How the government can still track your vehicle without GPS

The U.S. Supreme Court unanimously ruled today that law enforcement agencies can't track a vehicle by planting a GPS device without a warrant. It's a win for civil liberties, but the court punted on the bigger question of how far police can go to follow the increasingly vivid electronic trail left by driving.

In the case decided today, federal agents planted a GPS tracker to a Jeep Grand Cherokee owned by a suspect in a drug case, following his movements for 28 days and generating enough data to fill 2,000 pages. The suspect, Antoine Jones, was later convicted of drug trafficking and sentenced to life in prison.

An appeals court sided with Jones and threw out the conviction, finding the government had violated the Fourth Amendment's bar of unreasonable search and seizure because it had not asked for a warrant before planting the GPS. The Supreme Court agreed, saying the government violated Jones' constitutional rights when it physically attached the GPS unit to his Jeep.

But the court's ruling focused on how the government attached the unit, and struggled with whether the government could have legally followed Jones if it never touched his vehicle, and only used electronics to track him. "It may be that achieving the same result through electronic means, without an accompanying trespass, is an unconstitutional invasion of privacy," Justice Anthony Scalia wrote in the majority opinion, "but the present case does not require us to answer that question."

While GPS tracking units have been used for years, the abundance of technology in and outside of vehicles means investigators have many other options -- some of which require court oversight, and others that don't.

Start with your cell phone. Criminal cases routinely use cellphone signals to prove a suspect's movements. Talking on a cellphone creates a record of your location down to a few dozen feet in most cities. Even when not in use, cellphone companies can be required under subpoenas to "ping" handsets without an owner's knowledge to mark their locations. And the GPS chips built-in to most handsets offer yet another, more detailed alternative.

Thanks to apps like Apple's "Find My Phone," cellphones can also act as tracers in emergencies — like the case of a woman in Indiana last week, whose iPhone led police to her stolen Dodge Durango within minutes of its theft from a gas station. Even when car thieves make a clean getaway, in many cities they're snagged by license plate scanners -- special cameras connected to databases that can flag stolen or suspect vehicles.

General Motors' OnStar service has long been used as a stolen-vehicle recovery device; the company faced an uproar last year when it suggested it might let third parties use location data about its subscribers. With many cellphone functions now being built into new models, including Internet access in luxury cars, a growing number of vehicles will be broadcasting a signal whenever they're turned on.

Four Supreme Court justices suggested in a separate opinion today that the key to future cases will be what people expect when they use their devices; tracking public movements would be OK, but minute-by-minute pinging for weeks or months would be unacceptable. The rest of us will keep driving with our phones, our GPS maps and our emergency roadside assistance units in the gray area between the lines.