The Fourth Amendment protects Americans from unlawful searches and seizures. Among other things, this means that if law enforcement suspects someone of a crime, they need a probable cause warrant before they can search a person's home, car or otherwise invade their privacy.
But this does not seem to be the case for tracking cellphone GPS activity. A recent 6th U.S. Circuit Court of Appeals decision held that law enforcement does not need a warrant to track a person through cellphone GPS signals.