Preliminary Uncorrelated Encounter Model of the National Airspace System

Airspace encounter models, covering close encounter situations that may occur after standard separation assurance has been lost, are a critical component in the safety assessment of aviation procedures and collision avoidance systems. Of particular relevance to Unmanned Aircraft Systems is the potential for encountering general aviation aircraft that are flying under Visual Flight Rules (VFR) and which may not be in contact with air traffic control. In response to the need to develop a model of these types of encounters, Lincoln Laboratory undertook an extensive radar data collection and modeling effort. This report describes the structure and content of that encounter model. Bayesian networks are used to represent relationships between dynamic variables and to construct random aircraft trajectories that are statistically similar to those observed in the radar data. The model described in this report is one of three that are currently under development by Lincoln Laboratory. An encounter with an intruder that does not have a transponder, or between two aircraft using a Mode A code of 1200 (VFR), is uncorrelated in the sense that it is unlikely that there would be prior intervention by air traffic control. The uncorrelated model described in this report is based on transponder-equipped aircraft using a 1200 (VFR) Mode A code observed by radars across the U.S. In addition to representing VFR-on-VFR encounters, this model is representative of encounters between a cooperative aircraft and conventional non-cooperative aircraft similar to those that use a VFR transponder code. A second uncorrelated model is also being developed based on true primary-only (non-cooperative) aircraft tracks, though the quality of such tracks is inferior to that available from the cooperative 1200-code tracks used here. Finally, a correlated encounter model is also under development to represent situations in which it is likely that there would be air traffic control intervention prior to a close encounter. The correlated model will apply to aircraft that are using a discrete (non-1200) mode A code. A separate electronic file is provided by Lincoln Laboratory that contains the statistical data required to generate encounter trajectories. Details on how to interpret the data file and an example of randomly constructing trajectories are provided in Appendices A and B, respectively.