From the ACLU:
When the government demands that your cell phone carrier reveal information about where you’ve been, or where you’re going to go, they should get a warrant. Location information is exactly the kind of sensitive information that the Fourth Amendment is supposed to protect. That’s why we filed a “friend of the court” brief in the Northern District of California on Tuesday urging a federal magistrate judge to tell the government to come back with a warrant when asking for historical cell site location information.
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