On the train returning to Armonk, N.Y., from a recent shopping trip in Manhattan with her friends, Britney Lutz, 15, had the odd sensation that her father was watching her.

He very well could have been. Ms. Lutz's father, Kerry, recently equipped his daughters with cellular phones that let him see where they are on a computer map at any given moment. Earlier that day, he had tracked Britney as she arrived in Grand Central Terminal. Later, calling up the map on his own cellphone screen, he noticed she was in SoHo.

Mr. Lutz did not happen to be checking when Britney developed pangs of guilt for taking a train home later than she was supposed to, but the system worked just as he had hoped: she volunteered the information that evening.

"Before, they might not have told me the truth, but now I know they're going to," said Mr. Lutz, 46, a lawyer who has been particularly protective of Britney and her sister, Chelsea, 17, since his wife died several years ago. "They know I care. And they know I'm watching."

Driven by worries about safety, the need for accountability, and perhaps a certain "I Spy" impulse, families and employers are adopting surveillance technology once used mostly to track soldiers and prisoners. New electronic services with names like uLocate and Wherify Wireless make a very personal piece of information for cellphone users — physical location — harder to mask.

But privacy advocates say the lack of legal clarity about who can gain access to location information poses a serious risk. And some users say the technology threatens an everyday autonomy that is largely taken for granted. The devices, they say, promote the scrutiny of small decisions — where to have lunch, when to take a break, how fast to drive — rather than general accountability.

"It's like a weird thought I get sometimes, like `he definitely knows where I am right now, and he's looking to see if I'm somewhere he might not approve of,' " said Britney Lutz. "I wonder what it will be like when I start to drive."

Still, personal location devices are beginning to catch on, largely because cellular phones are increasingly coming with a built-in tether. A federal mandate that wireless carriers be able to locate callers who dial 911 automatically by late 2005 means that millions of phones already keep track of their owners' whereabouts. Analysts predict that as many as 42 million Americans will be using some form of "location-aware" technology in 2005.

Wireless companies and start-up firms are weaving the satellite system known as G.P.S., or Global Positioning System, which was begun by the United States military in the 1970's, into the cellular phone network and the Internet to sell products and services that provide location information.

After fixing an individual's location relative to a network of G.P.S. satellites orbiting 12,000 miles above the earth — or, more crudely, by the time it takes signals to bounce off nearby cell towers — personal locator services transmit the constantly updated information to a central database, where customers can retrieve it through the Internet, telephone or pager.

Until recently, one of the main civilian uses of G.P.S. was in devices issued by the criminal justice system to track offenders as a condition of their parole or probation. The new generation of tracking devices has moved well beyond that population and now takes many forms, from plastic bracelets that can be locked onto children to small boxes with tiny antennae that can be placed unobtrusively in cars.

"We are moving into a world where your location is going to be known at all times by some electronic device," said Larry Smarr, director of the California Institute for Telecommunications and Information Technology. "It's inevitable. So we should be talking about its consequences before it's too late."

Some of those consequences have not been spelled out. Will federal investigators be allowed to retrieve information on your recent whereabouts from a private service like uLocate, or your cellular carrier? Can the local Starbucks store send advertisements to your phone when it knows you are nearby, without your explicit permission?