(Originally posted on Daily Kos.)

We recently submitted a brief (PDF) to the 3rd Circuit Court of Appeals in a case about the legal limits of the government using people's cell phones to monitor their whereabouts, a.k.a. "cell phone tracking." The case is about what requirements must be met in order for the governmentto obtain a record of someone's past movements from the telephone company. The government argues (PDF) that it is entitled to this information whenever it shows the information is "relevant and material to an ongoing criminal investigation." The ACLU and our coalition partners the Electronic Frontier Foundation, the Center for Democracy & Technology and the ACLU of Pennsylvania argue that judges should be free to require the government to show probable cause, which we believe is required by the Fourth Amendment.

If you're like most people, you're probably not aware that your cell phone can be used to track your movements. It can. If your phone is fairly new, the odds are it has a GPS chip inside. The same technology that allows car navigation systems to know where your car is can be used to track your movements through your cell phone. But even if your phone doesn't have a GPS chip, it still has to connect to the cell phone network somehow. It transmits to the nearest cell network tower and, because the location of those towers is known, it's possible to approximate the location of you and your phone.

If you carry your phone with you, then you can be tracked. Your phone can be used to map your movements in real time as you move around. Depending on how long your phone company retains location information, it may also be possible for the government to obtain records from your phone company showing where you've been over the past weeks, months, and possibly even years.

The ACLU believes that people have a reasonable expectation of privacy in their movements, and that the Fourth Amendment requires the government to get a warrant before obtaining this sort of tracking information. Unfortunately, the government disagrees. In court papers filed by the Obama Justice Department, the government argued that "there is no reasonable expectation of privacy in such information, and, accordingly, no Fourth Amendment-protected privacy interest."

The government made a similar argument (PDF) back when President Bush was in charge, albeit more bluntly: "One who does not wish to disclose his movements to the government need not use a cellular telephone."

The ACLU is fighting hard to ensure that carrying a cell phone doesn't mean sacrificing your privacy. In addition to this brief, we have also filed a lawsuit asking a federal court to require the Justice Department to disclose records revealing the full extent of its warrantless cell phone tracking. With over 260 million people owning cell phones (or similar devices), this is a privacy issue that affects us all.

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