According the the 4th District court of appeals in Wisconsin, police don't need a warrant to track cars with GPS.  The court said that the police were able to track car with GPS because the devices were able to provide information about the location of  car withing public space and that when the vehicle went someplace private, like a parking structure or a personal garage, that someone observing the car could be able to tell the exact same information.  Because the tracking was deemed to resemble physical surveillance so closely, the court ruled that police could use GPS as an information gathering technique like they would be able to use physical surveillance - without a warrant.

The court also urge the legislature to make laws to regulate the way that police could use GPS tracking technology because of the privacy implications it could have, but for now they gave their stamp of approval to the police in using this type of technique to track suspects.