Where Are You?(Giving Globally)(The Technologist; Mobile Phones)(global positioning systems in mobile phones )

Byline: Benjamin Sutherland

For only $2.99 a month, Sam Altman says he can "increase our depth of vision." Altman's Mountain View, California, company, loopt, recently inked a deal with the mobile-phone carrier Sprint that will enable a GPS cell phone to track the current location of all your friends. Once you download loopt mapping software to your phone, dots representing the current locations of your friends appear on a digital map. (The friends, of course, must first agree to be tracked.) You can zoom in and out to pinpoint their exact positions, and if one of them sends you an SMS, the message appears in a bubble connected to the corresponding dot. "This is the next level up" in social-networking sites, says Jordy Mont-Reynaud, a programmer in San Francisco. He has loopt alert him each time a buddy comes within a mile.

The service is becoming available now largely because the hardware for determining a person's "geolocation" is getting cheaper. Many cell phones contain GPS chips. Loopt and other start-ups are reluctant to disclose growth figures, but analysts say demand, already considerable, may soon spike. Like much social-networking technology, this one could be a classic "snowball" technology -- when your friends know where everybody is, you want to know, too.

Another approach is to have cell-phone users file ...