(Originally posted on Philly.com)

If you own a cell phone, you should care about the outcome of a case scheduled to be argued in federal appeals court in Philadelphia tomorrow. It could well decide whether the government can use your cell phone to track you — even if it hasn't shown probable cause to believe it will turn up evidence of a crime.

The American Civil Liberties Union, the Electronic Frontier Foundation and the Center for Democracy and Technology will ask the court to require that the government at least show probable cause before it can track your whereabouts.

Although most people don't realize it, cell phones double as tracking devices. Newer phones contain GPS chips, the same technology that allows car navigation systems to know where you are and give directions ("Turn right now"). But even older phones that don't have chips can be tracked by knowing the location of the cell towers they use to connect to a network.

There's no question that cell phones and cell-phone records can be useful for police officers who need to track the movements of those they believe to be breaking the law. And it is important for law enforcement agents to have the tools they need to stop crimes. However, it is just as important to make sure such tools are used responsibly, in a manner that safeguards our personal privacy.

But documents obtained by the ACLU and the EFF as part of a Freedom of Information Act lawsuit show that the government takes advantage of this technology to track cell phones as extensively as possible — often without first obtaining warrants — except in states where courts step in to establish boundaries.

In fact, this issue gained national attention during last year's gubernatorial race in New Jersey. Documents turned over in our lawsuit revealed that the U.S. Attorney's Office — under Chris Christie, now the governor — was tracking cell phones without probable cause, in violation of a Justice Department recommendation.

The decision reached by the Philadelphia-based 3rd Circuit U.S. Court of Appeals will not only bind federal courts throughout Pennsylvania, New Jersey, and Delaware. It will also be a key source of guidance to courts around the country as they grapple with this issue.

We hope the court will send a message that merely carrying a cell phone should not make people more susceptible to government surveillance. No one wants to feel as if a government agent is following her wherever she goes — be it a friend's house, a place of worship, or a therapist's office — and innocent Americans shouldn't have to feel that way.

The government has argued that "one who does not wish to disclose his movements to the government need not use a cellular telephone." This is a startling and dismaying statement coming from the United States. The government is supposed to care about people's privacy. It should not be forcing the nation's 277 million cell-phone subscribers to choose between risking being tracked and going without an essential communications tool.

What's at stake in the case is not whether it's OK for the government to track the locations of cell phones; we agree that cell-phone tracking is lawful and appropriate in certain situations. The question is whether the government should first have to show that it has good reason to think such tracking will turn up evidence of a crime.

We believe it should. This case is not about protecting criminals. It's about protecting innocent people from unjustified violations of their privacy.

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