Yes, that was what I was thinking, over altitude, GPS shuts off. Also I was wondering if freezing the crap out of the batteries might affect their overall charge capacity.

Reading the antics of the folks over that theregister.co.uk and their high altitude experiments (granted its mostly a lot of puns around LOHAN) what struck me was they proceed very methodically in their planning. A reasonable guide for anyone wanted to get into the space balloon business I think.