In a split 5-4 decision, the U.S. Supreme Court struck down the idea that law enforcement agencies could track a person's whereabouts by GPS without a court order.

Justices Antonin Scalia, Anthony Kennedy, Clarence Thomas, Sonia Sotomayor, and Chief Justice John Roberts voted to uphold the ban on warrentless GPS tracking on Fourth Amendment principles. Allowing police agencies to attach GPS tracking systems without a written court warrant would violate the legal protection of "persons, houses, papers, and effects, against unreasonable searches and seizures," Justice Scalia wrote in the Court's ruling.

Justices Ruth Bader Ginsburg, Stephen Breyer, Elena Kagan, and Samuel Alito sided with the U.S. government, saying that the tracking would not have violated the Fourth Amendment.

Justices Say GPS Tracker Violated Privacy Rights [New York Times]
Law enforcement needs warrant for GPS tracking, high court rules [Cnet]
Supreme Court says police must get search warrant to use GPS tracking devices [Engadget]