Note: The Required ACCURacy can be the default value function of the FMS flight area, a value retrieved from the Navigation Data Base or value entered on FMS MCDU progress page.
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5. The navigation and guidance system design
Getting to grips with RNP AR

5.1.4. GPS AUGMENTATION 
The augmentation of GPS availability may be needed in certain environments. 
Different techniques are implemented to augment GPS PRIMARY function availability, they
are labeled “coasting” to describe the capability to keep navigation integrity with a temporary
loss of GPS integrity.

. GPS/IRS Coasting with Litton System
When GPS integrity is lost (less than 4 satellites available), the Litton GPS/IRS hybridization algorithm can continue to compute a HIL. The analysis of the Litton AIME system shows that Horizontal Integrity Limit (HILGPIRS) remains lower than HAL=0.2 NM up to a maximum of 7 minutes following loss of GPS.   The GPS PRIMARY function remains available and the GPS PRIMARY LOST indication will only appear up to a maximum of 7 minutes after loss of GPS.
. FMS Coasting  with Honeywell System
The Honeywell HIGH system has no GPS/IRS coasting. When less than 5 GPS satellites are in view, Hybrid RAIM is not available (HIL=NCD).  When HIL is not available (i.e. HIL=NCD) or HIL > HAL with no GPS satellite detected failure, the FMS computes an alternate integrity (“equivalent” HIL) for a limited period of time, based on an internal IRS drift model and  the MIX IRS position updated with the last GPS/IRS position. As per design of current FMS, the minimum IRS drift model is 0.28NM. Thus, FMS coasting is not available for Required ACCURacy < 0.14.  For Required ACCURacy equal or higher than 0.14, the FMS provides a GPS/IRS position in GPS PRIMARY mode during a limited time. Analysis of the Honeywell HIGH-ADIRU / FMS 2 system shows that “equivalent” HIL remains lower than HAL = 0.28 NM up to a maximum of 6 minutes maximum following GPS integrity loss. The GPS PRIMARY function remains available and the GPS PRIMARY LOST indication will only appear when alternate integrity is lost.
5.1.5.   NAVIGATION ALERTS 
An amber message, "GPS PRIMARY LOST", is displayed at the bottom of the ND and in the scratchpad of the MCDU, in all modes when GPS/IRS updating is abandoned due to loss of GPS integrity detected or GPS/IRS updating is kept but estimated accuracy of GPS/IRS position is not satisfying the Required ACCURacy criteria.   For the latter case, loss of expected navigation accuracy, the “GPS PRIMARY LOST” message is associated with “ NAV ACCUR DOWNGRAD” message on ND and in the scratchpad of the MCDU and “LOW” accuracy on MCDU PROGRESS page.   The display of GPS PRIMARY LOST or NAV ACCUR DOWNGRAD message on both navigation systems in approach will require the crew to perform a go around.
　
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