8 Horizontal and Vertical TC Types
A limited number of basic horizontal and vertical TC types are accommodated in DO-242A of the MASPS to enable representation of common trajectory flight segments for flight path prediction. It is expected that future revisions of the MASPS will accommodate additional TC types, depending on evolution of airplane avionics and on application needs, e.g. additional lateral types such as hold patterns and additional vertical types such as waypoint altitude constraints. Some of the TC types such as Direct-to-Fix transitions and Fly-by-Turns are needed to represent non-precision trajectories where the inertial path over the earth is not entirely predictable. Other TC types such as Course-to-Fix, Track-to-Fix and Radius-to-Fix turns are needed to represent precision RNP flight legs. (In the future, intent integrity concepts may be introduced to monitor conformance to horizontal and vertical RNP bounds.6 This version of the MASPS simply uses precision and non-precision TC types.) The vertical TC types include maintain or level at a Target Altitude (which may also be represented in the TS report), and traditional Top-of-Climb and Top-of-Descent trajectory changes. Estimated altitudes are provided when transitioning towards a target altitude at a lateral trajectory change. Altitude constraints are also provisioned as a future TC type.
8.1 Horizontal TC Types
8.1.1 Geodesic Path (Straight Course) to Fix Lateral Transition
The Geodesic Path to Fix transition includes both Course to Fix (CF) and Track to Fix (TF) leg types. The lateral path is defined by a course or track angle to a 2-dimensional waypoint that defines the endpoint TCP (see Figure 9). This TC type is typically followed by a routing change,
i.e. a Direct to Fix (DF) transition or a Radius to Fix (RF) turn. The case where a CF or TF leg ends with a Fly-By Turn is a separate case since more parameters are needed to represent Fly-By turn cases. From the viewpoint of the transmitting aircraft, CF and TF leg types are somewhat different since the latter represents a transition between a “from” waypoint toward the “to” waypoint / TCP point. However, from the receiving system viewpoint there is no difference between a CF and a TF leg ending at a TCP, since the “from” waypoint is only implicitly represented by the Track to TCP. Thus, both cases are combined into a single TC type. Time-to-Go to TCP is also required in order to properly sequence this and other flight segments.
Endpoint
Track to TCP
Figure 9. Geodesic Path to Fix Lateral Transition
8.1.2 Fly-By Turn Transition (Including CF or TF to Fly-By Turn Segment)
The Fly-By Turn TC report implicitly represents two flight segments, i.e. a straight segment such as a Course-to-Fix directed toward the fly-by waypoint, and the actual fly-by turn transition to the track-from course. Figure 10 shows the defining elements of a fly-by turn, other than turn radius and turn center. Fly-by turns are considered non-precision leg types since the start-of-turn point and end-of-turn points constructed using turn radius are rough estimates of turn behavior,
i.e. the actual path over earth can be substantially different due to winds and flight technical error. However, fly-by turns save message bandwidth compared to the use of explicit TCPs for start and end of turn segment. Required elements include the fly-by latitude, longitude and time-to-TCP (time to fly-by point sequencing), and track-to TCP, turn radius, and track-from TCP. Turn direction (one bit indicator) is also available for some systems and may be desirable for ADS-B transmission, but is not required for path reconstruction. Since end-of-turn is implicit, the TC report is sequenced when the track angle state captures the track-from TCP.
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Figure 10. Fly-By Turn Transition Showing Turn Start and Turn Endpoints
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