The U.S. Supreme Court published a decision yesterday in United States v Jones, in which it held unanimously (although with three separate opinions using different reasoning to reach the same conclusion) that the use of a GPS monitoring device to conduct long-term surveillance of an individual requires a warrant. When the Court granted certiorari and the case was argued last November, legal analysts suggested the ruling could be the most significant Fourth Amendment case in recent memory, but the arguments among the justices authoring the three opinions (Scalia, writing for the Court and joined by four of his colleagues; Alito, joined by two others; and Sotomayor) as to the best way to interpret key precedents leave unresolved some fundamental questions about the constitutionality of electronic surveillance methods.
In the D.C. Circuit appellate case that was the basis of the appeal to the Supreme Court, the crux of the government's argument was that use of a GPS device to conduct surveillance did not violate an individual's reasonable expectation of privacy, applying Justice Harlan's test from his concurring opinion in Katz v. United States. The Supreme Court appeared to find the reasonable expectation of privacy issue moot, concluding simply that placement of a GPS tracking device on the suspect's vehicle (an "effect" for purposes of applying the Fourth Amendment") constituted a search, and rejecting the government's assertion that without a reasonable expectation of privacy, no search occurred. The court's opinion refutes the government's apparent suggestion that applying Katz somehow substitutes for common-law approaches based on trespass rights. The strength of the argument applied here is that the trespass perpetrated by the police involved a personal effect, an item explicitly protected in the Fourth Amendment.
Justice Alito's concurring opinion reaches the same decision on the facts of the case as the official opinion but relies solely on Katz to do so, due to the many difficulties associated with relying on trespass law and pointing out that court precedent has established that the fact that a trespass occurred is neither necessary nor sufficient to establish a violation under the Fourth Amendment. Justice Alito further argues that the two separate events (installing the GPS device and using the device to gather information) alone would not constitute a search. With the specifics of this case, the court did find that the combination of physical trespass with an intent to gather information unquestionably constitutes a search and therefore invokes the protections of the Fourth Amendment. The problem with this approach, as both Justice Alito and Justice Sotomayor note in their concurring opinions, is that if GPS monitoring can be performed without a technical trespass, the court's argument in this case would provide little protection.
This ruling stops short of addressing another issue raised during the appellate process regarding whether the length of time during which a subject is under continuous surveillance has any bearing on his or her reasonable expectation of privacy. Other cases involving electronic surveillance devices, notably including United States v. Knotts, addressed limited or short-term tracking, leaving open the question of whether more prolonged use would require a different application of constitutional principles. Justice Alito, relying solely on Katz to base his concurring opinion argument, determined that "longer term GPS monitoring in investigations of most offenses impinges on expectations of privacy." Justice Scalia expresses some frustration with Justice Alito's reasoning, suggesting that under the Harlan standard, prolonged continous electronic surveillance would be permissible under current constitutional interpretation. Justice Sotomayor provided a closer consideration of the nature of GPS tracking technology and potential evolution of societal expectations of privacy, which are a key element in the Harlan test under Katz. She goes so far as suggesting that "it may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties" and to reject the prevailing assumption that secrecy is a prerequisite for privacy. It appears a more comprehensive ruling on GPS tracking and other forms of electronic monitoring will have to wait for a case with a set of circumstances that limits the Court to considerations of the reasonableness of such monitoring, with or without regard to the length of time over which it occurs.