Police in Wisconsin need no warrant to electronically track the moves of motorists not suspected of any crime, according to a ruling handed down yesterday by the state court of appeals. The decision came as a three-judge panel unanimously declined to overturn the stalking conviction of Michael A. Sveum which had been based largely on evidence provided by a Global Positioning System (GPS) tracking device secretly installed on Sveum's car while it was parked on his private driveway.

Sveum argued that this tracking was illegal because the Fourth Amendment protected him against unreasonable searches. The state countered that police are free to attach a GPS device to tracks a vehicle that uses public roads because the same information would be available to any human officers stationed to observe the streets.

"We agree with the state that neither a search nor a seizure occurs when the police use a GPS device to track a vehicle while it is visible to the general public," Judge Lundsten wrote for the court.

In this case, police did obtain a warrant before using the GPS device, but the appellate judges insisted no such warrant was even necessary. The court based its reasoning on a pair of twenty-five-year-old US Supreme Court cases, US v. Knotts and US v. Karo, which covered the legal limitations of using a primitive transmitter as a tracking device.

"Knotts and Karo teach that, to the extent a tracking device reveals vehicle travel information visible to the general public, and thus obtainable by warrantless visual surveillance, the use of the device does not normally implicate Fourth Amendment protections," Lundsten wrote. "It follows that no Fourth Amendment violation occurred here simply because the police used a GPS device to obtain information about Sveum's car that was visible to the general public."

Although the device tracked Sveum's car to its garage, it did not track movements within the garage. Thus, it did not tell police anything they could not have learned from conducting physical surveillance to determine when he entered or departed from his property, the court reasoned. The privacy consequences of the ruling forced the appeals panel to issue a forceful recommendation that the legislature change the law to limit police power to conduct warrantless tracking operations.

"We are more than a little troubled by the conclusion that no Fourth Amendment search or seizure occurs when police use a GPS or similar device as they have here," Lundsten wrote. "So far as we can tell, existing law does not limit the government's use of tracking devices to investigations of legitimate criminal suspects. If there is no Fourth Amendment search or seizure, police are seemingly free to secretly track anyone's public movements with a GPS device... Consequently, we urge the legislature to explore imposing limitations on the use of GPS and similar devices by both government and private actors. Such limitations would appear to be consistent with limitations the legislature has placed on electronic intercepts of communications."

New Hampshire adopted a law in 2007 banning GPS surveillance on highways not related to a specific criminal investigation. A copy of the appeals court decision is available in a 100k PDF file at the source link below.