This morning, you left the house tagged with a tracking device that the government can use to find out where you have been and where you are going.

I'm talking, of course, about your cell phone. Mobile phones communicate continuously with cellular towers in order to receive calls, sending out a signal registering its existence and identity with the provider's nearest towers. The provider stores this cell-site data, which can be triangulated to determine the customer's physical location.

While most courts considering the issue have held that police need "probable cause" to track your movements, a new decision (.pdf) last week out of the U.S. District Court of Massachusetts holds that law enforcement need show only "relevance to an ongoing investigation" to get a historical record of your past movement (something like the Jeffy trail in The Family Circus cartoon).

Why are courts treating past and prospective tracking so differently, and should they?

The problem starts with a basic congressional assumption that real-time information in transit is more private than stored information – a bias that is enshrined in various laws that protect wire and electronic communications. Congress has imposed stronger limits on how real-time information is accessed and used.

The Pen Register statute grants the government access to real-time (in-transit) signaling (to/from) information upon an assertion that the information requested is "relevant to an ongoing criminal investigation." While cell-site data could be considered signaling information, Congress protected location information further under the Communications Assistance for Law Enforcement Act (CALEA), which specifically prohibits using the Pen Register statute as the sole authority for obtaining physical-location information.

So the government argues that the Stored Communications Act is additional authority for its obtaining cell-site information with a mere showing of relevance. The SCA gives the government access to stored records or other information pertaining to a subscriber of an electronic communications service (not including the contents of communications) if the government "offers specific and articulable facts showing that there are reasonable grounds to believe that ... the records or other information sought are relevant and material to an ongoing criminal investigation."

This standard is very similar to the standard for government access under the Pen Register statute, although under the SCA the government must produce evidence, and not merely certify, that the information is relevant. Still, most courts have rejected this argument, because the SCA applies only to stored information, and utterly lacks the procedural safeguards (such as time limits and provisions for sealing, renewal and periodic reports) typical of prospective surveillance statutes.

In the absence of a statute permitting government access, the majority of courts have ruled that law enforcement can only track your future cell-phone movements with a warrant based on probable cause. Some of these decisions have further suggested that a warrant is constitutionally required, regardless of what the statutes say, because you have a reasonable expectation of privacy – protected by the Fourth Amendment – for your location information.

These constitutional privacy concerns were given short shrift earlier this month in the U.S. District of Massachusetts ruling. The new case considers when the government can track your historical, rather than current, movements. The judge held that cell information which reveals your past movements is squarely covered by the SCA, so the Pen Register statute and the additional CALEA limitations do not come into play.

Further, the court rejected the idea that your past movements are protected by the Fourth Amendment. Its ruling was based on two Supreme Court cases involving tracking devices. In one, United States v. Knotts, police placed a tracking beeper in a can of chemicals to find out where the defendant was transporting it. The Supreme Court found no Fourth Amendment violation because the suspect's car was visible on public highways, no more private than it would have been to an officer's naked eye.

In contrast, in United States v. Karo, the Supreme Court held that police violated the Fourth Amendment when the tracking beacon allowed police to monitor the suspect within the protected zone of a private home. With historical tracking by cell phone, the Massachusetts district court said the case was more like Knotts than like Karo, because there was no indication the information would give away anything that happened within a protected zone.

Something is deeply screwy with this scheme. While police may be able to trail a suspect's car with the naked eye, only psychics can track our past behavior without this cell-site data. There is also a difference of scale. Allowing police to place a tracking device on a suspect's car is quite different from allowing them to track any innocent person who's activities are merely "relevant" to an investigation.

We will continue to see problems like this as courts try to apply existing privacy laws to new technologies. Congress has not proven itself to be much better at predicting technological changes and protecting privacy, either, but something should change. Otherwise, the phone in your pocket might as well be a trail of bread crumbs.

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Jennifer Granick is Civil Liberties Director at the Electronic Frontier Foundation.