2.  
A stable TTG must be obtained prior to generating intent messages for TC reporting. A TTG value is considered “stable” if the estimated TTG based on past information is consistent with the current TTG value, i.e. the difference between the estimated and current TTG value is less than some threshold value. (Specific rules for TTG stability will be determined during TC report format validation testing.)

Given that the above conditions are satisfied, an A2 level system should, as a minimum, broadcast TC+0 reports whenever the ADS-B participant is within 4 minutes TTG to the next trajectory change point, or as needed to meet the acquisition range requirements for A2 equipage as specified in Table 8. Similarly, an A3 level system should, as a minimum, broadcast TC reports whenever the ADS-B participant is within 8 minutes TTG to the affected trajectory change point, or as needed to meet the acquisition range requirements for A3 equipage as specified in Table 8. (In other words, an A3 system should broadcast all TCPs within 8 minutes TTG to the extent that is feasible for that participant.) These broadcasts should continue until the current flight segment is sequenced or a major change in intent occurs which requires reinitiating TC report intent broadcasts.
In addition to the above conditions for intent broadcasting, it is important for level A3 systems to achieve continuity of intent as active flight segments are sequenced. This may be achieved with minimum additional broadcast of TC reports by adhering to a maximum of one TC report with TTG greater than 8 minutes. If the TTG to TC+0 report is greater than 8 minutes, then only TC+0 reports should be provided. In the event that TTG to the first TCP (TCP+0) is less than 3 minutes, then it is desirable to broadcast a TC+1 report for intent continuity, even if the TTG to TCP+1 exceeds 8 minutes. The overall objective is to achieve at least 3 minutes TTG continuity of intent when feasible, and to prevent indiscriminant broadcast of TC reports that are not operationally relevant.

11 Trajectory Change Report Management
11.1 TC Report Synchronization and Refresh
It is assumed that ADS-B systems will require multiple messages to construct a complete TC report sequence when outputting multiple TC reports. It then becomes necessary to ascertain that whenever a TCP is sequenced or intent information is changed, that the TC reports are appropriately synchronized and that all TC reports are currently valid and have the correct TC sequence number. In order to achieve proper synchronization, all broadcast messages related to TC report intent need to contain some mechanism for validating TC report messages that originated together as a coherent group of sequenced TCP data, and for rejecting old TC report data that originated prior to the latest changes in intent information.
The means adopted of achieving TC report synchronization for DO-242A is to report a two-bit TC cycle number for all TC report related messages. All TC reports which are output at a common time of applicability should be checked to assure that the cycle number for the underlying messages is current and common to all TC reports, i.e. any intent data which contains an old cycle number should be purged and not reported with current TC report data. In the case where the change consists of sequencing (passing through) the TCP+0 point, a TC Management Indicator is provided in the TC+0 report to signal that the data in the TC+1 report, TC+2 report, and TC+3 report can be reused by decrementing the TC sequence number and updating the common time of applicability, i.e. TC+1 report becomes TC+0 report, etc. The TC cycle number is also updated in each refreshed report, so that the re-sequencing process is not repeated until a new TC cycle number is reported.
The TC cycle number should be incremented each time a major change in intent is detected by the ADS-B transmitting subsystem, i.e. the TC cycle number would cycle from 0 to 1 to 2 to 3 to 0 again as the transmitted intent sequence or intent data is changed. Simple changes in estimated values such as estimated altitude at a waypoint are not considered major changes in intent, nor would addition of a TC report with sequence number higher than those currently being reported. Major changes of intent typically would result in TC report re-sequencing or would involve changes in TC type associated with a pilot input, e.g. a “direct to” clearance that bypasses one or more current trajectory change points. The message synchronization process must assure that only currently valid TC report data is being reported and that each TC report at a common report time has a unique sequence number.
　
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