FWIW today's smartyphones all have a GPS chip and antenna, so can function as a standalone GPS even sans a cell signal. One has to anticipate and download the appropriate basemap before leaving town however, otherwise the location will appear on an otherwise blank screen, rather limiting one's navigation.

My experience navigating by phone is it's great from the aspect of the big, sharp screen but lousy from the lack of easy-to-use controls and tools, and horrid power consumption. The GPS antenna isn't a match for a "real" GPS unit, either.

Cheers,
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--Rick