In a new decision this week, the New Jersey Supreme Court ruled unanimously on Thursday that cops must get a warrant before tracking a cell phone’s location. The ruling (PDF) only applies to residents of New Jersey, though it's part of a growing body of case law that is shaping judicial approaches to the contentious issue.

Montana recently became the first state in the US to pass a law requiring a warrant for cell phone tracking, less than a year after California’s governor vetoed a similar bill. In May 2012, the Florida Supreme Court ruled that law enforcement in the Sunshine State could seize a phone but that the cops need a warrant to actually search it. Meanwhile, the federal Fourth Circuit Court of Appeals is currently hearing a tracking case from Maryland suspects accused of armed robbery.

Many of these cases—notably the Maryland case—rely on an established concept under federal law known as the “third-party doctrine.” This notion says that when a customer has voluntarily disclosed their own location to a mobile carrier (the "third party") in the normal course of using a phone, then the customer no longer has a reasonable expectation of privacy of that information. Therefore, says the doctrine, location data can be accessed by law enforcement without a warrant.

But the judges in the Garden State don’t see it that way. As they wrote in their decision:

Historically, the State Constitution has offered greater protection to New Jersey residents than the Fourth Amendment. Under settled New Jersey law, individuals do not lose their right to privacy simply because they have to give information to a third-party provider, like a phone company or bank, to get service. See State v. Reid, 194 N.J. 386, 399 (2008). In addition, New Jersey case law continues to be guided by whether the government has violated an individual’s reasonable expectation of privacy. Applying those principles here, we note that 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.

The case dates back to a 2006 investigation of burglaries in the Middletown Township. As those burglaries were being investigated, the police requested and received location information from T-Mobile, the suspects' mobile phone provider. At no point, however, did the police feel the need to obtain a warrant.

The court concluded:

We consider the expectation of privacy that should be accorded the location of a cell phone in that context. Using a cell phone to determine the location of its owner can be far more revealing than acquiring toll billing, bank, or Internet subscriber records. It is akin to using a tracking device and can function as a substitute for 24/7 surveillance without police having to confront the limits of their resources. It also involves a degree of intrusion that a reasonable person would not anticipate.

So what becomes of this case now? The state’s highest court sent it back to the lower appeals court to determine whether or not, as the State of New Jersey argued, an “emergency aid” exception applies in this particular instance.

That question will be decided by a new two-part test: