The Airborne Collision Avoidance System X (ACAS X) is an onboard flight safety system designed to replace the current Traffic Alert and Collision Avoidance System II. It is fully compatible with the new airspace procedures and technologies of the FAA’s Next-Generation Air Transportation System, which aims to reduce gridlock in the sky and at airports. ACAS X detects nearby aircraft by receiving sensor measurements from onboard surveillance systems, estimates the relative position and speed of these aircraft, weighs the costs of all actions the pilot could take and decides on a single best action. If a collision avoidance alert is necessary, ACAS X sends the information to the pilot via the flight deck displays. A version of this system, ACAS Xu, is tailored for use on unmanned aircraft.