The American Civil Liberties Union, the ACLU of Wisconsin and the Electronic Frontier Foundation filed a friend-of-the-court brief with the Wisconsin Supreme Court on Friday urging the justices to find that, when the police place a global positioning system tracking device on a car to obtain information on its location and movements without a valid warrant, they violate the Wisconsin constitution.

“I think most people would be shocked if they knew the police could attach a GPS unit to their car or truck just out of curiosity about their movements,” said Amelia Bizzaro, from Henak Law Office, cooperating counsel for the ACLU. “The constitutional requirement that police have probable cause to believe a person is violating the law before doing a search is designed to prevent just such fishing expeditions.”

The case arises out of an investigation by the Madison police of a man suspected of violating a restraining order, but its implications affect all citizens. The police secretly attached a GPS unit to the underside of the suspect’s car and tracked his movements. When the police sought to use evidence from the GPS tracking in a criminal prosecution, the suspect asked the court to throw the evidence out because the order giving police permission to use the GPS was defective.

The Wisconsin Court of Appeals, however, said that the police do not even need court permission to attach a GPS unit, because doing so is not a “search or seizure” and thus is not subject to any limitations under the Fourth Amendment, which prohibits unreasonable searches and seizures and generally requires that police get a warrant before conducting a search. This means that the police, or any government agency, can track any citizen’s movements by attaching a GPS unit, without any suspicion that the citizen has done anything wrong and without any oversight by the courts.

“GPS technology allows the government easily and inexpensively to collect incredibly detailed information about a person’s comings and goings,” said Jennifer Granick, Civil Liberties Director of the Electronic Frontier Foundation. “The police can sift through the GPS data to see where and when you go to church – or don’t, what stores you frequent, what events you attend and the address at which you parked the car last night. This incredible power to peer into people’s private lives should not be entrusted to the police without constitutional safeguards.”

The parties in the case, State of Wisconsin v. Sveum, argue about whether the Fourth Amendment of the U.S. Constitution applies. In their friend-of-the-court brief, the ACLU and EFF argue that, whether or not the U.S. Constitution applies, the Wisconsin Supreme Court should conclude that Article I, Section 11 of the Wisconsin Constitution protects this state’s citizens from such intrusive police surveillance. Lawyers on the friend-of-the-court brief include Amelia Bizzaro of Henak Law Office in Milwaukee, Catherine Crump of the national ACLU, Jennifer Granick of the EFF, and Larry Dupuis of the ACLU of Wisconsin.