Authorities must establish probable cause and secure a warrant before obtaining information from cell phone providers that can indicate the round-the-clock whereabouts of customers, a federal judge in Brooklyn ruled yesterday.

Fourth Amendment protections against unreasonable searches apply to the so-called cell-site-location records as surely as judges of a previous generation found that they applied to people using pay phones, Eastern District Judge Nicholas G. Garaufis determined yesterday. In fact, he said, cell phones have all but rendered pay phones obsolete as a means of communication and are rarely out of the reach of users.