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The good news is, all nine justices of the United States Supreme Court agree that the police need a proper search warrant if they are going to secretly attach a GPS tracking device to your car for the purpose of gathering evidence against you.

The bad news is, the majority opinion was written by a justice who thinks the Fourth Amendment only protects you from things that involve physical contact with you or your property. And he thinks that because, when the Bill of Rights was ratified, in 1791, such a physical invasion was all that the language could have referred to.

So, rather that make it clear to law enforcement agencies at all levels that a search is a search is a search, the ruling handed down in the voice of Justice Antonin Scalia will leave both prosecutors and defense lawyers wondering about just how much electronic intrusiveness the law will allow.

The hopeful news is that, in their various concurring opinions, a bare majority of five other justices noted that Scalia's original-intent approach is wholly unequal to the task of applying 18th century ideals to 21st century inventions.

In the case at hand, federal and local investigators allowed their search warrant to expire before actually attaching a tiny GPS gadget to a Jeep owned by one Antoine Jones, a D.C. nightclub owner suspected of trafficking in narcotics. The electronic trail he left was part of the evidence used to convict him — and hand down a life sentence.

But the court of appeals and, now, the Supreme Court later ruled that the verdict must be tossed out because the tracking method used amounted to a warrantless search.

The effect of the ruling is the right one. Police who want to know where you go 24 hours a day, yet aren't worried enough about your immediate threat to society to put an around-the-clock tail on you, should have to convince a judge that they already have enough reason to suspect you of committing a specific crime to justify a search warrant. It is not only a basic tenet of Anglo-American law, it is easy enough to do that there is no good reason not to require it.

The problem is that Scalia's majority ruling deliberately decided not to decide what limits, if any, apply to modern surveillance methods that do not require anyone to stick a bug on your car. It leaves the door open to all manner of Star Trek gizmos that would allow police to delve into your every move, every financial transaction, every communication, just because they felt like it.

Our only hope, for now, is that police agencies tempted to so ferret into your actions and associations will understand that the Supreme Court is still looking over their shoulder.