Try putting your iPad into Airplane mode then switching the GPS or Location Services on. You should still be able to receive the GPS fix but you won't get the "assisted" data from the cell
service.
Basically GPS is totally separate from the cell/mobile signal so should not have anything to do with a SIM card or cell signal.
Depending on the receiver in the iPad you may be able to receive signal from the US GPS system and the Russian GLONASS system along with the European Galileo system as it increases. With all these satellites the extra accuracy gained from "assisted GPS" becomes less important.
I do this with my Android
phone all the time on planes so I can see where I'm flying over, how high and at what speed using just the GPS.
Hope this helps
Keiron