## 1 Introduction
Regarding maned aviation, aircraft receiving air traffic control instructions will be dynamically segregated from UAS operations via a Dynamic Airspace Reconfiguration mechanism, as outlined in [23; 24]. Aircraft flying without air traffic control instructions may operate in U-space, provided they meet the e-conspicuity requirements of Reg. 666. The traffic information service provides UAS operators with a comprehensive view of traffic around the unmanned aircraft (UA) under their control. The ultimate responsibility for avoiding any collision hazard rests with the UAS operators, based on the information from the traffic information service. However, SESAR envisages that, as demand increases, a U3 (advanced) tactical conflict resolution (TCR) service will be introduced to support denser UAS operations by providing tactical separation [25]. In fact, SESAR is currently funding research projects, such as SPATIO [26], to deliver solutions for tactical conflict resolution with a high level of maturity by 2026. In line with this approach and considering the DAA system described in ED-267, we assume that the RWC function is provided by the U-space tactical conflict resolution service, while the CA function relies on airborne systems. Both DO-396 and ED-267 recognise Air-to-Air Radar (ATAR) as a key enabling technology for DAA. The reference standard for DAA ATAR systems is RTCA DO-366, Minimum Operational Performance Standards (MOPS) for Air-to-Air Radar for Traffic Surveillance [27]. However, DO-396 notes that ATARs compliant with DO-366 may have size, weight, and power (SWaP) requirements that are incompatible with small drones. In addition to SWaP constraints, another significant difference between DO-396 ATAR systems and those used in larger UAS DAA systems, such as ACAS Xu, specified in EUROCAE ED-275 [28], is the radar field of view (FOV). While traditional systems typically offer a FOV of \(\pm 110^{\circ}\) relative to the longitudinal axis of the UAS, DO-396 extends this to a full \(360^{\circ}\) coverage around the aircraft. Cost is also a critical factor: an ACAS Xu system compliant with ED-275 far exceeds the cost of a typical multirotor or small fixed-wing UAS. Considering that antennas are major contributors to the size, weight, and cost of radar systems, the design of affordable, lightweight antennas with a low aerodynamic profile and \(360^{\circ}\) coverage around the host aircraft emerges as a clear research priority for enabling routine UAS operations using collision avoidance (CA) systems. In this paper, we propose and prove an innovative antenna architecture concept that addresses the identified research need. The proposed architecture consists of a circular array of 2N sectorial cavity antennas that generate overlapping fixed radiation patterns, each shifted by \(\pi/N\) radians around the host aircraft. Each pair of adjacent sectors synthesises a monopulse radiation pattern pointing in a predefined direction (i.e., the sector boresight). We compute the off-boresight angle of a target (also referred to as a threat) by processing the sigma and delta channels of the monopulse patterns, as performed in State-of-the-Art Mode S radars [29]. The phase of the delta channel is then used to determine the sign of the off-boresight angle. In this way, we can accurately estimate the angular position of a threat across the full \(360^{\circ}\) range around the ownship, using compact radiating structures that are easily integrable into the UAS airframe. We have proved the proposed concept through a combination of simulations and experimental techniques. As outlined in Section 3.1, this solution offers advantages over current State-of-the-Art radar antennas used in UAS CA systems, which generally rely on either directive antennas with limited fields of view, printed antennas that cannot handle the power levels required to meet relevant standards, or rotating antennas that are mechanically complex and therefore expensive. The remainder of the paper is organised as follows.