With mobile phones becoming as prevalent as conventional phones (there are 195 million cellular subscribers in this country), wireless companies are starting to exploit the phones' tracking abilities. For example, companies are marketing services that turn phones into even more precise global positioning devices for driving or allowing parents to track the whereabouts of their children through the handsets.

Not surprisingly, law enforcement agencies want to exploit this technology, too -- which means more courts are bound to wrestle with what legal standard applies when government agents ask to conduct such surveillance.

Cellular operators like Verizon Wireless and Cingular Wireless know, within about 300 yards, the location of their subscribers whenever a phone is turned on. Even if the phone is not in use it is communicating with cellphone tower sites, and the wireless provider keeps track of the phone's position as it travels. The operators have said that they turn over location information when presented with a court order to do so.

The recent rulings by the magistrates, who are appointed by a majority of the federal district judges in a given court, do not bind other courts. But they could significantly curtail access to cell location data if other jurisdictions adopt the same reasoning. (The government's requests in the three cases, with their details, were sealed because they involve investigations still under way.)

"It can have a major negative impact," said Clifford S. Fishman, a former prosecutor in the Manhattan district attorney's office and a professor at the Catholic University of America's law school in Washington. "If I'm on an investigation and I need to know where somebody is located who might be committing a crime, or, worse, might have a hostage, real-time knowledge of where this person is could be a matter of life or death."