Supreme Court Chief Justice Roberts doesn't like the idea of the government having easy access to five years' worth of his personal location data ("this newfound tracking capacity runs against everyone"---not just suspected criminals!). So we learned last week in Carpenter v. United States, which held that the government violated the Fourth Amendment when it accessed 127 days of Mr. Carpenter's cell site location information (CSLI) without a warrant.
This was a search because it invaded Mr. Carpenter's reasonable expectation of privacy in "the whole of his physical movements." Mr. Carpenter's expectation was reasonable despite the third-party doctrine (that is, despite the fact that he knowingly shared his location information with the phone company). The doctrine takes a pretty hard hit in this opinion, though it remains the law at least in some limited arenas for now.
The bottom line:
"Before compelling a wireless carrier to turn over a subscriber's CSLI, the government's obligation is a familiar one---get a warrant."
That's all good news, but how far does it go? It's not easy to tell from the opinion. The majority emphasizes the "deeply revealing nature" of historical CSLI and explicitly says that its decision "is a narrow one." But there's plenty of food for thought in both the majority's opinion and Justice Gorsuch's dissent (Justice Gorsuch would scrap the Katz reasonable-expectation-of-privacy analysis, and suggests that our property interest in our digital information is sufficient to give it Fourth Amendment protection).
Want to dive deeper? Here are links to some worthy early analyses:
Harvard Law Review Blog