In a CNN essay, the ACLU's Catherine Crump notes that if police can use GPS devices to track vehicles without obtaining a warrant—a question that is currently before the Supreme Court in the case U.S. v. Jones—they may not need judicial authorization to make similar use of people's mobile phones:

Everyone with a cell phone is already carrying a device that the government can use to track his or her location….

It doesn't matter whether your phone is a smartphone or whether you use it to make calls; as long as your phone is turned on, it registers its location with cell phone networks several times a minute, and all U.S. cell phone companies hold on to that data, some of them for years.

This kind of tracking is extremely invasive, because if the government knows where you are, it knows who you are. As the Jones appellate court explained in its ruling that the government violated the Fourth Amendment, "A person who knows all of another's travels can deduce whether he is a weekly churchgoer, 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."

Cell phone tracking can reveal our private associations and relationships with one another. The government could make note of whenever people being tracked crossed path or spent time together, showing who our friends, associates and lovers are.

The Justice Department sometimes gets warrants to track location, and some local police departments make it a policy, which shows that it's not an unworkable requirement. But state and federal judges across the country have made conflicting rulings on what standards are required for the government to obtain tracking information from cell phone companies.