If the government put a GPS monitor on your car and used it to track every vehicular movement of yours for four weeks, do you think that would violate your Fourth Amendment rights? The government would like to be able to do that kind of thing without getting a warrant, and the Supreme Court will soon decide whether it can.

On November 8th, the Court will hear oral argument in U.S. v. Jones. Yours truly was the lead author of Cato’s amicus brief in the case, which may have a significant effect on how Fourth Amendment law intersects with new information technologies for decades to come.

In 2004, suspecting that Antoine Jones was dealing drugs, the FBI secretly attached a GPS tracking device to his car without a valid warrant. The FBI used this device to monitor and record the car’s movements, noting its location every ten seconds when it was in motion, for nearly a month before finally arresting Jones. The U.S. Court of Appeals for the D.C. Circuit found that the FBI’s action was unconstitutional because it violated Jones’s “reasonable expectation of privacy”—the two‐​part Fourth Amendment standard developed in the landmark case of Katz v. United States. Though he traveled on public roads, the totality of his movements was available to nobody and thus was private.

Our brief argues that the government’s conversion of Jones’s vehicle into a surveillance device was an unreasonable seizure under the Fourth Amendment. Even though he didn’t lose a “possessory” interest in his car, the government invaded Jones’s various property rights, including the right to exclude, the right to manage, the right to use, and the right to the profits. Similarly, using his car to collect detailed data on his movements over this extended period without getting a warrant was an unreasonable search. The data reflecting his movements would never have come into existence without the government attaching its GPS device to his car. These are tough, interesting issues arising in the new circumstances created by information technology.

We spent as much time in the brief on the “reasonable expectations of privacy” test. The product of one Justice’s lone concurrence in the Katz case, it holds that if a person has an actual (subjective) expectation of privacy and that expectation is one society is prepared to accept, then the Fourth Amendment protects the object of that expectation.

Courts have never faithfully applied this test, and for good reason: it’s a doctrinal mess that reverses the Fourth Amendment’s focus. Courts have second‐​guessed what the citizenry thinks in terms of privacy rather than examining government action to see if it is reasonable. Under “reasonable expectations” doctrine, things that are left in plain view are always available to the government while things that are hidden—well, the Court will look to see whether keeping it private comports with “reasonable expectations.”

The majority ruling in Katz rested on physical and legal protection that Katz had given to the sound of his voice when he entered a telephone booth. Because Katz had secured the privacy of his conversation, the government wasn’t allowed to access it using a wiretap—not without a warrant. That’s the rule the Court should apply here. The government can’t use uncommon surveillance technology to access private information, including private information about things that happened “in public,” without a valid warrant.

With information technology still rapidly increasing in power, it is critically important that the Supreme Court update Fourth Amendment law while maintaining its consistency with ancient property principles. Doing so will ensure that technology doesn’t render the Fourth Amendment’s protections for our “persons, papers, houses, and effects” quaint.

You can read more, and our brief, on the Cato​.org page about U.S. v. Jones.