An Evaluation of Alert Thresholds for Detect and Avoid in Terminal Operations

Abstract

The Minimum Operational Performance Standards (MOPS) for Detect and Avoid (DAA) Systems outline a well-clear and alerting definition for Unmanned Aerial Systems (UAS) transitioning to and from Class A airspace or special use airspace [1]. This Phase 1 DAA well-clear (DWC) and alerting definition however, may produce losses of well clear (LoWC) and alerting undesirable to UAS operators when operating in the terminal environment, as the expectation of separation between aircraft in the terminal environment is reduced [2]. The ranges at which alerts are excited by normal terminal operations have implications on how a DAA system switches between an en-route and a terminal area DWC and alerting definition. In order to investigate this, an open loop fast time simulation study of UAS operations in a terminal environment was conducted by researchers at NASA Langley Research Center in order to investigate a terminal area DWC and alerting definition. Encounters were modeled with a UAS on a straight-in approach and traffic on various segments of a nominal visual traffic pattern to investigate the impact of integrating Detect and Avoid (DAA) for UAS into the terminal environment. The definition of well-clear was varied between the Phase 1 en-route DWC definition and a prospective terminal area DWC definition. Intruder performance, UAS performance, and flight trajectories were varied to represent a broad range of operations around an airport. Excitation of the Phase 1 MOPS alert thresholds for both the enroute and terminal area DWC were recorded through the collection of aircraft positions relative to the runway at alert threshold. Results indicate that both the terminal and en-route DWC definitions would excite early alert thresholds for aircraft on the 45° traffic pattern entry segment, indicating that the current alerting criterion would cause temporary alerts that may be undesirable to UAS operators. Late alert thresholds would also be excited for aircraft on the 45° entry segment using the en-route DWC definition defined in DO-365. Alerting for traffic on downwind was dependent on the Horizontal Miss Distance (HMD) threshold and the traffic's lateral distance from the runway centerline. These results have implications for how DAA systems will transition between the terminal area and en-route DWC.