As the highest court in Massachusetts considers whether cell-site data is private in the context of the Fourth Amendment, we filed an amicus brief arguing that when the police want to be able to recreate your every step—figuring out your patterns of movement, where you've been and with whom—they must obtain a search warrant.

Cell site information—a phone company's data about which antenna or tower a cell phone (and ultimately its user) connects to—is becoming more precise and revealing. As more people use cell phones and Internet enabled smartphones to communicate, the number of cell sites across the country has grown at a rapid pace. As more cell sites are responsible for smaller geographical areas, it becomes easier to pinpoint a person's precise location. This growing precision means police are increasingly acquiring cell site information as a routine part of any criminal investigation.

In the case before the Supreme Judicial Court ("SJC"), police obtained an order authorizing the disclosure of two weeks worth of cell site information. The trial court suppressed the evidence, finding that since people have a reasonable expectation of privacy in their movements, police needed to obtain a search warrant supported by probable cause before accessing this data. Most critically, the trial court found that the "third party doctrine," the idea that people don't have an expectation of privacy in information voluntarily disclosed over to other people, didn't defeat a person's expectation of privacy in their movements. Most recently, the Fifth U.S. Circuit Court of Appeals reached the opposite conclusion, ruling the third party doctrine defeated any privacy expectations in cell site records since the information belongs to the cell phone provider and a user voluntarily conveys their location to the provider in order to use their phone.

In our amicus brief, we urge the SJC to affirm the trial court, arguing that people maintain a reasonable expectation of privacy in their location—even their public movements—since society would deem it unlikely that anything more than small, discrete movements would be observed at a time.

Location privacy has been a hot topic this year, and while federal courts and Congress have been slow to address the significant privacy concerns with cell tracking, individual state courts and legislatures have stepped up to fill the void. Earlier this summer, the New Jersey Supreme Court ruled in State v. Earls that historical cell site information allowed the police to transform a cell phone into a tracking device, meaning police needed to obtain a search warrant to get access to the data. (Earls reached that result because New Jersey had abolished the third party doctrine as a matter of state law.) Montana and Maine passed legislation requiring police obtain a search warrant before tracking a person's location through their cell phone or electronic device.

Even the SJC itself has been a leader on location privacy. Earlier this year, it ruled in Commonwealth v. Rousseau that a passenger in a car had standing to challenge GPS surveillance because everyone, regardless of whether they are the car's owner or not, has an expectation of privacy in their location.

We hope that the SJC will extend Rousseau, recognize that the third party doctrine does not apply to invasive cell site monitoring, and require police obtain a search warrant to track a person's location through their cell phone.

Special thanks to Kit Walsh of the Berkman Center for Internet and Society at Harvard Law School for serving as our local counsel.