RFI Appendix A.pdf

9/22/2023 Appendix A: Addi�onal informa�on may be helpful in submi�ng their responses:

In some applica�ons the characteriza�on of the error distribu�on in terms of accuracy such as a standard devia�on or the 95th percen�le can be sufficient. However, when PNT systems are used for safety cri�cal applica�ons, addi�onal informa�on about the tails of the error distribu�ons are needed.

Such applica�ons usually define an upper bound, referred to as the Alert Limit (AL), and will have an associated probability p (nominally p=1e-7) for the error to not exceed this AL. In other words, the measurement error will be below AL 99.99999% of the �me. The integrity informa�on requested about the PNT system corresponds to nominal intended opera�onal condi�ons (i.e. Fault-free condi�ons), and seeks the AL value(s) that will bound the measurements 99.99999% of the �me? Usually this is done by collec�ng extensive measurements with extrapola�on of the error distribu�on tails or by a combina�on of system design guarantees + measurements.

With respect to con�nuity: a well-designed PNT system will have mapped Fault condi�ons with associated monitors that puts out an alarm when one of these Faults are detected. The term Fault here is in the sense of having a system malfunc�on or changes/challenges in the environmental or opera�on condi�ons that modifies the nature of the error distribu�on on which accuracy and integrity informa�on were based. The con�nuity requirement over a period T, ensures that if a system is Fault-free at some star�ng �me 𝑡𝑡𝑡𝑡 it will con�nue to be Fault-free and therefore not issue an alarm between �me 𝑡𝑡𝑡𝑡 and 𝑡𝑡𝑡𝑡 + 𝑇𝑇, or more precisely, have a very low probability of encountering a fault and issue such alarm between 𝑡𝑡𝑡𝑡 and 𝑡𝑡𝑡𝑡 + 𝑇𝑇. The mo�va�on for this, is that for an opera�on relying on a PNT system to perform its intended func�on, the operator has to have sufficient confidence that if the system is available (i.e. opera�ng under the nominal condi�on error distribu�on) at the start of the opera�on it will con�nue to be available smoothly throughout the dura�on of the opera�on 𝑇𝑇 with a low probability of issuing an alarm in the middle of that opera�on. That is, the PNT system must have a low probability of flickering between the available and unavailable states throughout opera�on. For example, in avia�on, if the naviga�on system is being relied on in the approach phase for say (e.g. LPV approach) and starts to flicker in the middle of the descent, it can impact the efficiency and safety of the opera�on.

While other applica�ons can have different �me scale requirements the concept is s�ll applicable.