The law requires, in most situations, that the police get a warrant in order to gather cellphone location information kept by cellphone providers. The U.S. Supreme Court established this privacy rule for all the country in the 2018 case Carpenter v. United States. (585 U. S. ____ (2018).)
In Carpenter, the Supreme Court explained that cellphones are constantly scanning for cell sites, which are sets of antennas mounted on towers and other high points like light posts and buildings. Phones look for the best signal, which the closest cell site usually offers. A typical cellphone produces a time-stamped record every time it connects to a cell site. That kind of record is called “cell-site location information” (CSLI). With CSLI you can get a really good idea of where someone’s phone has been.
In 2011, police investigating a series of robberies in Michigan and Ohio obtained cellphone records for several suspects, including a man named Carpenter. They had applied for and received court orders for the records, which showed months’ worth of information as to where Carpenter (really, his phone) had been. Basically, all the officers had to do to get the records was show that they had a reasonable basis to believe the information was “relevant and material to an ongoing criminal investigation.” (18 U.S.C. §2703(d).)
If the police get evidence without a warrant when a warrant is legally required, the evidence will normally be suppressed. The issue the U.S. Supreme Court faced in the Carpenter case was whether the prosecution should have been allowed at trial to use the location records to show where Carpenter had been in relation to some of the robberies. (The trial resulted in conviction and a long prison sentence.)
The U.S. Supreme Court held that the kind of court order the police got in Carpenter isn’t enough to get CSLI from a cellphone provider. It said that, to get that type of data, the government normally needs a warrant supported by probable cause. Such a warrant requires more proof of the relevance of the desired information to a crime than the Carpenter court order did.
The Supreme Court did allow for the possibility that emergency circumstances (like the need to address a bomb threat) could justify the police not getting a warrant for CSLI. It also took pains to note that its holding was narrow—that the decision didn't necessarily apply, for instance, to real-time CSLI or data regarding all the devices that connected to a given cell site over a certain period of time. (The case didn’t probe the topic of “stingrays.”)
The Court’s holding that warrants are generally required for CSLI was consistent with the trend suggested by recent decisions dealing with the relationship between technology and privacy. The Court had held in 2012 that the police can’t arbitrarily slap a GPS on a car and track the vehicle’s movements (United States v. Jones, 565 U. S. 400), and in 2014 that officers normally need a warrant to search the cellphone of someone they’ve arrested (Riley v. California, 134 S. Ct. 2473).