Can you be found anywhere, anytime?

Back on the East Coast, Skyhook Wireless in Boston has latched onto the same concept but is using another signal to do the job: wi-fi. Wireless fidelity is the opposite end of networking from GPS. Instead of a few big satellites in orbit, wi-fi boasts millions of small low-power transmitters that allow users to log onto the Internet wirelessly. The operating range of these signals is rarely more than a couple of hundred feet, but that's enough for them to spill out of buildings onto the streets. Skyhook says it has mapped the exact location of more 1.5 million wi-fi signals in the 25 largest American cities. Many of them overlap.

Walking down the street in Boston, for example, "you're pretty much being buried in five or 10 access points at any given time," says Ted Morgan, president and founder of Skyhook.

Just as with GPS or TV, multiple wi-fi signals can be used to determine a location, Mr. Morgan says. "Every positioning system uses the same concept, that if you have three or more reference points, you can use math to figure out where you are."

Skyhook doesn't log onto anyone's computer, he's careful to point out. "It's completely passive," he says. "We're just catching waves going around."

As with TV signals, wi-fi blankets urban areas, a nice complement to the strength of GPS in open territory.

Morgan expects GPS and wi-fi systems to appeal to different needs. But some users, such as companies with fleets of delivery trucks, may want to combine them for maximum coverage. UPS, for example, already equips its trucks with a powerful combination of Bluetooth (very short range wireless), wi-fi, cellular, and GPS technologies to collect and transmit data about locations and deliveries. "I think ultimately what you'll have is a collection of technologies all leveraging each other's strengths," Morgan says.

These advances also may be used to invade a person's privacy. As a result, Americans need to be more vigilant, some experts say. As GPS and other tracking technologies become more accurate and less expensive, people need to guard against "usage creep" from good purposes, like 911 calls, to less good ones, says Annalee Newitz, a policy analyst at the Electronic Freedom Foundation in San Francisco. Information about the whereabouts of people is going to be available, she adds. "We can't shake our fists and say, 'We don't want that!' when it's already happened," Ms. Newitz says.

Regardless, people already should be questioning who has access to their location data and for what purpose. "We want to be sure this data is being disposed of as quickly as possible," she says.

• 1978: The US military launches first of what eventually become 24 GPS satellites.

• 1982: Russia launches its competing GLONASS system.

• 1983: President Ronald Reagan offers first GPS for civilian aircraft, bringing it out of military realm.

• 1999: US amends rules so GPS can locate cellphones.

Sources: RAND Corporation; FindLaw; Britannica Online

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