I've seen 'passive' GPS's re-radiate through their antenna on the original frequency and knock out all other GPS's in a 2 mile radius ! I was on a seismic survey ship at the time and it took us about 2 days to figure out that a 'passive' GPS was transmitting. Turned out to be a fault with the antenna connection, so this is probably why they are not permitted electronic items. Mind you, on one flight to Singapore, my mobile phone managed to turn itself on in my bag, stayed on for 8 hrs of the flight, and didn't do anything. Since then I've taken to removing the battery !