Iphone will never loose reception of gps signal, regardless of the absence of wifi or 3g signal, if you attach it to a gps receptor. There are some of those available in the market, some of them fully compatible, others working only with jailbroken phones. I use the Dual Craddle, officially recognized by Apple, which not only make gps signal reliable but also works as a second battery. Do your research and decide if any of these devices are suitable for you.