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ATCMI value of two indicates that all current TC reports are to be marked as invalid. This value can be used to signal ADS-B applications that TC intent is no longer available, or as a precursor to reissue of new TC intent after a major change in ADS-B participant intent. Upon receipt of a TCMI value of two, the report assembly function should set the horizontal and vertical data available fields of each TC report to ‘data unavailable’ so that ADS-B applications will no longer use any previously received TC intent data.
ATCMI value of three indicates a major change in intent in one or more TC reports with sequence number greater than zero. In this case, the TC+0 report is updated and refreshed as indicated above, except that the track-from TCP value may have changed. If the effect of the intent change invalidates the current track-from TCP in TC+0, then the ADS-B transmit subsystem should send the updated value of track-from TCP for TC+0 report updating. All subsequent TC+n reports should then be marked invalid as described above for a TCMI value of two.
A TCMI value of four indicates a major change in TC+0 reporting with subsequent TC+n reports still valid, i.e. this value indicates that new intent messages are forthcoming or have just been received that contain major intent changes for the active flight segment. Upon receiving a TCMI value of four, the current TC+0 report should be marked as invalid and all subsequent TC+n reports updated and refreshed as described above, except that there could be a change in the track-to TCP value for the TC+1 report. If the effect of the intent change invalidates the current track-to TCP in TC+1, then the ADS-B transmit subsystem should send the updated value of track-to TCP for TC+1 report updating.
A TCMI value of five indicates that a new TCP has been inserted prior to the current TC+0 report, i.e. the active flight segment has been changed to insert a horizontal routing change or a vertical constraint prior to the currently active TCP. Figure 13 shows an example scenario where a new TCP has been inserted in order to change path length and time of arrival at subsequent TCP points. Upon receiving a TCMI value of five, the current TC+n reports are given incremented n+1 sequence numbers (limited to a maximum value of four), and updated with incremented TC cycle numbers and times as described above. The current TC+0 report field is then marked as invalid until newly issued TC+0 report data are received. If the effect of the intent change invalidates the current track-to TCP in TC+0, then the ADS-B transmit subsystem should send the updated value of track-to TCP for updated TC+1 reporting.
12 Conclusions and Future Plans for Intent Consideration
12.1 TS and TC Report Format Validation
Although considerable effort was expended in developing and evolving the TS and TC report formats for ADS-B intent broadcast in DO-242A, this work did not include simulation or detailed analysis of newly introduced ADS-B intent formats. Future simulation and flight test studies of proposed operational concepts using intent broadcast are needed to validate the formats and intent structure developed to date, and to further evolve ADS-B intent standards for future revisions of the ADS-B MASPS. This work needs to be coordinated with the operational groups developing intent-based operational concepts in order to further mature the use of airborne intent for surveillance and separation assurance applications. TS and TC report elements implemented in DO-242A can either be supported by current avionics or may be substituted with commonly available information. Additional coordination is needed with avionics manufacturers to ensure that all TS and TC report elements, including those provisioned for later MASPS versions, can be fully supported and are available for information exchange over standard data buses.
12.2 Intended Airspeed Reporting
DO-242A of the MASPS limits intent reporting to horizontal and vertical target states and trajectory change points. Other types of intent, such as target airspeed and target vertical rate, were not considered for TS reporting in DO-242A, since there seems to be less agreement as to the importance and operational utility of such data. There are some applications such as in-trail approach monitoring where intended airspeed may be extremely valuable for intent reporting,
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