A federal appeals court on Friday reversed a man's drug conspiracy conviction because authorities failed to get a warrant before using a global positioning system device that tracked his movements for a month.

Lawyers for the defendant, Antoine Jones, successfully argued that police officers violated the Fourth Amendment prohibition against unreasonable searches with the GPS system they secretly installed on his car.

In a 3-0 ruling, the U.S. Court of Appeals for the District of Columbia Circuit found that the use of the GPS device was a search and that the GPS data were essential to the government's case.

By combining GPS information "with Jones' cell-phone records the government was able to paint a picture of Jones' movements that made credible the allegation that he was involved in drug trafficking," said the decision by Appeals Judge Douglas Ginsburg.

The judges concluded that the police action was a search that first required a warrant because it defeated Jones' reasonable expectation of privacy.

Jones, who was owner of a Washington, D.C., nightclub, was convicted of conspiracy to distribute and to possess with intent to distribute cocaine and cocaine base.

In closing arguments to the jury, a prosecutor made use of the GPS data, arguing that it showed the defendant going to a certain address and that "there's no reason anyone goes there other than drug activity."

Ginsburg added that "the government had also stressed in its opening remarks, which would color the jury's understanding of the whole case, that the GPS data would demonstrate Jones' involvement in the conspiracy."

The judges rejected the government's argument that the court need not consider whether Jones' expectation of privacy was reasonable. The government based its unsuccessful argument on a 1983 Supreme Court ruling that the use of a beeper device to aid in tracking a suspect to his drug lab was not a search.

In Jones' case, the appeals court said the circumstances were far different because they involved prolonged surveillance.

"Here the police used the GPS device not to track Jones' movements from one place to another, but rather to track Jones' movements 24 hours a day for 28 days as he moved among scores of places, thereby discovering the totality and pattern of his movements from place to place to place," wrote Ginsburg.

"Society recognizes Jones' expectation of privacy in his movements over the course of a month as reasonable, and the use of the GPS device to monitor those movements defeated that reasonable expectation," the court ruled.

Ginsburg, the author of the opinion, is an appointee of President Reagan. The other judges on the case were David Tatel, an appointee of President Bill Clinton, and Thomas Griffith, an appointee of President George W. Bush.

"The case brings the Fourth Amendment into the 21st century," said attorney Dan Prywes, who filed a brief in the case with the American Civil Liberties Union of the National Capital Area and the Electronic Frontier Foundation.

Prywes said the ruling will prevent the police from "arbitrarily tracking anyone they wish for weeks or months on end."