All nine justices of the Supreme Court ruled on Monday that police officers violated the Fourth Amendment rule against unreasonable search and seizure when they attached a GPS device to a suspect's car and tracked it for 28 days without a warrant. But the court was split down the middle on the reasoning. Four justices focused on the physical trespass that occurred when the police attached the device, four focused on the violation of the suspect's "reasonable expectation of privacy," and the final justice, Sonia Sotomayor, endorsed both theories.

The case involved a suspected drug dealer. The feds got a warrant to track his car with a GPS device and then "installed a GPS tracking device on the undercarriage of the Jeep while it was parked in a public parking lot." But agents installed it a day after the warrant had expired and in a location not authorized by the warrant—making the surveillance warrantless. (The feds also had to access the Jeep again a couple weeks later in order to change the GPS tracker's battery "when the vehicle was parked in a different public lot in Maryland.")

At trial, the GPS data was used to link the defendant to an alleged drug stash house that "contained $850,000 in cash, 97 kilograms of cocaine, and 1 kilogram of cocaine base." The defendant was sentenced to life in prison. The Supreme Court considered the question of whether the GPS tracking had been conducted legally.

Two theories

While the result was unanimous, the reasoning was not. A five-judge majority led by Justice Scalia, and including most of the court's conservatives, focused on the physical trespass involved in attaching the device to the car. "The Government physically occupied private property for the purpose of obtaining information," Scalia wrote. "We have no doubt that such a physical intrusion would have been considered a 'search' within the meaning of the Fourth Amendment when it was adopted."

The focus on physical trespass is significant because it suggests that GPS surveillance by other means—such as by obtaining data from the GPS device already included in many of our cell phones—would not violate the Fourth Amendment.

Three of the court's liberals signed a concurrence by Justice Alito, a conservative, that would have taken a stronger pro-privacy stance. Alito argued that extended warrantless tracking itself violates the Fourth Amendment regardless of whether the government committed a trespass to accomplish it.

Alito focused on the famous case of Katz v. United States that established the "reasonable expectation of privacy" test for violations of the Fourth Amendment. He argued that the trespass here was of little consequence to Fourth Amendment analysis, and that what really matters is that the defendant had a reasonable expectations that the details of his movements over a 28-day period would be private.

Scalia responded to this critique in his opinion. "Unlike the concurrence, which would make Katz the exclusive test, we do not make trespass the exclusive test," he said. "Situations involving merely the transmission of electronic signals without trespass would remain subject to Katz analysis."

Of course, because Scalia chose to rule on narrow trespass grounds, he doesn't actually explain how the "reasonable expectation of privacy" reasoning would apply to GPS tracking. That leaves this important body of law unsettled, which is worrying because it's becoming increasingly common for the police to obtain cell phone location data without a warrant.

Sotomayor attacks the third-party doctrine

Justice Scalia's opinion is the majority opinion only because Justice Sotomayor, an Obama appointee, signed onto it. But in addition to endorsing Scalia's position, she also filed a separate concurrence in which she endorsed both Scalia's concerns about physical trespass and Justice Alito's broader concerns about the dangers of warrantless GPS tracking.

"As Justice Alito incisively observes, the same technological advances that have made possible nontrespassory surveillance techniques will also affect the Katz test by shaping the evolution of societal privacy expectations," Sotomayor wrote. "Under that rubric, I agree with Justice Alito that, at the very least, 'longer term GPS monitoring in investigations of most offenses impinges on expectations of privacy.'"

Justice Sotomayor also raised an issue that neither Scalia or Alito addressed: the third party doctrine. That's the theory that we lose Fourth Amendment protection when we disclose information, such as bank records, cell phone locations, or the contents of our email inboxes, to a third party such as Bank of America, Verizon, or Google, respectively.

Sotomayor called the third-party doctrine "ill-suited to the digital age, in which people reveal a great deal of information about themselves to third parties in the course of carrying out mundane tasks. People disclose the phone numbers that they dial or text to their cellular providers; the URLs that they visit and the e-mail addresses with which they correspond to their Internet service providers; and the books, groceries, and medications they purchase to online retailers."

Sotomayor's discussion of the third-party doctrine has no legal significance, since she was the only one to sign onto her concurrence. But it could prove to have greater significance in the long run. The existence of at least one justice who is skeptical of the doctrine will inspire privacy advocates to raise objections to the idea in future cases. And one of those cases is likely to reach the high court at some point in the future.