The U.S. Supreme Court provided that a GPS tracking device affixed to an individual's vehicle constitutes a search for the purposes of the Fourth Amendment.  When there is a "search", then that search must be reasonable under the U.S. Constitution, and, in some cases, a search warrant is necessary.  In all cases in which a search occurred by law enforcement, probable cause (or reasonable suspicion) must exist to warrant that search.

United v. Jones, 565 U.S. ___ (Jan 23, 2012)