## Materials and Methods
### Concept of Operations  
Based on what is indicated above, three main capabilities are provided by ASACAS implementation: separation assurance (SA), collision avoidance (CA) and situational awareness (SitA). Here, we provide a description of the interactions between the system and the RPAS remote pilot in order to outline the ASACAS's intended application and benefits. The RPAS remote pilot is aware of traffic information by means of an HMI that allows them to build their situational awareness related to the surrounding traffic, including the indication of the consolidated traffic tactical picture around the remotely piloted aircraft (ownship) and the associated level of severity of each surrounding aircraft in terms of loss of separation and collision risk with respect to the ownship. To consolidate the traffic picture and evaluate the associated risks, the system uses ADS-B IN-provided inputs, as well as ownship navigation data provided by the onboard navigation sensor (GPS, Global Positioning System). The overall ASACAS concept of operations is represented in Figure 1, where ATC indicates the air traffic control service provision facility, GCC is the RPAS ground control station where the remote piloted vehicle station is located, and FCC indicates the flight control computer hosting the on-board ASACAS software (as well as the additionally needed software for autopilot and low-level aircraft control). From an operational point of view, the ASACAS continuously performs the monitoring of traffic in order to provide an enhanced situational awareness to the remote pilot and to identify a possible loss of separation and/or collision risks. In the presence of a predicted loss of separation with respect to one or more traffic vehicles, the system is expected to provide information and automatically elaborate a separation recovery maneuver to the remote pilot. The remote pilot is then in charge of evaluating the feasibility of the maneuver with respect to the constraints affecting the flight and not considered by the ASACAS (it is worth noticing that the system performs the elaboration of the separation recovery maneuver without considering any constraint related to, for instance, existing no-fly zones, segregated areas, fixed obstacles, or flight plan constraints). If needed, based on the current tactical picture, the remote pilot can interact with the ATCo (air traffic controller) in order to ask for clearance for the implementation of the proposed maneuver or to negotiate it. Based on the outcomes of the remote pilot and/or ATCo evaluations, the maneuver can be accepted, and, in this case, the remote pilot can decide to delegate its automatic implementation to the aircraft autopilot or to implement the maneuver. In the presence of a predicted risk of collision with respect to one or more surrounding traffic vehicles, the ASACAS is expected to provide information to the remote pilot and to automatically elaborate collision avoidance maneuver with respect to the most dangerous vehicle, according to the proper prioritization criterion. The collision avoidance maneuver, due to its emergency nature, is expected to be automatically implemented by the automatic guidance system. Nevertheless, the remote pilot is provided with the possibility of aborting the maneuver and taking the direct control of the vehicle at any moment. The ASACAS is also continuously performing a TCAS compatibility check that is aimed to provide a prediction, based on the current trajectory of the ownship and traffic vehicles, of the resolution advisory activation in the surrounding traffic vehicles equipped with TCAS [30]. Based on the outcome of this check, the expected behavior of the ASACAS with respect to the identified loss of separation and collision risks is accordingly modified in order to ensure that no maneuvers are generated by the ASACAS that could create nuisances to the TCAS-elaborated maneuver.