AFTL section 1.22.20 FCOM 4.02.20 (FMGS Principles - Navigation) provides a full description of FMGS position computation.
During flight preparation, GPS PRIMARY will be indicated on the MCDU PROG page and the ND. This message should be cleared using the CLR pb. This is a positive confirmation to the crew that GPS is accurate.
The GPS workings are transparent to the crew and will only require attention in the case of a fault or a downgrading of the position information e.g. GPS PRIMARY LOST or GPS 1 (2) FAULT. If the GPS status changes , a message will be displayed on the MCDU and ND [GPS PRIMARY (white) or GPS PRIMARY LOST (amber on ND)]. The amber GPS PRIMARY LOST cannot be cleared from the ND and is to remind the pilots that GPS is not available. Navigation accuracy up- or downgrade will be shown in the same way as already used for non GPS navigation.
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The following is a list of MCDU pages associated with the use of GPS and a brief description of their use.
MCDU PAGE FUNCTION
SELECTED NAVAIDS Allows deselection or selection of GPS.
GPS MONITOR Display of GPS positions and other GPS derived information.
IRS 1 (2) (3) GPIRS Position for each IRS
PROG When GPS PRIMARY is shown, indicates that GPIRS is used for FM position calculation. Navigation accuracy is shown.
PREDICTIVE GPS* Displays information about the predicted availability of GPS at destination ETA or at a particular waypoint.
ARRIVALS Allows selection of GPS approach.
* PREDICTIVE GPS page not relevant when Litton GPS fitted.
Full descriptions of the above pages can be found in FCOM 4.03.20 (Pilot Interface - MCDU Page Description).
The full GPS standard will permit predictions to be made regarding the feasibility of a GPS approach at destination. The calculation involved relies on the number of, and the position of, the satellites at ETA. The "time window" is pilot modifiable.
The current status of GPS permits approaches to Cat 1 limits at best. Individual certification authorities are discussing GPS accuracy, reliability and so forth. Thus, the minima and acceptance of GPS in each country must be checked before using GPS as a prime means of navigation in the approach phase.
There are two types of GPS approach :
GPS OVERLAY APPROACH
The aircraft performs a managed approach along the trajectory of a published non precision approach using GPS position information in GPS PRIMARY navigation mode. Before beginning the approach a check of GPS PRIMARY and HIGH accuracy must be made which replaces the navigation accuracy check. Additionally, before the FAF check GPS PRIMARY and HIGH accuracy with a RNP of 0.3 nm or less. Raw data must be displayed and monitored at all times. FCOM 4.05.70 (FMGS Procedures) details the procedures to be used. If raw data indicates that the aircraft is not on the required flight path the pilot must use HDG/V/S or TRK /FPA to correct the flight path.
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GPS STAND ALONE APPROACH
The aircraft is guided along the trajectory of an approach the waypoints of which are not referenced to any ground base navigation aid. As yet, only a few GPS stand alone approaches are published and they require an operational approval.
If GPS PRIMARY is lost during a non precision approach, a GPS PRIMARY LOST message in amber will be displayed accompanied by a triple click. If this occurs or there is a navigation accuracy downgrade, the approach may only be continued in managed navigation as long as raw data continue to indicate satisfactory guidance.
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