The Buffalo News has written an editorial on warrantless GPS tracking.
The sub-headline seems to sum up the position pretty well: "Government's capricious use of GPS violates the Fourth Amendment." The editorial goes on to state, "It's not that law enforcement agencies can't use modern technology to snoop on people they believe need to be snooped on. It's just that they should not be allowed to do so without first employing an old-fashioned tool called a search warrant."
Unfortunately, the editorial seems mostly focused on exigent circumstances, such as the ticking time bombs featured on the television show 24. But it seems like most of the warrantless uses of GPS tracking devices are not in extreme time constrained circumstances, but rather as part of a well thought out investigatory strategy. Law enforcement agencies – with the support of some lower courts – believe that no expectation of privacy is violated by using these devices without a warrant on public property.
See here and here on the blog and here for a draft of an article I am publishing on the topic this winter. As I noted before, I think the warrantless use of GPS tracking devices by law enforcement presents significant Fourth Amendment issues.