I purchased a "certified pre-owned" iPhone 5 about a month ago and the GPS is horribly off without Wifi. This makes it neigh impossible to use the phone as a GPS while driving, trying to track my run, etc., which are all major features of having an iPhone. If I'm at home, turn off the wifi, and let it sit for ten minutes to acquire a signal, it does no better. In short, without wifi, it doesn't work. How can I be sure my GPS chipset is not defective?