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In June 2018, the Supreme Court decided in a groundbreaking ruling that law enforcement must obtain warrants before demanding cell phone companies hand over information showing where we鈥檝e been in the past and when.

This month, courts in New England will consider cases that test what that landmark ruling means in other surveillance contexts where police seek to search sensitive digital location data without a warrant. While the 2018 Supreme Court decision concerned historical location data stored by cellphone companies, it provides a roadmap for the protection of all manner of location data. Courts considering warrantless police access to digital data should heed its example.

On Wednesday, the Massachusetts Supreme Judicial Court 鈥� the state鈥檚 highest 鈥� will hear arguments in three cases dealing with location privacy. The court will be asked to answer three questions: Must police obtain a warrant to track a cell phone鈥檚 location in real time? Can police officers access historical location information derived from an ankle shackle without a warrant? And does it violate constitutional rights to put an ankle shackle on someone who is on probation without any individualized justification for the monitoring?

Likewise, the Maine Supreme Judicial Court is presently considering a case involving whether police violate the Fourth Amendment when they obtain a person鈥檚 real-time cell phone location data without a warrant.

The courts in these cases and others can look to the Supreme Court鈥檚 June 2018 decision in Carpenter v. United States for answers to these questions.

Carpenter, which was argued by the ACLU, concerned information revealing where the ACLU鈥檚 client, Timothy Carpenter, had traveled with his phone over a period of months. The police wanted the information to link Carpenter to the scene of various robberies, which had occurred months prior to the investigation. The cops obtained these records without a warrant in what the Supreme Court ruled was an unconstitutional search.

In his ruling in that case, Chief Justice John Roberts wrote that cell site location information is protected by the Fourth Amendment in part because it affords the government 鈥渘ear perfect surveillance, as if it had attached an ankle monitor to the phone's user.鈥� The Carpenter ruling echoes a from 2012, in which the Supreme Court ruled that placing a GPS tracking device on a car implicates the Fourth Amendment.

The Supreme Court acknowledged in those rulings that privacy principles need to adapt for the digital era. (The Massachusetts Supreme Judicial Court ruled similarly in a location tracking that helped to lay the groundwork for Carpenter.) It might be constitutionally appropriate for a police officer to follow someone around town without a warrant. But 21st century technology enables persistent, real-time, and even retroactive tracking of not just one of us, but of all of us 鈥� merely with the click of a button.

The New England cases provide an opportunity for courts to apply Carpenter to different, but no less important facts. Two of the cases, in Massachusetts and in Maine, deal with real-time tracking of phones instead of the historical location data at issue in Carpenter. Another Massachusetts case, , concerns law enforcement鈥檚 warrantless search of a government database containing detailed historical location records of a suspect. A third Massachusetts case, , asks whether the government can mandate the ankle shackling of all 鈥渘on-contact鈥� sex offenders for the duration of their probation for the purposes of location tracking 鈥� even if there is no individualized reason to justify the tracking.

The ACLU of Massachusetts filed friend of the court briefs in and , arguing law enforcement in both cases violated the constitutional rights of Mr. Almonor and Mr. Johnson by performing warrantless searches of their location information. The ACLU of Maine and the national ACLU filed a similar brief in .

In the Johnson case, police investigating a string of robberies searched five months of Mr. Johnson鈥檚 archived GPS location data, obtained from an ankle monitor he had previously worn as a condition of his probation. Police performed this warrantless search of Johnson鈥檚 data in the state鈥檚 electronic monitoring database, despite the fact that his probation term had ended more than a year before the warrantless search. The search ultimately resulted in his arrest.

The amicus brief, co-signed by the ACLU and the state鈥檚 public defender office, the Committee for Public Counsel Services, argues that this search violated Article 14 of the state constitution. The Massachusetts high court has previously held that Article 14, which served as the inspiration for the Fourth Amendment to the United States Constitution, requires that even probation officers need individualized suspicion and judicial approval before they may search probationers. And where 鈥� as in this case 鈥� the search is conducted by police, the same court has ruled that it must meet the traditional constitutional standard of a warrant supported by probable cause.

Almonor and 翱鈥橠辞苍苍别濒濒 involve challenges to law enforcement鈥檚 warrantless acquisition of real-time location information from cell phones. Without seeking or obtaining warrants, police in both cases directed suspects鈥� cellphone carriers to 鈥減ing鈥� their phones to consistently reveal their locations in real time. Thanks to these warrantless searches, police traced Mr. Almonor to a private residence and Mr. 翱鈥橠辞苍苍别濒濒 to a Motel 6. Both were arrested at those locations.

The ACLU鈥檚 friend of the court briefs, co-signed by the Electronic Frontier Foundation and the criminal defense lawyer associations of Massachusetts and Maine, argue that these searches violated the defendants鈥� constitutional rights. Electronically tracking a person鈥檚 cell phone in real time reveals locations within constitutionally protected spaces, such as homes or hotel rooms, that are normally shielded from warrantless police searches. It also can reveal a person鈥檚 鈥減rivacies of life,鈥� as the Supreme Court wrote in Carpenter, by laying bare 鈥渢he whole of their physical movements鈥� over time. More fundamentally, cellphone tracking without a warrant gives police the unprecedented power to pluck any person鈥檚 precise location out of thin air, and thus, as Carpenter warns, threatens to facilitate a 鈥渢oo permeating police surveillance.鈥�

The trajectory of judicial decisions in recent years is clear: If law enforcement wants to use high-tech tools to track our physical locations, either in real time or historically, they must first get a warrant.

Despite that clear trajectory, whenever statutes or courts have not explicitly and in detail spoken to a specific method of surveillance, law enforcement often continues engaging in constitutionally questionable electronic surveillance. Indeed, even statutory protections sometimes don鈥檛 prevent abuse: Maine has a strong law requiring police to get a warrant before tracking a cell phone except in emergencies. In Mr. 翱鈥橠辞苍苍别濒濒鈥檚 case, however, police claimed an emergency when there wasn鈥檛 one, sidestepping the state鈥檚 warrant requirement.

The cases before the Supreme Judicial Courts of Massachusetts and Maine will, we hope, provide both the public and law enforcement with more clarity about what is permissible under the law and what is not. In a post-Carpenter world, it should be abundantly clear that law enforcement needs warrants to mine sensitive personal location data.