The US Court of Appeals for the District of Columbia has rejected the federal government's contention that agents can conduct continuous GPS tracking of suspects without a warrant. In its ruling last Friday, the court held that such warrantless surveillance violated the Fourth Amendment's proscription against unwarranted searches and seizures.

The ruling came in US v. Maynard, in which two Washington, DC men, Antoine Jones and Lawrence Maynard, were convicted in a joint trial in 2008 of possessing and conspiring to distribute more than 50 pounds of cocaine. The men appealed their convictions, arguing that the government's evidence against them had come from a GPS device unlawfully attached to Jones' Jeep that tracked his movement continually, day and night, for a full month. The use of the device without a warrant violated their rights against unreasonable search and seizure, the pair successfully argued.

"It is one thing for a passerby to observe or even to follow someone during a single journey as he goes to the market or returns home from work," Judge Douglas Ginsburg wrote on behalf of the three-judge panel that reviewed the case. "It is another thing entirely for that stranger to pick up the scent again the next day and the day after that, week in and week out, dogging his prey until he has identified all the places, people, amusements, and chores that make up that person's hitherto private routine."

Federal prosecutors argued that several other appellate courts had allowed the use of GPS devices without a warrant, but the DC appeals court held that those cases had not involved extended continuous surveillance. That sort of unfettered use of GPS can reveal personal information and threaten one's reasonable expectation of privacy, the court held.

"Prolonged GPS monitoring reveals an intimate picture of the subject's life that he expects no one to have short perhaps of his spouse," Judge Ginsburg wrote. "The intrusion such monitoring makes into the subject's private affairs stands in stark contrast to the relatively brief intrusion at issue," in the other cases.

The Electronic Frontier Foundation (EFF) and the American Civil Liberties Union (ACLU) both filed amicus briefs urging the court to strike down the practice. The two groups welcomed the court's decision.

"The court correctly recognized the important differences between limited surveillance of public activities possible through visual surveillance or traditional 'bumper beepers,' and the sort of extended, invasive, pervasive, always-on tracking that GPS devices allow," said EFF civil liberties Director Jennifer Granick. "This same logic applies in cases of cell phone tracking, and we hope that this decision will be followed by courts that are currently grappling with the question of whether the government must obtain a warrant before using your cell phone as a tracking device."

"GPS tracking enables the police to know when you visit your doctor, your lawyer, your church, or your lover," said Arthur Spitzer, legal director of the ACLU-National Capital Area. "And if many people are tracked, GPS data will show when and where they cross paths. Judicial supervision of this powerful technology is essential if we are to preserve individual liberty. Today's decision helps brings the Fourth Amendment into the 21st Century."