Several different states have introduced bills to try and make covert GPS tracking by civilians illegal. Bills have been announced in New Jersey and two different counties in New York following the recent political uproar over political candidates tracking each other via hidden GPS devices. While police officers have to go through a legal minefield of regulations and court contradictions to get GPS tracking on a suspect, private citizens have far fewer restrictions.
For private citizens, tracking a vehicle is completely legal if: you own the vehicle, the device is placed on the outside of a vehicle (not hard-wired), the vehicle is in a public location (not on private property) or if the travel information could be obtained by physically trailing the vehicle. Of course, at the end of the day, laws vary slightly from state to state and judge to judge. There isn’t a court precedent for GPS tracking for private or police tracking that I could find.
These bills are aimed primarily at private tracking with GPS and aim to make it illegal to track a vehicle without the permission of the owner. Tracking your own property will continue to be legal. Both bills also say they intend to add exceptions to the law to allow employers to track company vehicles and to allow parents to track their children.