The Supreme Court will hear a case involving a man named Antoine Jones whose car was bugged with a secret police GPS device.

Without his knowledge, the police tracked the location of his automobile for a month.  They didn't obtain a warrant in the court of law to do so.  They argue they don't need a warrant because the whereabouts of vehicles on public roads are public knowledge. 

The counterargument, however, is that such persistent and comprehensive surveillance of his vehicle is anything but public.  Ultimately the courts sided with Jones, even though he was indeed intending to sell cocaine.

Judge Douglas Ginsburg of United States Court of Appeals for the District of Columbia Circuit, who ruled in favor of Jones, said:

Prolonged GPS monitoring reveals an intimate picture of the subject's life that he expects no one to have... The intrusion such monitoring makes into the subject's private affairs stands in stark contrast to the relatively brief intrusion at issue.

Previously, US courts have at times upheld evidence obtained through warrantless GPS tracking.  Police officers understandably don't always have time to obtain warrants before tracking, especially if they're witnessing suspicious behavior in real time.   

However, some judges have argued that once the tracking becomes sustained and prolonged, immediacy is no longer an excuse and a warrant is needed. 

Still, there is no clearly cut rule - by the length of the tracking, the type of data obtained, or other metrics- regarding the use of warrantless GPS tracking by authorities.

In the last two decades, the US judicial system has worked to interpret existing laws to establish rules for the many new technologies that were invented.

In this case, it's a matter of interpreting the Fourth Amendment (which protects against unreasonable searches and seizure) in the context of government GPS and location-tracking.

What the Supreme Court decides will likely provide clarity and precedence on this increasingly relevant and pervasive issue.