## 1 Introduction
Even if the RPAS domain is very relevant in terms of both current application and future further developments, as outlined above, the presence of such new actors in the airspace has led to a new and very important issue in terms of safety: the safe integration of RPAS with conventional manned vehicles in the common airspace. This issue becomes more relevant in view of the expected activation of the urban air mobility (UAM) transport segment, which will probably start by using conventional rotorcraft with conventional pilots and will then evolve through the RPAS paradigm towards fully automated vehicles for passenger (and freight of course) transport into wide urban areas over the next few years or decades. This suggests a double issue in terms of safety: (1) the integration of RPAS with conventional manned traffic in usual airspaces without using segregation while being under the control of conventional ATM and air traffic control (ATC) systems; (2) integration of the RPAS and, from the perspective of UAM vehicles, into an overall ecosystem devoted to unmanned traffic management (UTM) in the lower airspace (very low level (VLL) operations) ecosystem that is now being designed and implemented according to the U-Space paradigm [11]. A considerable number of research and development activities aimed at supporting and allowing for the integration of RPAS in the unsegregated airspace have been undertaken worldwide in the last few decades. In Europe, for instance, the INOUI (Innovative Operational UAV Integration) and ICONUS (Initial ConOps for UAS in SESAR) projects were carried out as the first ones addressing the issue. The findings of ICONUS [12] indicated that there are several classes of obstacles for RPAS integration in the unsegregated airspace: regulatory obstacles, such as common standards, interoperability, and airworthiness; procedural obstacles, namely the planning of operations, separation methods and minima, approach procedures, and emergency procedures; social and organizational issues, namely human factors, public acceptance, and liability insurance; and system limitations, notably the detect and avoid system, the command and control link, and the safety, security, and reliability aspects linked to those systems. Further complication have arisen due to the large number of RPAS types, as well as the dimension and the variability of missions. RPAS can range from few grams (and the further reduction of weight is a clear trend) to vehicles that can weigh as much as general aviation aircraft; missions can vary from usually scheduled to unscheduled flights. Depending on such a large number of issues, widespread initiatives have been conducted all over the word, often in a non-coordinated fashion. Based on that, in Europe, the SESAR program has devoted efforts to support the integration of RPAS together with conventional manned vehicles in the non-segregated airspace. Starting from the roadmap for the integration of RPAS prepared by the European RPAS Steering Group (ERSTG) [13], the SESAR Joint Undertaking (SJU) completed the RPAS Integration Definition Phase, which identified the guidelines for the RPAS integration topic to be included in the SESAR2020 program, thus driving the R&D development in ATM in Europe in these years. Consequently, SJU sponsored dedicated projects under the global program for RPAS Demos in order to investigate and collect data from flight tests and experiences about RPAS integration issues. One of such projects was the RAID (RPAS-ATM Integration Demonstration), which carried out both real-time validations and flight demonstrations for the integration of RPAS vehicles in ATM.