Huh. I'm kind of surprised, especially since (when you get down to it), GPS tracking is just the modern-day version of tailing a suspect. (As long as the suspect stays in public, no warrant is needed. If the suspect goes somewhere private, you just wait for them to come back out.)
Except tailing a suspect takes continuous manpower, and police wouldn't do it long term unless they *really* knew the guy was up to something. As in probable cause enough to get a warrant.
I know. Sorry, I didn't fully explain my though process.
You can tail someone without a warrant, so long as the person is in public (i.e. a place where an officer has a valid right to be). When it comes down to it, there is no difference (legally)* between tailing a person with a car and putting a GPS on their vehicle. In both cases, all that the police know is where the suspect is. A GPS actually provides less information, because the police cannot see what the suspect is doing, only that the suspect (or the suspect's car, anyway) is at a given location.
*OK, not anymore. The USSC has spoken. But that is the argument I have heard.