Abstract
Small unmanned aircraft systems operating in low-altitude airspace are expected to share volume with crewed aviation, urban air mobility platforms, rotary-wing traffic, and dense obstacle fields while meeting regulatory constraints on collision risk and interoperability. Existing airborne collision avoidance concepts such as those derived from ACAS frameworks were originally formulated for higher-altitude, cooperative, and transponder-equipped traffic, under assumptions about surveillance performance, pilot response, separation minima, and airspace structure that do not directly hold for many low-altitude use cases. This paper examines the compatibility between ACAS-based collision avoidance concepts and detect-and-avoid architectures suitable for small unmanned aircraft in low-altitude environments, focusing on technical interactions among surveillance modalities, algorithmic decision logic, maneuverability limits, and communication constraints. The assessment considers to what extent ACAS-style advisories, state representations, and thresholding mechanisms can be consistently mapped into autonomous detect-and-avoid functions that must operate without pilot-in-the-loop and often with non-cooperative or partially cooperative traffic. The discussion covers scalability in dense operations, latency and integrity bounds, and the implications of low-cost sensors with limited performance. The analysis highlights conditions under which ACAS-based logics remain structurally compatible, conditions where direct transplantation leads to inconsistent or degraded behavior, and where tailoring of alerting thresholds, action spaces, and coordination concepts is required. The goal is to clarify the technical alignment and misalignment between these frameworks without prescribing a unique architecture, supporting reasoned integration choices for low-altitude detect-and-avoid systems.