As her daughter enjoyed a weekend road trip, Donna Butler sat back home 120 miles away at her personal computer and watched a blue dot tick slowly across the screen.
But not slowly enough.
"They were going 85 on the interstate where the speed limit is 70," said Butler, who interrupted 17-year-old Danielle's getaway to let her know, " 'I will personally come up there and drive you home.' "
It would have been easy to find her. Whenever Danielle is away from her central Florida home, her mobile phone uses a global positioning system to transmit her precise location, which her mother can track online.
Developed originally as a military tool, GPS is used widely by drivers, hikers and boaters to figure out where they are. A new generation of relatively cheap GPS-equipped devices can tell others too -- allowing people for the first time to keep constant tabs on their rebellious teens, wandering spouses or loafing employees.
That prospect comforts mothers like Butler, but it concerns some who see ever more powerful and invasive technology eroding a sense of personal privacy.
"If your supermarket offers you the chance to take a few cents off a loaf of bread in exchange for tracking every purchase you make with one of their cards, you do it," said Jonathan Zittrain, co-founder of the Berkman Center for Internet and Society at Harvard Law School.
"TiVo quietly makes note of your TV viewing habits. Will we be willing to carry a GPS locator so we can order a pizza with the push of a button and know it's on its way right to us?"
Although GPS was added to cellphones so that 911 emergency calls could be tracked, 15 million Nextel Communications Inc. subscribers can now buy the locator service for personal or business use. Next year the approximately 23 million Sprint Corp. wireless users will be able to sign up. It costs about $15 a month to turn on the service.
Among the first to sign up was James Kinney, to keep track of workers at his Kinney Construction Inc. in Orange. His employees are required to carry the phones during the workday.
Shortly after handing out the phones last year, office manager Kristy Collins was demonstrating the system for a supervisor.
"We looked at the map on the computer that showed all the little dots where a crew was working a job," Collins said. "But one dot was way over in another spot. The guy was at home instead of on the job."
Management professor Lucas Introna, who specializes in workplace surveillance issues, said GPS tracking provided just enough information to breed discontent.
"In an office or a factory situation, a manager who might walk by has access to a whole range of situational information," said Introna, who teaches at Lancaster University in Britain. "But when a worker far away knows that every move they make is monitored by someone -- without information about just what they are doing -- it takes on a punitive sense."
Kinney didn't disagree. "The guys hate it," he conceded, even though the worker caught at home was able to show that he had gone to pick up materials needed for the job.
GPS, which uses a network of orbiting satellites to fix precise locations on Earth, was developed for the military. But as soon as the first satellite in the system was turned on in 1978, academics were testing its capabilities. By the early 1980s surveyors were using GPS in their work.
GPS has proved to be one of the most popular consumer uses of space technology. So far this year, nearly 3.9 million new cars came with factory-installed GPS navigation systems, according to research company CMS Worldwide. In 2008, that number is forecast to reach 6.5 million.
Hand-held GPS units for hikers, bicyclists and runners have steadily fallen in price and are now available for about $100.
Satellite tracking for the non-military market got its first big boost in 1988 when then-fledgling Qualcomm Inc. of San Diego introduced a system that allowed fleet managers to spot where their vehicles were anywhere in the country.
Consumer GPS tracking gear was soon to follow, popping up in shops and eventually on websites that often had "spy" as part of their names.
"I would say that 60% of my sales are to women who say, 'I think my husband is cheating on me,' " said Greg Shields of Cincinnati, who operates the Spygear Store on the Web and sells a $500 unit designed to be magnetically attached to the bottom of a vehicle. "The rest are men who want to track employees."
The unit is removed after several days and plugged into a personal computer to produce a map that can be zoomed down to the street level to show not only where the vehicle has been but also its speed and all starts and stops. Shields also sells a $1,200 device that sends the signals back to a personal computer for real-time tracking.
Customers, including a woman in Phoenix who recently bought a device from him, have been satisfied with the operation of the units if not the results.