The Supreme Court ruled unanimously that tracking someone’s vehicle using GPS is a search under the Fourth Amendment, and thus requires a warrant. Antonin Scalia wrote:
“We hold that the government’s installation of a GPS device on a target’s vehicle, and its use of that device to monitor the vehicle’s movements, constitutes a ‘search’ ” under the Fourth Amendment’s protection against unreasonable searches and seizures, Scalia wrote.
That it took the Supreme Court to decide that placing a GPS tracking device on someone’s car—for a month—should require a warrant is a statement to just how bad a state we are in.
The 9th Circuit Court of Appeals ruled on a similar (and more egregious case) in 2010, where the DEA not only tracked a person’s car without a warrant, but trespassed on his property to place it on his car. They ruled in favor of the government.