Citing "the seismic shifts in digital technology" since the last time it considered the issue, the U.S. Supreme Court has ruled that people have a reasonable expectation that their cellphone's location data will be private. A reasonable expectation of privacy means that the Fourth Amendment's prohibition against unreasonable searches and seizures applies. Therefore, law enforcement must obtain a warrant before seeking cellphone location data except in emergency situations.
On Wednesday, the U.S. Supreme Court heard arguments about whether police should be required to get a warrant before accessing location records for a suspect's cellphone. Law enforcement has been gaining access to these records by getting an ordinary court order, which requires a lower standard of evidence than a warrant.
When police ask a judge for a search warrant, they are required to explain why they have probable cause to search the person or location in question. The Constitution requires it.