Tracking movements in public is more complicated issue, because generally when you go from place to place in public, you can not say you expected this move to be private - it can be plainly seen by anybody who happens to be in the same street as you. It is very different from what happens in your home, where you can expect nobody (outside of your household) can see you. Note I'm not advocating any legal opinion here, just saying "home" case is much more obvious thing than "tracking" case. In general, US people (by means of Congress) can also put limits on the police or the NSA beyond constitutional limits, i.e. say even if the 4th amendment did not ban GPS tracking or email intercepts, we (the US people, by means of Congress) prohibit the police or NSA from doing so without judicial warrant. Unfortunately, US people seem to be unable or unwilling to elect representatives that are interested in doing so, so the only fallback remaining is the Supreme Court, which is not really the best way to solve complex cases, as they can go either way.