IMHO the IRS works out latitude by recording the X axis rate, which should be 15.04xCos Latitude (deg/hr). But this is only correct if the system X axis is pointing at true north. This is not necessarily so until azimuth alignment is complete, so the internal calculation of latitude is only approximate. The sysytem must have a very, very accurate latitude input, and this comes from the entered PPos data of ramp position. If this varies from the rough internal calculation by, usually, one degree, the system will call for a check, as it will if the entered position is different from the recorded shutdown position.
Azimuth alignment is done by taking the Y axis rate, which would be zero if the azimuth were spot on. If it is not, the Y rate will be 15.04xCosLatxSin of the error angle. As latitude gets nearer 90deg Cos Lat gets near to zero, and the error reading gets so small that it becomes unusable. That is why azimuth alignment - not the latitude calculation - does not work above 70deg latitude.
Dick W