A federal judge has ruled that subscriber data captured from cellphone towers is protected by the US Constitution's Fourth Amendment guarantee against illegal searches and seizures.

The decision is part of a sea change from half a decade worth of previous rulings, in which police weren't required to obtain search warrants based on probable cause before accessing the subscriber information. US Magistrate Judge Stephen Wm Smith of the Southern District of Texas said recent changes in case law and rapidly evolving mobile technology required a departure from the outcomes in that long line of cases.

“In 1789 it was inconceivable that every peripatetic step of a citizen's life could be monitored, recorded, and revealed to the government,” he wrote in a decision that was released late last month but only noticed in the last few days. “For a cell phone user born in 1984, however, it is conceivable that every movement of his adult life can be imperceptibly captured, compiled, and retrieved from a digital dossier somewhere in a computer cloud. Now as then, the Fourth Amendment remains our polestar.”

The ruling – which seemed to make reference to the year the Constitution went into effect and the George Orwell novel – is a huge victory for privacy advocates, who have long argued that historical cell-site information gives the government the ability to track users' location each time they make a call or send a text message. In this case, however, it would appear the government was seeking to electronically surveil targets “whether the phone was in active use or not,” Smith said.

The government's request for permission to capture 60 days worth of tower data didn't sit well with the judge, who likened the electronic record to “a continuous reality TV show, exposing two months’ worth of a person’s movements, activities, and associations in relentless detail.”

The decision follows August's landmark decision in which a federal appeals court bashed warrantless GPS surveillance, ruling FBI agents should have obtained a search warrant before planting a GPS device on the vehicle of a suspected drug dealer. A few weeks later, a federal judge in New York ruled cell-tower data was also protected by the Fourth Amendment, rebuffing investigators who said there was no reasonable expectation such data is private.

The American Civil Liberties Union, hailed Smith's decision.

“The court reached this conclusion both because cell tracking reveals information about constitutionally protected spaces such as the home, and because the prolonged nature of such surveillance is very invasive,” Catherine Crump, of the ACLU's Speech, Privacy and Technology Project, blogged.

A PDF of Smith's ruling is here ®