On Friday, a unanimous panel of the U.S. Court of Appeals for the District of Columbia Circuit decided (PDF) that the Fourth Amendment requires the government to obtain a warrant when it uses a GPS tracking device to monitor someone's movements for an extended period of time. The court held that we have a reasonable expectation of privacy in the sum of our movements over time, recognizing that continuous surveillance reveals a highly intimate picture of a person's life — for example, "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."
If we think about all the details of our lives that would be exposed by tracking all of our movements for the past month, it might seem obvious that the Fourth Amendment should protect our privacy in that information. The government had argued, however, that under a Supreme Court decision from over a quarter of a century ago in which the police used a primitive beeper to help them follow a car during a single trip between two locations, individuals never have a reasonable expectation of privacy in their movements on public streets.
In a friend-of-the-court brief, the ACLU of the National Capital Area and the Electronic Frontier Foundation argued that times and context have changed since then. The court agreed. The GPS technology that is now available permits the police to easily and inexpensively track people's movements 24 hours a day for indefinite periods of time, allowing them to watch over the totality of people's lives as they move from place, to place, to place. And by tracking many people and plotting their movements on a map — as is now technologically possible — the government could easily learn whose lives intersect with whose, and when and where. The resulting invasion of privacy is far greater than that from the visual surveillance practices of the past. It's truly a specter of Big Brother.
Friday's decision is important because it strikes the right balance between technological progress and privacy by requiring the police to obtain a warrant to use GPS devices — and its reasoning should apply also, for example, to cellular technology that the police can use to track people's whereabouts using their cell phones. The decision also recognizes that the whole package of data about us implicates greater privacy interests than the sum of its parts. This is a significant victory for privacy in the digital age, where more and more pieces of information about us are available in the public sphere.