It is about time…
Updated: Apr 27, 2020
If you are involved in commercial air transport, it might be a good idea to get used to the terms RBT, RTA, CTA and TBO...
They all belong to the framework of 4D-Navigation, the concept of controlling aircraft not only geometrically, but also in terms of time. Additionally, the aviation community braces for a paradigm shift in how Air Traffic Control (ATC) directs aircraft enroute. The industry acronym for this is “Trajectory Based Operation” (TBO) .
Figure 1: Time is of the essence...
It is often tagged as a new and innovative idea to control the flow of air traffic precisely in the time-axis, but actually this concept is very old. Indeed NASA investigated such an idea as early as 1975 . There was little progress since then, until recent years when it became more and more apparent that the current ATC system will need to dramatically change in order to cope with future demands. Not only in terms of speed control, but also how it handles the coordination of an aircraft’s trajectory.
“Clearance-based” vs. “Trajectory-based”, what is the difference?
In today’s clearance-based world, an ATC flight plan is agreed upon as part of a pre-departure IFR clearance. But the moment the aircraft departs, the airborne version of the flight plan and the version at ATC start to deviate from each other. They have to be “manually” synchronized by issuing specific clearances, e.g. “cleared direct XY”, or “fly Mach 0.79”. This concept has severe limitations and creates a tremendous amount of work for everyone involved. What if, there was a single “master trajectory” that ATC would use to make decisions and the airborne avionics would use to steer the aircraft? This is exactly what the fundamental idea behind the paradigm shift towards TBO is all about .
Figure 2: Clearance-based ATC vs. TBO 
Figure 2 above describes the difference: Under "clearance-based" ATC, it can be hard for the flight crew to comprehend ATC's intention, especially when being "RADAR vectored". On the other hand, weather enroute can lead to "free-style" avoidance maneuvers that are somewhat obscure to ATC in the sense that that the aircraft's further routing is ambiguous to say the least. All this would be rectified, if ATC and the airborne avionics would use the same, negotiated trajectory in "real-time", as shown in the lower part of Figure 2.
TBO concepts are being researched on both sides of the Atlantic, under the NEXTGEN program in the US, and under Single European Sky ATM Research (SESAR) in Europe. While they share the key idea, the implementation differs slightly . The basic concept is simple: Ensure that every aircraft leaves at a specific time, follows an agreed route and arrives at “metering point”, at a given time. This process shall span the entire journey from “gate-to-gate”. No holdings, no queue - well, at least that is the plan…
In Europe, the entire TBO concept evolves around the so-called Reference Business Trajectory (RBT), an agreed “path” of an aircraft in space and time from “gate A” to “gate B”, involving all parties such as airports, ground services, Air Navigation Service Providers (ANSP’s) etc. The SESAR program divides the introduction of 4D-Navigation into two phases: “Initial 4D” (i4D) and “4D”, the latter being the final development stage. The i4D phase involves trajectory sharing between the aircraft and ATC . The large-scale demonstration project for this phase is called “Demonstration of air traffic management Improvements Generated by 4D Initial Trajectory Information Sharing” (DIGITS). It is encouraging to see, that in 2019 the first European airline received aircraft that are compliant with the 4D TBO avionics requirements and therefore the real-world testing is under way  . The so-called “Automatic Dependent Surveillance-Contract Extended Projected Profile” (ADS-C EPP), which represents the future routing currently stored in the airborne avionics incorporates up to 128 waypoints and creates a “real-time” shared situational awareness for the flight crew and ATC . Figure 3 below shows a general overview of the TBO concept, based on the US planning. The i4D stage in Europe can be thought of the “4D trajectory downlink” part in Figure 3. This picture also shows how ATC will use different means of communication, depending on the aircraft's TBO capability. For TBO-equipped aircraft, communication with ATC is almost exclusively via ADS-C and Controller Pilot Data Link Communication (CPDLC). A more stategic communication loop is also formed via the ACARS network, connecting the airline operational control with the Collaborative Decision Making (CDM) part of Air Traffic Management (ATM). Conventional aircraft will use voice communication, and CPDLC without TBO (not shown in picture).
Figure 3 : US proposal for TBO concept 
The fully-fledged 4D TBO concept will involve a negotiated trajectory and “Controlled Time of Arrival” (CTA), which represents an ATC-given requirement to pass a certain point within a specific time-window . A key point of the European implementation is that the CTA negotiation is initiated by the aircraft. This takes into account specific limitations of the aircraft (performance, current wind, cost index etc.) . Specifically, the aircraft will downlink a range of feasible arrival times, referred to as “ETA min/max”. This time window is then the basis for the calculation of the CTA, which in turn is uplinked back to the aircraft. Under the US concept, it is foreseen that the CTA negotiation is initiated by the ground system . For both systems, the downlink occurs via ADS-C whereas the uplink is based on CPDLC. This combination is sometimes referred to as “FANS-C” . The process can be repeated, should unexpected changes (e.g. weather) occur. The aircraft’s FMS incorporates a “Required Time of Arrival” (RTA) function, which enables the aircraft to comply with a CTA constraints  .
From a global perspective, we are in a very interesting timeframe, as initial trajectory sharing is being tested in different locations and the transition to TBO is planned for the next decade . It remains to be seen, how smooth the implementation takes place, especially in Europe, where the air traffic management landscape is still very fragmented…
 Lee, Neuman & Hardy, 4D-Area Navigation System Description and Flight Test results, Ames Research Center, NASA, 1975
 Enea & Porretta, A COMPARISON OF 4D-TRAJECTORY OPERATIONS ENVISIONED FOR NEXTGEN AND SESAR, SOME PRELIMINARY FINDINGS, 28th International congress of the aeronautical sciences, 2012
 AIRBUS, FANS-C on A320 of EasyJet, https://www.airbus.com/newsroom/press-releases/en/2019/03/airbus-delivers-first-fansc-equipped-a320-to-easyjet.html , 01/2020
 EU commission, DIGITS, https://cordis.europa.eu/project/id/731818 , 01/2020
 Royal Netherlands Aerospace Center (NLR), Trajectory Based Operation, https://nlr.org/areas-of-change/shift-clearance-based-trajectory-based-air-traffic-control/ , 01/2020
 ICAO AIR TRAFFIC MANAGEMENT REQUIREMENTS AND PERFORMANCE PANEL (ATMRPP), Global TBO concept, version 0.11
 NASA, An Advanced Trajectory-Based Operations Prototype Tool and Focus Group Evaluation, NASA/TM–2017-219670
 ICAO, 2016-2030 Global Air Navigation Plan, Doc 9750-AN/963 Fifth Edition, 2016
 Jung & Lee, 4D Trajectory Modeling for ATC Simulation, Asia-Pacific International Symposium on Aerospace Technology, Seoul, Korea, 2017