Investigating Effects of Well Clear Definitions on UAS Sense-And-Avoid Operations in Enroute and Transition Airspace

Abstract

Unmanned aircraft systems (UAS) will be required to equip with sense-and-avoid (SAA) systems in order to fulfill the regulatory requirement to remain "well clear" of other air traffic. This study investigates the effects that different well-clear metrics have on the rate of well-clear violations and evaluates the distribution of distances between aircraft at a wellclear violation in high-altitude enroute airspace. The first analysis determines the predicted rate at which violations of well clear would occur between UAS and manned aircraft operating under instrument flight rules, indicating the frequency with which a sense-andavoid system would create a nuisance alert. This analysis is done both with and without an algorithmic model of air traffic control (ATC) separation provision services. The second analysis determines the relationship between time-based well-clear metrics and the range at which the violation would occur, a relationship that may inform the required SAA surveillance range and the frequency with which violations would occur despite ATC separation standards still being maintained. The analyses are carried out using a fast-time simulation capability of the entire US air traffic system over a single day, including 3000 UAS and more than 50,000 manned aircraft. Results indicate that, without any separation provision, a UAS would encounter a manned aircraft with a range tau (defined as the ratio of the relative range to range rate) of 60 seconds only every six hours. Approximately 75% of such encounters would occur outside the ATC separation standard of 5 nmi.

Well-Clear Recommendation for Small Unmanned Aircraft Systems Based on Unmitigated Collision Risk

“…With risk as the primary evaluation metric, we considered many different mathematical forms of well clear. Formally, well clear could be defined as any combination of spatial components [4]: horizontal separation r, vertical separation Δh, or temporal components [5,6] τ, τ mod , and distance modification (DMOD). DMOD is spatial threshold used to control the behavior of τ mod to account for specific types of encounters, such as slow closure encounters, and τ mod is a temporal variable defined as the modified time till closest point of approach (CPA):…”

“…Upon violation of the well-clear boundary, there should be sufficient time to execute a maneuver and avoid excessive concern for proximate manned aircraft. This consideration is not unique for sUAS and was addressed for the large UAS as well [4]. The recommended well-clear vertical separation of 250 ft enables sufficient time to resolve a collision avoidance geometry with even a modest relative vertical rate ( _ h).…”

Online Risk-Based Supervisory Maneuvering Guidance for Small Unmanned Aircraft Systems

“…It is noted that the safe horizontal range to separate UA from other traffic often changes in mid-air encounters, in terms of various relative velocities between UA and traffic. Thus, to unify a safe separation threshold in risk assessment, a time-based range tau (τ) is introduced to evaluate the risk severity of potential collisions as [12]…”

“…Thus, another modified range tau (τ mod ) is used to provide a minimum range, Distance MODification (DMOD), ‡ at which to alert regardless of the calculated τ. This τ mod is defined as [12,13] τ mod = − r − (DMOD 2 /r) r = − r 2 − DMOD 2 r r ( if r r 0) (10) ‡ DMOD: an absolute distance threshold to alert the collision risk in mid-air encounters, e.g., DMOD = 4000 ft to alert the loss of well clear.…”

Path Planning in the Local-Level Frame for Small Unmanned Aircraft Systems

“…As shown in Figure 2, the choice for this volume is a hockey-puck of radius d s and height h s that commonly includes a horizontal distance of 500 ft. and a vertical range of 200 ft. [1,33,34]. Accordingly, a collision event is defined as an incident that occurs when two aircraft pass less than 500 ft. horizontally and 100 ft. vertically.…”