PHARE took as its starting premise that changes in technology and the process of ATC would be required if capacity and productivity gains were to be achieved while maintaining or improving safety. The approach to characterising the ATC system, comprising the man and the machine, taken by PHARE was to view it as successive iterations of firstly, providing/updating information on current and future aircraft positions and secondly, taking and communicating decisions on the basis of that information. Seen in this way, the evolutionary path for ATC is one of steadily improving the accuracy and integrity of the information base and making the decision process more optimal until at some point the physical capacity of the airspace as defined by the minimum separation criteria is reached.

The key to the evolutionary path envisaged was seen to lie in the flight deck systems supporting the pilot, the ATC systems supporting the controller and the associated communication facilities between the two. It requires the aircraft to have progressively more accurate navigational performance in space (2D, 3D) and ultimately in space an time (4D). It requires the ground system to be continuously informed of each aircraft’s current position and its intentions and it requires ATC to exploit this high quality information through more optimal decision making.

This, in short, is the optimal air-ground traffic management system envisaged and researched by PHARE.