Huge electronic privacy news out of Washington. In an historic unanimous decision on United States vs. Jones, the United States Supreme Court found that “the Government’s attachment of the GPS device to the vehicle, and its use of that device to monitor the vehicle’s movements, constitutes a search under the Fourth Amendment.” That means that the federal government will now need a probable cause warrant to affix a GPS device to a car.

Kashmir Hill, Forbes’s ace privacy writer, summarized this decision succinctly in a tweet linking to the decision: “Gov lost. Privacy won.”

“The decision, in what is arguably the biggest Fourth Amendment case in the computer age, rejected the Obama administration’s position that American’s had no privacy in their public movements,” wrote David Kravets in Wired: “Warrant required for GPS tracking, Supreme Court rules.” Kravetz observed how long it’s been since a similar case made it to the nation’s highest court:

During oral arguments in the case in November, a number of justices invoked the specter of Big Brother if the police could secretly attach GPS devices on Americans’ cars without getting a probable-cause warrant.

The last time the high court considered the Fourth Amendment, technology and privacy in a big-ticket case was a decade ago, when the justices ruled that the authorities must obtain search warrants to employ thermal-imaging devices to detect indoor marijuana-growing operations, saying the imaging devices carry the potential to “shrink the realm of guaranteed privacy.”

“While the result was unanimous, the reasoning was not,” observes Timothy Lee in ArsTechnica: “Supreme Court holds warrantless wiretapping unconstitutional”

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

Justice Sotomayor straddled the line. She signed onto the majority opinion, but 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, referring to the famous case of Katz v. United States that established the “reasonable expectation of privacy” test for violations of the Fourth Amendment. “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.'”

The Center for Democracy and Technology, which was an active participant in the jurisprudence surrounding the case, released the following statement on the ruling:

“The Supreme Court today made it clear that it will not allow advancing technology to erode the Constitutional right of privacy,” said Gregory T. Nojeim, Director of CDT’s Project on Freedom, Security and Technology.

The Justice Department had argued that the GPS device, because it tracked the person’s movements only on the public streets, did not raise any concern under the Constitution’s Fourth Amendment, which generally requires a warrant for searches and seizures.  Not a single Justice agreed with the government on that issue.

Instead, all nine agreed that, under the facts of the case, the Constitution required a warrant issued by a judge.  Five Justices agreed that any use of GPS planted by the government was a search generally requiring a warrant, effectively settling that issue.

The case also has implications for tracking individuals using cell phone tower data.  Five Justices held that a warrant would have been required on the facts of this case even if the government tracking did not involve planting a GPS device.  “Cell phone triangulation can be just as precise as GPS,” Nojeim said. “Congress should build on this opinion by writing a statute that draws a bright line requiring the government, except in emergencies, to get a warrant before turning your cell phone into a tracking device.”

CDT has helped to coordinate a coalition of major Internet companies, think tanks and advocacy groups from across the political spectrum calling on Congress to require a warrant for cell phone tracking.

CDT filed an amicus brief in the Supreme Court case, arguing that warrant is required for GPS tracking.

“Wow,” tweeted electronic privacy and security researcher Chris Soghoian. “Justice Sotomayor in Jones concurrence (pg 5): it may be necessary to reconsider the 3rd party doctrine,” he continued, which is that there is “no reasonable expectation of privacy for data held by ISPs & telcos.”

In the decision, Sotomayor wrote that it is ‘ill suited to the digital age.’

“So 4 supreme court judges embraced the mosaic theory (but not by name),” tweeted Soghoian. “4 weeks of GPS tracking by gov not OK, but a lesser amount might be. Also interesting to see Sotomayor cite last year’s OnStar privacy firestorm as evidence that the public is not cool with covert GPS tracking. Majority opinion by Sup Ct paves way for more gov tracking of cellphones, which gov still claims it can do (w/single tower data) w/o warrant.”

Expect more tech policy and privacy writers to be all over this one, all week.