With GPS update

In IRS ONLY

5.1.3. GPS PRIMARY AND REQUIRED ACCURACY
GPS/IRS mode is the basic navigation mode as long as accuracy and integrity criteria are satisfied:
.
EPE < Required ACCURacy, and 

.
HILGPIRS < HAL

EPE: Estimated Position Error calculated by the FMS
HIL: Horizontal Integrity Limit
HAL: Horizontal Alarm Limit

The HIL is delivered by the RAIM or the AIME, it is the maximum error with an undetected
satellite failure. It is a measure of GPS position integrity.
In approach, and in terminal area the HAL = MIN (UIL, 2xRequired ACCUR). 
UIL represents the Upper Integrity Limit, which is a function of the FMS flight area.
The FMS OPC (Operational Program Configuration) file defines the UIL and the default
Required ACCURacy values for FMS terminal and FMS approach areas. The transition from
the FMS terminal area to the FMS approach area will occur 5 NM before the first approach
waypoint of the FMS flight plan.
In FMS terminal area the RNP default value is 1 NM and the UIL is 1 NM. If an RNP value <

0.5 NM is manually entered on MCDU or extracted from the navigation database, the HAL will
be 2xRNP.
In FMS approach area the RNP default value is 0.3 and the UIL is 0.3 NM. 
If for example RNP 0.1 NM is manually entered the HAL will be 2xRNP= 0.2 NM.

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5. The navigation and guidance system design

The EPE and the Required ACCURracy (=1xRNP) are indicated on the MCDU PROG page.
PR E D I C TI V E

EPE
<GPS GPS
R EQ U IR E D  A C C U R
E S T I MA T E D
0.15 NM HIGH 0.08NM

The EPE is a radial estimation of the navigation error, significantly more conservative than the
statistically demonstrated NSE (see chapter 4).
When in “GPS PRIMARY” navigation mode the Estimated Position Error (EPE)  compared to
a 1xRNP threshold is computed as follows:

22
EPE =
(HFOM ) +(δSchuler Bleed thru error ) +(Δlatency × GS)2
Horizontal Figure Of  Merit (HFOM) is an output of the ADIRU hybridisation algorithm and is a 2σ (or 95%) value of the position accuracy. Δlatency is the latency of the FMC = 0.74s GS = Aircraft Ground Speed δSchuler Bleed thru error = 15.511 m or 0.00837 Nm
The modes of the navigation function and associated interfaces are summarized in the table below:
EPE<Required ACCUR  EPE>Required ACCUR 
HILGPIRS<HAL  GPS PRIMARY Accuracy “HIGH” NAV Mode: 3IRS/GPS  GPS PRIMARY LOST Accuracy ”LOW”      NAV Mode:  3 IRS/GPS 
HILGPIRS>HAL  GPS PRIMARY LOST Accuracy “HIGH” NAV Mode: 3IRS/DME-DME or 3IRS/VOR-DME or 3IRS => depending on radio accuracy  GPS PRIMARY LOST Accuracy “LOW” NAV Mode: 3IRS/DME-DME or 3IRS/VOR-DME or 3IRS => depending on radio availability 
　
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