The doubling or tripling of airspace capacity that will be needed over the next several decades will require that tactical separation guidance be automated for appropriately equipped aircraft in high-density airspace. Four-dimensional (4D) trajectory assignment (three-dimensional position as a function of time) will facilitate such automation. A standard trajectory specification format based on XML (Extensible Markup Language) is proposed for that purpose.
The trajectory specification proposed is intended for communicating trajectories between air and ground. Pilots or Airline Operations Centers (AOCs) should be able to use it to downlink requested trajectories, and ground systems should be able to use it to uplink assigned trajectories. The basic requirements are that it be:
- Able to precisely specify any “reasonable” 4D reference trajectory,
- Able to precisely specify error tolerances relative to the reference trajectory,
- Based on a global Earth-fixed coordinate system,
- Parametric and reasonably compact,
- Based on a text format readable by humans, and
- Suitable for an international standard.
Trajectories are composed of a series of straight segments connected by turns of specified radius. A trajectory predictor is used to generate the reference along-track distance and altitude as a function of time. The trajectory predictor could be a function of the Flight Management Computer onboard the aircraft, or it could be in a ground-based ATC computer if a suitable trajectory predictor is not available onboard the aircraft. Flight technical error tolerances in the along-track, cross-track, and vertical axes determine a bounding space, at each point in time, in which the aircraft is required to be contained. The tolerances could depend on the traffic situation and could be relaxed in sparse traffic. Periodic updates in the along-track axis will adjust for errors in the predicted winds.
The stringent regimen implied by assigned 4D trajectories may seem to contradict the notion of “free flight,” but it actually does not. The objective is not to restrict routing options any more than necessary, but rather to keep precise and reliable track of intent. Rather than trying to predict the trajectory of each aircraft, trajectories will be assigned, and conformance will be mandated.
A technology called Cockpit Display of Traffic Information (CDTI) uses GPS/WAAS and ADS-B to provide an onboard visual representation of the local traffic environment. An advanced CDTI could also show assigned trajectories for improved situational awareness.
An important safety benefit of this regimen is that the traffic will be able to fly free of conflicts for at least several minutes, even if all ground systems and the entire communications infrastructure fail.