Originally Posted by FlapsOne
Indeed, with GPS primary, there is no need to enter TO SHIFT.
I will dispute this, but am happy to be corrected. It is a common school of thought and I have never understood it!
On T/O the aircraft uses an FM position that is the mix IRS with a leaning toward a GPS bias. The result being the GPIRS position.
If the GPS fed directly into the IRS, T/O wouldn't be important. But it doesn't.
Without inserting a shift, there will be error in the MIX IRSs position. If it is significant enough, there will be a GPS/POS disagree on T/O- which will be ignored anyway.