In what was considered a major victory for personal privacy advocates, the U.S. Supreme court has restricted law enforcement's ability to use cellphone location data in the course of criminal investigations. A 1979 precedent had given some the impression that this data could be obtained using a mere court order. The recent ruling makes clear that police ordinarily need a warrant for the information.
Modern cellphones can be tracked at least to the nearest cell tower and often more specifically. That location information is logged by cell service providers and can give law enforcement more insight than you might expect. When people carry their phones most of the time, cellphone location data can provide a timed record of virtually everywhere they have been.
In the case before the court, the police used cellphone location data to tie a defendant to the areas where a string of Michigan and Ohio robberies occurred. However, the officers did not get a warrant before obtaining the location data.
Requiring a warrant is meant to protect the rights of the defendant. Judges are to issue warrants only after independently verifying that the officers seeking the warrant have probable cause to believe that a crime has been committed, is in progress, or is about to be committed. Probable cause is a higher standard than is required for other court orders.
However, with the notable exception of medical records, police can obtain records held by third parties like banks and cell service providers without a warrant. This is due to a 1979 Supreme Court case holding that people have no reasonable expectation of privacy in business records held by third parties. Without a reasonable expectation of privacy, the Fourth Amendment generally doesn't apply.
This case, however, recognized "seismic shifts in digital technology" since 1979. The majority of the court found that people do have a reasonable expectation of privacy in cellphone location data. This is because the data is "is detailed, encyclopedic and effortlessly compiled."
"When the government tracks the location of a cell phone," wrote Chief Justice John Roberts for the court, "it achieves near perfect surveillance, as if it had attached an ankle monitor to the phone's user."
The third-party business records rule remains in place, so police can obtain most such records without a warrant. Also, the court made clear that a warrant would not be required to obtain cellphone location data in an emergency situation.
Still, the court recognized that the information gathered by a modern smartphone is far more detailed and meaningful than the phone company records of 1979.