Pam Dixon, executive director of the World Privacy Forum in San Diego, said the decisions show how courts "are looking for limits on the use of GPS" by law enforcement in the absence of any clear federal law on the issue. "Both judges were troubled by the implication of the uses of these devices. Both judges agreed that federal law is needed on whether use of GPS devices constitutes search in the law enforcement context," Dixon said.

The decisions come at a time when an increasing number of law enforcement agencies around the country are using GPS devices to track suspects in investigations. Several states have laws governing the requirements a prosecutor or police would need to meet in order to use a GPS tracking device. The requirements include authorities needing to explain specifically what information they hope to get from the tracking, or showing probable cause or reasonable suspicion.

So far, the courts have appeared to be split on how to handle challenges to such tracking, said John Verdi, senior counsel at the Electronic Privacy Information Center (EPIC) a Washington D.C. advocacy group. "There is no clear Supreme Court guidance on this issue," nor has there by any from any federal courts, Verdi said. As a result, state courts have been left to decide what to do using their own state constitutions, he said.

Some courts have ruled that an individual travelling in a vehicle on a public road should have little reasonable expectation of privacy, Verdi said. He pointed to a 1983 court case in which government agents placed a a pager-like tracking device in a five-gallon drum of chloroform and then followed the vehicle transporting the container using visual surveillance and the tracking device. In that case, the court ruled that the use of the device had only augmented their sensory abilities without violating any of the individual's rights.

In its decision last week, Wisconsin's court of appeals noted that if all that a tracking device did was to reveal vehicle movement, information that could have been obtained by warrantless visual surveillance, no Fourth Amendment violation was involved. Similarly, the police action of attaching a GPS devices to a car and subsequently tracking it does not constitute a search and seizure, the court ruled.