With all the reports of law enforcement collecting tons of location info
from telcos without a warrant, as well as a bunch of court rulings that seem to chip away at what's left of the 4th Amendment, it's somewhat surprising to see a magistrate judge say that police cannot use a warrant to find out your location
from a mobile operator, for the purpose of arresting you. From Orin Kerr's summary of the ruling:
Here’s the basic reasoning of the opinion. First Judge Gauvey creates what a appears to be a new distinction in Fourth Amendment law: a distinction between (a) Fourth Amendment rights in location at a given time, and (b) Fourth Amendment rights in movement over time. According to Judge Gauvey, individuals have a reasonable expectation of privacy in both. There is a reasonable expectation of privacy as to a person’s location if a person cannot be visually observed in that same way. And there is a reasonable expectation of privacy in movements, which Judge Gauvey seems to be taking from the DC Circuit’s Maynard/Jones “mosaic theory” case (which the Supreme Court recently agreed to hear). Judge Gauvey then reasons that if everyone has this Fourth Amendment right, people who have warrants out for their arrest have this right to privacy, too. For that reason, the information held by the phone company as to the location of the phone user is protected by the Fourth Amendment.
Judge Gauvey then considers whether the Fourth Amendment allows a warrant to be issued based on probable cause that the information will help execute an arrest warrant. She concludes the answer is no: A Fourth Amendment warrant requires probable cause that evidence or contraband is located in the place to be searched or that a person who committed a crime is in the place to be searched. Mere probable cause to believe that location information would help the police execute a warrant is not enough under the Fourth Amendment.
Kerr suggests that the case law actually disagrees with the judge in this case, and even the judge appears to admit that she thinks the Supreme Court might
disagree, but she said without specific guidance from the Supreme Court, she believes her ruling is correct.