FM position determination
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Join Date: Nov 2001
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FM position determination
FM = FMC/FMS/FMGS/FMGC...(the Flight Management system)
If FM 1 and FM 2 use the same source to calculate its positions, then why do :
- FMS 1 (3IRS GPS)
- FMS 2 (3IRS GPS)
- GPIRS
positions all differ ?
Especially FMS 1 and FMS 2 if both using 3IRS GPS
If FM 1 and FM 2 use the same source to calculate its positions, then why do :
- FMS 1 (3IRS GPS)
- FMS 2 (3IRS GPS)
- GPIRS
positions all differ ?
Especially FMS 1 and FMS 2 if both using 3IRS GPS
Join Date: Mar 2005
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each FMGC uses one IRS and one GPS.
FMGC 1 uses IRS 1 and GPS 1. FMGC 2 uses IRS 2 and GPS 2. IRS 3 is used as a backup and for the MIX IRS (not used when GPIRS navigating). It is an hybrid position using the best of both, accurate GPS positioning and accurate IRS GS and track. Since the IRS are different and the GPS are different too (don't know how much different they can be except for the antenna location...) then the computed position can be slightly different. As a note, GPIRS position is calculated in the IRS itself, not in the FMGC, if I recall correctly.
When there is no GPS each FMGC autotunes two DMEs to update position. They are not necessarily the same, and the receivers have differences, too. Even the MIX IRS and the BIAS vector are different for each FMGC. Differences are not significant within DME/DME coverage, but after a long period of IRS ONLY navigation you can see the roll bar fully deflected in the slave PFD.
Only in this manner, segregation of equipments and computers, we can truly have navigation redundancy.
As a note, when you insert a VOR/DME in the RAD NAV page you are not really "hardtuning" since the DME interrogators can interrogate and track up to 5 stations simultaneously. In SELECTED NAVAIDS you can see which navaids is your FMGC using for radio updating. Opposite FMGC might be using different ones. However you need to leave at least one RAD NAV in auto when radio update is VOR/DME, since a VOR receiver can only track one VOR at a time. If you hardtune both equipments to a different one than the FMGS autotuned you will see a message in the scratch pad like "TUNE FTP".
FMGC 1 uses IRS 1 and GPS 1. FMGC 2 uses IRS 2 and GPS 2. IRS 3 is used as a backup and for the MIX IRS (not used when GPIRS navigating). It is an hybrid position using the best of both, accurate GPS positioning and accurate IRS GS and track. Since the IRS are different and the GPS are different too (don't know how much different they can be except for the antenna location...) then the computed position can be slightly different. As a note, GPIRS position is calculated in the IRS itself, not in the FMGC, if I recall correctly.
When there is no GPS each FMGC autotunes two DMEs to update position. They are not necessarily the same, and the receivers have differences, too. Even the MIX IRS and the BIAS vector are different for each FMGC. Differences are not significant within DME/DME coverage, but after a long period of IRS ONLY navigation you can see the roll bar fully deflected in the slave PFD.
Only in this manner, segregation of equipments and computers, we can truly have navigation redundancy.
As a note, when you insert a VOR/DME in the RAD NAV page you are not really "hardtuning" since the DME interrogators can interrogate and track up to 5 stations simultaneously. In SELECTED NAVAIDS you can see which navaids is your FMGC using for radio updating. Opposite FMGC might be using different ones. However you need to leave at least one RAD NAV in auto when radio update is VOR/DME, since a VOR receiver can only track one VOR at a time. If you hardtune both equipments to a different one than the FMGS autotuned you will see a message in the scratch pad like "TUNE FTP".
