The ruling represents a serious complication for law enforcement nationwide, which increasingly relies on high tech surveillance of suspects, including the use of various types of GPS technology.
A GPS device installed by police on Washington, D.C., nightclub owner Antoine Jones' Jeep helped them link him to a suburban house used to stash money and drugs. He was sentenced to life in prison before the appeals court overturned the conviction.
Associate Justice Antonin Scalia said that the government's installation of a GPS device, and its use to monitor the vehicle's movements, constitutes a search, meaning that a warrant is required.
"By attaching the device to the Jeep" that Jones was using, "officers encroached on a protected area," Scalia wrote. He concluded that the installation of the device on the vehicle without a warrant was a trespass and therefore an illegal search.
All nine justices agreed that the GPS monitoring on the Jeep violated the Fourth Amendment's protection against unreasonable search and seizure.
Scalia wrote the main opinion of three in the case. He was joined by Chief Justice John Roberts and Justices Anthony Kennedy, Clarence Thomas and Sonia Sotomayor.
Sotomayor also wrote one of the two concurring opinions that agreed with the outcome in the Jones case for different reasons.
Justice Samuel Alito wrote, in the other concurring opinion, that the trespass was not as important as the suspect's expectation of privacy and that the long-term duration of the surveillance impinged on that expectation of privacy. Police monitored the Jeep's movements over the course of four weeks after attaching the GPS device.
"The use of longer term GPS monitoring in investigations of most offenses impinges on expectations of privacy," Alito wrote in an opinion joined by Justices Ruth Bader Ginsburg, Stephen Breyer and Elena Kagan. Sotomayor in her concurring opinion specifically said she agreed with Alito on this conclusion.
Alito added, "We need not identify with precision the point at which the tracking of this vehicle became a search, for the line was surely crossed before the four-week mark."
Regarding the issue of duration, Scalia wrote that "we may have to grapple" with those issues in the future, "but there is no reason for rushing forward to resolve them here."
Alito also said the court should address how expectations of privacy affect whether warrants are required for remote surveillance using electronic methods that do not require the police to install equipment, such as GPS tracking of mobile telephones.
A federal appeals court in Washington had overturned Jones's drug conspiracy conviction because police did not have a warrant when they installed a GPS device on his vehicle and then tracked his movements for a month. The Supreme Court agreed with the appeals court.
The case is U.S. v. Jones, 10-1259.