Analysis of the Tau concept used in aircraft collision avoidance through kinematic simulations

Abstract

The airborne Traffic Alert and Collision Avoidance System (TCAS) is the current standard for detecting and avoiding potential mid-air collisions. Based on information received from nearby aircraft, TCAS issues Traffic Advisories (TAs) to alert pilots about potential conflicts. If an impending collision threat arises, TCAS provides Resolution Advisories (RAs) involving vertical maneuvers (such as Climb or Descend) to help resolve the threat. The TCAS logic primarily uses a parameter Tau, which is a measure of the time to the closest point of approach, in terms of range or vertical separation, between the two aircraft. In this paper, we use particle kinematics-based simulations to generate two-aircraft mid-air encounters with different geometries and closure rates, and we assess and compare the performance of collision threat detection criteria that employ conventional and modified definitions of Tau as well as a criterion that closely models the logic of TCAS. We analyze encounters between jet airliners, as well as those between jet airliners and small general aviation aircraft in urban airspace. Low closure rate encounters are also considered. Performance of different threat detection criteria is assessed by calculating the range and vertical separation between aircraft at the instant of RA generation. The modified Tau definitions provide greater separations than the conventional ones. The criterion that closely models the threat detection logic of TCAS provides intermediate levels of separation.