In January the Supreme Court unanimously ruled that tracking a suspect's movements by attaching a GPS transmitter to his car counts as a "search" under the Fourth Amendment. But because the majority opinion emphasized the physical intrusion needed to surreptitiously install the transmitter, it did not resolve the constitutional implications of surveillance using cellphones, the tracking devices that Americans voluntarily carry in their pockets and purses.
In the absence of clear guidance, a recent report from the American Civil Liberties Union (ACLU) suggests, law enforcement agencies are making up the rules as they go along, often obtaining location data from cellphone carriers without a warrant even for routine investigations. Last week a House subcommittee considered a bill that would address this threat to privacy by requiring a warrant for geolocational surveillance, regardless of the method used.
While the Supreme Court's decision involved surveillance that required a trespass on the target's property, five justices seemed to agree the real issue was the sensitive information collected by continually tracking his car for 28 days. As the U.S. Court of Appeals for the D.C. Circuit observed in the same case, "A person who knows all of another's travels can deduce whether he is a weekly church goer, a heavy drinker, a regular at the gym, an unfaithful husband, an outpatient receiving medical treatment, an associate of particular individuals or political groups—and not just one such fact about a person, but all such facts."
Cellphone tracking can be even more revealing, since people take their phones everywhere, including private indoor locations. Furthermore, carriers retain location records for months or years, creating a trove of personal data that law enforcement agencies can peruse at will if there is no requirement for judicial authorization.
"There have always been facets of American life that have been uniquely safeguarded from the intrusive interference and observation of government," the ACLU's Catherine Crump told the House Judiciary Committee's Subcommittee on Crime, Terrorism, and Homeland Security last week. "Geolocation surveillance threatens to make even those aspects of life an open book to government."
Crump was testifying in support of the Geolocational Privacy and Surveillance (GPS) Act, a bill introduced by Rep. Jason Chaffetz (R-Utah) that would require the government to obtain a probable-cause warrant before intercepting or demanding geolocation data, except in emergencies and cases involving foreign intelligence. That rule is considerably more protective than the Justice Department's current policy, which is to seek a warrant only for real-time tracking of cellphones using GPS or triangulation (a technique that helps locate a phone within the sector served by the nearest base station).
But as Crump observed, "this is a meaningless distinction," since investigators can convert live tracking into historical records simply by waiting a minute or two before looking at the data. In any case, the Justice Department's rule bizarrely implies that examining six months of location records is somehow less intrusive than tracking a cellphone in real time for a day.
Furthermore, as University of Pennsylvania computer scientist Matt Blaze noted in his testimony on the GPS Act, the sectors served by each cellphone base station are becoming smaller and smaller as carriers strive to keep up with increasing demands on their networks. That means it may be possible to identify a target's specific location without GPS or triangulation, simply by knowing the closest base station, which is information cellphones automatically collect.
While the federal approach to cellphone tracking makes little sense, the ACLU reported last month that local policies "are in a state of chaos, with different towns following different rules—or in some cases, having no rules at all." Examining documents from more than 200 law enforcement agencies, the ACLU found that only a few had a general policy of seeking a warrant for cellphone tracking. Some do warrantless tracking only in life-threatening emergencies, but many do it routinely.
Our privacy deserves more respect. The GPS Act would provide it.
Jacob Sullum is a senior editor at Reason and a nationally syndicated columnist. Follow him on Twitter.
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