A federal appeals court ruled for the first time Friday that police cannot use a Global Positioning System device to track a person's movements for an extended time without a warrant, clearing the way for the Supreme Court to decide the privacy impact of the new surveillance technology in products such as cellphones and vehicle-navigation systems.

The decision, by a three-judge panel of the U.S. Court of Appeals for the D.C. Circuit, created a split with federal circuit courts in New York and California that have upheld warrantless GPS tracking of a vehicle by law enforcement. Feeding the national debate, a half-dozen state courts have issued conflicting rulings, while police across the country embrace GPS tools in hunting drug dealers, sexual predators and violent criminals.

In striking down the drug conviction of Antoine Jones, former co-owner of a District nightclub called Levels, the D.C. court said the FBI and District police overstepped their authority by tracking his movements round-the-clock for four weeks, placing a GPS monitoring device on his Jeep after an initial warrant had expired.

U.S. Circuit Judge Douglas H. Ginsburg, writing for a unanimous and ideologically diverse panel that included judges David S. Tatel and Thomas B. Griffith, said such surveillance technology represents a leap forward in potential government intrusion that violates constitutional protections against unreasonable searches.

"A single trip to a gynecologist's office tells little about a woman, but that trip followed a few weeks later by a visit to a baby supply store tells a different story," Ginsburg wrote.

He added, "A person who knows all of another's travels can deduce whether he is a weekly churchgoer, a heavy drinker, a regular at the gym, an unfaithful husband, an outpatient receiving medical treatment, an associate of particular individuals or political groups -- and not just one such fact about a person, but all such facts."

Bill Miller, spokesman for U.S. Attorney Ronald C. Machen Jr. of the District said, "We're studying the opinion and have no further comment."

Jones's attorney, Stephen Leckar, along with the American Civil Liberties Union of D.C. and the Electronic Frontier Foundation, which filed friend of the court briefs, called the case an important constitutional precedent ready for Supreme Court review.

"This case is really a big step toward bringing the Fourth Amendment into the 21st century," said Arthur Spitzer of the D.C. ACLU. "The technology of the 21st century needs to be judged on its own terms, and not in terms of what some early 20th-century technologies meant."

Kevin Bankston, senior staff attorney for the Electronic Frontier Foundation, said the case has important implications for cellphone GPS tracking. The federal government has mandated that U.S. cellphone carriers make nearly all their phones trackable for help in 911 emergencies. However, companies say that the federal law that allows them to turn over data to law enforcement without subpoenas is prone to abuse.

Although federal magistrate judges typically require warrants for GPS-enabled cellphone tracking, the issue is before a federal circuit court for the first time in Philadelphia, Bankston said.

In the Jones vehicle-tracking case, civil libertarians say police should have obtained a judge's approval for a warrant based on probable cause that he was committing a crime. Police argue that officers can freely trail a person on public thoroughfares, and using technology to do the same thing saves taxpayer money and police resources.

The Supreme Court in 1983 held that the use without a warrant of a "beeper"-like transponder to track five gallons of chemicals carried by a suspect in his car from Minneapolis to a drug lab at a lakeside cabin in Wisconsin was permissible. The car's driver had no expectation of privacy because he drove on public roads and "voluntarily conveyed to anyone who wanted to look" where he was going.

However, the Court also noted that evidence about the suspect's car monitored only a single one-way trip, and withheld judgment about whether a warrant would be needed for "dragnet-type" or "twenty-four hour surveillance" by law enforcement. In Friday's opinion, the D.C. appellate judges focused on the unprecedented reach of new technology, making surveillance possible continuously and cheaply.

"Practical considerations prevent visual surveillance from lasting very long. Continuous human surveillance for a week would require all the time and expense of several police officers," Ginsburg wrote.