerabilities in the aviation system. Aimed at six key aviation systems, such as CNS/ATM system, avionics system and airport management system, they used analytic hierarchy process and fuzzy logic system to analyze the impact of eight important attack types on the system and assess the system security risk. By improving the consistency and accuracy of risk attributes, the risks faced by the system can be better managed. [PERSON] from Boeing and [PERSON] from University of Washington [2; 18; 19] proposed a novel CPS framework for aviation systems applicable to aircraft, aviation users, airports, and ATM and analyzed its challenges and solutions. Their research showed that data communications and networks that guarantee the safe flight of air vehicles were key enablers for civil air transportation systems to meet the aviation needs in the next 20 years and beyond, and they analyzed the security threats and challenges faced by aviation communication networks and the open problems in security authentication in aviation communication networks. Professor [PERSON] et al. [20] from Coastal Carolina University proposed a framework for vulnerability assessment of aviation CPS, which was used to assess and prevent security threats and vulnerabilities in the computer system and network of aviation CPS.