Trajectory-based operations
Combining the foundational building blocks of SWIM, A-RNP, and time-based flow management.
By David Bjellos
ATP/Helo. Gulfstream G650
Senior Contributor
Trajectory-based operations (TBOs) represent a foundational concept within air traffic management (ATM) systems, enabling aircraft to operate along negotiated 4-dimensional (4D) trajectories defined by latitude, longitude, altitude, and time.
In 2004, the US Federal Aviation Administration (FAA) initiated system-wide upgrades to its Next Generation Air Transport System (NGATS) program, which we now know as NextGen. When combined with Advanced Required Navigation Performance (A-RNP) and improved airborne collision avoidance systems, TBOs will improve significantly trajectory predictability, airspace capacity, and overall operational efficiency.
In this article we will examine the operative framework of TBOs and explain how A-RNP capabilities support its implementation, particularly in high-density and complex airspace environments.
We are approaching quickly the final implementation of components that will enable trajectory-based operations within the US NAS. The vision of “free flight” from the 1990s will be realized with TBOs through an expanded system of digital communications called DataComm. CPDLC is one form recognizable by the pilot population, but dozens more exist within the architecture of air traffic management, including system-wide information management (SWIM) and en-route automation modernization (ERAM).
Traditional ATM continues to rely on tactical control and ground-based surveillance, where controllers issue vectors and altitude instructions to maintain separation. This increasingly outdated paradigm limits system efficiency because aircraft trajectories are modified constantly in real time. TBOs shift this model toward strategic trajectory management.
In a TBO environment, the aircraft and ATM system share and manage a pre-negotiated trajectory that is updated continuously through data exchange (data communications, or DataComm). The objective is to optimize traffic flow while maintaining safety and minimizing controller intervention.
A-RNP plays a critical, enabling role in this transition. Existing RNP defines the level of lateral and track adherence accuracy required for a specific operation, incorporating onboard performance monitoring and alerting.
By guaranteeing precise adherence to defined flightpaths, A-RNP provides the navigation integrity required to execute tightly managed trajectories within the TBO framework. For an example, see graphic for FNC (Madeira, Portugal) Approach RNP Z Rwy 05 on page 18.
TBO concept
TBO centers on the management of 4D trajectories from gate to gate. Each flight is assigned a trajectory profile that specifies spatial and temporal (time) constraints. These trajectories are shared across multiple ATM stakeholders, including flight operations centers, flow management units, and air traffic controllers.
Implementation relies on several technological components, such as System Wide Information Management (SWIM) networks for terrestrial data exchange, Time-Based Flow Management (TBFM) tools for arrival or departure sequencing, data communications such as Controller-Pilot Data Link Communications (CPDLC), and advanced Flight Management Systems (FMS) capable of time-controlled navigation.
The integration of these systems allows real-time trajectory negotiation and updates throughout the flight. For example, arrival times at inbound (STAR) metering fixes and departure (SID) path terminators can be adjusted strategically during the flight rather than through tactical vectoring.
Trajectory-based operations (TBOs) within the FAA National Airspace System (NAS) represent a paradigm shift from clearance-based, sectorized control toward a time- and trajectory-managed environment driven by systemwide data exchange, performance-based navigation, and predictive analytics. At their core, TBOs operationalize a shared 4D trajectory (latitude, longitude, altitude, and time) as the primary unit of planning, negotiation, and execution.
The building blocks of NextGen
SWIM is the backbone of NextGen. As part of the NextGen portfolio of programs, SWIM is critical to ensuring all participants can communicate with each other. It will allow airline operations, air traffic managers and controllers, military, and other stakeholders to share information in near real time. It’s essentially DataComm hardware/software improvements for ground-based stakeholders.
SWIM will complement (but not replace) human-to-human with machine-to-machine communication and improve data distribution and accessibility regarding the quality of the information exchanged.
ICAO Doc 9854, Global Air Traffic Management Operational Concept, envisages the application of SWIM to promote information-based ATM integration. It is important to understand that SWIM is a global initiative, while NextGen is the US application of SWIM.
The second building block is A-RNP – a navigation specification that enhances aircraft precision, safety, and operational efficiency by combining innovative RNP capabilities. It optimizes flightpaths in challenging, mountainous, or congested airspace, using features like scalable RNP, Radius-to-Fix (RF) legs, and parallel offsets to improve airport access.
A-RNP procedures extend traditional performance-based navigation by enabling highly precise path definitions and containment boundaries.
Typical characteristics include scalable RNP, RNP values as low as 0.1 nm, RF curved path segments, vertical navigation with defined (sometimes multiple) path angles, and onboard monitoring and alerting.
These capabilities ensure that aircraft can follow accurately complex trajectories designed for terrain-constrained environments or high-density terminal areas.
Scalable A-RNP differs from existing RNP Authorization Required (AR) approaches in that the RNP value for multiple segments of the approach can be different, depending on the level of lateral clearance required.
Current RNP (AR) approaches retain the same value from initial approach point through to missed approach point (eg, RNP 1.0 or 0.3).
Scalability enhances terrain avoidance/track observance and allows for highly-defined paths through difficult terrain with high-confidence track adherence, such as SIR (Sion, Switzerland) or CMF (Chambéry, France).
Within a TBO framework, advanced RNP contributes in 3 principal ways. The first is trajectory accuracy.
Precise lateral and vertical navigation ensures the aircraft remains within the planned trajectory envelope, supporting the time-based constraints required for 4D trajectory management.
The second is predictability. Because aircraft adhere consistently to defined navigation performance standards, ATM systems can model aircraft behavior with greater confidence.
This predictability improves traffic flow management and arrival/departure sequencing. And third is airspace capacity and efficiency.
Advanced RNP enables closer route spacing and optimized descent profiles, including continuous descent operations (CDOs). These features reduce fuel burn, emissions, and controller workload.
However, there are operational considerations. In order for A-RNP to support TBOs through functional efficiency, several operational requirements must be met. Aircraft must be equipped with certified RNP avionics, and crews must be trained in RNP operational procedures.
Air navigation service providers (eg, state AIPs, Jeppesen, Lido, NAVBLUE) must publish validated procedures and maintain performance monitoring frameworks.
In addition, robust data communication infrastructure is required to support trajectory negotiation and updates (currently FANS 1/A+, 2, 3, and ATN B1/B2). DataComm and advanced RNP are the primary partners in TBOs, and are equally important.
The third building block of NextGen includes the components of time-based management (TBM). These are:
1. Required time of arrival (RTA). The objective of an RTA operation is for an aircraft to cross a defined point in space at a specified clock time. This time is also referred to as the RTA. Many corporate avionics suites currently have RTA capability but as yet are severely under-utilized. However, this will change as TBOs are introduced and implemented – much like CPDLC was a decade ago.
The beginnings of RTA can be seen in the latest version of ICAO Document 007, North Atlantic Operations and Airspace Manual. The term “estimated time over” (ETO) has replaced “estimated time of arrival” (ETA) and represents the FMS-calculated arrival. Through existing ADS-C, this can be accurate to the minute. Future ETO criteria will be improved further with sub-minute accuracy. More to follow here.
2. Interval management (IM). The IM operational concept comprises a set of operational applications in which the flight crew of a trailing aircraft follows FMS Interval Management (FIM) capability-generated speeds (FIM speeds) to achieve or maintain a spacing goal relative to an ATC-specified “lead aircraft.”
A comprehensive graphic of the multiple applications of ACAS X. The primary difference between ACAS X and existing TCAS II is the introduction of probabilistic vectors for traffic avoidance. TCAS II currently relies on legacy enterprise software and associated (outdated) algorithms for one-dimensional traffic avoidance maneuvers – specifically vertical, either up or down. ACAS X will eventually have two-dimensional options for traffic avoidance, ie left/right turns and up/down vertical components.
The use of IM operations will yield precise inter-aircraft spacing and improved spacing consistency by enabling more frequent speed adjustments than is currently possible. Precise and consistent spacing translates into increased arrival throughput when aircraft can be spaced closer to desired separation minimums without increasing the need for controller interventions.
IM combines ground-based and flight deck capabilities to provide air traffic controllers with another tool to manage traffic flows. Ground-based automation assists ATC in identifying and issuing IM clearances to merge and space aircraft safely and efficiently, and in monitoring the progress of those IM operations.
Meanwhile, flight deck capabilities allow the pilot to conform to the IM clearance by providing FIM speeds to achieve and maintain a designated spacing goal relative to another aircraft.
Note that in-trail spacing procedures have been in use across the North Atlantic High Level Airspace (NAT HLA) for more than a decade with excellent results. IM can be measured in miles in trail (MIT) or minutes in trail (MINIT).
The final pillar of NextGen is the integration of Airborne Collision Avoidance System X (ACAS X) into TBO.
The National Airspace System (NAS) is becoming crowded, so separating aircraft safely in the new paradigm of digital communications will be accomplished by ACAS X, and must be framed against 2 rapidly increasing and converging trends – aggregate airspace density and complexity, and the continued transition toward data-centric operations.
ACAS X is not merely an incremental upgrade to Traffic Collision Avoidance System II (TCAS II), but a foundational enabler of NextGen airspace operations.
By introducing probabilistic decision-making, reducing unnecessary interventions and supporting diverse aircraft types, ACAS X resolves key incompatibilities between legacy collision avoidance systems and modern ATM hardware/software.
Implementation of ACAS X will be one of the final steps prior to the introduction of TBOs.
Conclusion
TBOs represent a significant evolution in global ATM strategy, shifting ATM from tactical control to strategic trajectory planning.
They evolved from the 1990s concept of “free flight” with more robust guardrails and performance specifications. A-RNP provides the navigational precision and integrity necessary to execute these trajectories reliably.
As avionics capabilities, data communications, and ATM automation continue to mature, the integration of TBOs and advanced RNP will play a central role in increasing airspace capacity, improving operational efficiency, and enhancing safety in our increasingly complex, multi-dimensional air traffic environment.
TBOs will transform the NAS into a predictive, data-centric system where time-constrained trajectories replace tactical (controller) vectors.
Implementation success hinges on high-fidelity trajectory modeling, powerful data exchange via SWIM, and robust integration between airborne and ground-based systems.
Current implementation remains constrained by equipage and hardware/software interoperability challenges.
TBOs are truly foundational to achieving the myriad demands of future air traffic, both within the NAS and globally. Now, you just have to convince the boss to show up on time.
Senior contributor David Bjellos has been writing for PP since 2004. He is an active airman flying a G650 based in south Florida.