Interesting iPhone App if it works. I installed the Tom's GPS App on my iPhone 4S and turned airplane mode on to simulate no cell coverage. The App would not display a location until I turned airplane mode off. Then it immediately gave me a location. The problem as I see it with relying on a cell phone for "GPS" location data, is that the cell phone is not a GPS receiver.

From http://gps.about.com/od/glossary/g/A-GPS.htm:
Quote:
...Assisted GPS, also known as A-GPS or AGPS, enhances the performance of standard GPS in devices connected to the cellular network. A-GPS improves the location performance of cell phones (and other connected devices) in two ways:

-By helping obtain a faster "time to first fix" (TTFF). A-GPS acquires and stores information about the location of satellites via the cellular network (see almanac) so the information does not need to be downloaded via satellite.

-By helping position a phone or mobile device when GPS signals are weak or not available. GPS satellite signals may be impeded by tall buildings, and do not penetrate building interiors well. A-GPS uses proximity to cellular towers to calculate position when GPS signals are not available....
From the short down the block test I just did, once I determined the system would not give a location without access to the cellular network, I gave it a signal, got a fix and then went back into airplane mode. It took about 1 minute for the Toms GPS app to go blank. IOW, without the cell network, the app doesn't seem to work.

A-GPS assumes GPS signals may become weak or not available while the places people may really run into navigation problems that are life threatening are where cell signals are weak or not available. I will not be relying on a cellphone for navigation.

If others have different experience, please weigh in and let me know what I'm missing. TIA
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