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As part of the Advanced Air Transportation Technologies (AATT) project, NASA is investigating a free flight concept known as Distributed Air Ground Traffic Management (DAG-TM).3 In this environment, air crews would work collaboratively with air traffic service providers to resolve airspace and traffic conflicts and enable user-preferred routing. Intent information is needed to support separation assurance applications and ensure situation awareness for all parties.
ADS-B intent information is also intended to enable advanced air-ground applications such as sequencing and merging of terminal area flow streams4 and use of precision trajectory separation concepts for aircraft arrival and departure flows in congested airspace.5-6 For these applications, aircraft intent exchange allows collaboration between air and ground personnel on detailed trajectory plans. Mutual understanding of aircraft intent should help ground controllers take advantage of precise navigation capabilities typical of advanced Flight Management Systems (FMS). System benefits would include more efficient airspace use and user-preferred routing.
The type of intent information considered for ADS-B broadcast is limited to generic trajectory segment information that does not require detailed knowledge of airplane avionics, e.g. the use of standard lateral leg types for horizontal flight segments, and the use of climb, cruise and descent flight segments with specified end-points for vertical flight transitions. The overall objective is to describe intended trajectory segments in a generic way, avoiding the use of airplane specific guidance implementations and control modes.
The original ADS-B MASPS specifies only a limited range of intent information, i.e. the use of 3-D and 4-D TCPs as endpoints of the current and next flight segment, respectively. Several reasons for expanding the use of intent beyond that in DO-242 and ways in which these issues are addressed include:
(1)
The original ADS-B TCPs needed revision to reduce ambiguity in representing and predicting flight trajectories. One problem with DO-242 is that TCPs alone do not adequately describe either the current intended trajectory segment, or the intended trajectory change at the endpoint TCP. DO-242A replaces the old TCP and TCP+1 reports with a new Trajectory Change (TC) report. In addition to containing TCP-related information, the TC report provides information on connecting flight segments, conformance with the broadcast trajectory, and waypoint altitude constraints that may not involve a trajectory change.
(2)
ADS-B intent should better reflect the operational capabilities of existing and future aircraft avionics systems, i.e. to represent autopilot target values when flying in less automated tactical modes, and to include a wide range of aircraft automation systems ranging from current 2-D Area Navigation (RNAV) systems to existing and future FMS-based precision Required Navigation Performance (RNP) RNAV systems. DO-242A incorporates a new Target State (TS) report that provides short-term tactical intent information from the onboard system actively supporting aircraft guidance. Primary elements of the TS report include the target altitude (next level-off altitude or commanded holding altitude) and target heading or track angle (depending on whether the aircraft is controlled to air-referenced heading or ground-referenced track angle).
(3)
ADS-B systems will need expansion to better reflect longer-term intent, i.e. beyond that represented by next and next+1 TCPs. Some operational concepts envisioned for ADS-B could require trajectory prediction times in excess of ten minutes look-ahead or longer.7 Moreover, trajectory changes may occur quite frequently in the terminal area and more TCPs are required than in en route applications for short-term separation and flow planning. These changes are also consistent with recently formulated Eurocontrol ADS-B requirements.8 The TC report includes data management provisions to handle at least 4TCPs.
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