Laura M. Moy


I used to track cell phone location information for prosecutors.

Tomorrow morning, the Supreme Court is hearing arguments in Carpenter v. United States, a case that asks whether the Constitution protects the cell phone location data that wireless providers collect from their customers. When a law enforcement agency wants access to 127 days’ worth of this data, does it have to show a judge that it has “probable cause” to believe the records will reveal evidence of a crime? Or does it only have to meet a somewhat lower standard — that it reasonably believes the records are “relevant and material to an ongoing criminal investigation”?

Photo by Mike Mozart, licensed under CC-BY-2.0.

There are lots of other articles and op-eds right now about the issue that is before the court — whether we have an “expectation of privacy” in location information that we’ve voluntarily shared with our phone providers in order to get service, especially now that due to changing technology, the information we necessarily share with our phone provider is far more revealing than it used to be.

I want to talk about a closely related and equally important issue people aren’t talking about: How advancing technology makes it extremely difficult for defense attorneys with limited resources to do their job.

I have an unusual perspective on the issue because before I was a lawyer, I analyzed this type of location information full-time as an analyst for the Manhattan District Attorney’s Office. In fact, my experience at the Manhattan DA is what helped me make the decision to go to law school and ultimately to become a privacy lawyer, and I’ll explain why.

But before we get to that, allow me to explain exactly what information is at stake in this case. Every time your phone connects to the network, such as to make or receive a phone call or text message, or to transfer data via an app, your phone company makes a record of it. You can view some of this information. For example, if you have a detailed phone bill, it might list out all the calls you’ve made, and look something like this:

What you might not realize is that there are missing columns here — your phone company stores more information about your communications than what appears on your bill. One of the things it stores is the wireless cell towers your phone connected to during each communication — your phone’s “cell-site location information.” In other words, on the phone provider’s end, the records might look like this:

(This example only shows the cell sites your phone was connected to at the beginning and end of each event. That’s what I was working with a decade ago, but recently the government has admitted that it also seeks access to cell-site location information generated by apps running in the background — which could be a tremendous amount of additional information.)

I used to examine this stuff firsthand, so take it from me: If someone wants to know not only who you’ve spoken to, but where you’ve been, those extra columns are data gold. Those numbers correspond to specific antennas, or cell sites, operated by the phone company. By looking up the geographic location of those cell sites, an analyst can approximate your movements over time. This is especially the case if you make and receive a lot of phone calls and text messages while you’re on the move or if you live in an area where towers are really densely packed, and therefore tend to be communicating with phones that are very nearby, like in an urban area. Analysis of tower locations can reveal highly private details of a person’s life. Someone like me might be able to use cell-site location information to figure out where you live, where you work, where your kids go to school, whether you’re having an affair, and where you worship.

As a 23-year-old college grad, that’s exactly what I was doing. I advised prosecutors on what information they should request to make their case or figure out where someone had traveled and I helped generate court orders for that information. Then when that information came in, I analyzed it, mapped it, and occasionally presented it in court.

There are few things as riveting — and damning — in a courtroom as a really nice graphic that suggests a defendant was right by the scene of a crime when the crime took place. Judges and juries would stare intently as I introduced my maps, and defendants’ faces would fall. When it came time for defense attorneys to cross-examine me, they struggled to produce cutting questions. Underwater as they were with incredibly heavy caseloads, little or no technical expertise, and few of the analytical resources that the prosecution had, the defense attorneys I encountered in court invariably were forced to cross-examine me without having performed an independent analysis of the data.

Photo by Captain Roger Fenton.

The adversarial process in a trial is supposed to ensure that the evidence presented against someone accused of a crime — who could be deprived of their freedom for years or even decades to come — is carefully scrutinized, and that judges and juries are aware of evidentiary weaknesses. But because my evidence was technically complicated, that process broke down. I was seemingly invincible in the courtroom. Defendants and their attorneys felt blindsided by the evidence I presented and powerless to contest it.

It didn’t feel good to have that kind of power when it was clear that the power sprung from a failure of the legal system. That’s why I went to law school and, eventually, made my way into privacy law and policy. Because as technology develops, the law should keep up. Hopefully after listening to arguments tomorrow, the Supreme Court will agree and recognize that we have a right to privacy in our location, even as technology advances and more of that information is collected.

But in the longer term, we have to figure out how to deal with the growing challenges faced by defense attorneys who increasingly lack the necessary technical expertise to mount a defense. Among other things, defense attorneys are going to need the resources and trainings to develop their own technical expertise, as well as funding to hire outside experts when the need arises. And policymakers need to recognize that when they support law enforcement agencies’ acquisition of and access to advanced tech tools, they must support parallel enhancements for defense attorneys. Otherwise the already-long odds against criminal defendants — especially indigent defendants — will become completely insurmountable.

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