A lot of the early press reports on United States v. Jones reports that the Supreme Court held that the government needs a warrant to install a GPS device. But that’s not correct, actually. The Court merely held that the installation of the GPS was a Fourth Amendment “search.” The Court declined to reach when the installation of the device is reasonable or unreasonable. As the opinion explains on page 12 of the slip opinion:
The Government argues in the alternative that even if the attachment and use of the device was a search, it was reasonable—and thus lawful—under the Fourth Amendment because “officers had reasonable suspicion, and in-deed probable cause, to believe that [Jones] was a leader in a large-scale cocaine distribution conspiracy.” Brief for United States 50–51. We have no occasion to consider this argument. The Government did not raise it below, and the D. C. Circuit therefore did not address it. See 625 F. 3d, at 767 (Ginsburg, Tatel, and Griffith, JJ., concurring in denial of rehearing en banc). We consider the argument forfeited. See Sprietsma v. Mercury Marine, 537 U. S. 51, 56, n. 4 (2002).
So we actually don’t yet know if a warrant is required to install a GPS device; we just know that the installation of the device is a Fourth Amendment “search.”