The decision is Tracey v. Florida, and it answers “whether accessing real time cell site location information by the government in order to track a person using his cell phone is a Fourth Amendment search for which a warrant based on probable cause is required,” concluding that the answer is “yes.”

First, the court rejects the mosaic theory of the Fourth Amendment:

We . . . conclude that basing the determination as to whether warrantless real time cell site location tracking violates the Fourth Amendment on the length of the time the cell phone is monitored is not a workable analysis. It requires case-by-case, after-the-fact, ad hoc determinations whether the length of the monitoring crossed the threshold of the Fourth Amendment in each case challenged. The Supreme Court has warned against such an ad hoc analysis on a case-by-case basis . . .

Ad hoc, after-the-fact determination of whether real time cell site location monitoring constituted a Fourth Amendment violation presents . . . [a] danger of arbitrary and inequitable enforcement. Nor can we avoid this danger by setting forth a chart designating how many hours or days of monitoring may be conducted without crossing the threshold of the Fourth Amendment.

As for the court’s reasoning that it did accept, it’s unfortunately not entirely clear. The court’s opinion lists several concerns and factors, none of which seem to be dispositive. At the end, though, the court reaches this conclusion:

For all the foregoing reasons, we conclude that Tracey had a subjective expectation of privacy in the location signals transmitted solely to enable the private and personal use of his cell phone, even on public roads, and that he did not voluntarily convey that information to the service provider for any purpose other than to enable use of his cell phone for its intended purpose. We arrive at this conclusion in part by engaging in the “normative inquiry” envisioned in Smith. See Smith, 442 U.S. at 740 n.5. There, the Supreme Court cautioned that where an individual’s subjective expectations have been “conditioned” by influences alien to the well-recognized Fourth Amendment freedoms, a normative inquiry may be necessary to align the individual’s expectations with the protections guaranteed in the Fourth Amendment.

Moreover, we conclude that such a subjective expectation of privacy of location as signaled by one’s cell phone—even on public roads—is an expectation of privacy that society is now prepared to recognize as objectively reasonable under the Katz “reasonable expectation of privacy” test. See Katz, 389 U.S. at 361 (Harlan, J., concurring) (establishing the two-pronged “reasonable expectation of privacy” test). Therefore, we hold that regardless of Tracey’s location on public roads, the use of his cell site location information emanating from his cell phone in order to track him in real time was a search within the purview of the Fourth Amendment for which probable cause was required. Because probable cause did not support the search in this case, and no warrant based on probable cause authorized the use of Tracey’s real time cell site location information to track him, the evidence obtained as a result of that search was subject to suppression.

Justice Canady dissented based on the third-party doctrine as applied by the Fifth Circuit.

Tracey sets up a particularly interesting dynamic in the Eleventh Circuit, which has a cell-site case pending en banc in United States v. Davis. If the en banc Eleventh Circuit holds that cell-site data is protected, it sets up a clear federal circuit split with the 5th as well as a partial split with the 6th. On the other hand, if it holds that cell-site data is unprotected, it sets up a federal/state split in the same jurisdiction with the Florida Supreme Court. With Davis pending and the Fourth Circuit’s Graham case pending on this as well, I suspect Supreme Court review is likely relatively soon.

The Florida Supreme Court tried to minimize the split by limiting its opinion to real-time access rather than historical access. But it’s hard to imagine how that could make a constitutional difference. That distinction matters in the statutory context because the Stored Communications Act expressly regulates historical access but does not regulate real-time access. But I don’t see how it could matter for purposes of the Fourth Amendment question of what is a “search.”

Thanks to David Oscar Markus for the link.