ACAS X is a family of collision avoidance systems that use machine learning to optimize traffic alerts, resulting in dramatically reduced nuisance alerting and improved safety. Billions of virtual test flight hours have been run, resulting in ACAS X recognizing and appropriately responding to a wide range of flight configurations and more accurately alerting when a real threat is present. This technology has been specialized for a variety of aircraft—ACAS Xu for large unmanned aircraft, sXu for small unmanned aircraft, Xr for helicopters and urban air mobility, and Xa for commercial planes.