Significance of ‘GPS PRIMARY’ | Airbus A320

“GPS PRIMARY” is displayed when Flight Management Guidance System (FMGS) prioritizes GPS as the primary navigation source.

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FM uses following positions to navigate in decreasing order of priority:
IRS-GPS (GPIRS position) – this is the condition when GPS Primary is displayed on MCDU
IRS-DME/DME (Mix IRS and Radio position)

IRS-VOR/DME (Mix IRS and Radio position)
IRS only (Mix IRS position)

The “GPS PRIMARY” status combines two important criteria:

Accuracy Criterion (HIGH/LOW): This accuracy is checked by comparing EPU and RNP value on progress page. If the Estimated Position Uncertainty (EPU) is less than the Required Navigation Performance (RNP), the display will indicate “HIGH”; otherwise, it will indicate “LOW.”

Integrity Criterion: Integrity refers to the system’s capability to detect a failure and provide appropriate warning if such a failure occurs. This means that the GPS system is not only providing a position solution but also continuously monitoring its own performance to ensure that it is functioning correctly.

When “GPS PRIMARY” status is displayed, it signifies that the GPS navigation solution meets both the accuracy and integrity criteria, and thus, GPS is considered the primary and preferred navigation source. The flight crew can rely on the GPS-derived position for navigation purposes.

However, if the Estimated Position Uncertainty (EPU) associated with the GPS position exceeds a certain threshold (0.28 NM), indicating that the accuracy of the GPS position may be compromised, the FMGS will reject the GPS position and switch to an alternative navigation mode, such as IRS-DME/DME, IRS-VOR/DME, or IRS only, based on the prioritized positions for navigation.

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