How is this constitutional? You cannot, I repeat, cannot go onto a person's private property and collect evidence without a warrant unless there is a crime in progress.
The driveway has been ruled as a place where you cannot reasonably expect privacy, and therefore the police can do surveillance there, and that includes attaching a GPS unit.
http://www.time.com/time/nation/article/0,8599,2013150,00.html?hpt=T2
This doesn't violate your Fourth Amendment rights, because you do not have any reasonable expectation of privacy in your own driveway — and no reasonable expectation that the government isn't tracking your movements.
driveway = can't expect privacy
can't expect privacy/not private = no warrant needed
Whether or not this will hold up to the supreme court is another matter, but as of now, that's how it goes.