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Accuracy functions in direct connection with the satellite constellation in view of the aircraft. If the satellites are low on horizon, or not in appropriate positions, accuracy will be poor. It is provided as a "figure of merit".
If the GPS position fulfils both the integrity and the accuracy criteria, GPS PRIMARY is displayed on the MCDU PROG page and the GPS position is the best raw data position available.
SUMMARY
FM POSITION
Flight phase WITHOUT GPS PRIMARY WITH GPS PRIMARY
On groundbefore Takeoff MIXIRS GPIRS
Takeoff Updated at runway threshold (shift) (1)
In flight With RADIO Tends to RADIO GPIRS
 Without RADIO MIXIRS + BIAS GPIRS

(1) The FMS position update at take-off is inhibited with FMS2 when GPS PRIMARY is active.
USE OF FMS
Applicable to: ALL
The navigation accuracy is managed through several MCDU pages: PROG PAGE
This page indicates GPS PRIMARY.
The PROG displays the estimated navigation accuracy in green. This provides the EPE, if GPS PRIMARY LOST, or is computed by the GPS, if GPS PRIMARY is displayed
The PROG page displays the required navigation accuracy in blue (this can be changed). The required navigation accuracy thresholds are determined, depending on the flight phase, or can be manually entered. These thresholds are used to change from HIGH to LOW accuracy, or vice versa. These indications are used when flying within RNP airspace.
SELECTED NAVAIDS PAGE
The SELECTED NAVAID page is accessible from DATA/POSITION MONITOR/ FREEZE/SEL NAVAIDS. It has a DESELECT prompt, that enables the flight crew to prevent the FMS from using
 

  SUPPLEMENTARY INFORMATION
NAVIGATION ACCURACY
A318/A319/A320/A321
FLIGHT CREW TRAINING MANUAL 

the GPS data to compute the position, in the case of a major problem. GPS PRIMARY lost is then displayed on MCDU and ND. The GPS can be reselected using the same page.
PREDICTIVE GPS PAGE (IRS HONEYWELL ONLY)
The PREDICTIVE GPS page is accessible from PROG page. The GPS PRIMARY criteria depend upon the satellite constellation status (position and number) and this is predictable. The crew can assess the GPS PRIMARY status at destination or alternate.
ND/MCDU
A GPS PRIMARY message is displayed when GPS PRIMARY is again available. This message is clearable.
A GPS PRIMARY LOST message is displayed when GPS PRIMARY is lost. This message is clearable on MCDU but not on ND.
When the class of navigation accuracy is downgraded from HIGH to LOW (LOW to HIGH), a NAV ACCUR DOWNGRADE (UPGRADE) is displayed on ND and MCDU.
AIRCRAFT POSITION AWARENESS AND OPERATIONAL CONSEQUENCES
Applicable to: ALL
NAVIGATION ACCURACY INDICATIONS
The navigation accuracy indications are available on the MCDU PROG page. The following guidelines apply:
? If GPS PRIMARY is displayed, no navigation cross-check is required
　
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本文鏈接地址：A318/A319/A320/A321 機組訓練手冊 FCTM Flight Crew Training Manual(175)