## 1 Introduction  
Unmanned Aircraft Systems (UAS) are defined by the International Civil Aviation Organization (ICAO) as _'aircraft and their associated elements which are operated with no pilot on board'_[1]. This term refers to either a Remotely Piloted Aircraft System (RPAS) or a fully autonomous unmanned aircraft (i.e., one that operates without any human intervention during flight). Throughout this paper, we use the term Unmanned Aircraft (UA) specifically to denote the airborne component of a UAS. Although UAS have been employed in military operations for over a century--for example, during World War I [2] and the Vietnam War [3]--widespread interest in their civil applications did not emerge until the early 21 stcentury. Today, there is substantial anticipation surrounding a wide range of professional uses for UAS across various sectors--including urban mobility, delivery services, public safety and security, entertainment, agriculture, infrastructure inspection, and recreational activities. At the European Union (EU) level alone, the projected market demand is valued at several billion euros annually over the coming decades, with expectations of creating more than 400,000 highly skilled jobs by 2050 [4]. A large share of this market involves operations within what is commonly referred to as Very Low Level (VLL) airspace, typically located over densely populated urban and industrial areas. According to [5], VLL is defined as the airspace below the minimum altitude specified for Visual Flight Rules (VFR) operations. However, VFR operations may still occur within VLL airspace, provided they are authorized by the competent authority. While citizens are drawn to the anticipated economic and societal benefits of UAS operations, they also expect these operations to be as safe as traditional manned aviation and to pose no negative impact on the safety of crewed air traffic (see, for instance, [6]). In response, competent authorities worldwide have implemented measures to ensure that the safety of UAS operations is commensurate with the risks they pose. One such measure is Commission Implementing Regulation (EU) 2019/947 [7] (hereinafter referred to as Reg. 947), which establishes three risk-based categories of UAS operations: open, specific, and certified, representing increasing levels of operational risk. This regulation has been in force in EU Member States since July 2020 and is also applicable in other countries within the European Economic Area (EEA). Depending on the operational risk, UAS operators may be required by the competent authorities to implement specific mitigation measures. As a matter of fact, in the EU, UAS operators must conduct a safety assessment using the Specific Operations Risk Assessment (SORA) methodology [8] for operations that fall under the specific category, as defined in Reg. 947. According to SORA, operators are required to assign ground and air risk classes--referred to as GRC and ARC, respectively--based on the estimated level of risk [8]. Regarding the air risk, operators must apply mitigation measures to reduce the risk to acceptable levels unless the operation is classified under the lowest risk category (ARC-a). As part of these mitigation measures, operators may be required by the competent authority to employ Detect and Avoid (DAA) systems, with performance levels appropriate to the assessed risk. Although regulations for UAS operating in the certified category are still under development, DAA capabilities have already been identified as a key enabler for such operations, particularly in urban environments [9]. Detect and Avoid (DAA) comprises two functions that address the second and third layers of conflict management--namely, separation provision or Remain Well Clear (RWC), and Collision Avoidance (CA), respectively--according to [10; 11]. Currently, two parallel initiatives are underway to define the specifications for DAA systems.