Real time cell phone tracking reveals private, invasive, and increasingly precise information about our locations and movements. Whenever your phone is turned on—even if you enable its location privacy settings—your cell phone service provider is able to determine with increasing accuracy where your cell phone is located. For many of us who carry our phones throughout the day and sleep with them nearby, that also means that our cell phones can reveal where we are virtually all the time.

And what the cell phone company can learn, police can find out too. In a brief filed yesterday in a North Carolina case, the American Civil Liberties Union and the ACLU of North Carolina Legal Foundation argue that any time police seek to use cell phone location data, they should first obtain a warrant showing probable cause.

A 2011 investigation by the ACLU of North Carolina revealed that more than 50 law enforcement agencies across North Carolina have requested cell phone location data from service providers in order to track suspects, and many more have likely done so since. In 2014, for example, AT&T received 13,619 requests for real-time cell phone location information from government entities across the country. While some agencies seek a court order to obtain this sensitive data, many do not. And in North Carolina, there are currently no laws requiring law enforcement to obtain a search warrant based on probable cause before tracking someone’s cell phone location.

Your location data can reveal a lot about you. If the government knows where you are, they often know who you are. Your location can reveal what types of political and religious activities you attend, which doctors you visit, who you spend time with, whether you go to bars or Alcoholics Anonymous, and much more. When law enforcement seek to use location data as part of an investigation, they should first be required to get a probable cause warrant from a court  

That’s why the ACLU submitted a brief in the case of Paul Perry, which is now before the North Carolina Court of Appeals. Perry was arrested and charged with drug trafficking after police tracked his location in real time through cell phone data provided by AT&T. The location data allowed police to track him to a hotel in Raleigh, and even to figure out which part of the hotel he was in. Police did not apply for or receive a search warrant in Perry’s case.  

In our brief, we argue that the Fourth Amendment’s protection against unreasonable searches should not lose its significance in the face of rapidly advancing technology. In fact, when technological advances make it possible for the government to know someone’s location down to a room in a building, the protections of the Fourth Amendment are more vital than ever.