A US appeals court ruled Friday that police cannot track a suspect using GPS technology without a warrant, tossing out the conviction of a man arrested for drug dealing.

The US Court of Appeals in Washington overturned the 2008 conviction of Antoine Jones, saying police violated his constitutional rights by tracking his movements with a satellite navigation system device installed on his vehicle without a warrant.

The three-judge panel ruled that Washington police effectively established their case by tracking Jones to locations of drug activity.

"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," the court opinion said.

"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."

The judges said the use of GPS, or Global Positioning System tracking, was a violation of the constitutional guarantee in the Fourth Amendment of unreasonable searches and seizures, saying it was an "intrusion" into "private affairs."

"This case itself illustrates how the sequence of a person's movements may reveal more than the individual movements of which it is composed," the court said.

"Having tracked Jones's movements for a month, the government used the resulting pattern -- not just the location of a particular 'stash house' or Jones's movements on any one trip or even day -- as evidence of Jones's involvement in the cocaine trafficking business."

They said the pattern of GPS data "was central" to the case against him.

Privacy activists applauded the 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 Jennifer Granick of the Electronic Frontier Foundation, which filed a brief in the case.

"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."

Arthur Spitzer of the American Civil Liberties Union said the decision strikes a blow in favor of privacy.

"GPS tracking enables the police to know when you visit your doctor, your lawyer, your church, or your lover," he said.

"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."

The court upheld the conviction of another man in the case, Lawrence Maynard, saying the GPS tracking was not central to his case.