The ADS-B intent revisions summarized in this document address the above issues. The resulting intent specifications are intended to be a basis for DO-242A implementation, and to serve as an incremental basis for future development of ADS-B applications.
2 Scope of DO-242A Intent Changes
One of the challenges in developing and evolving intent information for ADS-B is that most current aircraft avionics, including many advanced digital FMS-based systems, do not output much intent information on avionics buses for downstream use by avionics other than those directly used to communicate to the pilot or to navigate, guide, or control an airplane. Changes to the ADS-B MASPS address this situation in two ways: (1) allowing aircraft which output some intent information to communicate such intent when appropriate through the TS report and TC report formats, and (2) providing intent provisioning in the report formats for future evolution and introduction of more comprehensive intent data. DO-242A provides an incremental approach to intent broadcasting by allowing for partial broadcast of limited intent while accommodating evolution to more comprehensive intent data on both an individual aircraft basis as avionics systems are upgraded, and with further intent evolution anticipated in future revisions to the ADS-B MASPS.
The newly introduced TS reports allow for broadcast of Target Altitude,and Target Heading or Track data used for current path guidance. Since full implementation of target state data may depend on FMS or autopilot mode information not currently available on any avionics bus, DO-242A allows for partial implementations of target states based on information which is available for input to an ADS-B transmit system. For example, if only autopilot-based selected altitude is available for TS reporting, then it is allowed to broadcast such information with appropriate status indicators, even if the aircraft’s next intended level off altitude may be an unknown FMS target value. However, the fact that the aircraft is only capable of broadcasting selected altitude and autopilot modes is transmitted in the TS report, to avoid interpreting selected altitude as the probable next level-off state.
The TC reports introduced in DO-242A consist of a number of horizontal and vertical flight segment and TC types which are commonly used, have standard segment and TCP parameters, and are available as potential outputs on an ARINC data bus, e.g. the 702A trajectory bus.9 The horizontal flight segment types include Course-to-Fix (CF), Track-to-Fix (TF), and Direct-to-Fix (DF) leg types, and Fly-By and Radius-to-Fix (RF) turn segments. (See Section 8 for further explanation of these leg types.) Fly-over turns can also be modeled by appropriate use of the above leg types in conjunction with a DF or TF flight segment to model the turn transition to a specified end-fix. The vertical flight segments include initial climb to Top-of-Climb, flight at cruise altitude to Top-of-Descent, i.e. start of the descent phase, and some level-off transitions. In addition, target altitude as the intended end of a vertical transition is allowed as a TCP. RNAV systems that only output 2-D TCPs are also allowed, i.e. the vertical TCP components are marked as “not-available”.
Some parameters and leg types that are important for intent broadcast and are not currently available as inputs on a data bus, or are not sufficiently developed, are provisioned in the TS report and TC reports, but are not fully implemented in DO-242A. Broadcast space is allocated for these elements, but manufacturers are not required to support them at this time. This provisioning should facilitate an easier path to implementation as future research demonstrates their utility in an operational environment. Examples of provisioned elements include operational intent validity (used for conformance monitoring), altitude constraint parameters (“At” and “At and Above/Below”), and leg parameters such as turn radius which may not be available for some RNAV / Lateral Navigation (LNAV) systems. The validity data would provide guidance system status for TS report target values, and navigation system conformance for TC reports and are considered essential for critical separation assurance applications. Current FMS / Vertical Navigation (VNAV) systems provide the ability to specify altitude constraints at specified waypoints or fix locations which may constrain the FMS planned vertical trajectory. Broadcasting of such constraints is important for predicting vertical trajectory level-offs and changes in vertical path to meet such constraints. However, these constraint points are not generally available from FMS systems, and are not available on an ARINC data bus today. Consequently, these parameters and leg types are to be provisioned for later version ADS-B MASPS adoption.
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