A federal appeals court in Washington, D.C., heard arguments last week about whether police should have to get a warrant before putting a GPS device on a suspect’s car. It is a cutting-edge civil liberties question that has divided the courts that have considered it. GPS devices give the government extraordinary power to monitor people’s movements. The Washington court should rule that a warrant is required.

Antoine Jones was charged with being part of an interstate drug conspiracy. The government obtained evidence against Mr. Jones by putting a GPS device on his Jeep. It obtained a court order to install the GPS device, but the defense said the order was faulty, and tried to get the evidence collected by the device thrown out. The government responded that the evidence was admissible because it did not need to get a court order at all.

The Supreme Court has not considered the question of whether the police need a court order to install a GPS device. The government has tried to draw an analogy to a 1983 case in which the court ruled that the police do not need a warrant to use a radio beeper to track a vehicle on public roads, but the circumstances were different. In that case, the police were conducting visual surveillance of a particular suspect’s movements, and a beeper augmented the officers’ senses. A modern GPS device is a far more potent means of tracking people than a beeper.

Lower courts have reached different conclusions. A panel of the Chicago-based United States Court of Appeals for the Seventh Circuit ruled in 2007 that a warrant is not required for remote surveillance by a GPS device, although it said that if the police began to use the technique on a large scale it might violate the Fourth Amendment.