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. If your really brave you can do an INS update by flying over a navaid and do a manual update, but that's not the same as you had implied. For all of you who that anyway, forgive me for being redundant.
Thanks for clearing that up- sorry I don't know a huge amount about INS systems...although I had heard the phrase 'GPS updated' before and assumed that that was what it meant. I just tacked on the question about INS because I was curious.