Jameel Jaffer and Alexander Abdo:

The case, Carpenter v United States, arises out of the government’s prosecution of Timothy Carpenter for a series of armed robberies carried out in south-eastern Michigan and north-western Ohio several years ago. In the course of its investigation of the crimes, the government ordered Carpenter’s cellphone provider to turn over data it had collected relating to Carpenter’s movements. In response, the provider produced 186 pages listing every call that Carpenter had made over a 127-day period, as well as coordinates indicating where Carpenter had been at the beginning and end of each of those calls.

Importantly, it turned over these records even though the government had not obtained a warrant based on probable cause. Carpenter asked the court to suppress the government’s evidence under the fourth amendment, which protects the right to privacy.

Many cellphone users have only a vague understanding of the extent to which providers monitor their movements, but these companies now track us much more closely than even the most committed human spies ever could. Cellphones function by connecting to antennas – “cell sites” or “cell towers” – that provide cellular service. Those cell sites, which are owned and operated by the cellular companies, are programmed to record which phones connect to them, and when. They also record the direction from which the connecting phone’s signal is received and, often, the distance of the phone from the cell site.

So-called “cell site location information” is becoming ever more precise, because the cellular network is becoming ever more dense. The analytical tools that can be brought to bear on this information are also becoming more sophisticated, meaning that investigators can draw reliable conclusions from smaller and smaller amounts of data. It’s precisely because the information is so rich, of course, that the government is interested in accessing it.