I have The Trimble hunt pro on my iPhone and I can't get it to work without cell service.
I struggled with that for a few trips at first. What I found out is you have to turn airplane mode off. Apparently turning on airplane mode disables the gps along with the phone service. I turn airplane mode off for a few minutes to check my location, then turn it back on when I'm done to conserve battery life. I leave the phone on, use the camera, video, music, and view the maps also with it and can get about 3 days out of the battery. If you turn it off and only turn it on to use the gps it will last a lot longer.