## 1 Introduction  
In the last few decades, remotely piloted aircraft systems (RPAS) have emerged as relevant and increasingly diffused new actors in air traffic, covering flights that range from very low-level operations to higher flight levels. The first appearance of RPAS was in the military framework due to the inherent advantages deriving from the use of unmanned aerial vehicles (UAVs) in dangerous operations in threat environments. Advantages such as the possibility to use the full range of the performance of the platform, the lack of humans on board, and the reduced costs associated with mission execution including risk for human pilots, motivated significant efforts towards the development of military RPAS [1]. After that, RPAS also started to be considered in the civilian aviation framework with the aim of achieving similar advantages in civil applications in terms of costs reduction and the substitution of human pilots in performing repetitive tasks and long endurance operations that can be efficiently delegated to automation in aircraft guidance. In addition, the parallel development of concepts such as the ones of small air transport (SAT) [2] and personal air transport system (PATS) [3] is linked to RPAS concepts and technologies. The SAT concept is now addressed in terms of the research and development of solutions allowing for single pilot operations that have the potential to ease the introduction of this aviation transport paradigm as a relevant industry, especially, for instance, in areas of Europe where surface transport infrastructures are not extensively implemented enough and the costs for implementation would be excessively high due to the presence of geographical barriers. In such a situation, a solution to increase mobility capabilities would be to implement the extensive use of regional small airports, which are numerous in those areas, by wide number of SAT vehicles that would be piloted under single pilot operations to reduce flight costs and the needed number of qualified crew. In such a framework, research activities are ongoing under the transversal SAT work package in the Clean Sky 2 EU-funded program, including activities specifically devoted to design of enabling technologies for single pilot operations in SAT vehicles that are carried out in the Cost Optimized Avionic System (COAST) project [4; 5]. In this framework, in order to reduce the single pilot workload, detect and avoid technologies are fundamental and also more relevant in the case of the delegation of the separation responsibility to the flight segment, as envisaged under specific circumstances by the SESAR (Single European Sky ATM Research) air traffic management (ATM) Target Concept [6]. Based on that, specific design activities are ongoing in the COAST project addressing an automatic dependent surveillance-broadcast (ADS-B)-based tactical separation system supporting the pilot in the separation task [7; 8] and a step forward in terms of the design of an overall integrated mission management system that automatically enables flight, including self-separation tasks, in the case of pilot incapacitation [9]. It is clear that these technologies represent a cross fertilization of the detect and avoid (DAA) technologies coming from the RPAS domain and the unmanned vehicle automation domain in general. Therefore, the PATS approach, which is intended to be implemented in this new and revolutionizing aviation sector, targets the total autonomy of the vehicles in cases of both freight and passenger transport, thus motivating dedicated efforts addressing the design of technologies such as ones for 4D automatic navigation, situation assessment and supervision, swarms guidance, separation assurance and collision detection, fault detection, and isolation [10]. For this aim, RPAS technologies play a fundamental role as the baseline for cross fertilization and further improvement of full autonomy in the PATS domain. Additionally, a fundamental role is played, once again, by DAA technologies.