Does the mobile device have to have either cell service or wifi access to fly? I have an iPhone 6 which worked fine but yesterday during flight the battery died (screen went blank). I am considering purchasing an iPad mini (bigger screen) to do the job. My question is if I take this thing out in the middle of nowhere with no wifi connection and no cell svc, will it work. I don't want to buy an iPad with cell service contract, that's what my phone is for.  Thanks.

Cell signal is not required.  WiFi on the tablet/phone is required if you purchase and using the DJI Phantom 3 Standard model.  The advanced and Professional use a direct cable to the tablet/phone so a WiFi is not required.

But no Cellular signal is required at all.
Even if your phone dies, you can still use the controller to fly your bird.  The conteoller also has a RTH (return to home) toggle switch to initial the RTH in the event the tablet/phone dies or the app crashes.

Also, please insure to close all other apps on the phone to increase battery life.

Will work just fine with no internet or wifi.  only thing will be you will not have a graphical map with no internet.   you can turn on the setting in the general settings in the dji app for" cache map in the backround" and you can manually load up maps while you are on wifi so you will have them if your going to known flying areas.   

I fly with an ipad mini 4 wifi.  i just turn my iphone into a mobile hotspot and connect my ipad to it while flying so i just download the maps from wifi to my phone.  

you should connect the app to wifi from time to time anyway to check for app updates and firmware updates no matter what but that's just common sense

If you buy an ipad, consider purchasing one with cell capability.  You do not have to activate the cellular service.  Wifi only ipads do not have a GPS.  Cellular equipped ipad do have GPS and cellular service is not required to use the GPS.  I picked up a like new used mini a few months ago with cellular for under $200.  It works great and I love the screen size.

I use an iPad Air 2 without cell function but I use a Garmin  Glo ($99) for GPS functionality.  It works great with P3P.  In fact the GPS accuracy is much better than what you get with the cellular version of the iPad.  The Glo uses Bluetooth to link to iPad.

Richard in Bois Posted at 2016-3-22 11:30
I use an iPad Air 2 without cell function but I use a Garmin  Glo ($99) for GPS functionality.  It w ...

That is a nice piece of knowledge right there.  Have to get a Garmin Glo now.

Thanks everyone,  question answered.