PROG, RNP AND THE DISPLAYED POSITION ESTIMATE
PROG combines phase-related information with navigation monitoring. REQUIRED is the reference requirement used for the accuracy comparison; ESTIMATED describes the system's position estimate. The supplied guide calls that estimate EPE, Estimated Position Error. Discussions using ANP terminology must be tied to the actual quantity displayed rather than used to rename every estimate as an independently certified performance result. [66, PDF pp. 178-179,197-198,239,486-502]
1. Read the navigation state. Inspect GPS PRIMARY or the applicable navigation/accuracy indication. In the source, GPS PRIMARY reflects the relevant accuracy/integrity conditions, while EPE is computed from available navigation inputs. A green route line alone does not establish that state. After alignment, a navigation-source change or a reported discrepancy, read the actual monitoring information before continuing to rely on the map. [66, PDF pp. 178-179,197-198,239,486-502]
2. Compare requirement and estimate. Read REQUIRED and ESTIMATED in their units, then HIGH/LOW. The cited standard compares EPE with the required value and describes downgrade/upgrade messages. Do not increase the required RNP value to make LOW become HIGH when the procedure requires the tighter value. That would change the comparison rather than improve the aircraft's position knowledge. [66, PDF pp. 178-179,197-198,239,486-502]
3. Add an independent reference. Enter an identified navaid/fix on BRG/DIST and compare with suitable raw bearing/DME indications. Verify the navaid and source first. A great-circle measurement to a database point and a raw radio measurement provide different evidence, which is why their agreement can be useful. Selecting the PROG reference does not alter the active route. [66, PDF pp. 178-179,197-198,239,486-502]
4. Read phase information separately. CRZ, OPT and REC MAX provide different references. Planned cruise is a preparation input; optimum is an economic/performance result, and recommended maximum is not a clearance or automatic target. The active phase can change what is displayed. In descent, do not repair a deliberately blanked cruise reference merely because it was visible during cruise. [66, PDF pp. 178-179,197-198,239,486-502]
5. Handle a downgrade through the applicable procedure. Check reference identity, contributing source/position information and actual approach requirements. Clearing a scratchpad message does not restore navigation capability or change a procedure's requirement. Avoid exceptional manual position updating as a normal programming fix. The completion point is an explainable navigation state and a suitable operational plan, with the requirement and estimated performance still honestly represented. [66, PDF pp. 178-179,197-198,239,486-502]