| 4DT is four Dimension Trajectory, the four being latitude, longitude, altitude and time. The basic premise is that time is added to the traditional three dimensional control of aircraft. Note that RNP represents the superior method of controlling the first three dimensions. An early objective of 4DT is to permit aircraft to fly the optimal profile on descent and approach to an airport. Utilizing time sequencing, controllers are able to give clearance for approach and landing long before aircraft reach their destination. In 4DT trials at Stockholm Arlanda Airport, SAS 737-700s are arriving over the runway threshold within seconds of their assigned time. AVTECH, a Naverus partner, is the world leader in 4DT technology. Their website can be found by clicking here. In the future, GPS-based RNP may be the basis for aircraft-to-aircraft separation. Aircraft paths will come particularly close in terminal operations and while we have GPS- based ADS-B for monitoring, the primary method for airspace management will be advanced coordination of aircraft trajectories and dependance on robust navigation performance throughout the operation. Another potential use for RNP is speed-based enroute traffic lanes. This would permit aircraft to operate at their optimal speed using the same airspace currently allocated for jet ways. | 
