On my smart phone, a Motorola something-or-other, I can select between true gps (a couple different aps) and Google Maps. Google maps runs off of the cell service, so it only works when there is service, and it is only approximate as to location, so that type of ap does not work well for trail rides.
The GPS aps are, in fact, true GPS and get their info from satellites. As has been said, deep canyons, and sometimes even heavy cloud cover, will occasionally block the signal, but it will immediately update as soon as it gets a clear shot.
As for battery life, different phones are different in that regard. Some phones will run the battery dead while using GPS even when it's plugged in if other services are also being used! To maximize your battery life, make sure you turn off services like bluetooth, wireless, and 4G while you are on your ride. You have to remember to turn them back on, though, when you're done. You can also buy accessory battery packs now, that plug into your phone's port and will add several hours to your phone's battery life.
I've been thinking that one of those GPS units used by joggers and bicyclists would be just the thing. You strap it on your arm (or maybe on a pack horse) and it records mileage, speed, maps, etc. Batteries last longer than on a phone. Anybody know if there's one optimized for horse trail riding yet?
I'm going to look into that cooking pot charger thingie! Thanks for posting that PaintedHorse!