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WITHOUT GPS PRIMARY  PRINCIPLE
The FMS position is computed from the three IRS positions, that are combined to provide a MIXIRS position. The radio position is also combined, if two DMEs, a VOR/DME or a GPS supplemental are available. The GPS supplemental is considered to be an additional form of NAVAID, and can be accepted, if it falls within the radio position or the MIXIRS position.
INITIALISATION
Refer to NO-020 ADIRS INITIALIZATION TAKE-OFF
Each FMGC uses the MIXIRS position as its position, until the thrust levers are pushed forward to TOGA. The FMS position is then updated to the runway threshold coordinates. The difference between the MIXIRS position and the FMS position is referred to as the TO BIAS. The TO BIAS is added to the MIXIRS position, for the subsequent FMS position.
FMS position updating at take off
 
IN FLIGHT
The original TO BIAS is continuously updated with the current radio aid.
 

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

updating BIAS principle
 
If the radio position is lost, the system uses the updated BIAS to determine the FMS position from the MIXIRS position.
NAVIGATION ACCURACY
The FMS computes the Estimated Position Error (EPE). The EPE is an estimate. To compute the EPE, the FMS considers the immediately available navigation means in the FMS position computation and applies defined tolerances for each of them. These tolerances assume that the navigation means are working properly. They ignore any possible excessive IRS drift or erroneous locations of NAVAIDs. The MCDU PROG page displays the HIGH/LOW indications, according to the EPE. These indications reflect the probable accuracy of the FMS navigation compared to the determined accuracy criteria.
WITH GPS PRIMARY
PRINCIPLE
The GPS interfaces directly with the IRS that outputs a GPIRS position. When a GPIRS position
is available, it overrides the RADIO position, if available. Therefore, the FMS position tends toward the GPIRS position.
INITIALISATION
Refer to NO-020 ADIRS INITIALIZATION TAKE-OFF
The FM position is automatically updated at the runway threshold. With FMS2, this automatic position update is inhibited.
IN FLIGHT
The FM position tends to the GPIRS position as long as the GPS satellites are available.
 

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

NAVIGATION ACCURACY
The GPS position is characterized by two parameters:
? integrity
? accuracy
The integrity is a direct function of the number of satellites in view of the aircraft. If five or more satellites are in view, several combinations of the satellite signal may be used to process "several positions" and to carry out reasonableness tests on the satellite signals themselves.
　
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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(174)