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.

Other courts though have held decidedly less benign views on the use of GPS. Courts in Washington and Oregon have in past cases held that law enforcement authorities require warrants to conduct GPS tracking. In coming to these decisions the judges have argued that the kind of surveillance enabled by GPS devices do not just augment existing abilities, but allows access to a large amount of additional information on an individual, Verdi said.

Chief Judge Lippman wrote that GPS devices are "vastly different and exponentially more sophisticated" devices than pagers or other tracking devices. "Constant, relentless tracking of anything is now not merely possible but entirely practicable."