Google is good to aerial scout the areas you are looking to go into. Figure out land terrain features and all or even print and take with you just in case.

Bull camps advice is spot on. Service areas depend with providers and I have used the android app on my phone in urban areas and had to wait ten minutes just for the signal to find my relative area. Get a handheld gps unit designed for that purpose, or if you want I hear the international satellite phones they have out now have much better gps systems built in but plans and phones are expensive.