Last year a federal appeals court panel ruled that law enforcement agencies needed a warrant to collect cellphone location data. The U.S. Court of Appeals for the 11th Circuit said "cell site location information is within the subscriber's reasonable expectation of privacy."
Jacob Sullum previously wrote about the case. It revolved around a criminal investigation where the FBI obtained a court order (not a warrant) under looser rules to collect phone location data that connected the defendant to various crime scenes. The guy, named Quartavius Davis, was convicted of armed robbery and sentenced to life in prison. He fought the constitutionality of the FBI having such loose access to the data and convinced a three-judge panel a warrant was necessary.
Unfortunately, the full 11th Circuit Court of Appeals yesterday reversed the ruling. Sullum had previously noted the panel made its decision in such a way as to get around the "third party doctrine," the privacy exception that allows law enforcement to get easier access to personal data because it's in the hands of service providers, like phone companies, Internet companies, et cetera. The full court ruling (pdf) put the focus back on the third party doctrine, and also noted that the police didn't even actually track Davis' phone. They tracked the cell towers that were routing Davis' calls to show that he was in the general vicinity of where the crimes took place. There was also no "real-time" tracking. This was all information gathered from Davis' cell company after the fact. The majority ruled that Davis does not have an expectation of privacy for this third-party data, and the FBI followed all applicable laws to collect it.
The next stop, should Davis pursue it, would be an attempt to get the Supreme Court to consider the case. As the Wall Street Journal notes, though, this new ruling is in alignment with rulings from two other federal appeals courts, so there's no "split" for the justices to resolve as yet.
As for actual real-time cellphone tracking, I wrote earlier in the week about how the Department of Justice is considering some more transparency and actual due process (maybe) in how location tracking technology is used. Read about that here.