In recent years, the courts have struggled to decide whether the government needs a warrant to access historical records about a cell phone user's location. Some courts have found that when users turn on their cell phones, they "voluntarily" transmit their location to their cell phone providers and thereby waive any expectation of privacy.

On Monday, Judge Nicholas Garaufis of the Eastern District of New York soundly rejected this line of reasoning. The federal government had asked the courts to order Verizon Wireless to turn over 113 days of location data about a suspect's cell phone. It did so under a provision of the Stored Communications Act that only requires law enforcement to show that the records are "relevant and material to an ongoing criminal investigation."

Does the government violate the Constitution when it obtains location data without meeting the Fourth Amendment's "probable cause" standard? Some courts have found that it does not. But in a 22-page opinion, Judge Garaufis analyzed and rejected these other courts' arguments, holding that law enforcement needs a warrant to obtain months of location data.

"The fiction that the vast majority of the American population consents to warrantless government access to the records of a significant share of their movements by 'choosing' to carry a cell phone must be rejected," he wrote. "In light of drastic developments in technology, the Fourth Amendment doctrine must evolve to preserve cell-phone user's reasonable expectation of privacy in cumulative cell-site-location records."

In recent decades, questions about how to apply the Fourth Amendment to electronic communications has focused on two key concepts. The third-party doctrine is the idea that Americans waive their Fourth Amendment rights when they disclose information to a business like a phone company or bank. For communications entrusted to third parties, the courts have traditionally extended Fourth Amendment protection only to the contents of communications—the audio of a phone call or the text of a letter—and not to non-content metadata about the call, such as the number dialed or the address on the envelope.

Some judges have tried to shoehorn cell phone location data into this analytical framework. But Judge Garaufis refused to do so, ruling that the distinction between content and non-content information didn't make sense in this context. "There is no meaningful Fourth Amendment distinction between content and other forms of information, the disclosure of which to the Government would be equally intrusive and reveal information society values as private," he wrote.

Instead, he offered two reasons to think the Fourth Amendment protects cell phone location data. First, the third-party doctrine should not apply to "widely used communication technologies in which service-provider intermediaries receive and store private user information incident to the service."

And second, "established normative privacy considerations support the conclusion that the reasonable expectation of privacy is preserved here." In other words, when a user signs up for a cell phone, he's not consenting to his cell phone provider disclosing a complete record of his movements to the government.

The decision is a refreshing change from the long line of cases that have tried to apply decades-old Fourth Amendment principles to 21st Century communications technologies. The extent of tracking enabled by cell phones was unthinkable when those earlier cases were decided. Judge Garaufis recognized that the legal standards need to evolve along with changing technologies. We hope other judges will follow his lead.