To say that cell phones are an integral part of our personal and professional life might be the biggest understatement of the 21^st century. Today's smartphones can do virtually almost anything— browse the web, take pictures, play music, watch videos, send messages.

How smart are they? Today's smartphones have more computing power than all of NASA did when it started sending astronauts to the Moon.

And with that computing power, police can track your movements with frightening precision.

William Phillip definitely knows this now, and this rapid technological advancement almost put him in the prison for the rest of his life—almost.

What is CSLI?

As most people know, modern smartphones tap into the closest cell site to find the best wireless signal. In fact, they tap into these sites several times a minute when their signal is on. Each time the phone connects to a cell site, the phone generates a time-stamped record known as cell site location information (CSLI).

Locating a cell phone based on data from a single cell tower can place the phone (and, of the course, the suspect) in a broad area, but it can't pinpoint it. As the phone connects to more towers, accuracy improves. By using what's known as “cell tower triangulation,” police can determine a phone's location within an area of about ¾ of a mile.

Not surprisingly, in a densely populated area like Seattle (where cell towers are close together), police have an easier time zeroing on a location, as compared to more rural areas like Eastern Washington, where towers are far apart.

Phillips takes a drive ... to prison

After a long, extensive, and rather fascinating investigation, Auburn police arrested William Phillip for first-degree murder.

In brief, police determined that Phillip traveled from his home in Portland to Auburn and murdered Seth Frankel, a man who was dating Phillip's ex-girlfriend. After committing the murder, Phillip returned to Portland that evening. During the investigation, police obtained a warrant for Phillip's CSLI records, which showed that Phillip was within blocks of Frankel's home near the time of the murder.

A jury eventually convicted Phillip of murder, but on appeal, the Court of Appeals reversed Phillip's conviction. The Court ruled that the warrant for Phillip's CSLI records were not supported by probable cause--in other words, police didn't have enough evidence at that point in the investigation to justify the warrant.

Undeterred, the State tried a different tactic. Clearly, the CSLI records were crucial to the government's case, so the State asked the trial court to issue a subpoena to get Phillip's CSLI records from AT&T. The judge granted the State's motion.

Phillips appealed again. And he won again in the Court of Appeal, in State v. Phillips. 

Get A Warrant

The Washington Constitution is more defendant-friendly than the defendant-friendly Fourth Amendment. In Washington, your privacy rights are paramount. Unless law enforcement gets a warrant, they cannot disturb your private affairs (i.e. seize your property, search your home, or search you). If they don't, and they find evidence that supports your guilt, they can't use this evidence against you at trial. This is called the “exclusionary rule,” one of the most defendant-friendly provisions in all of criminal law.

In granting the State's subpoena, Phillip's trial judge determined that he had a lower expectation of privacy in cell phone towers than he did in—for example—his personal cell phone or his apartment. Therefore, the State didn't need to get a warrant. The Court disagreed—strongly.

Today CSLI records “provide an intimate window into a person's life. If you go to church, your cell phone knows it. If you visit a strip club, your cell phone knows it. When you visit your family your cell data knows it. As the Court put it, what could be more intimate and private than your personal, sexual, or religious preferences?

For these reasons, individuals have a reasonable expectation in the privacy of their physical movements, so they have a reasonable expectation of privacy in their cell phone's CSLI records. And because you have a reasonable expectation of privacy in your cell phone records, police need to get a warrant. The government can't circumvent the warrant requirement by applying for a subpoena.

The Court of Appeals quashed the subpoena. To use Phillip's CSLI records at trial, the State would have to prepare a more detailed warrant application or go forward without them.