12.3 Additional TC Leg Transition Types
The TC leg types that were considered for DO-242A are limited to basic leg types for horizontal and vertical transitions. There are other leg types that are potentially available from FMS systems, e.g. procedure holds, Mach/CAS cross-over speeds on climb and descent, planned changes in vertical rate or flight path angle, longitudinal deceleration prior to meter fix entry, etc. One potential leg type is an “Interpolated Track to Fix” type that would be similar to a Track to Fix type, except that the ADS-B transmit subsystem could potentially interpolate additional TCP points in order to assure that a TCP is available for broadcast within operationally relevant time limits for TCP broadcast. Expansion of TCP leg types will be re-examined for future MASPS use based on operational value and future development of separation assurance operational concepts.
12.4 RNP based Intent Integrity Monitoring
The extent to which intent data can be used for critical separation assurance applications will depend on the integrity of such data, i.e. the reliability of trajectory path following and staying within specified bounds of the intended path. The RNP RNAV MASPS6 specify integrity containment bounds for path following which can serve as a basis for intent integrity metrics for ADS-B reporting, provided such aircraft are RNP qualified. In future versions of the ADS-B MASPS, it is expected that RNP metrics and altitude “windows” may be used to express aircraft capability to stay close to the broadcast path, and to fly within specified trajectory bounds. This version of the MASPS did not include RNP integrity metrics since operational concepts for trajectory based separation assurance are not considered sufficiently mature and only limited operational experience is available to assess the value of RNP systems. The material below summarizes the overall concept of RNP containment integrity and conformance monitoring.
In the horizontal plane, RNP accuracy and integrity bounds are used to describe the expected lateral path deviation and the allowable lateral path deviation for path conformance. For example, an RNP-1 RNAV system is certified to stay within 1 NM of the intended lateral routing at least 95 % of the time, including turn maneuver periods. The RNP integrity bound for conformance monitoring is twice the accuracy value, i.e. a conformance warning is generated by the RNAV system if the aircraft deviates from the intended lateral path by more than 2 NM. If TC intent data is to be used for critical separation assurance applications, such as detecting and resolving flight path conflicts, then it may be necessary to expand TC report data to incorporate lateral RNP RNAV capability and a lateral RNP conformance flag (element 7g of Table 4) for assessing the integrity of horizontal TC report data. The transmitted conformance flag would indicate that the aircraft was capable of detecting a loss of RNP containment, and that the current lateral path deviation was within allowable limits for lateral path conformance. Since the broadcasted intent data could potentially result in misleading predictions of the future intended aircraft path, conformance monitoring on the ADS-B receive side may be necessary as well. Figure 14 illustrates the concept for user conformance monitoring of lateral path predictions for a horizontal turn maneuver. In this example, the aircraft is moving along an intended path toward the left side TCP Start-of-Turn point to the right side TCP End-of-Turn TCP point. As the aircraft approaches the RNP route bound, a conformance alert is generated, cautioning the data user of a potential integrity error in the broadcast path. When the aircraft flies outside the intended RNP containment region a conformance warning is generated, indicating an intent integrity error.
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