Last week a Wisconsin appeals court ruled that police do not need a warrant to track someone's movements by secretly attaching a GPS device to his car. The District 4 Court of Appeals reasoned that attaching the device is neither a "search" nor a "seizure," and the information gleaned from it also could be obtained through perfectly lawful observation of the car's movements, without intruding into any private areas. But the court added that it was "more than a little troubled" by that conclusion and urged Wisconsin legislators to restrict the use of GPS technology by police.
I too am troubled by this result, especially since attaching a tracking device to someone else's car without his permission is a violation of his property rights. At the same time, the Fourth Amendment does not apply to all forms of surveillance or all invasions of privacy. It's pretty clear that surveillance cameras are constitutional, for example, as long as they are located in and trained on public spaces. The best approach in this case might be a statute that makes GPS bugging a criminal offense without a warrant.