Analysis of Quantitative Terminal Area Well-Clear Volumes for Unmanned Aircraft Systems

Abstract

As the use of Unmanned Aircraft Systems (UAS) increases, the need to address integration challenges within the National Airspace (NAS) becomes increasingly more important. RTCA Special Committee 228 (SC-228) has begun to tackle these integration challenges by developing a set of Minimum Operational Performance Standards (MOPS) for Detect and Avoid (DAA) as part of a Phase I effort [1]. A large portion of these requirements are written for the purpose of ensuring that the DAA system performs a similar function to that of a manned pilot’s ability to “see and avoid”, and also remain “well clear” of other aircraft. For pilots of manned aircraft, “well clear” is a subjective spatial separation, achieved by means of pilot training and on-board situational awareness. In the case of a DAA system, the Pilot in Command (PIC) or UAS itself must rely on sensor information and ground station aural/visual alerts to provide the necessary situational awareness to remain “well clear”. In order to define an objective interpretation of “well clear”, the Science and Research Panel (SaRP) developed a mathematical definition that involves both a spatial and temporal aspect [2]. However, DO-365, and the quantitative well clear definition described therein, only addresses class 3-5 UAS performing en route operations within class D, E, and G airspace. With DO365 as a starting point, RTCA SC-228 has begun its Phase II effort which will tackle additional aspects of UAS DAA operations in the NAS, namely the terminal area. The terminal area poses several unique challenges for DAA. It is a denser environment, with more closely spaced aircraft. The terminal area also tends to encompass more complex and dynamic encounters, given inherent arrival/departure routes and the VFR traffic pattern. Additionally, the terminal area poses several unique operational considerations, necessitating close coordination with Air Traffic Control (ATC) and adherence to standard procedures so as to not disrupt the airspace. Given these complexities, this paper provides results illustrating that the well clear definition used for DAA Phase I MOPS compliant operations is likely unsuitable for the terminal area, as it would lead to a large number of alerts due to intruder aircraft performing nominal operations. Several terminal area encounter sets are evaluated in a variety of geographical locations and airspace classes to gain insight into typical separations at closet point of approach (CPA). Results of this evaluation are provided herein and are used to inform several alternate quantitative well clear definitions for the terminal area. These alternate definitions are assessed for their potential suitability for terminal area operations by evaluating metrics such as violation rates, separation at time of violation, alert rate, and separation at time of alert. All alerting related analysis is performed using NASA’s DAIDALUS algorithm [3]. This paper places particular focus on whether or not a temporal component is appropriate for the terminal area, or if a spatial-based definition is suitable. This analysis aims at providing a suggested quantitative well clear definition for the terminal area and also supporting the development of the RTCA DAA Phase II minimum operational performance standards.