You no longer have to worry about police officers tracking you via a hidden GPS transmitter attached to your vehicle, at least not without probable cause. Wired is reporting that the United States Supreme Court has ruled that doing so violates the 4th amendment of the constitution that protects citizens from "illegal search and seizure."
The specific case being argued centered around an alleged drug dealer from the District of Columbia. Police monitored his vehicle for nearly a month and used this data to help get a conviction back in 2008. His lawyers argued that most of the information presented in court was illegally obtained and today the Supreme Court agreed with that decision.
The Obama administration argued that using these methods was not a violation and actually contended that they could put a GPS tracker on the vehicle of every Supreme Court member without a warrant. What's even more scary is that the administration told the court that these devices are used to track thousands of people a year.
Although the question of whether law enforcement can affix a tracking device to a vehicle has been answered, the court did not make any decisions on whether pre-existing GPS devices, such as those provided by services like OnStar, require the same procedures to use.