## 1 Introduction
On one hand, the Radio Technical Commission for Aeronautics (RTCA) has developed a Minimum Operational Performance Standard (MOPS) for the Airborne Collision Avoidance System sXu (ACAS sXu) [12], also known as RTCA DO-396. This standard specifies the requirements for systems to be installed on small, unmanned aircraft (sUA) with a wingspan of less than 25 feet with the goal of assisting remote pilots in avoiding collisions with a variety of intruders, whether manned or unmanned, and whether cooperative or non-cooperative--referred to as the ownship. Note that the term sUA comes from DO-396 and should not be confused with other similar weight-based definitions commonly used in the literature, such as the classification of small UAS with a maximum take-off weight below 55 pounds. On the other hand, the European Organisation for Civil Aviation Equipment (EUROCAE) has developed an Operational Services and Environment Description (OSED) for DAA in Very Low-Level (VLL) operations, referred to as EUROCAE ED-267 [5]. This OSEDdefines a system designed to protect ownships operating in the specific category (as per Reg. 947) from intruders that meet the e-conspicuity requirement set forth in Commission Implementing Regulation (EU) 2021/666 [13] (hereinafter Reg. 666). Additionally, RTCA Special Committee (SC) 147 is developing the ACAS Xr concept, which is tailored to the specific characteristics of rotorcraft, including those intended for Urban Air Mobility (UAM) and Advanced Air Mobility (AAM) operations [14]. The system in DO-396 does not separate the RWC and CA functions but instead provides a single layer of warning alerts and directive guidance, based on protection volumes tailored to the intruder type. In contrast, ED-267 defines two distinct functions: RWC, which can be provided by either onboard or ground-based systems, and CA, which is handled exclusively by onboard equipment. In parallel, the concept of Unmanned Aircraft Systems (UAS) Traffic Management (UTM) emerged in the mid-2010s as \"a specific aspect of air traffic management that safely, economically, and efficiently manages UAS operations through the provision of facilities and a seamless set of services, in collaboration with all parties, involving both airborne and ground-based functions\" according to the International Civil Aviation Organization (ICAO) definition in [15]. Based on ICAO's UTM framework, several concepts of operations (ConOps) for providing UTM services have been developed worldwide (e.g., [16; 17]). In the EU, the regional implementation of the UTM concept is known as U-space, which is being developed by the SESAR Joint Undertaking (JU), under the appointment of the European Commission, through the CORUS and CO-RUS-XUAM research projects. These efforts led to the publication of the latest version of the U-space ConOps in 2023 [18]  
Based on the U-space ConOps, the European Commission has established a comprehensive regulatory framework that requires UAS operators to use certain foundational U-space services (U1), such as e-registration and e-identification [19], along with an airborne geo-awareness function, as stipulated by Commission Delegated Regulation (EU) 2019/945 [20], as amended by [21] (hereinafter referred to as Reg. 945). Furthermore, Commission Implementing Regulation (EU) 2021/664 (hereinafter Reg. 664) introduces the concept of U-space airspace as a designated UAS geographical zone (as per Art. 15 of Reg. 947) [22], where UAS operations can only occur with support from various U1 and U2 (initial) U-space services. These include at least network identification, flight authorization, geo-awareness, and traffic information, along with common information services that may be provided either by a federation of entities or a single designated organization (at the discretion of Member States). This regulation applies to all UAS operation categories defined in Reg. 947, with the exception of the lowest-risk open category and operations conducted under Instrument Flight Rules (IFR).