I find this a very interesting thread; I love to know whats going on behind the scenes in aircraft systems, but for me Polar operation is outside my experience; all my flying was short / medium haul around Europe.
Anyway, I was reading Mr Boeings
Polar Route Navigation By Model page. Wherein the following statement is made for the 747-400:
"When the computed position enters the region north of 84 deg north latitude or the region south of 84 deg south latitude, each FMC shifts down from the triple-mix inertial reference system (IRS) position to a single inertial reference unit (IRU) position. The SPLIT IRS OPERATION message is displayed on the control display unit (CDU) scratchpad message area. This reversion to the single IRS position is necessary because longitude and latitude singularity converge at the poles."
I understand roughly what this is saying, but I don't get the why. Since there is clearly some polar nav expertise present in this thread, I was wondering is anyone could expand on the rationale behind it please?
Thanks,
pb