4D air traffic control for non-4D-equipped aircraft

Current concepts for 4-dimensional (4D) time-based air traffic control automation systems are generally designed to control aircraft so that they cross a final control waypoint at a scheduled time, but the trajectory between the initial waypoint and the control waypoint is not explicitly specified. Instead of this type of single-point 4D control, it may be advantageous to use closed-loop control to reference 4D trajectories. The design of a compensator to help air traffic controllers and pilots accurately and efficiently control aircraft to buffered 4D reference trajectories is presented in this paper. The main technical challenge is to develop a control algorithm that efficiently achieves 4D control for aircraft while maintaining acceptable air traffic controller workload levels. The approach is to soften 4D trajectory clearances by allowing aircraft to be within a well-defined error buffer region around the trajectory and to try and synchronize advisory updates with the reference trajectory advisories such that the aircraft remains within the buffer. A bias estimator is used to remove steady-state errors. The control algorithms are developed and a simple example of a 4D control scenario is presented.