In United States v. Jones, the United States Supreme Court unanimously held that attaching a GPS device to a defendant's car to track their movements constitutes a "search" under the Fourth Amendment.
Antoine Jones was a drug defendant convicted of conspiracy to distribute cocaine after police installed a GPS device on his Jeep. While cars generally are held to a lower standard than other private property, Justice Scalia opined that "the government physically occupied private property for the purpose of obtaining information. We have no doubt that such a physical intrusion would have been considered a 'search' within the meaning of the Fourth Amendment when it was adopted." Therefore, the police were required to have a warrant before the GPS device was attached, leading the Supreme Court's blog to call the case's outcome a "big loss for the Federal government." In her concurring opinion, Justice Sotomayor added that even in the absence of a trespass, the Fourth Amendment applies when a defendant's reasonable expectation of privacy has been violated.
The ACLU lauded the decision, exclaiming that a "majority of the court acknowledged that advancing technology, like cell phone tracking, gives the government unprecedented ability to collect, store, and analyze and enormous amount of information about our private lives."