ok let me see if I can say this straight
:Assume a configuration of 3 IR's and 2 GPS recvrs. add to that FMGS1 and FMGS2. this is what happens,each IR comes up with a position( measures gravity on alignment subsequently figures out the lattitude compares this to the last memorised posn and the present lat long entered through the mcdu init page which is either bay co-ords or arp and all that on full align only. quick align simply resets g/s to zero) meanwhile each GPS displays its own derived posn(on gps posn monitor pg).Now things start to warm up. Each IR will independently choose GPS1 or GPS2 data and come up with a GPIRS posn.lets pause and review what we have: 3 GPIRS POSN and 3 IRS POSN.
The three IRS agree upon a common posn called MIXIRS POSN(if one of the IRS posn is too far out from the other two that IRS is left out of the calculation). Now each FMGC is presented with 1 MIXIRS POSN and 3 GPIRS POSNs.( take a break!! I need one
) At this point the FMGC picks anyone of the three GPIRS POSN( in the decreasing priority sequence of a)on side-GPIRS1 for FMGC1.b)GPIRS3 c) opposite side GPIRS) and does the following
amoungst many others
)
-Correct the posn bias of the mixirs posn and thereby contain IR drift(not displayed directly)
-Display a calculated FM POSN
-When T/O pwr set update a/c posn on the rwy to minimise posn errors in the r-nav sid.
when you loose gps then posn is calculated with mixirs posn, updated with radio posn(DME-DME,VOR-DME, etc...)
And when you loose radio updates aswell
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NOW is when you are well and truly cast a drift
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airbus recommends on pwr up like first flt of the day....full alignment.
at all other times if residual g/s<5kts...........no action
if residual g/s on any one IRS>5kts............quick align all three.
full align takes ten min at higher lats 45+. at equator full align takes about 6 min