When the government tracks the location of a cellphone it achieves near perfect surveillance, as if it had attached an ankle monitor to the phone's user," wrote John Roberts, the chief justice of the Supreme Court, in a 2018 ruling that prevented the government from obtaining location data from cellphone towers without a warrant. "We decline to grant the state unrestricted access to a wireless carrier's database of physical location information," Chief Justice Roberts wrote in the decision, Carpenter v. United States. With that judicial intent in mind, it is alarming to read a new report in The Wall Street Journal that found the Trump administration "has bought access to a commercial database that maps the movements of millions of cellphones in America and is using it for immigration and border enforcement." The data used by the government comes not from the phone companies but from a location data company, one of many that are quietly and relentlessly collecting the precise movements of all smartphone-owning Americans through their phone apps. Many apps - weather apps or coupon apps, for instance - gather and record location data without users' understanding what the code is up to. That data can then be sold to third party buyers including, apparently, the government. The courts are [an] imperfect venue for protecting Fourth Amendment rights. The Carpenter ruling applies only to location data captured by cellphone towers and not to location data streamed from smartphone apps.