I never cease to be amazed at the folks who think OnStar is a true satellite service. Yes, it uses GPS navigation, but it communicates via cellular service, so just as you said, no tower, no communication. And as others have commented, there are still weird and unexpected gaps in cellular service in the U.S. You don't need to be in the middle of a national forest to find yourself out of range. OnStar provides lots of convenient services, but dependable emergency communication in fringe areas is not one of them.