Airspace Encounter Models for Conventional and Unconventional Aircraft

Collision avoidance systems play an important role in the future of aviation safety. Before new technologies on board manned or unmanned aircraft are deployed, rigorous analysis using encounter simulations is required to prove system robustness. These simulations rely on models that accurately reflect the geometries and dynamics of aircraft encounters at close range. These types of encounter models have been developed by several organizations since the early 1980s. Lincoln Laboratory's newer encounter models, however, provide a higher-fidelity representation of encounters, are based on substantially more data, leverage a theoretical framework for finding optimal model structures, and reflect recent changes in the airspace. Three categories of encounter model were developed by Lincoln Laboratory. Two of these categories are used for modeling conventional aircraft; one involving encounters with prior air traffic control intervention and one without. The third category of encounter model is for encounters with unconventional aircraft—such as gliders, skydivers, balloons, and airships—that typically do not carry transponders. Together, these encounter models are being used to examine the safety and effectiveness of aircraft collision avoidance systems and as a foundation for algorithms for future manned and unmanned systems. Because of the extreme time criticality and the potentially catastrophic consequences of error in the operation of collision avoidance systems, civil aviation authorities such as the FAA and Eurocontrol require rigorous safety studies to gain confidence in system effectiveness before deployment. The analysis process includes flight tests and simulation. Although a flight test can evaluate a collision avoidance system in actual operation, only a few situations can be examined due to time, cost, and safety constraints. Simulation analyses use Monte Carlo techniques to estimate the robustness of a given collision avoidance system across a wide range of encounter situations. Central to Monte Carlo simulation analysis is an encounter model that describes the types of encounter situations typically occurring in the airspace. An accurate representation of these encounters is required so that the collision avoidance system being tested is exposed to a realistic set of problems to resolve. This paper describes the highest-fidelity models to date of aircraft encounters, based on hundreds of times more data than was used to construct previous models. The primary function of an encounter model is to generate random encounter situations between two aircraft, capturing the potentially hazardous events that may occur in the actual airspace. The encounters represented by the model are those involving aircraft in the final stages before a collision, typically covering a period of one minute or less. The model assumes that prior safety layers—e.g., airspace structure and air traffic control (ATC) advisories or vectors—have failed to maintain standard separation distances between aircraft. A situation generated from an encounter model describes the initial relative positions, velocities, and attitudes of two aircraft and subsequent maneuvers that may take place before the aircraft reach a point of closest approach. A dynamic simulation using the encounter model then propagates the aircraft positions based on the model, applies sensor and algorithm models to determine whether a collision avoidance command is issued, and then tracks the resulting outcome. This paper begins with a background on prior U.S. and European encounter models and discusses the different categories of models introduced in our work. The remainder of the paper describes the model construction process and the data used to build the models. The paper concludes with a discussion of some applications of the models to collision avoidance system development and safety analysis.