GPS for 5D3

I now have 2 iPhone apps to test for their ability to add gps to the photos. Gps4Cam and GeoTag Photos Pro. But both have the same drawback - if there is no cell service there is no gps information since the iPhone does not have "real" gps but rather only the gps it can get from a data signal. So, if I am out making photos in the middle of a US National Park, I probably cannot expect to get any gps data to add to the photos.

I was really hoping for something less expensive than the GP-E2 to use to insert real gps data into the images but perhaps there is nothing else. The iPhone app is probably a good short-term solution but, since I frequently camp and hike in National Parks, it leaves a pretty big gap in my geotagging ability.