Drug dealers, beware. Your pay-as-you-go phones probably have GPS. And, according to a federal appeals court in Cincinnati, police can track the signal they emit without a warrant.

The U.S. Court of Appeals for the Sixth Circuit ruled that the Drug Enforcement Administration committed no Fourth Amendment violation in using a drug runner's cellphone data to track his whereabouts. The DEA obtained a court order to track Melvin Skinner's phone, after finding his number in the course of an investigation of a large-scale drug trafficking operation.

The DEA didn't know much about Mr. Skinner or what he looked like. They knew him as Big Foot, the drug mule, and they suspected he was communicating with the leader of the trafficking operation via a secret phone that had been registered under a false name. Agents used the GPS data from his throw-away phone to track him, and he was arrested in 2006 at a rest stop near Abilene, Texas, with a motorhome filled with more than 1,100 pounds of marijuana.

Mr. Skinner was convicted of drug trafficking and conspiracy to commit money laundering. On appeal, he argued that the data emitted from his cell phone couldn't be used because the DEA failed to obtain a warrant for it, in violation of the Fourth Amendment.

The question in the case was whether Mr. Skinner had a reasonable expectation of privacy in the data his phone emitted. It's a question that several courts are wrestling with. Federal law enforcement authorities, as in this case, say that investigators don't need search warrants to gather such information.