The U.S. Supreme Court today agreed to hear a lawsuit that will determine whether police need to obtain a judge's approval before installing GPS trackers on Americans' automobiles.

A ruling, which is expected by next year, will establish whether a warrant signed by a judge is required before law enforcement can engage in the practice of tracking a driver's every move on the roads. The Obama administration argues that no warrant is needed.

The case that will be presented to the justices arose out of a criminal prosecution of Antoine Jones and Lawrence Maynard, two suspected cocaine dealers who ran a nightclub in Washington, D.C. Jones said the warrantless use of a GPS device to track every movement of his vehicle over the course of a month violated the Fourth Amendment, which generally says that warrantless searches are "unreasonable."

In August 2010, the U.S. Court of Appeals for the District of Columbia agreed (PDF), tossing out Jones' conviction "because it was obtained with evidence procured in violation of the Fourth Amendment."

"A reasonable person does not expect anyone to monitor and retain a record of every time he drives his car, including his origin, route, destination, and each place he stops and how long he stays there; rather, he expects each of those movements to remain 'disconnected and anonymous,'" circuit judge Douglas Ginsburg wrote.

Even though police are planting GPS bugs on Americans' vehicles thousands of times a year, the legal ground rules remain unclear, and lower courts have split on whether a warrant should be required. Once relegated, because of their cost, to the realm of what spy agencies could afford, GPS tracking devices now are readily available to jealous spouses, private investigators, and local police departments for just a few hundred dollars.

A brief (PDF) submitted by the Justice Department asking the justices to hear the case says that no American has "a reasonable expectation of privacy in his movements from one place to another," even if technological advancements "allow police to observe this public information more efficiently."

Prosecutors also argue that the lower court's decision "seriously impedes the government's use of GPS devices at the beginning stages of an investigation when officers are gathering evidence to establish probable cause" for a search warrant. On the other hand, however, they're nothing stopping the FBI or other police from tailing a suspect the old-fashioned way: in another vehicle.

The ACLU applauded the Supreme Court's decision to hear the Justice Department's appeal. "Sensitive information about who we are and where we go should not be available to any police officer simply because he is curious about someone," said Catherine Crump, staff attorney for the ACLU Speech, Privacy, and Technology Project. "It must be subject to the judicial oversight that has always been a foundation of liberty in America."

As a response to law enforcement's aggressive use of warrantless tracking--the Los Angeles Police Department even has a dart it can fire at and implant in a passing car--legislation has been introduced in the U.S. Congress and state legislatures to require judicial oversight.

On June 15, Sen. Ron Wyden (D-Ore.) and Rep. Jason Chaffetz (R-Utah) introduced bills that would provide new legal protections for "geolocation information," meaning data that can locate a person through a wireless device or a GPS tracker placed on a vehicle. In March, CNET was the first to report on an earlier draft of the measure. (See a Q&A with Wyden.)

California has enacted a law saying the use of "an electronic tracking device to determine the location or movement of a person" implicitly requires a warrant for police use of a GPS. A handful of other states have taken similar steps, enacting penalties for some uses of electronic tracking devices and excluding their evidence from criminal proceedings unless police obtain warrants.

This morning's order (PDF) from the Supreme Court agreeing to hear the case asks lawyers for both sides to address this question: "Whether the government violated respondent's Fourth Amendment rights by installing the GPS tracking device on his vehicle without a valid warrant and without his consent."

Complicating the privacy risks of tattletale cars are a pair of U.S. Supreme Court cases decided two decades ago. Those cases, U.S. v. Knotts and U.S. v. Karo, established that police don't need court approval to track suspects through a crude radio beeper.

But the Knotts decision, which involved a single car trip, said only that someone "traveling in an automobile on public thoroughfares has no reasonable expectation of privacy in his movements from one place to another"--and did not deal with the kind of second-by-second tracking, without limit, that the Obama administration and other law enforcement agencies believe is perfectly permissible.