Automatic Dependent Surveillance(21)

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10 Minimum Intent Acquisition Range and Reporting Requirements
10.1 Transmission Update and Acquisition Range Requirements
DO-242 requirements on update rate for TCPs are partly implicit and are not directly related to the functional requirements for applications, i.e. “The rate shall be sufficient to ensure continuous positive assessment by the receiving aircraft at least 2 minutes prior to reaching the closest point of approach for class A2 equipage (5 minutes… for class A3)”.1 In addition, TCP update rates as a function of range are specified in Table 3-4 as equal to the coast interval for state vector reports, with 95% confidence of reception.1 Moreover, most TCP intent data is static or slowly changing until the time to TCP is imminent or the TCP point is sequenced.
The update requirements for TS and TC reports are specified, as in DO-242, as a function of range and in terms of the update interval TU for 95% reception probability of a single TS or TC report. Table 8 summarizes the new DO-242A minimum requirements for update interval as a function of range. TC reporting requirements are not implemented in DO-242A, but proposed values are specified in an appendix. It is recommended that TS and TC information be updated more frequently if there has been a recent major change in intent or a newly initiated intent broadcast, e.g. the recommended update interval TU for TS and TC reporting at 40 NM range after an intent change is 12 seconds compared to 18 seconds below.
Table 8. ADS-B Update Requirements for Intent Reporting (Minimum 95% Update Interval Requirements in Seconds)
Report Type  R ≤ 20 NM  R=40 NM  R=50 NM  R=90 NM  R = 120 NM  Notes 
Equipage Class  A2  A2  A2  A3  A3  (1) 
Required  Required  Desired  Required  Desired 
TS Report  12  18  23  (2) 
TC Report (Proposed)  12  18  23  41  54  (2) 

Notes for Table 8:
1.  
For a Level A2 system, 40 NM acquisition range reception is required, 50 NM is desired. For a Level A3 system, 90 NM acquisition range reception in the forward direction is required, 120 NM acquisition range reception forward is desired.

2.  
Formula for update interval (TU)isTU = max (12, 0.45*Range). This formula allows for up to a 15% loss in range to update intent reports, with 95% confidence.

3.  
Table 8 is based on an air-air en route scenario between two aircraft closing at 1200 knots, which is considered a worst case for deriving range requirements for ADS-B conflict alerting.

4.  
The coast interval for report validity is two times the update interval TU at the last reported range for that ADS-B participant. If no new intent data is received within the coast interval, the associated data are considered invalid.

 

10.2 TS and TC Report Broadcast Conditions
TS reports should be broadcast whenever the ADS-B participant is a Level A2 or A3 system, the flight director or autopilot is engaged consistent with the axis of the target states being broadcast and when either of the following conditions apply:
1.  
Target altitude or an acceptable substitute for target altitude is available from the aircraft automation system, or

2.  
Target heading or target track is available from the aircraft automation system.

The following conditions govern the determination of TC report broadcast status for each TCP. These conditions can be applied independently to the horizontal and vertical axis parameters:
1.  
If the transmitting aircraft does not have an autopilot or flight director engaged, then no TC reports are generated. If the aircraft only supports a single axis autopilot or flight director, then the complementary axis data fields for TC reports are marked “unavailable”.
　
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