Quote Spooky: "Small point of order here but GPS nor any ground based nav aids actually update INS. The INS, IRS ADIRU etc. only interface with the respective FMS and do not receive any updates directly from any ouside sources."
Depends on the system. On the three large aircraft types I have flown which had GPS updating, the IRSs all used very different methods of updating position. My current type uses the FMC to produce a blended position which is weighted depending on the quality of the information fro a Multi Mode Receiver which has an inbuilt GPS along with a VOR, DME and ILS. The FMC tells you on the progress page which position it is using, it's nearly always 'GPS Primary'. The type before that used an IRS position which was updated by GPS and the type before that had an IRS with a GPS embedded in it, so the IRS position itself was a blended position.
From my experience, I would doubt a GPS set would disrupt the aircraft electronics at any stage of the flight. But of course you should pay regard to the current airline policy. Mine tells the passengers, 'No electronic items are to be used until the seat belt signs are turned off after take off - and they must be turned off again when the seat belt signs are back on for landing".
But also, I haven't found a GPS set which works in a cabin. Not that I've tried very often!