On January 23 the U.S. Supreme Court ruled unanimously that law enforcement authorities do not generally have a right to affix a GPS tracking device to a suspect's car without first obtaining a valid warrant. Of the many things that can be said about the case, which has been called the most important Fourth Amendment test in a decade, perhaps the most sobering in the long run will be this: the decision is based on technology assumptions that are rapidly becoming irrelevant.
The case, formally known as United States v. Jones, has its roots in the technologically distant past of 2005, when smart phones, tablets, mobile apps, social networking and license plate cameras had not yet become ubiquitous—when it was still possible to make a trip to the grocery store without leaving a megabits-long trail of digital footprints.
The question before the Court turned in significant part on the physical trespass involved in placing a GPS tracker on a suspect's car. Nowhere is this clearer than in the majority opinion, delivered by Justice Antonin Scalia, explaining why the Court ruled against the government (pdf):
"It is important to be clear about what occurred in this case: The Government physically occupied private property for the purpose of obtaining information. We have no doubt that such a physical intrusion would have been considered a 'search' within the meaning of the Fourth Amendment when it was adopted."
However, the majority opinion stops well short of addressing the most pressing privacy issues that accompany today's location tracking technologies. First, there are many more ways today to perform surveillance without physically trespassing on private property, a point explicitly recognized by Justice Samuel Alito in a concurrence joined by three other justices. And, as Justice Sonia Sotomayor wrote in another concurring opinion (pdf), "the Fourth Amendment is not concerned only with trespassory intrusions on property."
Second, the assumption that tracking a person's location requires advance planning and action is fast becoming outdated. Increasingly, and likely irreversibly, to be engaged in the world means leaving detailed digital records of almost everything we do.
Mobile phone service providers log the list of cell sites to which our cell phones connect throughout the day. Mobile apps, more than half a billion of which were downloaded in the U.S. during the last week of December alone, gather data on the usage patterns of our wireless devices. In addition, mobile apps often track device location to the accuracy of a specific residence or office building, undermining the oft-cited claim that the data gathered is not "personal." Much of this data is collected and then sold with our consent, in accordance with privacy policies that few of us read before accepting, to a complex ecosystem of mobile application providers and advertisers. License plate cameras record our automobile trips. When we walk into a store, restaurant, office building, or sit in a taxi, images of us are recorded and date-stamped.
In the past it would have been impractical to archive all of this information. Not anymore. Retail hard drive costs are over one million times cheaper today than in the mid-1980's, and currently stand at roughly 5 cents per gigabyte. About $50 worth of storage can hold the information identifying the location of each of one million people to 4.5-meter accuracy at five-minute intervals, 24 hours a day for a full year. Data from video surveillance cameras is more voluminous, but storage cost trends are making it easier to archive that information as well.