Privacy-rights advocates had an important court victory last week, but the issue of police use of Global Positioning System tracking technology remains dangerously unsettled.

The U.S. Court of Appeals for the D.C. Circuit overturned the conviction of a cocaine dealer because prosecutors had relied heavily on information gleaned from a GPS device that was planted in his car without a warrant.

This is the first time that a court has ruled this kind of surveillance is illegal, and it has broad implications. The federal government has mandated that cell phones have GPS equipment so that public safety officials can track the origin of 911 emergency calls.

Without a clear standard about when police can use GPS for surveillance, the government would be able to track the movements of any cell phone user, creating a record of every move without every having to get a warrant.

Prosecutors in the recent appeal argued that police were getting information that anyone could have observed, so a warrant had not been necessary.

The judges, however, disagreed, finding that when you look at the totality of a person’s movements over an extended period of time, you have information that no individual could ever reasonably observe. When delving that deeply into a person’s activities, the police should first go before a judge to show why the surveillance is necessary, just as they would when getting a wiretap.

The court made the right call in this case. This issue appears headed to the U.S. Supreme Court, where we hope the justices reach the same sensible conclusion.