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· FCOM 4.05.70 (Approach)
04 - INSTRUCTOR’S ACTIONS
Briefing of the following key points.
MAIN
· GPS PRIMARY, an operational concept
· FMGS position calculation
· MCDU pages associated with GPS
· GPS overlay approach
SECONDARY
· GPS deselection
· Check of NOTAMs for satellite availability, prior to flight
A319/A320/A321
FLIGHT CREW TRAINING MANUAL
1.02.30 Page 1
REV 21 MAY 98
NORMAL OPERATION BRIEFINGS
GLOBAL POSITIONING SYSTEM
CHAP02\30
05 - TRAINEES’ ACTIONS
The GPS is a satellite based radio navigation aid. GPS PRIMARY is an Airbus term defining
an operational concept. It means that adequate GPS accuracy and integrity are provided
and that GPS is available as the basis for the FM position. The GPS fitted to the Airbus
family of aircraft monitor the integrity of the position information provided, and give
indications of when this position information cannot be relied upon.
The position calculated by the twin GPS receivers is added to the IRS calculated position
producing a GPIRS position. This is then fed to the FMGCs and a FM position is produced.
At take off, even with GPS, the FM position is updated to runway threshold (+ T/O SHIFT if
applicable). The FMGCs will use the GPIRS position as the FM position so long as GPS
PRIMARY is indicated on the MCDU PROG page.
Reasonableness tests on the GPIRS and IRS positions are carried out and any
unreasonable position is disregarded for the purpose of FM position calculation. The table
below summarizes how FM position is derived with and without GPS PRIMARY.
FM POSITION
WITHOUT GPS WITH GPS
On ground before T.O. Mix IRS position GPIRS
At T.O. Runway threshold (+ T/O shift) GPIRS
Flight MIX IRS & Radio Position
(tending towards Radio Position)
GPIRS
Flight without GPS or Radio Position
update
MIX IRS position + Last memorized FM position,
gradually tending towards IRS position.
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.
A319/A320/A321
FLIGHT CREW TRAINING MANUAL
1.02.30 Page 2
REV 21 MAY 98
NORMAL OPERATION BRIEFINGS
GLOBAL POSITIONING SYSTEM
CHAP02\30
05 - TRAINEES’ ACTIONS (CONT’D)
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
　
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