Risk-Based Method for Determining Separation Minima in Unmanned Aircraft Systems

Risk assessment is a key issue in enabling the use of unmanned aircraft systems (UASs) in nonsegregated areas, especially in very low-level airspace and urban areas. The specific operations risk assessment (SORA) methodology represents an important milestone in performing the risk assessments required by aviation safety agencies to UAS operators in specific operations. However, the SORA is a qualitative method used for UASs operating inside a well-bounded operational volume. This paper proposes a quantitative method that can not only be used in a closed volume but also in an airspace shared by several UAS missions and even general aviation. The basis for this is providing a separation volume (a “bubble”) to prevent collisions that is calculated using a risk-based approach. The method consists of setting a target level of safety, which is accomplished using a tradeoff between strategic and tactical mitigations. A probabilistic methodology for performing quantitative risk assessment of strategic and tactical mitigations is provided, and the dependence of the separation distance is carefully analyzed. All factors affecting the separation distance are identified, and their contributions to collision risk are probabilistically estimated. The method takes into account specific factors relating to UASs such as trajectories, separation methods, and performance. As a result, the method allows numerical determination of a separation distance for a given target level of safety and an operational scenario.