I'm using my phone as a GPS navigator all the time (and used them for many years). No cell coverage necessary at all if you know what you are doing. There are plenty of various software options (paid and free) available, which capabilities significantly better than those of any handheld civilian GPS units I ever heard about. The only issue with them - they need more power, so a solar panel is a must have on a multi-day hike (I've got Goal Zero Nomad 7 - adequate enough in sunny California).