Interoperability of Horizontal and Vertical Resolution Advisories

To operate in civil airspace, unmanned aircraft systems (UAS) are expected to maintain safe separation from other aircraft. Self-separation and Collision Avoidance Systems (CAS) designed for unmanned aircraft are under development to meet this requirement. To maintain airspace safety, these systems must interoperate safely with CAS onboard manned aircraft. Whereas manned aircraft CAS such as TCAS and ACAS Xa issue vertical resolution advisories (RA) that may direct aircraft to climb or descend, new UAS systems may issue horizontal RAs that direct aircraft to turn left or right, potentially leaving UAS free to maneuver vertically during the collision avoidance timeframe. These vertical maneuvers may negatively interact with manned aircraft employing a vertical CAS, representing a potential safety risk. This paper summarizes a study conducted to determine the extent of this safety risk and whether the vertical dynamics of UAS should be constrained to ensure interoperability. Fast time simulations were conducted to determine the collision risk of encounters between an aircraft equipped with TCAS or ACAS Xa and a UAS equipped with ACAS Xu: a CAS developed for unmanned aircraft. A worst-case-scenario approach was taken in which the UAS altered its vertical rate towards the intended path of the manned aircraft during the collision avoidance timeframe. The results show that ACAS Xa was safer and more robust to the UAS’s vertical maneuvers than TCAS. A vertical coordination scheme was also evaluated and was shown to reduce collision risk. These results will contribute to the drafting of interoperability recommendations for UAS collision avoidance and self-separation system behavior in encounters with manned