It is easier than ever to know where people are. Sharing locations on Facebook, tracking via the Snapchat map, and using the "Find my Friends" feature on the iPhone—everyone wants to publicize where they are. But are they as eager to share when the government is watching? We expect not. But would you be surprised to learn that law enforcement has frequently gathered location information from wireless carriers without obtaining a warrant, and then used it to track down and arrest criminal suspects?

Your wireless network provider knows a lot about where your device is at all times. Cellphones work by establishing a connection with cell towers. Each tower projects unique directional signals so when a cell phone connects to a tower, the carrier records this information. The information can then be used for a variety of purposes, including network management.

As cellular networks continue toward a 5G future with thousands of new "microsites" and smaller coverage areas, location information collected by carriers rivals GPS as a way to nearly pinpoint a device's location—which is more often than not your location. Thus, while the records might be valuable to law enforcement, does sharing them trigger privacy concerns under the Fourth Amendment? In a landmark 5-4 decision issued June 22, the Supreme Court said because over time, these records for any given phone can track personal movements in a detailed, encyclopedic, and effortless manner, law enforcement must have a warrant to obtain these records from a provider.

We have written previously on this case, Carpenter v. U.S., which was argued before the Court in November of 2017. Carpenter was part of a gang of armed robbers targeting Radio Shack and (ironically, as the Court observed) T-Mobile stores. The issue in the case was whether or not the warrantless search and seizure of cellphone records revealing the location and movements of Carpenter over the course of 127 days was permissible under the Constitution's Fourth Amendment, which secures persons, houses, papers, and effects against unreasonable searches and seizures. The Sixth Circuit Court of Appeals said it was and upheld his conviction, applying the third party doctrine we discuss below.

Law enforcement was not permitted to simply take the records. The lower court said they were required to use a subpoena. Under a federal law (the Stored Communications Act), a court may approve a subpoena and order disclosure of records if investigators show there are reasonable grounds to believe they are "relevant and material to an ongoing criminal investigation." The standard for a search warrant is far higher: probable cause that a crime has been committed.

Carpenter argued for a rule that labeled long-term periods or aggregations of cell site location information as a search that requires a warrant. For Carpenter, were the rule permitted to stand as it was (no warrant requirement), the government would essentially have access to a time machine allowing them to go back in time and track an individual's locations, without probable cause that the suspect had committed a crime. Our cellphones would become retroactive tracking devices. Carpenter emphasized that when a phone routinely connects to a network it is not a voluntary transfer of information by the user. Indeed, most people have no idea that companies are recording this information.

Law enforcement argued that the routing information the government obtained from the cellphone company functions as a business record because the business is gathering the data as part of providing their service. Therefore, because information has been communicated to a third party as part of the transaction, prior cases implied the government can acquire it without having probable cause.

Lower courts were having difficulty applying the third party records standards to digital age records and new technology, but most appellate courts had permitted access to the information via a subpoena alone.

In a 5-4 decision, the Supreme Court sided with Carpenter, ruling that the third party doctrine does not apply to these cellphone records because obtaining the records without a warrant was a search as to Carpenter and therefore, going forward, a warrant is required before accessing the location information recorded by a cell provider.

This is the second time the Court has grappled with privacy issues related to our now ubiquitous cell phones. Four years ago, in Riley v. California, the Court said our personal wireless devices contain "privacies of life" and that cellphones are almost a "feature of human anatomy." Thus, the Court said police need a warrant to review the contents of a cellphone taken during an arrest.

Now, the Court followed a similar path by noting that since a cellphone creates a cell-site record by merely operating, virtually any activity on the phone generates location data. Therefore, due to the "deeply revealing nature of [the data], its depth, breadth, and comprehensive reach, and the inescapable and automatic nature of its collection..." the acquisition of the cell-site records will be treated as a search that requires a warrant.

Four justices dissented, arguing mostly that the Fourth Amendment could never limit review of records that had never belonged to Carpenter. Nothing of his was "searched." But consider the implications if the decision had gone the other way. Would law enforcement have the ability to gather (from Waze, Google and the like) electronic business records created by our cellphones when we use them for highway navigation, then issue retroactive speeding tickets based on what the records revealed?

As the majority explained in Carpenter, privacy interests require that the law resist turning our cellphones into virtual ankle monitors.

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