iMapMy (either Android or iOS) has that feature without any additional hardware. If you can leave your phone's GPS off and still use the feature on the Garmin, that would alleviate a lot of battery life concerns, though (although my new phone has WAY better battery life than my old one used to). You'd still need to be in cell phone range, which is the #1 drawback of any of this stuff. I just can't bring myself to carry around the weight of a SPOT.