When "the Government trespassorily inserted the information-gathering device" on a car, it was a search within the meaning of the 4th Amendment.

Says the Supreme Court, this morning, in United States v. Jones. Scalia writes the privacy-protecting opinion, joined by Roberts, Kennedy, Thomas, and Sotomayor.

It is important to be clear about what occurred in this case: The Government physically occupied private property for the purpose of obtaining information....

The text of the Fourth Amendment reflects its close connection to property...

Consistent with this understanding, our Fourth Amendment jurisprudence was tied to common-law trespass, at least until the latter half of the 20th century....

Our later cases, of course, have deviated from that exclusively property-based approach. In Katz v. United States, 389 U. S. 347, 351 (1967), we said that “the Fourth Amendment protects people, not places,” and found a violation in attachment of an eavesdropping device to a public telephone booth. Our later cases have applied the analysis of Justice Harlan’s concurrence in that case, which said that a violation occurs when government officers violate a person’s “reasonable expectation of privacy”....

Despite the deviation — which worked to protect people using public phones and so forth — the Court rejects the Government's argument that Jones had no "reasonable expectation of privacy" with respect to the underside of his Jeep and where the Jeep was when it was driving about on the public roads. The Katz test was about extending the scope of an individual's privacy, not cutting back on traditional property-based protections.

There's no dissent, but Alito writes a concurring opinion which is joined by Ginsburg, Breyer, and Kagan. Alito characterizes the majority of using "18th-century tort law" to interpret the 4th Amendment and says the question should be analyzed in terms of reasonable expectations of privacy.