Required Navigation Performance - Application of Performance Monitoring and Alerting To Aircraft

Application of Performance Monitoring and Alerting To Aircraft

Although the TSE can change significantly over time for a number of reasons, including those above, the RNP navigation specifications provide assurance that the TSE distribution remains suitable to the operation. This results from two requirements associated with the TSE distribution, namely:

• the requirement that the TSE remains equal to or better than the required accuracy for 95% of the flight time; and
• the probability that the TSE of each aircraft exceeds the specified TSE limit (equal to two times the accuracy value) without annunciation is less than 10 −5.

Typically, the 10−5 TSE requirement provides a greater restriction on performance. For example, with any system that has TSE with a normal distribution of cross-track error, the 10−5 monitoring requirement constrains the standard deviation to be 2 x (accuracy value)/4.45 = accuracy value/2.23, while the 95% requirement would have allowed the standard deviation to be as large as the accuracy value/1.96.

It is important to understand that while these characteristics define minimum requirements that must be met, they do not define the actual TSE distribution. The actual TSE distribution may be expected to be typically better than the requirement, but there must be evidence on the actual performance if a lower TSE value is to be used.

In applying the performance monitoring requirement to aircraft, there can be significant variability in how individual errors are managed:

• some systems monitor the actual cross-track and along-track errors individually, whereas others monitor the radial NSE to simplify the monitoring and eliminate dependency on the aircraft track, e.g. based on typical elliptical 2-D error distributions.
• some systems include the FTE in the monitor by taking the current value of FTE as a bias on the TSE distribution.
• for basic GNSS systems, the accuracy and 10−5 requirements are met as a by-product of the ABAS requirements that have been defined in equipment standards and the FTE distribution for standardised course deviation indicator (CDI) displays.

It is important that performance monitoring is not regarded as error monitoring. A performance monitoring alert will be issued when the system cannot guarantee, with sufficient integrity, that the position meets the accuracy requirement. When such an alert is issued, the probable reason is the loss of capability to validate the position data (insufficient satellites being a potential reason). For such a situation, the most likely position of the aircraft at that time is exactly the same position indicated on the pilot display. Assuming the desired track has been flown correctly, the FTE would be within the required limits and therefore the likelihood of the TSE exceeding twice the accuracy value just prior to the alert is approximately 10−5. However, it cannot be assumed that simply because there is no alert the TSE is less than twice the accuracy value: the TSE can be larger. An example is for those aircraft that account for the FTE based on a fixed error distribution: for such systems, if the FTE grows large, no alert is issued by the system even when the TSE is many times larger than the accuracy value. For this reason, the operational procedures to monitor the FTE are important.