Yesterday the New Jersey Supreme Court unanimously ruled that police generally need a warrant to obtain cellphone location data from wireless service providers. Responding to a challenge brought by Thomas Earls, a burglar whom Middletown police tracked by repeatedly asking T-Mobile where he was, the court emphasized the potentially sensitive nature of information about cellphone users' current and past whereabouts:

Disclosure of cell-phone location information, which cell-phone users must provide to receive service, can reveal a great deal of personal information about an individual. With increasing accuracy, cell phones can now trace our daily movements and disclose not only where individuals are located at a point in time but also which shops, doctors, religious services, and political events they go to, and with whom they choose to associate. Yet people do not buy cell phones to serve as tracking devices or reasonably expect them to be used by the government in that way. We therefore find that individuals have a reasonable expectation of privacy in the location of their cell phones under the State Constitution....

We conclude that Article I, Paragraph 7 of the New Jersey Constitution protects an individual’s privacy interest in the location of his or her cell phone. Users are reasonably entitled to expect confidentiality in the ever-increasing level of detail that cell phones can reveal about their lives. Because of the nature of the intrusion, and the corresponding, legitimate privacy interest at stake, we hold today that police must obtain a warrant based on a showing of probable cause, or qualify for an exception to the warrant requirement, to obtain tracking information through the use of a cell phone.

Although the language of the New Jersey Constitution's privacy clause is very similar to the Fourth Amendment's, the state supreme court reads it as providing more protection. Of particular relevance in this case, the New Jersey Supreme Court has explicitly rejected the idea that information divulged to private parties for specific purposes is fair game for government snooping. Last year, concurring in U.S. v. Jones, the case in which the U.S. Supreme Court unanimously ruled that tracking a suspect's car by attaching a GPS device to it amounts to a "search" under the Fourth Amendment, Justice Sonia Sotomayor wisely recommended reconsideration of that "third party doctrine," which has increasingly alarming implications in an age when so much personal material is stored in remote locations outside of people's homes.

The majority opinion in Jones hinged on the trespass required to attach a GPS device. But five justices also indicated that they believed the surveillance at issue in that case, which involved tracking a car's movements for a month, violated reasonable expectations of privacy. It is therefore possible that in a future case the Court will rule that warrantless cellphone tracking violates the Fourth Amendment. In the meantime, unless Congress acts, the legal constraints on the use of cellphone location data outside of New Jersey (and Montana, which requires a warrant by statute) will remain a matter of dispute. Last year an ACLU survey of law enforcement agencies found that police practices in this area vary widely, with some departments seeking warrants for cellphone data but many others demanding the information without judicial authorization.

The widespread practice of unilaterally demanding cellphone location data highlights a problem with the "reasonable expectations" test: The more warrantless surveillance the government gets away with, the less privacy people come to expect. At the extreme, when the government is monitoring your every move, you have no expectation of privacy at all, reasonable or not. Hence the longer this issue remains unresolved, the less privacy protection we are apt to get. 

Because it was announcing a new rule in State v. Earls, the New Jersey Supreme Court said the warrant requirement won't apply to past cases. It may not even make an important difference in this case, because the court said it's possible that the cops' queries to T-Mobile were covered by an emergency exception. The police said they were concerned that Earls might harm his girlfriend after discovering that she was cooperating with the cops. The case now goes back to the Appellate Division of the Superior Court for consideration of that issue.